Svetan Ratchev (Ed.) **IFIP AICT 620**

# **Smart Technologies for Precision Assembly**

9th IFIP WG 5.5 International Precision Assembly Seminar, IPAS 2020 Virtual Event, December 14–15, 2020 Revised Selected Papers

# IFIP Advances in Information and Communication Technology 620

Editor-in-Chief

Kai Rannenberg, Goethe University Frankfurt, Germany

### Editorial Board Members

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More information about this series at http://www.springer.com/series/6102

Svetan Ratchev (Ed.)

# Smart Technologies for Precision Assembly

9th IFIP WG 5.5 International Precision Assembly Seminar, IPAS 2020 Virtual Event, December 14–15, 2020 Revised Selected Papers

Editor Svetan Ratchev University of Nottingham Nottingham, UK

ISSN 1868-4238 ISSN 1868-422X (electronic) IFIP Advances in Information and Communication Technology ISBN 978-3-030-72631-7 ISBN 978-3-030-72632-4 (eBook) https://doi.org/10.1007/978-3-030-72632-4

© The Editor(s) (if applicable) and The Author(s) 2021. This book is an open access publication.

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# Preface

Since its inception in 2003 the International Precision Assembly Seminar (IPAS) has created a strong international community which meets regularly to share new developments in a wide range of assembly theory and practice, report exciting research results and discuss high-impact industrial applications. The seminar also provides a forum for debating new ideas and formulating new lines of research.

This book includes a selected set of papers presented at the 9th IPAS, which was originally planned to take place in the beautiful winter surroundings of Kitzbühel, Austria between 15 and 17 March 2020. Due to the Covid-19 pandemic, it was instead held as a virtual seminar on 14 and 15 December 2020.

The main theme of the 9th IPAS seminar was the application of smart technologies for precision assembly with specific emphasis on human-centred assembly processes and systems. The book is structured into six sections: keynote paper, assembly design and planning, assembly operations, design of assembly cells and systems, human-centred assembly and assistive methods for assembly. The selected papers reflect on the dramatic impact of digital technologies in assembly with specific focus on how such technologies can be leveraged to improve assembly process performance and promote new types of human-in-the-loop assembly systems.

The seminar was sponsored by the International Federation of Information Processing (IFIP) WG5.5 and the International Institution of Production Engineering Research (CIRP). The organisers would like to express their gratitude to the members of the International Advisory Committee for their support and guidance and to the authors of the papers for their original contributions and enthusiastic and active participation in the seminar. We would like to thank Professor Hamideh Afsarmanesh, chair of the IFIP WG 5.5, and Prof. Dr.-Ing. Jörg Krüger, chair of the STC-A of CIRP, for their continuous support, encouragement and sponsorship of the seminar. Our special thanks also go to Nancy Martin for her exceptional handling of the administrative aspects of the seminar and for managing the communication with all delegates, organisers and publishers in what has been a very challenging time.

January 2020 Svetan Ratchev Christian Wögerer

# Organization

#### International Advisory Committee

J. Barata UNINOVA, Portugal A. Delchambre ULB, Belgium M. Desmulliez Heriot-Watt University, UK M. Dickerhof Karlsruhe Institute for Technology, Germany F. Dietrich TU, Berlin G. Dini University of Pisa, Italy A. Drouot University of Franche-Comté, France K. Ehmann Northwestern University, USA I. Fassi STIIMA, Italy P. Felli Free University of Bozen-Bolzano, Italy M. Krieger CSEM, Switzerland P. Lambert ULB, Belgium M. Lanz Tampere University, Finland R. Leach University of Nottingham, UK P. Lutz ENSMM, France B. Nelson ETH, Switzerland M. Onori KTH, Sweden D. Pham University of Birmingham, UK G. Putnik University of Minho, Portugal A. Raatz Leibniz University of Hannover, Germany J. Segal University of Nottingham, UK M. Tichem TU Delft, Netherlands T. Tolio Milan Polytechnic, Italy

# Contents

#### Keynote Paper

and Jeremy A. Marvel

#### Assembly Cells and Systems

Stefan Thalhammer, Timothy Patten, and Markus Vincze

#### Assistance Methods in Assembly

# **Keynote Paper**

# **Augmented Reality in Assembly Systems: State of the Art and Future Perspectives**

M. Dalle Mura and G. Dini(B)

Department of Civil and Industrial Engineering, University of Pisa, 56122 Pisa, Italy<EMAIL_ADDRESS>
**Abstract.** Assembly represents a fundamental step in manufacturing, being a time-consuming and costly process, on which the final quality of the product mostly depends. Augmented Reality (AR) may represent a key tool to assist workers during assembly, thanks to the possibility to provide the user of realtime instructions and information superimposed on the work environment. Many implementations have been developed by industries and academic institutions for both manual and collaborative assembly. Among the most remarkable examples of the last few years are applications in guidance of complex tasks, training of personnel, quality control and inspection. This keynote paper aims to provide a useful survey by reviewing recent applications of AR in assembly systems, describing potential advantages, as well as current limitations and future perspectives.

**Keywords:** Augmented reality · Assembly systems · Manual assembly · Collaborative assembly

# **1 Introduction**

Assembly represents a key element in the whole fabrication process. The total cost of a product, the time requested for completing it and its quality strictly depend on the efficiency and accuracy of execution of the different assembly steps.

Very often, these operations can be complex and require fine adjustments in order to obtain an acceptable final result. The sequence can be lengthy, with many parts to be assembled together in a specific order, so as to assure the product to function as it should.

For these reasons, the workers should be skilled and trained to do that within the cycle time imposed by the production rate. In many cases, the sequence can be dependent on the variant to be assembled and can request the consultation of paper manuals, reference tables that could lead to time inefficiencies, distractions and, therefore, safety problems.

Typically, in manual assembly, the tasks are performed by human operators aided by tools or by semi-automated machines. Set up and maintenance operations should be provided and planned in advance for all equipment, so as to allow workers a proper use and avoid problems related to unexpected breakdowns, with consequent inactivity of the production system.

A relevant problem in assembly is also represented by human errors. Obviously, an error should be prevented by using different methods such as training sessions, sensing devices or poka-yoke solutions. These approaches are often expensive and in many cases do not give a complete assurance of avoiding these inconveniences. Human errors in assembly may result in increased production wastes or higher processing time and costs, also worsening the quality level of products because of manufacturing defects. To avoid possible damage to the entire production system, the probability of human errors in assembly should be reduced.

That said, the need of integrating the traditional manual assembly with a tool able to enhance the efficiency and the efficacy of the process becomes evident. The operator must be supported and guided in his/her actions through the accomplishment of assembly operations by non-invasive devices, which allow the user not to deflect attention and distract from the process. The tool should also be connectable to the hardware employed in the system, such as sensors, and able to access and consult software databases, in order to help the operator in executing the tasks as efficiently and effectively as possible.

Augmented Reality (AR) technology could be particularly suitable to address these issues. The possibility of replacing paper manuals with interactive instructions allows the worker to interact with both the real environment and the virtual information, also receiving a feedback on the operations made.

### **2 Basics on Augmented Reality**

Augmented Reality (AR) can be defined as a hardware-software system able to overlay virtual images or objects on the real world visual, in order to give to the observer information that he could not obtain by using only the interaction of his/her physical senses with the workplace.

The main objective of an AR system is therefore to create virtual elements in the optical path linking the user's eyes with the real scene in front of him/her. To do that, the following steps have to be implemented:

#### **2.1 AR Techniques**

The composite image can be generated using 3 different approaches which deeply characterize and influence the performance of a specific application:

**Fig. 1.**  This device represents a very important element of the whole AR system since it constitutes the mean that gives to the human the perception of an augmented workplace.

In order to fulfill the previous conditions, an AR device should be:

Many solutions have been proposed in the last decade by different manufacturers and tested by researchers and industries. Four categories can be identified to be used in a workplace, depending on their position with respect to the user and to the real environment:

Some of these types of devices are best suited to operate with certain visualization approaches of AR contents, as presented in Table 1. In particular HMDs can effectively operate either in video see-through and optical see-through modes.

**Table 1.** AR devices capability to be used in the three composite image generation approaches.

Other manufacturers are proposing innovative and more powerful devices, nowadays under development, such as AR contact lenses (or smart lenses) to be used as accessory to smartphones or as a separate device. Smart contact lens technology aims to project images into the wearer's eyes, through little display, camera and antenna inside the lens, where sensors are controlled by wearer's blinking [1]. Other researches are working on the development of new AR hardware, such as head-worn projectors [2] and Virtual Retinal Displays (VRD), which create the augmented images by projecting laser light directly into the worker's eye.

These last solutions, even if they aim at obtaining a more immersive impact to the user, at the current stage of development are far from a practical use and the absence of risks for the worker has to be proved.

#### **2.3 AR Applications in Production Engineering**

In recent years, AR techniques are spreading out in any field, from retail to military, from healthcare to education, gaming, travel, tourism, etc.

Production engineering, in particular, represents an area having a more and more increasing interest in this kind of tools able to be used in many activities, such as:

As said before, assembly represents a typical area of production engineering where this kind of technique may be profitably used, due to the strong need of supports and interactive tools in performing complex and articulated tasks, which can be encountered in assembly procedures.

The aim of this survey is therefore to provide a comprehensive insight on the state of the art of AR applications in assembly systems, as well as some current open issues that need to be overcome.

#### **3 Applications in Manual Assembly Systems**

Many applications have been experimented in manual assembly systems, mainly to support operators in performing correctly an assembly sequence. Extensive surveys have been published in 2012 and 2016 [4, 5], which demonstrate that from the beginning of this century, augmented reality is receiving increasing attention by researchers working on assembly problems.

In this wide literature scenario, a first rough subdivision can be made considering that the contributions can be addressed to "real industrial" and to "demonstrative" cases. The former concerns the implementation of an AR systems for supporting a real industrial problem (e.g.: assembly of an engine or a car body) and represents the main topic of this survey paper, the latter represents the experimental demonstration of procedures by using simple parts or toys, used as benchmarks.

These last contributions are often used to test specific hardware or software, such as in [6, 7], where the use of a smartphone, a Microsoft HoloLens and an Epson Moverio BT-200 smart glasses are compared with the use of paper-based instructions. Nevertheless, the contribution demonstrates that, on the one hand, users are able to solve the task of assembly a Lego Duplo set fastest when using paper instructions, but, on the other, made fewer errors with AR support (Fig. 2).

**Fig. 2.** HMD used to support simple assembly tasks of LEGO® bricks (source: [7]).

**Fig. 3.** Guidance in taking an object from a bin (source: [9]).

Other "non-industrial" cases often concern the assembly of furniture, which are very good as demonstrators, such as the research described in [8], where Microsoft HoloLens are used. The tests show that good results are achieved for assembly with high complexity.

#### **3.1 Guidance in Assembly Tasks – Optical and Video See-Through Approaches**

Guidance in performing assembly tasks undoubtedly represents the typical objective of many AR applications. In most cases, optical or video see-through approaches are used to guide assembly operations.

An example of a system used to substitute paper list in assembly operations is given in [9] and illustrated in Fig. 3. The AR application is developed for a Microsoft HoloLens hardware and the information is presented to the worker via a head-mounted display. The system is able to show a virtual "tunnel" indicating to the worker the bin where the part is placed, demonstrating its time-efficiency both for single kit and batch preparation.

Other applications concern the use of AR devices for guiding the correct execution of assembly operations. In [10], a prototype system is developed for providing different modality of guidance based on the different cognition phases of the user, i.e. perception, attention, memory and execution, in order to give appropriate information during assembly tasks. The prototype adopts an HMD and a 3D bare-hand interface, free from any devices, such as mouse or keyboard. In experiments concerning a motorbike alternator assembly, users proved to execute operations more quickly and accurately in comparison to a digital documentation on an LCD.

The study presented in [11] is focused on an AR video see-through support that associates different visual features to explain assembly operations with different difficulty level: the more the task to perform is complex, the easier to understand is the instruction given by the visual feature (Fig. 4). The results show that the method outperforms paper instructions.

A voice-based interaction technique is integrated with an augmented reality system for car assembly and disassembly in the work proposed in [12]. The speech signal is used to interact with the augmented reality environment, made of the virtual car's components visualized in the real scene. The voice commands, converted into textual commands, leave both hands free, so as to allow the user to manipulate the various components.

**Fig. 4.** Example of instructions given to the worker concerning: a) a scheme of the assembly operation; b) the assembly location; c) information regarding the next step; d) the next part to assemble (source: [11]).

In addition to hearing, AR has been combined with other human senses to overcome some drawbacks that may occur when only using this technology. An example is presented in [13], where a real-time vibrotactile guidance method is developed to support assembly tasks. The system uses a web camera, located on top of the workstation, to recognize components and user hands, and a vibrotactile wristband to give the user a feedback on the action made in the form of vibration.

#### **3.2 Guidance in Assembly Tasks – Image Projection Approaches**

Image projection is often used in assembly workplaces for its intrinsic capability of leaving the worker free of moving without carrying AR devices on his/her body. These systems, as the previous ones, eliminate the need for hard copy or monitor-based work instructions, and create a sort of digital "operating canvas" on a work surface.

A projector-based AR tool is in general more simple with respect to a video seethrough or an optical see-through system and also more intuitive as regards its use by the operator, throughout every step of a manual process: what they have to do is follow the lights projected on the workbench or on the parts, as illustrated in Fig. 5 [14].

**Fig. 5.** Assembly instructions projected on the workbench and on the parts (source: [14]).

At Dassault Aviation, an image projection AR system named Diota's [15] is used to guide operators in complex assembly tasks performed inside the panels of fuselage structures. Instructions related to operations to be performed and machine types to be used are displayed on the assembly. Figure 6 demonstrates how the system works, also showing the obstruction generated by the presence of the worker, typical problem in these kinds of systems.

**Fig. 6.** AR projection systems used at Dassault Aviation (source: [15]).

Another approach of image projection is shown in Fig. 7 [16]. The system uses a panel placed near the operator, where instructions are projected to avoid major drawbacks of an HMD, such as poor comfort, need for special versions for users who wear eyeglasses and limited field of view. To step through the assembly sequence and review previous instructions, the worker uses foot pedals; in this way, hands are kept free from the AR device and the worker does not interrupt the assembly process.

**Fig. 7.** Presentation of picking information and assembly instruction on a projection-based AR assembly station (source: [16]).

#### **3.3 Guidance in Complex Assembly Tasks**

Guidance in complex assembly tasks is essential for performing this kind of operations and for avoiding errors or time consuming procedures. Usually, these tasks are accomplished by using reference manuals and forcing the worker to read several pages of paper instructions.

**Fig. 8.** Head-mounted display used at NASA for assembling spacecraft parts (source: [17]).

Typical complex assembly tasks can be encountered in aerospace industry. NASA uses HoloLens AR headsets [17] to faster build a new spacecraft and assemble the crew capsule Orion, as shown in Fig. 8. Boeing has developed smart glasses and an AR application for wiring harness operations. The system enables assemblers to easily see where the electrical wiring goes in the aircraft fuselage and this allows a significant reduction of errors and production time [18].

A particular situation of complex assembly is represented by performing operations in areas not directly visible by the user. The work proposed in [19] uses a tracking method to acquire information on the position of target objects in real time, to be combined with the digitized model of parts and human hands in the invisible area, superimposed on the assembly scene (Fig. 9). The method demonstrated an improved efficiency and accuracy compared to the assembly without augmented reality assistance.

**Fig. 9.**

Complex assembly tasks can be also encountered in mounting electrical components. A tutoring system for motherboard assembly has been developed in [20]; the prototype uses an HMD to teach novice users the assembling of hardware components on a computer motherboard.

#### **3.4 Order Picking**

Order picking is a fundamental process in any production plant, to find and take the correct parts and products from a warehouse, just before assembly tasks. Companies are continuously looking for new methods to improve such a costly process, for which AR can represent an innovative solution.

In an application made in automotive industry, workers can automatically receive all the information they need directly in their field of vision, such as storage locations or part numbers, by using smart glasses (Fig. 10). The camera mounted in this device can be also used as a barcode reader: when a worker selects the correct part, the barcode is shown in green, if not, it is illuminated in red [21].

**Fig. 10.** Smart glasses used in order picking operations (source: [21]).

#### **3.5 Quality Control and Inspection**

Quality control and inspection represent important steps in an assembly sequence. In many cases, they could request instruments and a comparison with reference values. For these reasons, AR systems are very suitable for this kind of application and can efficiently support operator in this critical phase.

An example is reported in Fig. 11, where an HHD using a video see-through system is employed to determine the correct position of the rear light of vehicle and its alignment with respect to the surrounding body panels [22].

**Fig. 11.** A video see-through method for inspection of parts after assembling (source: [22]).

Also Renault Trucks is using AR for quality control in assembly lines [23]. Quality control operators wear Microsoft HoloLens smart glasses to improve inspection operations during engine assembly, as shown in Fig. 12.

**Fig. 12.** Microsoft HoloLens smartglasses used for quality control of Renault Trucks engines (source: [23]).

In the aerospace industry, an AR tool for quality control was developed by Airbus Group, for fuselage section inspection during assembly. The tool consists of an HHD that quickly detects errors, in order to optimize inspection time, while reducing the risk of deviations in conformity [24].

#### **3.6 Integration with Sensing Devices**

Since the sole use of AR could not provide feedback signals for a closed-loop control on the tasks executed by the operator, in manual assembly lines AR-based systems are strongly requested to be integrated with sensors, able to give to the user appropriate physical values of the process [25]. Indeed, AR can be integrated with sensing devices to create a synergistic system in which the limits of one may be filled by the other. AR can thus be employed to guide the operator in carrying out the correct action and to provide a support to recover any committed errors.

For this purpose, the contribution described in [26] proposes a configuration of a manual assembly workstation integrating the use of a sensing device and augmented reality equipment, in order to guide the actions performed by the worker. Figure 13 shows the scheme of the system obtained by the integration of a force sensor, able to detect a part of assembly errors not relievable by the AR camera.

**Fig. 13.** A workstation integrating AR with a force sensor for error detection during assembly (source: [26]).

#### **3.7 Training**

Starting from the consideration that an AR system, in principle, can be used both for guiding expertise workers and for training unexperienced users, the difference between these two applications relies in the level of instructions given by the system.

In an AR system used for training, the sequence of instructions and the virtual objects used to guide the operator take into account the fact that he/she does not have any experience on a given assembly procedure. In this case, even very simple assembly tasks are explained in detail, showing also elementary information about the use of a tool or safety principles.

For example, the assembly line shown in Fig. 14, equipped with AR projectors, provides different assembly instructions according to the skill level of workers [27].

Other examples are described in [28], where the training platform has been tested on an actuator assembly, and in [22, 29] where an HMD-based training system is used at the BMW Group Production Academy on an engine assembly (Fig. 15).

**Fig. 14.** AR assembly line equipped with PDs (source: [27]).

**Fig. 15.** AR-assisted training developed at BMW Group Production Academy (source: [22]).

Training can be also efficiently performed through virtual assembly. An example of augmented reality system developed for training, which uses a low-invasiveness hand tracking device, is reported in [30]. The proposed methodology enables the user to interact with the virtual objects, acquiring the pose of all the fingers of both hands by a markerless optical triangulation, as reported in Fig. 16.

# **4 Applications in Collaborative Assembly Systems**

The recent introduction of collaborative robots in assembly lines has led to the development of new fields of study, mainly concerning the safety of the work environment shared between human and robot. Also in this regard, AR may be beneficial, particularly in providing virtual information on the real workplace, to enhance situational awareness for the worker, to support spatial dialog among human and robot and to enable even remote collaboration.

**Fig. 16.** Sequence of virtual assembly by augmented reality (source: [30]).

The main topics currently treated in the field of AR applications for human-robot collaboration are in particular the development of Human Robot Interface (HRI) and the enhancement of worker's safety perception.

An application of AR in Human-Robot-Interface is given in [31]. The research focuses on the study of an AR solution for increasing human confidence and trust in collaborating with robots. The work describes the design and development of an interface system that shows virtual animation of the robot movements to the operator in advance on the real workplace, in order to improve human safety.

**Fig. 17.** Application of a see-through AR system to assist an operator in a cooperative work with a robot (source: [32]).

A typical application concerning the worker's safety perception when working close to an industrial robot is given in [32]. The main information provided by the system are assembly related instructions, robot workspace and trajectory visualization. The system, as illustrated in Fig. 17, was tested on a real case study derived from the automotive industry and demonstrated to optimize the time for assembly and the worker's safety perception in an HRC environment. Other examples can be found in [33], where an AR system for collaborative cooperation between a robotic assembly unit and a human worker has been developed by EON Reality, or in [34], where virtual instructions are given to workers to perform simple assembly tasks in cooperation with a robot (Fig. 18).

**Fig. 18.** Virtual instructions given during a collaborative work between a human and a robot (source: [34]).

# **5 Other Potential Areas of Application**

AR techniques could be also useful in other areas concerning assembly process and assembly systems. Among them, the most promising and with the highest potential of development are:

# **6 Open Issues and Future Perspectives**

Despite the significant progress of AR technology and applications in recent years, some limitations to be solved still occur. The main limits of AR implementation in assembly systems concern the following aspects:

They represent limits but, consequently, issues to be enhanced by the research community and therefore future perspectives for AR to more effectively assist assembly in the near future.

#### **6.1 Hardware and Software Performance**

Hardware performance is a key-element for obtaining efficient solutions. Nowadays this aspect presents some limits, even if it is increasingly improving. Features such as comfort for the user, wider field of view, comprehension of the environment, and, most importantly, more natural human machine interface through NLP (Natural Language Processing), hand and eye tracking are very important and many manufacturers are working on improving them.

Another open issue is "latency", which refers to the error of misalignment among virtual objects resulting from the delay between the movement of the user and the moment in which the changed image is displayed, with respect to the new position of the user [11]. To solve this problem, Waegel and Brooks [35] developed a low latency inertial measurement unit (IMU) and proposed new 3D reconstruction algorithms.

#### **6.2 Tracking Methods**

Among tracking methods, AR systems for assembly applications mainly use the markerbased approach, thanks to its stable and fast detection of markers in the entire image sequence. Valuable features are accuracy, flexibility, robustness, computation efficiency as well as ease of use. However, significant drawbacks of this tracking method are incorrect registration in the field of view due to marker occlusion and impossibility of using markers on many industrial components for their small size.

Tracking should be enhanced to create a good orientation of virtual contents with respect to the user's field of view. For example, Microsoft HoloLens 2 has implemented some tracking additions and improvements which are very beneficial to the workers. Gesture recognition, eye and hand tracking working together may provide to workers a more contextual interaction with virtual objects.

Marker-less tracking is considered the natural successor of marker-based approach. Among these techniques is Simultaneous Localization and Mapping (SLaM), which allows to build up a map within an unprepared environment, simultaneously keeping track of the current location, by extracting scene features.

The importance of this issue is also demonstrated by another contribution [36], where a method using the combination of point cloud and visual feature mechanical assembly system is used to create a tracking method. The accuracy is presented through a pump assembly case study and the results show that the tracking trajectory estimated by the method is very close to the real one and better than other methods, obtaining a good alignment of objects.

#### **6.3 User's Acceptance**

As far as the user's acceptance is concerned, research conducted in real work environment shows that operators could prefer AR over conventional methods of instruction [37]. Users generally give positive feedbacks when experiencing AR, especially for the possibility for non-experts to perform complex tasks and for the reduced mental workload on the user. However, this applies to AR systems implemented with care and attention to details on the interface and the content within presented to the user [37]. Moreover, to feel for the user that a task is worth using the AR support, its complexity must be high enough, as discussed in [38].

#### **6.4 Authoring Procedure**

Another important open issue is represented by the complexity of authoring in AR applications: an effective AR system requires a deep analysis of several aspects related to the assembly procedure (assembly sequence, instructions to be given to the operators, tools and fixtures, work environment, etc.) and its implementation in the AR platform and software.

A proposal of standard guidelines has been given in [39], with advantages to obtain time saving, error reduction and accuracy improvement.

Another contribution in this field has been given in [37] with the creation of AR work instructions for assembly using a new form of authoring, based on an expert recorded performing the assembly steps, automatically processed to generate AR work instructions.

An authoring tool has been also developed by BMW Group in order to support and simplify the design of AR training programs [22].

#### **7 Conclusions**

This paper presents a comprehensive survey of AR research and development in assembly systems. Many successful implementations made by industries and academic institutions, both for manual and collaborative assembly, are described, as well as current open issues to be overcome.

According to the analysis of the state of the art conducted, it is possible to conclude that augmented reality, after an important and still existing experimental stage, is becoming increasingly utilized in industrial applications and the sector is in constant expansion. The potential contribution of this technology to facilitate faster and correct operations encourages the market to a continuous evolution of hardware and software systems, including aspects related to the portability and comfort of AR devices, for a best users' acceptance. Furthermore, the latest developments in tracking and authoring procedures aim at more robust and sophisticated systems.

# **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

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# **Assembly Design and Planning**

# **Application of a Standardized Design Procedure in the Development of Automated Micro-assembly Processes**

Gordon Saunders and Tobias Müller(B)

Fraunhofer Institut Für Produktionstechnologie, Aachen, Germany<EMAIL_ADDRESS>
# **1 Introduction**

Automated precision assembly of e.g. optical systems requires development efforts in manifold domains, such as part feeding, handling, alignment, bonding and quality control. The use of systematic design procedure enables the rapid and complete development of new applications and use cases using existing equipment. Combined with modular equipment and subsystems, the use of a standardized design process significantly reduces development time and therefore costs. A generic methodology based on the functional decomposition of assembly task requirements and a coherent synthesis of functional process building blocks can be an answer to reduced process ramp-up time, more stable processes and enable concurrent engineering for novel tool and process development.

The objective of this paper is to present a systematic design procedure for the development of automated production processes to the point of Technology Readiness Level 5, or TRL 5, as defined by the European Union's Horizon 2020 framework [1]. This development process includes the analysis of the assembly task, the identification and selection of promising process concepts, and the testing of said process concept feasibilities within the assembly environment. As a result, individual component processes solving the most substantial automation-specific issues are tested in a production-relevant environment. These results prove the practical validity of the assembly process on the component-level, thereby fulfilling the TRL 5 criteria [2]. Ultimately, the results of these component validation tests are combined into a cohesive system, laying the groundwork for the testing and validation of the entire process chain and paving the way to meeting TRL 6 criteria.

# **2 A Function-Based Design Procedure**

An assembly task must be separated into its constituent parts to allow for a dynamic and complete concept generation phase. Doing so allows for the restructuring of a once homogenous and complicated problem into many individual and clearly-defined subroutines. This clarity in purpose enables a more complete examination and problem-solving process for each of the individual functions inherent within the assembly task without inadvertently omitting functional considerations by working on too complicated of a design problem. In this respect, the design procedure presenting in this paper aims to decompose a typical micro-assembly process chain into its constituent process blocks, subtasks, and functions. These functions support their associated process subtasks which in turn make up their associated process blocks. Hence, these components can be viewed as hierarchical in nature and are depicted as such in Fig. 1 [3, 4].

**Fig. 1.** Functional decomposition hierarchy

This work follows an adapted procedural design strategy originally laid forth by Ulrich and Eppinger (2013) [3]. Given a specific assembly task, this adapted strategy follows a systematic and iterative procedure to develop a process design concept. That is, if the development process stalls at any given stage, work can be resumed at any previous stage using the same previously developed materials and preventing a complete restart.

The core principle of this design strategy revolves around the identification of customer needs and the hierarchical functional decomposition of the assembly task to generate process concepts. Using these techniques allows for an adaptable problem-solving procedure while ensuring a complete solution via systematic function matching [4]. Each step of the overarching procedural design strategy is summarized in Fig. 2 and further clarified in the following descriptive text.

**Fig. 2.** Design procedure for use in process development

*Identify Customer Needs* - The identification of explicit and latent customer- and taskrequirements through internal and external interviews, background research, as well as an examination of existing process concepts and the given assembly task.

*Establish Targets*- The transformation of previously identified customer and task requirements into appropriate functional requirements that a successful assembly process must satisfy.

*Generate Process Concept(s)* - The generation of process concept solutions that have the potential to satisfy the functional requirements of the assembly task.

*Select Process Concept(s)* - The refining of the pool of potential solutions by comparison and analysis of the suitability of each solution with regards to the previously identified functional and customer requirements.

*Test Process Concept(s)* - The feasibility testing of the subset of plausible process solutions.

*Specify Final Targets* - The determination of specifications for a complete process module and process chain based on the results of process concept testing and functional requirements.

*Plan Downstream Development* - The planning of a construction and testing process for a prototype of the combined process module.

# **3 The Design Procedure in Practice**

Ultimately, this procedure introduced in the previous section progresses from identifying basic needs of a given assembly task to providing a technically feasible proof of concept for the continued development of an automated micro-assembly process module. This section dissects and further explores each of these individual design steps in further detail while discussing the tools and practical applications of each step as it relates to a theoretical process design challenge.

#### **3.1 Identify Customer Needs**

In order to fully understand the needs of the customer and therefore the requirements of an automated micro-assembly process, the given assembly task must first be understood within its context. In other words, knowledge of the status quo of both existing assembly processes, as well as knowledge of automated micro-assembly systems in general, facilitates a thorough analysis of the options present in micro-assembly automation. This is an exploratory role undertaken by the designer as the customer alone cannot provide or enumerate a complete and trustworthy list of his or her own needs. Rather, the customer is only aware of his or her own explicit requirements and may be unaware of their interpretation within the context of automated micro-assembly process. Research of the status quo helps to reveal this hidden, or implicit, process requirements and, combined with knowledge of automated micro-assembly processes, provides a basis through which future process concepts are generated as a component of the overarching procedural design strategy in Fig. 2.

A review of the present status quo involves many approach vectors which can be simplified into two main categories: those pertaining to the given assembly task and those pertaining to automated micro-assembly in general. First, the analysis of the given assembly task can involve, among others, the inspection and documentation of the part to be assembled, the existing manual or semi-automatic assembly processes, the production processes of the component parts to be assembled, or the eventual use case of the assembly part. Such analysis should take care to document limitations and bottlenecks of existing process and interfaces. In particular, it must be noted that an automated assembly process only serves to replace the existing assembly processes, leaving the production routines and customer use cases unaffected. Therefore, special attention should be given to first, how initial component error can be mitigated through module design, and second, that the part quality is preserved throughout use.

The second main approach to need identification involves needs based on automated micro-assembly processes in general. In this respect, designers must be well-educated with regards to process and equipment varieties present in the field of automated microassembly. This familiarity provides multiple benefits: not only does it assist in the generation of concepts due to a breadth of options, but also allows the designer to interpret customer needs specific to automated micro-assembly and add additional unspoken needs to support the new demands of automation.

This research of the status quo can be accomplished via multiple means. Both internal and external interviews as well as site visits, academic background research, market research, and self-examination and experimentation with the existing process all assist the designer in the identification of customer needs. Additionally, limitations of the development or production environment can be considered as needs. Examples of these limitation-based needs include cost-limits, time-limits, and the reality of machine availability.

#### **3.2 Establish Target Specifications**

A typical micro-assembly process chain can consist of five primary process blocks: part feeding, handling, alignment, bonding and handling again. Within the part feeding process block, parts to be assembled are first transported and delivered to the working space. Following this, the handling process involves the gripping and movement of parts inside the working space. Handling can be separated into two primary blocks to account for differences in functional demands—one pertaining to the the handling of multiple components prior to joining and a second block pertaining to the handling of the joined part.

In contrast to the handling process, alignment focuses on the precise measurement and fine local manipulation of parts to match its desired configuration. The bonding process, performed in conjunction with the alignment process, involves the mechanics used to join parts into a final assembly. Collectively, these five process blocks are pictured in Fig. 3 [5].

**Fig. 3.** Typical micro-assembly process chain

These process blocks allow for a simplified view of the process chain but are divided on an inherently subjective basis. These overarching process blocks are too complex to allow for a thorough discussion of the inherent functional requirements present in each. For this reason, the process blocks are further reduced into their subordinate tasks, offering a more nuanced lens through which to view the process chain.

A useful exercise to refine the process chain and therefore define subtasks is to create a process map. In such a map, blocks representing subtasks are characterized by the inputs, outputs, unique functional requirements of each and are connected by the flow of inputs and outputs to and from blocks. Subtasks are intentionally represented as generalized black boxes in order to avoid assuming process concepts, and are connected by energy, material, and data flows with the use of arrows leading into and out of various subtasks such as in Fig. 4. Through this exercise, the intricacies of the process dependencies are extracted. Furthermore, as this process map is a decomposition of the standard micro-assembly process chain, it serves as a generalized foundation for microassembly solutions that can be further adapted to match the reality any given assembly task.

**Fig. 4.** Subtask block with charted inputs and outputs for use in process maps

The benefits of using subtask identification as an intermediary tool are not immediately shown but rather reflected later in the design chain. By refining and redefining the process chain using subtasks, functional requirements unique to a specific subtask are identified that may have otherwise been overlooked. These constituent functions serve to continually narrow the scope of the each process step and form the basic building block of the typical micro-assembly process chain. Functions at this level are single, task-specific, and solution-independent transformations of a given material, energy, or data input into a prescribed output. These functions are required actions that a subtask or process must fulfill and take the form of "the process module must \_\_\_\_." Examples of functions specific to the present assembly task include "the process module must *grip the part*" or "the process module must *calculate the required movement*."

The initial phase of function identification comes from three sources: the customer, the assembly task, and the decomposition of the subtasks found in the generalized process map. Functions are created from customer needs through a transformation in perspective. In this vein, a customer need that requires "batch sizes of at least 25 parts" is represented in two functional requirements, that "the process module must *accept a batch size of at least 25 of each part*" and "the process module must *store at least 25 assembled tools*". Meanwhile, task-based functions stem from the technical requirements of the assembly task as well as the procedural and safety requirements of the working environment. These first two sources account for the customization of the development process specific to a given assembly task. Meanwhile, the third source, the standard functional decomposition of typical micro-assembly subtasks, ensures that the customized requirements are implementable within an automated assembly process. In later stages of development, specific solution configurations may require an additional fourth source of functions and reconsideration of the identified functional components.

Following the work of Galvao and Sato (2005), this body of functional parts can be matched with their associated process subtasks and vice versa [6]. Should there be a perfect 1:1 matching of functions and subtasks, the marked intersections on the matrix would lay perfectly on the diagonal. However, in most systems that is not the case. Portraying the function-subtask relationship in this way enables the analysis of subtask and function interdependencies along the process chain. These interdependencies result from multiple subtasks sharing an individual function or from multiple functions sharing an individual subtask and are represented on the matrix by rows or columns with more than one point of intersection. This results in vertical or horizontal sections that cause clumps or deviations from a perfect diagonal line, such as the identified regions in Fig. 5. These regions are of particular interest in the concept generation phase, as these areas of interdependency identify complex problem areas and require solutions that satisfy multiple requirements simultaneously. Shared functions and subtasks are accounted for in the design of process concepts by ensuring that the shared function is satisfied in all associated subtasks and that subtask concepts balance the needs of multiple functions.

**Fig. 5.** Areas of functional interdependence for further investigation

Therefore, a function-subtask matrix is the ultimate result of the functional decomposition of the assembly task and has a two-fold purpose in the design procedure. First, it is a listing of all identified functional requirements which serve as a basis for generating and evaluating process concept solutions for their feasibility. Secondly, the matrix shows process interdependencies and identifies critical problem areas. As these areas of interdependent functions often form the most complex problems, they are the first problems to be accessed for feasibility in the development of a new process module.

### **3.3 Generate Process Concepts**

The concept generation step of the design procedure is focused wholly around solving the identified functional requirements. These functions provide both a springboard from which concept generation and discussion can begin as well as a metric by which the concepts can be later evaluated. However, to maximize designer creativity and therefore novelty of the solution reached, the concept generation phase is a purely theoretical phase wherein no idea is necessary false. Suitability determination and the development of in-depth testing procedures are omitted until the later selection and testing phases respectively.

Thus, for each identified critical problem area, existing solutions are identified and new concepts are developed. This process can take place using a variety of different approaches including but not limited to individual or group brainstorming, interviews with both internal and external experts, a search of published literature and patents, benchmarking of existing processes, or systematic exploration of technical solutions [3].

#### **3.4 Select Process Concepts**

In order to reduce the generated solutions to a testable process concept, the various solutions are screened via a qualitative selection matrix. The selection matrix is a novel approach to decision making in an engineering design environment as it provides a quantitative evaluation method to interpret otherwise qualitative data. Furthermore, a selection matrix aims to select the optimal solution variant out of a given set by analyzing each variant's ability to fulfill the functional requirements specific to each process block [3]. Each solution is rated based on its satisfaction of the functional requirements relative to the other variants. Rating each functional requirement on a scale and summing the grades of each solution allows for the scoring of the each generated concept and the determination of the most suitable solution such as in Fig. 6.

**Fig. 6.** Example of a standard selection matrix with discrete grading

In addition to strictly functional requirements, the selection matrix can include additional relevant metrics. In this way, the additional concepts of testability, compatibility with dependencies, and the consensus opinion of the experiment interviews are considered in the solution narrowing procedure [4]. Doing so expands on the functional adherence of the chosen process concept by ensuring that the process can be tested with the available time and equipment, does not inhibit or hinder dependencies elsewhere in the process chain, and is considered a reasonable and plausible option by those with experience in the field.

**Fig. 7.** Example of a selection matrix variation with varied grading and weighting

A major benefit in using a selection matrix is its customizability to the demands of any given assembly task or development environment. This customization is achieved through three primary means. First, the variation of evaluation parameters such as the previous examples of accounting for testability and dependencies allows for the consideration of any metric. Second, the grading scales can be adjusted based on each metric. In the example shown, a binary, discrete scale of 0, 0.5 and 1 was used to quantify subjective judgement of functional suitability. However, alternative grading systems such as ranking, or the transformation of quantitative data into a continuous grade scale can also be used. Lastly, functional requirements can be weighted with a blanket multiplier to denote those functional requirements of special importance such as in Fig. 7. Through these variations, selection matrices expand upon their ability to aid the designer but the parameters must be thoughtfully chosen as they affect the final selection.

#### **3.5 Test Process Concepts**

Following concept selection, functional requirements continue to be used as a basis and a metric by which process concepts are judged. Evaluating how test results adhere to the original functional requirements ensures that a final process module using those concepts will fulfill the functions required by the customer and the given assembly task. Thus, the previously selected concept variants that were determined to be most suitable for their associated functional requirements are implemented in prototypical process module subsystems corresponding to each problem area. In order to fulfill TRL 5 criteria, these subsystem prototypes are implemented and tested in production-relevant conditions.

The evaluation methodology used to determine concept suitability varies depending on the specific functional requirements of the subtask or subtasks the prototypical subsystem aims to fulfill. Thus, each problem area merits its own custom experimental methodology. However, in the design of these experiments, the determination of exact process uncertainties or production parameters are not the goal of these experiments. Rather, the testing phase seeks to determine only that the previously chosen concepts are feasible in a production-ready environment and that its associated functional requirements will be fulfilled. Thus, feasible results of these tests denote a production-ready proof of concept for the associated subsystem and enables the further development, integration, and refinement of the tested subsystem.

#### **3.6 Specify Final Targets**

In the specification of final targets the findings of each separate testing process are combined into a single, cohesive list of specifications a combined process module must fulfill. This phase is a purely analytical phase of the development process wherein requirements and dependencies inherent in the combined process module are identified by examining the performed tests and revisiting the decomposition and definition of functional requirements and subtasks.

In these testing phase, certain equipment and processes are determined as being necessary for final process feasibility. As these demands are found separately, they must be combined along with their dependent inputs and outputs and be recorded in a systematic way to enable the expedient development of combined process module incorporating the required elements of all preliminary tests. In addition, these equipment and process demands can impart additional functional requirements necessary for their functioning or impart changes to the subtask map relating to their place in the process chain. Thus, the decomposition and definition of subtasks and functions should be revisited and reevaluated at this stage in the development process to ensure that these documents and metrics remain relevant to the evolving design.

#### **3.7 Plan Development**

The continued development of an assembly process module is facilitated by the development of a combined process module concept based on the consolidated demands and requirements specified in the previous development phase. This combined process module represents one cohesive concept for the complete automated assembly process, including straightforward or standard solutions for the subtasks and functional requirements falling outside of the tested problem areas.

In the development of this combined process module, the competing demands of processes and equipment must be weighted and judged, in order to tactfully design a module without impeding previously solved functions. The determination of consistent reference frames, the minimization of travel times, and consideration of tool envelopes can all be deciding features in the combination of module subsystems with regards to physical location. Similarly, the rudimentary order of subtask functioning must also be determined. Thus, a combined process module and process chain concept is produced as a result of this development step.

#### **4 Conclusion**

This paper introduces a systematic design framework that enables the flexible but thorough development of automated assembly processes based on identified customer- and task-specific needs and their associated functions. The first step of this design procedure involves the identification of the base functions of the assembly task through the decomposition of a generic automated assembly process chain. These functions are integrated with the customer- and task-specific needs, forming a complete model of the requirements of a final process module. From these functions, problem areas are identified for testing by focusing on complex regions of interdependencies found within the process requirements. Process concepts corresponding to each of these problem areas are developed via a three-step procedure. First, solution concepts are generated through the exploration of existing techniques and technologies. Second, these solutions are screened by a qualitative evaluation of each concept's fulfillment of the requirements. Finally, the remaining concepts are systematically tested and evaluated for each's feasibility within its process block. These process concepts are merged into a combined process module incorporating the required elements of all preliminary tests. This process module represents one cohesive concept for the complete automated assembly process. Collectively, this investigation and practical validation of process components in a production-relevant environment determines a feasible automated assembly process concept and satisfies the requirements of Technology Readiness Level 5.

This module is not necessarily a production-ready design but rather a model for the future development of a final process concept. In order to expand this process to Technology Readiness Level 6 and beyond, the continued development of the module and assembly process involves the construction, testing, evaluation, and optimization of the module concept based on its fulfillment of the functional requirements of the task. Doing so validates the developed process on a system-wide level as opposed to the compartmentalized component level tests done earlier in the development procedure and enables the realization of the assembly process in a production environment.

# **References**

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# **Towards the Automated Coverlay Assembly in FPCB Manufacturing: Concept and Preliminary Tests**

Marcello Valori1(B) , Vito Basile1 , Simone Pio Negri<sup>1</sup> , Paolo Scalmati2, Chiara Renghini2, and Irene Fassi3

<sup>1</sup> Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing,

National Research Council, Via P. Lembo, 38/F, 70124 Bari, Italy

<EMAIL_ADDRESS>
<sup>2</sup> Somacis S.p.A, Via del Lauro, 7, 20121 Milan, Italy

<EMAIL_ADDRESS>
<sup>3</sup> Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, National Research Council, Via A. Corti, 12, 20133 Milan, Italy

**Abstract.** In modern electronics, flexible and rigid-flex PCBs are largely used due to their intrinsic versatility and performance, allowing to increase the available volume, or enabling connection between unconstrained components. Rigid-flex PCBs consists of rigid board portions with flexible interconnections and are commonly used in a wide variety of industrial applications. However, the assembly process of these devices still has some bottlenecks. Specifically, they require the application of cover layers (namely, coverlays), to provide insulation and protection of the flexible circuits. Due to the variability in planar shape and dimensions, the coverlay application is still performed manually, requiring troublesome manipulation steps and resulting in undetermined time-cycle and precision.

This paper aims at the improvement of the industrial process currently performed, by proposing an approach for the automation of Kapton coverlay manipulation and application. Since these products are commercially provided as a film with a protective layer to be removed, the peeling issue is addressed, representing a challenging step of the automated process; the results of a systematic series of tests, performed in order to validate the peeling strategy, are reported in the paper. The overall assembly strategy relies on the development of a customized multi-hole vacuum gripper, whose concept is presented and contextualized in the proposed assembly process by outlining a suitable workcell architecture.

**Keywords:** PCB assembly · Film manipulation · Flexible electronics

# **1 Introduction**

Modern electronics is moving fast, chasing market demands, towards miniaturization, flexible and stretchable electronics [1], embedded PCBs [2], molded interconnect devices (MIDs) and 3D electronic devices [3], environment friendly electronics [4]. Among these trends, flexible and rigid-flex PCBs are used in many applications due to their advantages compared to traditional rigid PCBs: by interposing flexible connections between rigid board portions, complex 3D arrangements can be achieved, complying with demanding volume constraints.

The main advantage obtained by the use of flexible printed circuit boards (FPCBs) is the possibility of 3D wiring, enabling the exploitation of the available volumes and complex routing requirements. In particular, exploiting their intrinsic lightweight and the possibility to be deformed and constrained into narrow volumes, flexible circuits are commonly used in biomedical applications [5], aerospace and smartphone industry. One fundamental feature of flexible circuits is the necessity of mechanical protection of the conductors; in order to comply with this requirement, coverlays are applied in addition to the solder masks. Among the various types of coverlay (film, screen printable, photoimageable) [6], film coverlay is generally preferred, due to its mechanical properties in terms of strength and durability. However, it is the most complicated solution from the point of view of the automation possibilities. Film coverlay thickness can span from 25μm to 127μm; the base material, polyimide (PI) or Polyethylene terephthalate (PET), is coated with an adhesive layer of epoxy resin or acrylic with thicknesses in the range 25 ÷ 75 μm. The adhesives are semicured, to be activated finally in the lamination phase. A further layer of release material is applied as adhesive layer protection for delivery and handling before application.

Figure 1 shows an example of rigid-flexible PCB layer stack-up. Figure 2 shows two examples of rigid-flexible circuits developed by the PCB manufacturer Somacis, Italy, as well as the effects of a final "routing" phase, referred in this case to the full-thickness trimming of the external frame to actualize the flexibility. Light brown, flexible circuits can be distinguished from green rigid circuits; the different color is due to coverlay surface. In some cases, flexible circuits enable signal connections between rigid circuits (Fig. 2, a), while in other cases they embody sensors or other devices (Fig. 2, b).

**Fig. 1.** Rigid-flexible PCB stack-up. Left: schematic of the layers. Right: a detail of the assembled circuit section acquired by a Zeiss Axio Imager M2m microscope. (Courtesy of Somacis, Italy).

In Fig. 3, a flexible circuit portion is shown, before and after the application of the coverlay; as highlighted, coverlay application is limited to flexible areas of the circuit, specific for each board design. The actual flexibility is obtained when, furtherly, the

**Fig. 2.** Rigid-flexible circuits. a) Industrial application board, 14 layers (10 rigid + 4 flexible), thickness 2 mm. b) Device for human epilepsy monitoring. c) and d) The external frame furtherly eliminated, enabling the deformation of flexible circuits. (Courtesy of Somacis, Italy).

external frame, that is necessary for automated manipulation during the assembly of the other components, is removed. It is worth to highlight that the removal of the coverlay protective film can be troublesome, increasing process time, thus production costs, and inducing visual and mental stress for the worker.

**Fig. 3.** Coverlay application. Flexible circuit before (a) and after (c) the application of the coverlay (b). In c) the red countour highlights the frame to be trimmed out and the blue line indicates the flexion axis of the final device. (Courtesy of Somacis, Italy). (Color figure online)

Flexible circuits layers are laminated together with rigid layers, according to the designed build-up (Fig. 4, left). In PCB fabrication, indeed, the layer stack-up is preassembled and then subjected to lamination and curing (pressure-heating cycle), aimed at the activation of the adhesive on coverlays and flexible circuits and the polymerization of epoxy resin in prepreg foils (Fig. 4, right).

The state-of-the-art application process of film coverlays is characterized by the following steps (Fig. 5): a) laser trimming of the specific contour, b) manual removal of the protective film, c) manual positioning on the PCB, d) manual application of one

**Fig. 4.** PCB fabrication: lamination and curing cycle. Pre-assembled stack-up (left) and 20 positions loader of a Lauffer press. (Courtesy of Somacis, Italy)

or more welding points to avoid displacements, e) lamination and curing for adhesive activation and/or polymerization of prepreg layers. Another option for increasing process accuracy is represented by the use of laser drilling after lamination: the coverlay film is applied without pre-drilling; after lamination, laser drilling is performed in order to selectively eliminate the coverlay film and obtain the designed geometry [6]. However, with this approach, there are no advantages for the automation of the peeling phase.

**Fig. 5.** Coverlay assembly: state-of-the-art process phases.

In panel-based processes of electronics industry, automating the peeling of flexible materials still represents an issue [7]. In this paper, a new concept is proposed for automated manipulation, positioning and assembly of coverlays in flexible and rigidflexible circuit manufacturing, addressing in detail the peeling issue. Section 2 presents the conceived assembly procedure including gripping, peeling of the protective film and assembly on the current layer of the stack-up. Section 3 illustrates the concept of the automated assembly system with its architecture and main equipment, including gripper architecture. Section 4 reports the experimental tests performed for the peeling step of a typical coverlay and finally, conclusions are presented in Sect. 5.

#### **2 The Proposed Assembly Approach**

In this Section, a strategy for coverlay automated assembly is proposed, identifying firstly the process requirements, then the gripping features and the peeling strategy and, finally, the process phases are described.

#### **2.1 Requirements**

The requirements of the assembly procedure are determined based on the manual process currently carried out and the material properties.

**Flexible Component Manipulation.** Film coverlays are characterized by extremely low thickness. The system shall be able to manipulate these components, avoiding bending and ensuring flat positioning on FPCB surface.

**Compliance to the Geometry.** Depending on the specific FPCB design, coverlays are characterized by different dimensions and planar shapes, thus, the gripping strategy shall enable the compliance with a wide variety of geometries.

**Removal of the Protection Film.** In the assembly task, the protection film must be removed before coverlay application; therefore, a methodology for the peeling of such a film shall be implemented.

**Thermal Bonding:** Film coverlay shall be constrained to the FPCB by applying one or more bonding points, by locally activating the epoxy resin with a soldering tool.

**Performance.** Accuracy and repeatability of the assembly process shall be lower than the accuracy limit determined for flexible circuits with High Density Interconnect (HDI) PCB (100 μm) [6].

### **2.2 Gripping**

In the manipulation of flat surfaces characterized by extremely low thickness combined with low stiffness materials, thus, deformable, the issue of preserving component configuration, avoiding undesirable bending, arises. This aspect can be even more relevant during the positioning phase, in which folds and air bubbles must be avoided. The use of a flat gripping surface as interface with the component represents a valid option to address these aspects.

Gripping strategy shall also comply with the requirement of the selective pick. Compliant grippers usually rely on the 3D shape of the object, in order to passively adapt to perform gripping, using different approaches and often implementing soft materials [8]; there are also commercial products available enabling the unconditional pick of objects with flat surfaces [9]. In the application dealt with in this paper, coverlays lie on a flat surface; due to the extremely low thickness, their 3D shape is not appropriate to be passively distinguished from the background by a compliant gripper. Thus, the necessity of a strategy for gripper "pre-shaping" arises.

Basing on these considerations, the gripper specifically designed for coverlay "pickpeel-place" is characterized by a flat interface surface and based on vacuum principle; a pattern of holes, representing themselves a matrix of suction cups, is realized in the flat surface. In order to avoid the uncontrolled pick of other patches lying on the same support, as well as the possible vacuum loss due to uncovered condition of one or more orifices, each suction cup is controlled individually, thanks to an upstream pneumatic valve module. With this approach, gripping pattern can be set depending on each specific coverlay contour, enabling the possibility of gripping different geometries.

#### **2.3 Peeling Strategy**

Peel strength represents one of the most important characteristics of coverlays, as it is a measure of component integrity in determined loading conditions. DuPontTM Pyralux® LF coverlay material, for example, based on a Kapton® polyimide film, exhibits a peel strength of 1.6 kg/cm after curing, defined by the provider as per the applicable standard [10].

On the other hand, peeling represents also one phase of the assembly, as the coverlay is provided coupled with a release material; it is also the most demanding challenge to address in order to accomplish the whole task, due to the uncertainties in the peeling dynamics combined with the geometrical variability of the patches. Peeling dynamics has been widely investigated by the scientific community, and there are a number of patented machines, designed to be implemented in roll-to-roll production for different applications, performing tape peeling. In the majority of cases, peeling is triggered by the action of rolls [11] or insertion means acting as blades [12], combined with high bending angles of the release material. However, circuit board industry is mostly characterized by panel-based processing and coverlays may have very different shapes and dimensions. Therefore, the necessity of implementing a specific material removal method arises, studied for components characterized by planar dimensions in the order of few square centimeters.

In order to trigger the peeling of a film, adhering to another due to a substrate of a bonding agent, a mechanical action should be applied. With decreasing thickness, this task becomes more demanding, requiring precision and tools with appropriate edges. The detachment is achieved when the force, properly applied, is sufficient to trigger local fracture of the bonding agent. In the approach hereafter presented, the issue is addressed by an inverted perspective: instead of applying a detaching force when precise positioning occurs, the method relies on driving the film towards a source of mechanical action, working continuously at a high frequency; the approaching is stopped when detachment takes place due to the contact randomly occurred between the film and the source of the force.

In the practical implementation, a rotary tool with appropriate cutting profile can be suitable. In our experiments, a rotary sanding drum was used as a trigger for the peeling of the release material. As shown in Fig. 6, such a method, called hereafter "prepeeling", exploits abrasive surface roughness to collide with the release material surface and detach it from the coverlay, thanks to the impulsive force obtained by high grinding speed and applied by one or more abrasive particles to the film to remove. Relying also on friction contact, mutual positions can be calibrated in order to guarantee that this force affects only the protective film. A specific setup, further described in this paper, was used to validate this method.

In Fig. 6, the most important process parameters are indicated. The *a* parameter is the unsupported overhanging portion of the coverlay (cantilever configuration) out of the clamp jaws, while *e* is the distance, along the x-axis, of the coverlay tip from the grinding tool axis, *r* is the radius of the sanding drum, rotating with angular speed ω and exerting a torque "*T*". Parameter *e* is straightly related to the approach angle ϑ to the cylindrical grinding tool, since *e* = *r* cos(ϑ). These parameters have been chosen due to their direct relation with the physics behind the peeling process. Indeed, *a* is related to

**Fig. 6.** Peeling trigger based on a sanding tool: a) the surface of the protective film approaches the rotating surface in a determined configuration; b) it is detached from the film coverlay. A magnification of the contact zone clarifies the definition of some parameters, considering the real operating conditions.

the coverlay flexural stiffness, so that stiffness decreases as *a* increases. Parameter *e* has influence on the peeling mechanism as it directly affects the direction of the interaction force applied by the tool on the component, due to the direct relation with angle ϑ. The combination of *r* and *T* determines the value of the force and ω affects the momentum. Finally, the whole process is closely linked to the roughness of the sanding drum.

Parameter *h* represents the distance, along the z-axis, between the sample and the grinding tool. The optimal set [*a*, ϑ, *h*] uniquely identifies the most suitable configuration of the coverlay and the rotating sanding drum for the pre-peeling phase. Regarding this aspect, it is relevant to point out that, in the practical implementation, coverlay samples preserve the curvature of the original material reel. The real *h* values differ from the theoretical value *h*\* = *r* sin(ϑ) as they are affected by coverlay intrinsic curvature. According to this, ϑ is to be considered as the angle spanned between the radius connecting the center of the sanding drum with the contact point P and the axis of the clamp (or, in the automated task, of the end-effector), rather than the surface of the coverlay itself, as shown in the detail in Fig. 6-b).

#### **2.4 Assembly Procedure**

The assembly procedure is then defined in accordance with the chosen gripping and peeling strategy. With reference to Fig. 7, the following phases are identified:

**a) Gripper Setup.** With reference to the specific geometry of the coverlay to manipulate, an off-line setup is performed in order to activate selectively the vacuum orifices, thus modeling the gripping surface.

**b) Coverlay Pick.** Once the position of the component to manipulate is detected by an external sensing source, i.e. a vision system, the gripper can approach it and perform the pick on the coverlay side. A portion of the coverlay is kept on purpose outside of the gripping envelope.

**c) Pre-Peeling.** The unconstrained component portion is approached to the grinding tool, by the protective film side, thus triggering the detachment of the two films.

**Fig. 7.** The phases of the proposed assembly procedure: a) Gripper setup, b) Coverlay pick, c) Pre-peeling, d) Peeling, e) Coverlay positioning, f) Thermal bonding.

**d) Peeling.** The whole protective film is pulled away by a suction cup, acting on the component portion detached in the previous phase.

**e) Coverlay Positioning.** The coverlay is placed on the PCB.

**f) Thermal Bonding.** The coverlay is bonded in one or more points in order to guarantee constrained precise positioning before lamination.

#### **3 Assembly System Architecture**

The whole assembly system under study is based on the approaches proposed for gripping and pre-peeling. In this section, a concept of the workcell and the design of the specific gripper for the task are presented.

#### **3.1 Workcell Description**

The workcell architecture for coverlay application, outlined in Fig. 8, is aimed at exploiting the proposed automation strategy to obtain the PCB assembly ready for lamination and curing. The inputs of the process are the flex or rigid-flex PCB with circuits and SMT (surface mounted technology) components installed in previous steps.

In the assembly concept, the task is customized off-line, depending on the coverlay contour: a specific software tool is required for the pre-shaping phase. This setup is fundamental for task completion, since an appropriate portion of the picked coverlay has to remain outside of the gripper envelope, enabling peeling triggering.

In the workcell concept there are two conveyors, one providing the laser-trimmed coverlays and one sliding the flex or rigid-flex PCBs to assemble. The "pick-peel-place" task is performed by a commercial manipulator, equipped with the specifically designed gripper. The implementation of a collaborative manipulator is foreseen, due to the advantages provided by including a human operator in the assembly environment [13], even for a mere on-site supervision. Within the collaborative robot scenario, several commercial models are suitable for the task, considering the extremely low payloads and the accuracy required, assessed 100 μm in Sect. 2.1. As an instance, in Figs. 8 and 9 the

**Fig. 8.** Possible architecture of the workcell for coverlay application in a generic assembly scenario: a) top view and b) front view.

gripper is equipped on a Universal Robot UR5e. Regarding component pick and place, two vision systems are placed on top of the two conveyors, with appropriate fields of view (FOVs) to detect, respectively, position and orientation of the coverlay to pick, and the release position on the PCB. Within the robot reach, the pre-peeling and peeling stations are included, corresponding respectively to a rotating sanding drum and a fixed vacuum gripper; a vision system is necessary for gripping position check.

**Fig. 9.** Design of the specific gripper. On the left, the gripper equipped on a UR5e; on top, right, gripper components are highlighted: on bottom, right, the interface surface is indicated.

#### **3.2 Gripper Architecture**

A specific gripper is designed in order to perform coverlay film manipulation and comply with all the assembly phases. The overall design is shown in Fig. 9 and each subsystems is hereafter described.

**Vacuum Pump.** A vacuum pump, dedicated to gripper actuation, is installed on robot frame. In the design reported in Fig. 9, a vacuum generator is included.

**Interface.** The interface with the object consists of a flat surface; tiny holes provide vacuum source and a rubber coating guarantees adhesion and avoids undesired impacts with the board during the component placement. The diameter of the holes, 2 mm, has been experimentally determined as a trade-off between gripping force and deformation of the material due to suction force.

**Valve Block.** The interface surface covers the suction compartments, which are independent each from another provided by the vacuum valve block. The valve block consists in a series of 32 tiny 3/2 normally closed, cartridge solenoid valves, commercially available, supplied by common manifolds in parallel configuration, controlling, downstream, one duct each one; only one duct is highlighted in the Figure. Each duct ends up with a suction compartment. With this configuration, the goal of selectively shaping the gripping surface is achieved by modeling the gripping surface, controlling each valve individually.

**Bonding Tool.** The bonding apparatus consists in a retractile, metal solder tool. It can slide on the side of the gripper, in order to bond the coverlay once it is located on the PCB, while it is maintained by the gripper in the correct position. One spindle drive actuated by a DC motor enables tool sliding, as shown in Fig. 9. The bonding tool slides out from a flat surface adjacent to the gripping surface, inclined to it in order to work as a foothold for the vacuum peeling tool during peeling completion.

# **4 Peeling Tests**

#### **4.1 Experimental Setup**

As stated above, the peeling of the coverlay is conceived in two steps: pre-peeling and peeling completion. In order to verify the effectiveness of the proposed method to trigger coverlay peeling, a series of experimental tests was performed. The following parameters are considered to mainly affect the process: thickness of the coverlay, grit designation and dimensions of the sanding drum, rotational speed, geometrical configuration of the coverlay/drum approach. In this phase, the feasibility is addressed, focusing on the individuation of the optimal geometrical configuration for a defined set of coverlay material (thickness), pre-peeling tool (grit designation and diameter) and rotational speed.

Coverlay material is DuPont Pyralux LF0110 made of 25 μm thick Kapton polyimide coated on one side with 25μm proprietary modified acrylic adhesive. The adhesive side is covered by a protective film with thickness of 40 μm. Coverlays are IPC certified by the supplier. Coverlay geometries are determined on the basis of an industrial case, currently object of study of the research group; such a geometry is rectangular with dimensions 25 × 40 mm; further materials and dimensions will be investigated in following experimental surveys. The samples are trimmed, which means full-thickness cut, by a 2D laser cut machine from a coverlay sheet with dimensions 610 × 454 mm, obtained by cutting a roll 610 mm wide and 76 meters long.

The laser trimming operation separates the coverlays from the sheet, then collected and loaded on a conveyor for the subsequent operation. The testing equipment is composed by two main parts: 1) apparatus for coverlay positioning; 2) pre-peeling stage. With regard to the first component, the coverlays samples are mounted on a three-axes high precision metric stage, actuated by three micrometer screws, with travel range of 15 mm on each axis and sensitivity 1 μm. The coverlay samples are held inside two small aluminum jaws mounted at the end of two cylindrical fixtures. In order to distribute the clamping force on a higher coverlay surface, two stainless steel plates with thickness 0.45 mm are used as interfaces between the clamp jaws and the coverlays. The pre-peeling stage consists of an electrical drilling tool, mounted horizontally. The sanding drum is a cylinder with an external diameter of 8.78 mm and height 12.7 mm (1/2 in.) used for smoothing applications, with grit designation P80, corresponding to average particle diameter of 201 μm. The tool shaft has a diameter of 3.2 mm. The setup is showed in Fig. 10.

**Fig. 10.** Peeling initialization experimental set-up. Frontal (a), top (b), side view (c).

Preliminarily, the geometry of the set-up is verified: vertical stroke of the coverlay, alignments between grinding tool axis and coverlay profile, zeroing of the z-axis coincident to the first contact between the coverlay protective layer surface (bottom) and the side of the cylindrical grinding tool, centered position of the grinding tool with the coverlay width, etc.

The series of experimental tests was based on the variation of parameters *a* and *e*; ω was set at 800 rpm and the same sanding drum - thus, same radius and same sanding grade - was used. *a* was set at 10 and 15 mm, while *e* was set at 0, 1, and 2 mm; for a = 10 mm also e = 1.5 mm was considered as a further set of variables. Each of the 7 parameter sets is tested with 5 repetitions (sample labels A, …, E). This value is reputed sufficient for the feasibility assessment. Each run is performed by setting the parameters (*a* and *e*) and driving the coverlay sample towards the grinding tool by slow vertical motion (approximately 2 mm/min), as schematically shown in Fig. 6; vertical motion is stopped when peeling is triggered. The outputs of each experiment are the peeling triggering success, the peeled area and the recorded value of the parameter *h*; the standard deviation characterizing this value for each set of runs is indeed used to assess the repeatability of the process. For each successful pre-peeling test, an image was captured and processed in order to calculate the peeled area, considered an important parameter, since peeling completion relies on the gripping of a film portion already peeled.

#### **4.2 Results and Discussion**

Figure 11 shows the results of image processing procedure previously described, obtained for two runs. The light brown portion of images are pre-peeled areas (Fig. 11, left). They have irregular contours which are identified by image processing algorithms (Fig. 11, right). The proposed procedure can also be automated and used real-time for peeled area detection and characterization. Accuracy can be increased by illumination and background optimization in the image acquisition step.

**Fig. 11.** Image processing applied to peeled samples. ROI (Region of Interest) original image (left) and processed image (right) for calculation of peeled area, in white. Runs reported: 100 A (a = 10; e = 0; sample A) on top and 150C (a = 15; e = 0, sample C) on bottom.

Table 1 reports the results obtained with the different parameter sets. At a first glance, it highlights that the grinding-based method can be a successful solution for peeling triggering in several configurations. It is important to point out that all the successful pre-peeling runs led to successful peeling completion, definitely validating the approach of splitting the peeling issue in two sub-phases.

Besides the validation of the approach, this preliminary series of tests was aimed at the definition of a reliable configuration for the automation of the pre-peeling phase,

**Table 1.** Experimental results of peeling triggering.

*Success Index*: number of successful pre-peeling runs; *PA: Peeled Area; TA*: *Total Area* (limited to the overhanging coverlay portion); <*h*>: mean *h* value and σh: standard deviation.

intended as the mutual position of an end-effector holding the coverlay and the grinding tool. The choice is the result of a hierarchical parameter analysis. The first parameter to consider is the *Success Index*: only configurations leading to 100% successful runs were considered. The Peeled Area PA (and, in particular, the ratio between PA and TA, the Total Area) was assessed as an important discriminating factor for the subsequent peeling completion phase; however, as already stated, all the successful pre-peeling runs were followed by successful peeling completion. From the pre-peeling automation perspective, acquired position standard deviation σ*<sup>h</sup>* is then relevant: since the *h* value indicates a position suitable for the robot end-effector, a lower σ*<sup>h</sup>* is directly translated in a better repeatability of the process, that represents a fundamental indicator in an automated task. According to this approach, the most suitable set of parameters for the batch of coverlays currently under examination, is represented by the "100" series (a = 10 mm; ϑ = 90°; h = 5.5 mm). It is worth to point out that, according to the discussion reported in Sect. 2.3, in the practical implementation, this particular configuration (ϑ = 90°) does not correspond to the protective film surface tangential to the sanding drum, due to the coverlay intrinsic curvature. As a further remark, in all the successful runs, no damages were observed on coverlay inner surfaces.

In the proposed approach, the process is considered stochastic, relying on the sand particles random distribution. The coverlay/particles contact frequency is directly related to the high rotational speed of the tool. Certain contacts lead to detachment, thus, success probability is strongly increased by high rotational speeds. However, coverlay/tool relative configuration is a fundamental factor to enable the engagement of the only protective film layer; in some cases, indeed, also the coverlay film is engaged itself, leading to a process fail. The experimentation led to the optimal configuration for the specific case-study.

### **5 Conclusions**

The presented research activity deals with the automation of a specific task in electronic assembly, still performed manually. The topic is of outstanding interest, as it represents a critical phase in the production of flex and rigid-flex PCBs, whose implementation is incredibly growing in a wide range of application fields. Coverlays work as stiffeners, insulators and mechanical protection of the circuits; film coverlays present the most suitable mechanical properties, but they are characterized by low accuracy, as their application relies on operator skills. Furthermore, the removal of the protective film is troublesome and stressing.

The automation of the coverlay application phase is addressed by proposing an approach for the protective film peeling triggering, introducing a pre-peeling phase, based on a common workshop sanding tool. The whole assembly process is then outlined and the concept design of a suitable gripper is presented. Finally, the results of a series of tests, aimed at validating the peeling strategy and defining an optimal position for the automated pre-peeling phase, are reported.

The results demonstrate that the proposed peeling strategy can represent a successful methodology to be implemented in an automated process. Geometrical parameters need to be properly configured to ensure process reliability, defining the optimal relative position to promote the separation by engaging only the support material layer. In our experiments, indeed, some configurations led to 100% pre-peeling success, followed by successful peeling completion.

In future developments, the results obtained will be further validated in other experimental surveys; in particular, the variation of tool rotation speed will be considered, as well as the use of different sanding drums, corresponding to the variation of tool radius and sanding grade, and different coverlay materials. At the same time, gripper design will be refined, followed by the supply of all the components and realization. By equipping the gripper on a manipulator, it will be possible to validate the automation of most of process phases. Finally, the robot will be integrated in a complete workcell, as outlined in the paper.

**Acknowledgments.** This research was funded by the Italian Government through the National Operative Program H2020 PON 2014-2020, within the project "High-performance electronic Embedded Systems (HELMS)", grant number F/050507/02-03/X32.

#### **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

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# **Resource Interface Matchmaking as a Part of Automatic Capability Matchmaking**

Niko Siltala(B) , Eeva Järvenpää , and Minna Lanz

Tampere University, Korkeakoulunkatu 6, 33014 Tampere, Finland<EMAIL_ADDRESS>
**Abstract.** This paper presents a case study for capability matchmaking method and specifically focuses on interface matchmaking process. This method can be utilised during production system design or reconfiguration by system integrators or end users. They will benefit from fast and automatic resource searches over large resource catalogues. The paper binds together the process around the capability and interface matchmaking, which are presented more in detail in our earlier publications. In this paper, the use of matchmaking process is exemplified, and a verification of the method is provided with two test cases.

**Keywords:** Interface matchmaking · Assembly system design · System reconfiguration

# **1 Introduction**

Responsiveness of manufacturing is an important strategic goal for manufacturing companies operating in a highly dynamic environment characterized by constant change. Such responsiveness and adaptivity is related to the need to reconfigure and adjust the production and corresponding production system as efficiently as possible to the required changes in processing functions, production capacity, and the dispatching of the orders. [1, 2] To do this, the production system needs an inherent ability to facilitate continual and timely change in its structure and in its functional operations.

Traditionally, the production system design and reconfiguration has been timeconsuming process done by the human designer. It includes search for suitable and connectable resources, and it relies heavily on the expertise and tacit knowledge of the system integrators and the end users of the system [3]. Meeting the requirements of fast adaptation calls for new methods and solutions that would drastically reduce the time and effort put into system design [2, 4], both in brownfield and greenfield scenarios. Plug and play interfaces, modern information and communication technologies, formal information models representing resources and products, as well as simulations and other computer-aided intelligent planning tools can all contribute to such methods and solutions [2, 4]. During the system design and re-configuration, new structural configurations are built to fulfil the functional requirements set by the product [4]. Similar to the design of modular products [5] – consideration of interfaces plays an important role in enabling the interchangeability and independence of resource elements. Thus, in order to achieve a feasible structural configuration, the combined production resources must have compatible interfaces.

Within the past two decades, there have been multiple different projects and research [6–8] trying to provide computerized support for system design and reconfiguration planning process. According to [6], the modular architecture paradigm for new production systems, which focuses on the clear functional decoupling of equipment module functionalities and the use of standardized interfaces to promote interchangeability, presents the possibility for developing automated system design and reconfiguration methods. Important steps towards modular assembly equipment and standardized hardware and control interfaces was made, for example, in EU-funded project EUPASS [7]. The recently finished project ReCaM [8], which results this paper also reports, aimed to develop a set of integrated tools for rapid and autonomous reconfiguration of production systems. The approach relies on a formal unified functional description of resources [9], providing a foundation for rapid creation of new system configurations through capability-based matchmaking of product requirements and resource offerings [10].

This paper will present, through two case studies, how the capability matchmaking process operates. Emphasis is placed on the interface matchmaking. First objective is to present how the interface concept for production resources can be utilized as a part of the capability matchmaking [11] procedure. Secondly, we will present, through two cases, how the capability and interface matchmaking work in practise and how they sort out feasible resource combinations for the system designer.

The paper is organized as follows. Section 2 introduces shortly our overall capability matchmaking approach and its associated concepts and data models. Section 3 focuses on the interface matchmaking process in general. In Sect. 4 we present with two cases how the matchmaking process advances and provides the matching resource combinations. Section 5 discusses how we have verified the automatically gained matchmaking results. The specific focus is placed on interface matching viewpoint. In the Sect. 6 we conclude the work done.

# **2 Capability Matchmaking Process**

#### **2.1 Information Models Involved**

The capability matchmaking relies on different information models that are used to describe the resources and products in a formalized way. For each different resource, a resource description (RD) file (in XML format) is created and published by the resource provider. These files are saved to different catalogues, either global or local, from where the potential users, in this case the capability matchmaking software, can then utilize the descriptions. The resource description represents the basic characteristics, capabilities, interfaces, and properties of the resource. It can contain links to documentation, CAD models and to illustrative figures. One essential part in the resource description is the capability description of the resource, which selects the relevant capabilities and their parameters from the Capability Model.

Central information model for the matchmaking is the Capability Model. The Capability Model is a formal ontology model which is used to model capabilities and their relations. Capability consists of concept name, such as "Drilling", "Milling", "Moving" and so on, and parameters, such as "payload", "speed" or "torque". The model consists of simple and combined capabilities, where combined capabilities are combinations of simple capabilities. For instance "Picking" is a combined capability which requires as an input "Moving", and some sort of "Grasping" capability. The capability model forms a capability catalogue, which is a pool of capabilities that can be assigned to resources to describe their functionality. [12]

The Resource Model Ontology imports the Capability Model and is used to describe manufacturing resources and their capabilities. An instance ontology of Resource Model can be automatically created and populated by reading in the information from RDs. The Resource Model can also be used to model systems composed of multiple resources. Based on the relationships between simple and combined capabilities, it is possible to identify potential resource combinations that can have a certain combined capability. [12]

The Resource Model imports another ontology, called Resource Interface Model, which describes the resource interfaces in detail. It is used during the matchmaking for checking the compatibility of the resources from interface perspective. In other words, it is utilized to assess if two resources can be connected physically together. It includes information about interface definition, including identifier, name, category, gender and purpose of the interface; Interface standard and its characteristics and the standardisation organization; Interface port implementation. [13]

Also, the product requirements are described using formal Product Model ontology. The Product Requirement Description (PRD) contains information about the parts, how they are connected to assemblies and what kind of processes and process parameters should be used. The Product Requirement Description uses the same concepts that are involved in the Capability Model. Thus, there is a link between the resource capabilities and process requirements of the product. [11]

#### **2.2 Overview of Capability Matchmaking Process**

Figure 1 represents the capability matchmaking process. The Capability Matchmaking is implemented as a service [14] that the System Designer may call through his desired planning system. When the designer wants to launch this service, he will have to give it certain inputs, for example the matchmaking search space. This input includes description of the resources that should be taken into consideration during the matchmaking, and description of the product (i.e. the PRD), for which a match will be looked for. Thus, the resources and product requirements need to be formally described before the capability matchmaking process can be initialised. These resources in specific search space are automatically processed and read in from RDs to form a Resource Model instance ontology. The input related to the resources may be either a description of the existing system layout (i.e. a resource hierarchy building a system layout) or pool of resources selected from one or various resource catalogues. In the latter case, the input is in form of a flat list of resources creating a resource pool. The Capability Matchmaking software will then process these inputs and provide the matchmaking result to the designer. The result includes information about the resources and resource combinations matching for each process step of the product requirement. This information can then be further processed in the system designer's planning tool where the desired resources are selected according to some company or situation specific criteria. [14, 15]

**Fig. 1.** Capability matchmaking process inputs and outputs

The capability matchmaking involves two aspects: Generation of new resource combinations and matching the capabilities of these combinations with the product requirements. The process flow is illustrated in Fig. 2 from left to right. First, the matchmaking system generates new resource combinations that have capabilities requested by the product, e.g. "Screwing". Next, the interface compatibility of the resources is checked based on the interface matchmaking rules [13, 16]. After that, the combined capability parameters are calculated for the remaining resource combinations based on the combined capability rules. Finally, when the resource combinations have been created and their combined capabilities have been calculated, these combined capabilities are compared to the characteristics and requirements of the product. For this purpose, capability matchmaking rules are used, which find out which resource combinations answer the requirements of the product. The combined capability rules have been introduced in [17] and matchmaking rules in [11]. Both have been implemented with SPIN rule language (SPARQL Inferencing Language).

**Fig. 2.** Internal process phases in capability matchmaking

#### **3 Interface Matching Process**

The objective of the interface matching is to evaluate if a finite set of resources is connectable all together. The interface matching process and algorithm has been explained in detail in [16], and data models, namely Resource Interface Model, and rules guiding the interface matching process in [13]. The interface matching process is independent evaluation procedure working as one phase of capability matchmaking. Once capability matchmaking finds a potential combination of resources, it calls the interface matchmaking procedure with the found resources as a set of tested resources. With the call capability matchmaking provides populated Resource Model ontology as argument. At the end of the call, the interface matching procedure responses with simple true or false response, if the given set of resources are connectable from interface point of view or not.

The interface matchmaking algorithm has two main phases – coarse level and fine level interface matching. The former focuses on the interface code and the gender. The first checks only that the string label representing the coded name of the interface standard is the same at both resource sides. The second defines the polarity of the interface and evaluates it with simple rule – male and female or two neutrals can be connected. Only the coarsely connected resources can continue to fine level interface matching. It utilises the detailed interface characteristics. Each interface contains zero to many additional characteristics. These are properties with name, value and operator, which describes a rule how this characteristic should be compared with a counterpart in order to have successful connection between the resources. Each characteristic is defined as optional or mandatory. All mandatory ones must match with counterpart before resources are deduced connectable at fine level. If the interface does not have any additional characteristics defined, the coarse level match is directly valid at fine level.

The algorithm is implemented with Java. It makes several SPARQL queries to Resource Model and Resource Interface Model ontologies, in order to collect information and reason out connectable resources. It also uses internal data structures, where buffered information and intermediate results are stored for later use. At the end of both interface matching levels, algorithm uses intermediate results to create network out of tested resources. If and only if a single connected network containing all resources is created, it is determined that these resources are connectable. Detailed description of the algorithm is provided in [16]. The software module implementing the interface matchmaking process is now integrated as integral part of overall capability matchmaking process.

# **4 Case Examples**

Two case examples are used to illustrate the capability matchmaking process, specifically focusing more deeply into the interface matchmaking phase. The first case focuses on screw driving and the second on pick and place operations. In practice, capability matchmaking processes all product requirements i.e. each process step one by one and provides output results for each process step at once. For illustration and simpler representation, the cases presented here focus only on a single process step and its requirements and results.

### **4.1 Screwdriving Solution**

This case focuses on a single process step, which defines the requirement for fastening two parts with a M6 sized screw with socket head cap (socket size 5 mm). The required end torque is 13..17 Nm. The matchmaking for this process step is illustrated in Fig. 3, which shows only a subset of our complete test data. First, the matchmaking will create new resource combinations from the available resource pool for the required capability at capability concept name level, i.e. resource combinations that could provide "Screwing" capability. The available resources in the resource pool are illustrated in the up left corner in the figure, while the first phase presents four different combinations created by the matchmaking software. While creating these combinations, matchmaking software will simultaneously check that the interfaces of the resources match at fine level, and that the resources can be physically connected. In case of the resource combination (b) on the second phase, the tool bit does not fit into the screw driver's tool interface and thus incompatible combination from interface perspective is filtered out.

After that, the matchmaking system will calculate the combined capability parameters for the remaining combinations and check that the capability parameters match with the parametric requirements of the product. During the combined capability calculation, the matchmaking system considers each individual resource and calculates the viable range for the whole combination. I.e. if a tool bit can bear only a certain torque, it can be limiting the overall torque for the whole combination. The combined capability SPIN rules are used for inferring the combination parameters and matchmaking SPIN rules are used to compare the parametric requirements of the product with the capability parameters. In this example case the matchmaking system will check that the screw type and screw head size matches with the tool size and that the provided torque is within the range of the required torque. Unsuitable resources and resource combinations are again filtered out. In this case the combination possibility (a) (in phase three) does not provide enough torque, as it limits in maximum to 6 Nm, and it is eliminated from this

**Fig. 3.** Capability matchmaking process for screwing case

result. In the end, only the two suitable resource combinations are left as suggestions in the matchmaking result.

Within our practical test data, the resource pool used for this specific case had 68 resources. 530 resource combinations providing "Screwing" capability where found, and 36 out of those was found compatible from the interface point of view. Combined capabilities were calculated for these and finally two resource combinations were found feasible for the given product requirement. These were presented in Fig. 3.

Table 1 shows interface details for the resources used in this case example. Interface port is a placeholder for interface(s) used for a specific function. Interface standard identifies the code of the standard used. Gender sets the polarity of the interface implementation. Additional interface characteristics define properties characterising the interface, such as size and shape in this example. Comparison operator defines how a characteristic should be used in the compatibility evaluation. In this case operator is 'SAME\_SET', which means that at both sides of the connection, the characteristic value(s) must be the same to allow successful connection.

In Fig. 3. we have the first resource combination (a) from resources 1, 5 and 9. The interfaces allows the combination (See Table 1) – driver (1) has fitting interface (IF1) with the tool bit (5/IF1) at standard, gender and all interface characteristics. Driver (1) has also fitting interface (IF2) with the operator (9/IF1) moving the driver. But later, the combination (a) fails to fulfil the capability parameter requirement for generating enough output torque. The second resource combination (1, 6 and 9) fails at interface matching. The gender of driver (1/IF1) and tool bit (6/IF1) does not match, and even it would, the interface characteristics are not fitting (See Table 1). This would be the case if driver (1/IF1) is tried to connect with tool bit (7/IF1), when the interface characteristic C.Size will prevent the connection. In case of the resource combinations (c) and (d) both interface and capability parameter requirement are matching, leading to propose resource combinations (3, 7, 9) and (2, 6, 9) as a match and potential solution for this product requirement.

#### **4.2 Pick and Place Solution**

Another case example focuses on pick and place case. Similar kind of case was discussed as test case in [13]. The handled part is cylinder shaped having diameter of 44 mm and length of 30 mm and it weights 50 g. The product requirement defines following needs for the process step: grasping is done externally, maximum allowed compressive force is 10 N, transportation is needed in XYZ-directions (linear movements), and positioning accuracy is 0.2 mm.

Within our practical test data, the same resource pool with 68 resources is used as in the previous case. It contains two grippes and seven moving devices. The capability name level matchmaking found 12 potential combinations providing "pick and place" capability, and four out of those were found compatible also from the interface point of view. Combined capability parameters were calculated for these and finally three resource combinations were found feasible for the given product requirement. In this case three different robot arms are found mating with one of the grippers. Resource combination (a) from resources 4 and 1; (b) from 5 and 1; and (c) from 6 and 1 (See Table 2). The resource (4) demonstrates use of several standard interfaces for same interface port. In this case the Schunk-SWS interface is implemented with help of an adapter plate, thus a tool fulfilling either of these standards can be connected to this interface port. In case of resource combination (c) the gripper has two interface ports where the connection can take place – 6/IF2 and 6/IF3. The current version of interface matchmaking does not count or reserve the interface ports, but match is returned if there is any suitable position for connection. The fourth resource combination (d) is a manipulator (3) mating with the gripper (2) (See Table 2), but it provides motion only in two degrees of freedom, thus getting neglected from the result.

#### **5 Verification of Capability and Interface Matchmaking Results**

Important part of our SW development process was testing and verification. We used automatic unit tests for our SW development, but the verification of the end results was done mainly manually. We defined manually the expected resource combinations, and inferred and calculated the combined capability parameters from the input resource pool and the product requirement in question. This defined a target result, which was then compared with the result of automatic capability matchmaking process. The verification contained mainly two evaluation criteria: a) all resource combinations/resources compared to manually deduced matches are found, and b) no any false positive matches are included. I.e. resource combination is included to the result, even it is not solving the product requirement from whatever perspective or its resources are not connectable together.

In the development phase, we verified separately and gradually the operation of the different phases of our capability matchmaking process. I.e. starting from name level matchmaking and ensuring gradually, that correct sub-set or sub-result was passed on

the next phase. This was mandatory because of complexity of our system and eventually timely long run of complete capability matchmaking process.

The verification was comprehensive and truthful, because of the use of representative resources even the amount of resources was limited. We had different kind of resources providing the same simple capability, with good mix of interfaces and capability parameter values. The finite set made it possible to determine and calculate manually all possible resource combinations and their combined parameter values. The processes (capability name matching, SPIN rules, and interface matching algorithm) are expected to operate deterministically, thus the matchmaking should behave similarly and find correct results when the search space (i.e. size of resource pool) is enlarged.

# **6 Discussion and Conclusions**

This paper presented interface matchmaking process and how it can be used to sort out resource combinations as a part of capability matching process. We have integrated the interface matching software module as integral part of the overall matchmaking process, and we have proven that the implementation of the prototype does work as desired. In this paper, the operation of the whole capability matchmaking process was demonstrated through two cases, focusing on interface matching. The presented cases illustrate how the capability matchmaking creates proposed resource combinations, and how interface and parametric matching filter out the non-fitting combinations, leaving eventually only the fitting matches as output. Sub-set of resources, which are involved within the cases, and the resources' interface definitions were described in the paper. The verification of results was discussed. The output of capability matchmaking corresponds with the manually deduced results for the given test resource pool containing 68 resources.

The capability matchmaking method can facilitate rapid system design and reconfiguration planning, by allowing computerised methods to find feasible system configuration scenarios to different product requirements. Use of automatic matchmaking reduces and speeds up the manual design efforts, as the system designer can focus his/her resource selection to truly connectable and fit resources, instead of searching for resources, and analysing their interfaces and properties. The matchmaking can be applied over large resource catalogues containing thousands of resources and their variants, which can be automatically screened to find the few appropriate resources and resource combinations. Additionally, the digital resource catalogues are expected to contain production resources from multiple vendors, which increases number of resources to study, but also increases the number of available alternatives. Thus, matchmaking opens possibilities for new and more innovative solutions to be found. The system designer is not bound to comfortable "old and known solution", which is almost solving the requirements, but he/she can select the optimum one.

As a future work, we have some ideas to continue the interface matchmaking procedure to still finer level of detail including resource matching at interface port level. This can extend the matchmaking procedure for suggesting also physical layouts, not only resource combinations. Testing of capability matchmaking tools and processes will be continued with more resources, and different assembly and manufacturing processes.

### **References**

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# **Investigation on the Convergence of the Genetic Algorithm of an Aerodynamic Feeding System Due to the Enlargement of the Solution Space**

Torge Kolditz(B) , Caner-Veli Ince , and Annika Raatz

Institute of Assembly Technology, Leibniz Universitaet Hannover, 30823 Garbsen, Germany<EMAIL_ADDRESS>
**Abstract.** To meet the demands for flexible assembly technology, an aerodynamic feeding system has been developed. The system autonomously finds the optimal configuration of four parameters – two angles of inclination, nozzle pressure and component speed – using a genetic algorithm, which has been presented in earlier work. To increase the flexibility of the feeding system, an actuator was implemented, that enables the variation of the nozzle position orthogonally to the moving direction of the components. This paper investigates the effects of the more flexible flow against the components on their behavior when passing the nozzle. Additionally, the nozzle position was implemented into the genetic algorithm as a fifth parameter. Therefore, the impact of the enlargement of the solution space of the genetic algorithm due to the implementation of a fifth parameter is investigated in this paper as well.

**Keywords:** Assembly · Genetic algorithm · Aerodynamic feeding

# **1 Introduction**

The buyer's market is changing, which places new demands on products. These demands include individual design, a high standard of quality and a minimum price. Added to this is the shortening of the product's lifespan [1]. Production must adapt to these demands while the industry is pursuing cost reduction in order to maximize profits. Secondary processes that do not make a direct contribution to assembly must therefore be kept lean, reliable and inexpensive. Apart from organizational and constructive measures, automation is one way to rationalize assembly processes [2].

The costs of an automated production line are largely generated by feeding and transport systems. The actual assembly process is responsible for about 20% of the costs [3]. Feeding plays an important role, as the objects are transported as bulk material for cost reasons. Bulk material is cheaper and easier to handle [4]. For the following process, however, the objects are required in a defined position. For this reason, a targeted orientation from the bulk material must take place so that the next process can be performed [5]. The feeding process can be divided into four subtasks [3].