# Design and Development of a Remotely-enabled Modular Release Mechanism for
Autonomous Underwater Vehicles††thanks: As of May 30, 2023, the work described
herein is covered under US Patent 11,661,153

Demetrious T. Kutzke ‡, Gustavo E. Miranda López‡,
Robert J. Herman‡, and Harryel Philippeaux Corresponding author: Demetrious T.
Kutzke (Demetrious.Kutzke1@navy.mil)The authors are with the Naval Surface
Warfare Center Panama City Division, Panama City, FL 32407, USA.

(September 18, 2021)

###### Abstract

We introduce a launch device, called the remotely-enabled modular release
mechanism, to augment rapid testing and prototyping of cooperative autonomy
maritime applications by facilitating autonomous deployment of an autonomous
underwater vehicle (AUV) from an autonomous surface vessel (ASV). While we
focus our development on a specific application of deploying an AUV from a
catamaran style ASV, the release mechanism can be adapted to different
deployable objects and towing vehicles, such as buoys and sensors for
oceanographic surveys or mono-hull ASVs. In this paper we explore a number of
hardware and software design considerations to facilitate ease of integration
with existing maritime autonomy systems. We expound on bench tests and in-
water tests used to explore the utility of the release system and diagnose
system issues. Additionally, we make a first-principles argument, based on a
hydrodynamics physics model, for assured deployment that is virtually
independent of sea state, making the release system a suitable alternative for
different maritime applications in varying environmental conditions.

##  1 Introduction

Cooperative autonomy solutions show promise for accomplishing traditionally
challenging tasks such as cleaning and containing oil spills, sensor network
deployments, medical supply deliveries, package deliveries, and natural
disaster recovery systems [38, 39, 36, 1, 25, 18, 14, 26, 16, 42]. Cooperative
autonomy here refers to multiple robots or autonomous agents working in a
cooperative fashion to accomplish a task or a number of subtasks. These
applications often require the coordination of distinct tasks across
heterogeneous agents or robot teams. This means that often one agent’s
schedule is coupled to another agent’s schedule through the task assignment.
These cross schedule dependencies present significant challenges to develop
task allocation and scheduling algorithms and represent an even more
challenging task to satisfy in practice [21, 23, 46, 10].

In maritime applications, for example, transporting, docking, and deploying an
autonomous underwater vehicle (AUV) from a manned or autonomous surface
vehicle (ASV) are common tasks that are required to perform some of the
aforementioned cooperative autonomy applications. Developing schedules around
these tasks falls broadly under the multi-robot task allocation set of
problems and doing so in practice can be very challenging, particularly for
underwater robotics since communication assumptions breakdown and require
alternative collaboration techniques [15, 19, 9, 34, 20].

Our contribution is the development of a technology that augments rapid
prototyping of cooperative autonomy solutions that need to autonomously deploy
something, be it an AUV, buoy, or a sensor from an ASV. It is appropriate to
think of the device, which we call the remotely-enabled modular release
mechanism or (RM)^2 as an experimental apparatus, the use of which allows
development teams to quickly test and integrate advanced autonomy without any
additional development time to acquire or build a custom solution.

Two critical ideas form the basis of this work: First, experimental autonomy
requires numerous technology enablers, not the least of which is algorithm and
vehicle specific hardware to “prove” or provide an experimental basis for
certain theoretical algorithms. Second, without the experimental apparatus in
place, there is no clear path that goes directly from theory and numerical
simulation to commercial application.

Cost and design considerations must be considered as well, since cooperative
autonomy solutions typically begin with a bulk of the research focused on the
theoretical and numerical developments of the underlying algorithms and do not
permit extensive experimental budgets. Moreover, there is little incentive for
academia or government to invest in the validation of “proven” cooperative
solutions, since there is little chance of publication success and little
applicability to the needs of government. Industry cannot justify investing in
theoretical algorithms, if there is no clear path to commercial success. To
mitigate these challenges which often leave technically rigorous and
innovative theoretical results relegated to simulation results, early stage
science and technology projects need affordable, low-cost alternatives for
proof-of-concept and experimental testing of theoretical autonomy projects.

The release mechanism we introduce in this paper offers just that—a low cost
alternative that allows rapid testing and prototyping of cooperative autonomy
for maritime applications. While we focus our development on a specific
application of deploying a AUV from an ASV, the release mechanism can be
adapted to different “deployable” objects and towing vehicles. We are
primarily concerned with a number of hardware design considerations and
software development to facilitate compliance with IoT industry standards. In
summary, our main contributions to the field of launch and recovery of
unmanned systems are

  1. 1.

An affordable, low-cost alternative for deploying AUVs from catamaran style
ASVs

  2. 2.

An argument for assured release under varying sea state conditions

  3. 3.

A control module written in the robot operating system for easy integration
with existing autonomy software

The paper is outlined as follows: In Section 2 we discuss the historical
development of release mechanisms for unmanned vehicles. In Section 4 we
discuss the hardware and environmental factors affecting design
implementation. In Section 5 we discuss industry standards for Internet of
Things data interfaces that affect software design considerations. Section 7
introduces the experimental testing that was conducted to determine the
efficacy of the release mechanism. We conclude in Section 8 with a discussion
on the results and future work.

##  2 Related Work

Many efforts have been dedicated to launch and recovery devices of AUVs from
surface vessels and underwater vessels such as submarines [43, 12, 11, 4, 5,
32, 33]. Often launch and recovery devices use a cable or winch mechanism to
both deploy and recover the AUV onto the vessel. For example, Renilson
describes a proposed solution to recover a AUV from a submarine that does not
require the use of torpedo tubes or slow maneuvering by the submarine to allow
the AUV to overtake the submarine. Instead, Renilson proposes an innovative
approach that utilizes a cable system to recover the AUV via a cable that
drops from the AUV and is captured by the submarine [35]. Alternatively, there
exist numerous studies on the hydrodynamics of launching AUVs from tube-like
structures. Zhang et al. provide a detailed account of the complexities of
deploying a AUV from a torpedo tube, including requirements on launch thrust
requirements throughout the deployment process [44]. Much of the focus on
docking systems assumes design considerations such as a fixed dock or certain
sea state considerations for tethered systems [17, 2]. To accommodate for
complex docking situations, some have propounded sophisticated algorithmic
approaches. Trslić et al. have considered an algorithm-focused approach that
utilizes fuzzy logic for localization control for docking remotely operated
vehicles (ROVs) to floating docks on offshore drilling platforms [41]. They
suggest that due to limited resource constraints on AUVs, alternative methods
must be utilized to predict heave motion of docking stations suspended on the
surface. Landstad et al. utilize dynamic positioning control to predict the
wave motion of the surface vessel that the remotely operated vehicle can use
to match and predict when to dock with the surface vessel [24]. More recently
Zhao et al. have provided a detailed study on a hydrodynamic model of the
coupling between an ROV and an ASV system. This coupling is of supreme
importance since as the ROV approaches the ASV for docking or capture, it
becomes increasingly susceptible to the wave drive motion of the AUV, making
docking an extremely challenging task from a control perspective [45]. Meng et
al. demonstrate the “capture rod” method, which utilizes a rod or cable that
extends from a docking station. The AUV then utilizes an acoustic signal for
homing [29]. These methods vary in sophistication, since recovery devices are
far more sophisticated and require a great deal more effort to recover a AUV
than to release or deploy a AUV system. Some have approached the problem from
an operational research perspective or even a machine learning perspective to
handle localization in the vicinity of the capture rod [40, 3]. Allotta et al.
have also explored docking considerations for underwater wireless recharging
stations. They too encounter similar hydrodynamic issues when looking at
docking robotics platforms on charging stations subject to undersea currents.

The preceding examples provide substantial inspiration for developing a
release system that assumes similar or simplified assumptions. We acknowledge
that this problem is trivial to a certain extent; however, previous attempts
at combining both launch and recovery systems have proven very complex,
costly, and challenging to adapt to legacy systems. In our work, we are not
concerned with recovery, since it is assumed that whatever is being deployed
can either be recovered by hand or will operate in the “persistent” autonomy
space, which is long duration missions that require little to no human
intervention. We also consider that the release mechanism should not be
invasive to the towing system, which according to Kouriampalis et al. can be
quite challenging from an operational and ship design perspective to
accommodate for launching and recovering AUV systems [22]. Since we are
targeting those end users who would benefit from a so-called “plug and play”
device, then we do not want to burden the developer with significant changes
to the available towing vessel.

Sarda and Dhanak introduce a design concept very similar to our release
system. Indeed, the similarities are striking [37]. There are some important
distinctions, however. First, they utilize a release method that requires the
nose and tail of the AUV to enter the water at the same time. During the
deployment phase the AUV is lowered from the payload tray of the towing
vessel, utilizing two winches attached below the payload tray of a wave
adaptive modular vessel (WAM-V). The AUV is suspended from underneath the
WAM-V during transit, which introduces additional control problems for the
autonomy software. We take an alternative approach and utilize the REMUS
vehicle’s neutral buoyancy to tow the AUV on the surface between the
catamarans. We utilize a very simple PVC style housing to contain the AUV
during transit. While the winch system of Sarda and Dhanak is a unique design
concept, we argue that winch systems add additional weight, resource
requirements (such as additional power sources), and reduce modularity by
placing restrictions on the towline. Second, Sarda and Dhanak have invested
significant effort into modeling and simulation of the multi-step process of
launch and recovery. Modeling and simulation (M&S) is important when
considering a process as complex as recovery of the AUV onto the towing
vessel. Their efforts are admirable and quite impressive but beyond the scope
of our discussion, since we have had the advantage of a decidedly less complex
scenario of launch or deployment, exclusively. Third, to our knowledge, Sarda
and Dhanak proposed but never implemented the launch and recovery system they
propounded in their work. To that end, they have not tested the system in an
experimental environment, whereas we have designed, built, and tested our
release mechanism in both laboratory and real-time test events.

##  3 The (RM)^2 System Description

The system works according to the architecture described in Fig. 6 and the
physical system sketch shown in Fig. 3. The user specifies a ‘DEPLOY_POSITION’
in the form of a comma delimited latitude and longitude pair, or a
‘DEPLOY_TRIGGER,’ which is a string alias, in the catkin script for either the
RM2Node directly or in the global catkin configuration for the user’s
software. The RM2Node subscribes to ‘POSITION’ which is updated by the
navigation software of the vehicle, typically published in a so-called local
coordinate system that maps a latitude and longitude to an origin and all
further position updates are in the local coordinates. Internally, RM2Node
converts all coordinates to local coordinates. RM2Node handles conversion
between coordinate systems utilizing the library MOOSGeodesy [31]. Once the
RM2Node registers an update on its subscribers, it checks if the difference
between the current position $\vec{x}^{\prime}$ and the desired deployment
location $\vec{x}$ is within the tolerance distance $\delta$ or if the trigger
has been updated to true. If either of these conditions is true, then the node
triggers a serial connection with the low-level Arduino controller, sending a
single ASCII encoded character ‘D’ to signal that the controller should
actuate the release mechanism. The physical release mechanism consists of a
linear actuator that drives a pin within the housing. A three dimensional
rendering is shown in Fig. 1. In Fig. 1(a), “A” marks the linear actuator, “B”
is the hook or shackle, “C” is the pin that maintains the hook in the locked
position, and “D” is the loop that ensures a consistent angle between the
tether and the hook at all times. Without the pin, the hook will fall open due
to gravity. The tether runs between “C” and “D.” The pin is attached to a
spring that is pulled using the actuator. In Fig. 1(b), there are three main
events that take place in succession of control. The first event, labeled as
“1” in Fig. 1(b), pulls the pin in the direction of the arrow. The second
event occurs once the pin is pulled, allowing the hook to move in the
direction of arrow “2.” In the third event, labeled as “3,” the tether slides
through the U-bolt and passes back through the handle of the AUV which is held
between the pontoons within the PVC housing. This is to ensure that the tether
remains with the WAM-V and does not foul the propellers on the AUV once
released.

An image of the PVC housing is shown in Fig. 2. The PVC is a commercial off
the shelf product, $9.5~{}\text{in}$ or $0.241~{}\text{m}$ in diameter. It is
$0.838~{}\text{m}$ in length. A $3.81~{}\text{cm}$ notch was cut in the nose
portion of the PVC to accommodate the tether that loops through the handle of
the REMUS 100. The notch down the underneath portion of the PVC is to permit
the ballast of the REMUS to slide in as well. The REMUS 100 will power on to
an active state, which starts mission execution, once a magnet activates the
side panel of the vehicle. A magnet is attached to the interior of the PVC
toward the aft part of the housing, which physically touches the side of the
AUV as the AUV exits from the housing. A summary of the (RM)^2 system
specifications is shown in Table 1.

Table 1: (RM)^2 physical dimensions and specifications Specification | Numerical value
---|---
Physical dimensions | $(35.56~{}\text{l}\times 16.51~{}\text{h}\times 12.7~{}\text{w})~{}\text{cm}$
Total mass | 3.95 kg
Linear actuator force | 1468 N
Linear actuator stroke | 5.08 cm

(a) Side view of internal actuator and spring with hook system

(b) Angled view of internal actuator and spring with hook system

Figure 1: (Color online) Renderings of the release mechanism’s internal
components Figure 2: (Color online) PVC housing for the REMUS 100. The picture
is oriented to show the nose portion of the housing

##  4 Mechanical Design Considerations

The primary mechanical design aspects considered when first developing the
system were size, weight, towing strength, and design for maneuverability. An
artistic rendering of (RM)^2 is shown in Fig. 3. The device was constructed to
interface with the WAM-VWAM-V is a registered trademark of Marine Advanced
Robotics, Inc. 16 Autonomous Surface Vessel, manufactured by Marine Advanced
Robotics, Inc [28]. The vehicles are manufactured with flexible pontoons that
adapt to varying sea state conditions, making the WAM-V a suitable vessel for
maritime autonomy research, since they are light-weight, easily transportable,
easy to assemble, and modular for adjusting payloads. The specific vessel
chosen is inconsequential to the development of (RM)^2, however, since it is a
relatively simple task to adapt (RM)^2 by changing the face plate (see Fig.
3(c)) to accommodate alternative vessels. The plate is bolted to the existing
hinge that fixes the payload tray to the catamaran arms.

(a) Top view

(b) Side view

(c) Side view close up

Figure 3: (Color online) Artistic renderings of (RM)^2 configurations

The size of (RM)^2 was constrained to minimize the occupied space on the
vessel, since we did not want to have to modify the vehicle directly or
utilize precious payload space for external sensing and attachments, all of
which would degrade modularity of the vessel. Moreover, care was taken to
avoid any fouling that might occur due to the tethers attached to the pontoons
and the tow cable attached directly to the vehicle. Fig. 3(a) shows how a
simple tether system is used to maintain the position of the AUV within the
PVC housing between the pontoons of the WAM-V. The tethers that attach the
housing to the pontoons are nylon braided rope and constrained to be about
$2/3~{}\text{m}$ in length with a diameter of approximately
$1.27~{}\text{cm}$. (RM)^2 is positioned fore on the towing vehicle rather
than aft to mitigate fouling in the motors. The weight of (RM)^2 was
constrained to occupy a negligible amount of payload weight, which could
affect maneuverability and potentially compromise the integrity of the vessel.
(RM)^2 measures approximately $3.9~{}\text{kg}$ in assembled mass, compared to
the overall payload capacity of the WAM-V which is $113~{}\text{kg}$ using the
four battery configuration. The towing strength was explored to ensure that
(RM)^2 could at least release a towed load equivalent to that of the tow
strength of the tether. Practically speaking, the tow strength is bounded
above by the tow strength of the vessel, which through experiment we
determined was far greater than the towed unmanned underwater vehicle or
approximately $110~{}\text{N}$ of tension in the towline. This tension was
approximated using a simplistic free body diagram shown in Fig. 4.

Figure 4: Free body diagram of the equilibrium forces incident on the AUV
under tow

There are four primary forces acting on the AUV: The drag force, labeled as
$\lvert\vec{F}_{d}\rvert$, acts on the AUV in the $-\hat{x}$ direction and is
proportional to the effective cross sectional area acted on by the fluid; the
gravitational force, labeled as $\lvert\vec{F}_{g}\rvert$; the force due to
tension in the towline, labeled as $\lvert\vec{F}_{t}\rvert$; and finally the
buoyant force that acts opposite to the direction of the gravitational force.
The buoyant force is proportional to the volume of the fluid displaced by the
submerged portion of the AUV’s hull. Assuming there is no net force on the AUV
and the WAM-V moves with speed $\upsilon$ in the $\hat{x}$ direction, then it
is a relatively straightforward task to calculate the tension in the towline.
For the forces in the $\hat{x}$ direction,

$\lvert\vec{F}_{t}\rvert\cos{\theta}-\frac{\rho}{2}C_{D}\sigma\upsilon^{2}=0,$
(1)

and

$\lvert\vec{F}_{t}\rvert=\frac{\rho}{2\cos{\theta}}C_{D}\sigma\upsilon^{2}.$
(2)

In (2), $\rho$ is the density of the fluid medium, in this case, surface
seawater which is approximately $1020~{}\text{kg/m}^{3}$; $\theta$ is
approximately $45$°; $C_{D}$ is the drag coefficient, which utilizing a half-
sphere to approximate the nose of the AUV is $0.42$; $\sigma$ is the effective
surface area, and in this case $\sigma\approx\pi
r^{2}/2=0.057~{}\text{m}^{2}$; and finally $\upsilon\approx 2.5~{}\text{m/s}$.
Substituting these values in (2) yields approximately $109~{}\text{N}$ of
force in the towline. The towline used was rated for nearly $2\times
10^{3}~{}\text{N}$ safe load, indicating that the total tension in the line
under tow is significantly below the rated tow capacity. Since there is no net
force, and the AUV is not completely suspended but only kept on the surface
between the pontoons then the $\hat{z}$ forces can be neglected without
concern for any payload effects.

###  4.1 Total Cost

Since the total funding level for an early stage development effort is usually
in the range of $10^{4}$–$10^{5}\text{K}$ USD, we considered commercial off-
the-shelf components for (RM)^2 that did not exceed $1\text{K}~{}\text{USD}$
in assembled cost to ensure that the acquisition of (RM)^2 was nearly
negligible when compared to the overall funding level [27].

###  4.2 Environmental Conditions

Generally, environmental conditions dictate the external housing for maritime
applications. Polymers and plastics perform the best in harsh salt-water
conditions but lack the durability and robustness to handle the effects of sea
state and varying towline tension considerations. These materials can be
structurally weak when subjected to external forces such as those involved in
towing. (RM)^2 was developed using a steel housing which after only a few
months of testing and exposure to salt-water conditions, showed significant
corrosion, as shown in Fig. 5. The corrosion occurred along the steel cotter
pins that held the linear actuator in place and some oxidization occurred on
parts of the aluminum exterior.

(a)

(b)

(c)

Figure 5: (Color online) Corrosion after in-water testing of (RM)^2

Though we did not consider corrosion resistance as an immediate concern for
rapid prototyping and testing, it is straightforward to implement corrosion
mitigation through a more intelligent selection of housing materials and
components. Stainless steel is known for its anti-corrosive properties, but it
can be costly. Alternatively, there are corrosion inhibitor aerosols that can
be purchased and applied to the housing directly to inhibit corrosion.

##  5 Internet of Things Architecture Considerations

With the proliferation of the Internet of Things (IoT) and advanced sensing
technology, collecting and aggregating data across heterogeneous sensor
networks has necessitated the development of architectures and industry
standards to ensure that sensors and computers can communicate across
different networking communication protocols, hardware systems, and software
systems.

Three architectures have garnered support from the robotics community:
Lightweight Communications and Marshalling (LCM), Data Distribution Service
(DDS), and the Robot Operating System (ROS). Both LCM and DDS offer a publish-
subscribe pattern that allows nodes to communicate directly, while still
maintaining loose-coupling. LCM is an open-source set of tools that targets
real-time systems that require high bandwidth (“fast”) network speeds for
delivering data. DDS is maintained by the Object Management Group (OMG). DDS
also maintains loose coupling between nodes and has a vast user group that
includes academic, None, however, has gained the widespread adaptation in
robotics as ROS. ROS was developed at Stanford University, with a bulk of
early development conducted by Willow Garage to create a generic framework for
facilitating robot communication. It utilizes the publish-subscribe (see
“Design Patterns” by Gamma et al. for a precise description of this pattern
[13]) that relies on a ROS master node (which is a server) that communicates
parameter updates to subscriber nodes.

Since ROS has garnered significant attention in the robotics community, it was
selected as the IoT protocol of choice for its widespread adaptation and
utilization, particularly in maritime robotics applications. By developing the
control software in ROS, (RM)^2 integrates with existing maritime autonomy
software written in ROS. Fig. 6 shows a high-level software architecture of
(RM)^2. The RM2Node utilizes a desired deployment location or deployment
trigger to call WriteToSerialPort which communicates with the low-level
Arduino controller by sending a character that actuates the release mechanism.
The algorithm is shown in Alg. 1.

1:procedure RM2Node($\\{\vec{x}~{}\textit{or}~{}\text{T}\\}$,$\delta$)

2: $\vec{x}^{\prime}=\textsc{ReceivePositionUpdate}()$

3: $\text{T}^{\prime}=\textsc{ReceiveBoolTriggerUpdate}()$

4: if
$\lvert\vec{x}^{\prime}-\vec{x}\rvert<\delta~{}\text{or}~{}\text{T}^{\prime}.\text{value}=\textit{true}$
then

5: $\textsc{WriteToSerialPort}(\text{`D'})$

6: end if

7: return true

8:end procedure

Algorithm 1 (RM)^2 algorithm Figure 6: Software architecture

##  6 A Theoretical Argument for Assured Release

We make an argument based on simplified assumptions that (RM)^2 will release
under certain towing vessel velocity considerations. In Fig. 7 we have drawn a
representation of the AUV under tow. The towing vessel moves in the $\hat{x}$
direction with speed $\upsilon$. We have also shown an accompanying force body
diagram on the center of mass of the AUV that includes all external forces
acting on the AUV while under tow. Assuming that there is no net force acting
on the AUV or any incident surface waves, then it is clear that the tension in
the line implies (RM)^2 will always release. However, under non-zero surface
wave conditions the towing vehicle must move with a speed of at least
$A\omega$, where $A$ is the wave amplitude and $\omega$ is the angular
frequency of the incident wave. To show this, we assume that an incident wave
or plane progressive wave perturbs the towing system, causing both a heave and
surge translation of the AUV in both $\hat{z}$ and $\hat{x}$ directions,
respectively. These perturbations would cause slack in the towline, which
would affect the ability of (RM)^2 to actually release the towed AUV. Under
free surface assumptions, one can show that the resulting fluid velocity field
is given by

$\phi(x,z,t)=\frac{gA}{\omega}\text{e}^{kz}\text{sin}(kx-\omega t).$ (3)

For a detailed derivation of (3) from wave mechanics, the interested reader
should consult the work of Newman [30]. The velocity of the potential field in
the $\hat{x}$ direction is given by $u=\partial\phi/\partial x$. Taking the
derivative yields

$u=A\omega\text{e}^{kz}\text{cos}(kx-\omega t).$ (4)

Notice that the maximum value of cosine is $1$. For the purposes of computing
the surge velocity the coefficient of the cosine function dominates the surge
velocity value. Also, assuming that $z<<k=2\pi/\lambda$ then effectively the
leading order for the surge velocity is

$u\approx A\omega,$ (5)

where $\omega=2\pi/\text{T}$, and T is the period of the wave. Utilizing
values for both wave amplitudes and wave periods that are encountered in a
typical testing environment, $A=0.50~{}\text{m}$ and a wind or gravity wave
with a period of $\text{T}=30~{}\text{s}$ then the towing vessel transit speed
$\upsilon$ must be at least $0.105~{}\text{m/s}$ or about $0.20~{}\text{kn}$.
The WAM-V vessel we used can achieve a maximum of $11~{}\text{kn}$. Clearly,
even at very slow operating conditions and moderate sea state conditions, then
there is sufficient tension in the towline to guarantee release of the AUV.

Figure 7: Simplified representation of the (RM)^2 system. Drawing is not to
scale

##  7 Experimental Testing

Experimental testing of the (RM)^2 prototype was conducted in two parts: bench
testing, to test simplistic assumptions about functionality and system
performance, and in-water testing to verify system performance when deploying
the AUV under realistic mission assumptions.

###  7.1 Bench Testing

Bench tests were conducted to ensure that (RM)^2 could accommodate different
sea state conditions. Specifically, we created conditions in a laboratory
setting to simulate both heave and surge translation. The experimental setup
is shown in Fig. 8. Heave conditions during an in-water test perturb the AUV
in the $\hat{z}$-direction, perpendicular to the direction of travel.
Effectively, this changes the angle between the vehicle measured along the
equilibrium or steady sea condition and the tether. This angle is shown in
Fig. 8, labeled $\theta$. This angle is challenging to measure directly.
Instead, we measured the height $h^{\prime}$ or the distance between the floor
and the testing weight, which was held at a controllable height. By varying
the height $h^{\prime}$, this effectively tests (RM)^2 under heave conditions.
Care was taken to ensure that the line was always taut, as slack in the line
would prevent (RM)^2 from releasing. This is not unsurprising, since any surge
translation without a constant velocity in the $+\hat{x}$-direction results in
slack and will not allow the device to release as expected. To keep the line
taut, at each angle, the distance $d$ was increased.

The bench test parameters are shown in Table 2. The distance between the
WAM-V’s payload tray and the steady state water line (the black solid line
labeled “Ground”) is equivalent to the bench height used in the laboratory or
$H$ in the figure. Although in theory the AUV could fluctuate an arbitrary
amount in the $\hat{z}$-direction, practically, this height is limited by the
tether length that affixes the PVC housing to the pontoons. The housing tether
lengths are $0.660~{}\text{m}$. This implies that AUV can only fluctuate with
wave heights of approximately $1.32~{}\text{m}$. The keen observer will note
that even this is practically limited by the tether length chosen as well,
since at the lowest possible trough of $-0.660~{}\text{m}$ as measured from
the mean water level, then $H+0.660=1.422~{}\text{m}$. This distance exceeds
the length of the tether used as shown in the table. Hence, the AUV would
effectively be suspended from the payload tray. To avoid this, we focused on
sampling heave conditions that did not exceed the length of the tether or
create unrealistic conditions like the AUV colliding with the payload tray of
the WAM-V. In this case, wave amplitudes $h^{\prime}$ between
$-0.50~{}\text{m}$ and $0.50~{}\text{m}$ were scaled to match with a shortened
tether to approximate the maximum and minimum angles produced in the real-time
system. In the real-time system, the maximum angle that is possible occurs at
the wave trough or $\theta_{\text{trough}}$, given by

$\theta_{\text{trough}}=\sin^{-1}\bigg{(}\frac{H+h^{\prime}_{\text{min}}}{l}\bigg{)}.$
(6)

Conversely, the minimum angle occurs at wave crest, given by

$\theta_{\text{crest}}=\sin^{-1}\bigg{(}\frac{H-h^{\prime}_{\text{max}}}{l}\bigg{)}.$
(7)

These equations do not apply in the experimental case, since the origin does
not occur at the “Ground” level shown in the figure. Instead, tether length
was chosen so that the maximum and minimum angles could be approximated within
the bench top height $H$. In the experimental case,

$\theta_{\text{trough,crest}}=\sin^{-1}\bigg{(}\frac{H-h^{\prime}_{\text{min,max}}}{l}\bigg{)}.$
(8)

In both instances of maximum and minimum angles, (RM)^2 successfully released
the tether after five total iterations at each height. Since the device
functioned as expected at both the maximum angle (the wave trough) and minimum
angle (wave crest), then passing through equilibrium the device should release
as well.

Figure 8: (Color online) Bench test configuration for release performance under simulated heave conditions Table 2: Real-time system dimensions and scaled experimental dimensions Parameter | Real-time | Experimental
---|---|---
tether length $l$ | 1.27 m | 0.537 m
mean surface level $H$ | 0.762 m | 0.762 m
wave trough depth $h^{\prime}_{\text{min}}$ | -0.50 m | 0.152 m
wave crest height $h^{\prime}_{\text{max}}$ | 0.50 m | 0.532 m
$\theta_{\text{trough}}$ | 83.57° | 83.57°
$\theta_{\text{crest}}$ | 16.66° | 16.66°

###  7.2 In-water Testing

In-water tests were conducted to determine how (RM)^2 performs when releasing
a REMUS 100 from the WAM-V. Fig. 10 shows the REMUS tethered to the WAM-V. To
validate laboratory tests, the service agent transport problem (SATP)
developed by Bays and Wettergren, was selected as a test case for autonomously
triggering the deployment of the REMUS from the WAM-V [7, 6, 8]. The SATP is a
generalized scheduler for coordinating move, dock, deploy, and survey tasks
across a team of ASVs and AUVs, where ASVs transport AUVs between survey
areas. The algorithm utilizes a mixed-integer linear program to allocate tasks
and near-optimally order waypoint transits. The mission schematic is shown in
Fig. 9. The WAM-V tows the AUV to a preset deployment location. En-route the
WAM-V utilizes the SATP algorithm to optimize the order of the service areas
and generate a ‘WAYPT_UPDATE’ publication in the form of an
$x_{1},y_{1}:\cdots:x_{n},y_{n}$ string that the AUV parses and follows once
deployed. This list is communicated over Ethernet to the AUV, utilizing a
directional wireless antenna. At the deployment location, the algorithm shown
in Alg. 1 executes and the AUV is deployed. At that point, the AUV follows the
optimized waypoint list and rendezvouses with the WAM-V at a preset location.

Ultimately, in-water tests proved fruitful for diagnosing a number of system
issues. The list of optimized waypoints was generated and received by the
REMUS vehicle; however, repeated testing showed that while the RM2Node
received both position updates and triggers to actuate, the device did not
actuate. After many repeated attempts it was determined that a faulty actuator
controller was to blame. After introducing the new actuator controller,
repeated bench tests showed that (RM)^2 functioned as expected under simulated
position and trigger updates. Ironically, funding levels did not permit
extensive in-water testing after the first actuator controller malfunction.

Figure 9: (Color online) Mission schematic of the service agent transport
problem under test

(a) Front view in-water test

(b) Rear view in-water test

Figure 10: (Color online) In-water test images showing the AUV in the housing
under tow by the WAM-V

##  8 Conclusion

We have outlined the development of a remotely-enabled modular release
mechanism called (RM)^2 that facilitates the autonomous release of an
autonomous underwater vehicle from catamaran-like vehicles. While we have
conducted bench tests and sea tests utilizing the WAM-V catamaran, (RM)^2 is
adaptable to a variety of different vehicle types and is not restricted to any
specific vehicle implementation. We have shown that through careful design
considerations (RM)^2 is suitable for a variety of different applications via
the standardized ROS interface built to control the low-level actuation
hardware. Since it is easily assembled and low-cost, (RM)^2 is suitable for
early-stage and proof of concept autonomy projects that require cooperative
autonomy between ASV and AUVs.

Future development will look at a feedback mechanism to inform the autonomy
software of release which differentiates between success and failure. This
will help diagnose system errors and provide a better troubleshooting
mechanism for developers.

## Declarations

D. T. Kutzke, G. E. Miranda López, and R. J. Herman declare that they have
submitted a patent application to the United States Patent and Trademark
Office to cover the release mechanism described in the paper. H. Philippeaux
has no financial conflicts of interests or competing interests to report.

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