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Volume: 04 Issue: 07 | July -2017 p-ISSN: 2395-0072


Akshaya D1, RK Karunavathi2
1 M.Tech Scholar Digital Electronics and Communication, Dept. of Electronics and Communication Engineering
BIT, Bangalore, Karnataka, India
2 Associate Professor Dept. of Electronics and Communication Engineering BIT, Bangalore, Karnataka, India

Abstract - FBG sensors are the optical sensors which are stable SLED source and optical Fabry-Perot filter. In every
sensitive to the external environmental conditions like interrogation system the interrogation monitor
temperature, pressure, strain. This feature of the FBG sensors communicates the software which is designed to set the
make them suitable for many applications where electrical basic settings of the interrogator and to measure the FBG
sensors are to be replaced due to their sensitivity to the sensor results. In the process of making portable and flexible
electromagnetic interference and other radio frequency interrogator system, one idea is to replace the software
sources. FBG sensors are compact in size and their application with that of the embedded system. This paper
performance are not affected by the electrical sources. The presents the method to replace software with that of the
sensed parameters of the FBG sensor should be analyzed using interrogator. For high resolution, high speed, and better
the interrogator monitor. To measure and test these external SNR an interrogation method based on wavelength-swept
parameters the computer based application is used, which is lasers [8] is used.
connected to the interrogator. In all most every application
where the FBG sensor is used to measure the external 2. BACKGROUND
parameters, interrogator used a specific software which is
installed in the PC. This paper aims to eliminate the use of 2.1 Basic principle of the FBG sensor
software to measure and test the FBG sensor results. Instead it
The fiber bragg grating sensors are optical sensors,
demonstrates the use of embedded system which
formed by writing gratings inside the optical fiber. The
communicates with the interrogator to perform measurement refractive index changes periodically inside the core of the
analysis. optical fiber when it is exposed to the UV radiations.
Key Words: FBG; Interrogator; Embedded system; In the interrogation system SLED is used as an optical source
Temperature; Strain; Stress. that illuminates the broadband light on the FBG sensor. The
FBG sensor works with the principle of fiber bragg wave
1.INTRODUCTION length. When a broadband light illuminates on the FBG
sensor, it transmits all other wavelength except the one
wavelength which is called bragg wavelength. It reflects the
In some applications external parameters like temperature, bargg wave length, and the shift in the bragg wave length
pressure, strain should be monitored continuously for their takes place based on the external parameters like
better performance. Several low cost FBG interrogator temperature, pressure and strain. The shift in the bragg
systems were developed [1-3] to perform wavelength wavelength gives the amount of external parameter that is
analysis of the FBG sensor. Compared to that of the electrical experienced by the FBG sensor. The bragg wave length is
sensors, FBG sensors have higher sensitivity to the above given as :
mentioned parameters and are insensitive to electrical
sources. This makes wide spread use of FBG sensors in many
applications. FBG sensors can also be used to measure
Where , is the bragg wavelength, n is the refractive index,
ultrasound, magnetic fields, acceleration and vibration [3].
is the spacing between the gratings. The reflected
Multiple FBG sensors are used in the aircrafts [4], civil
wavelength is given to the interrogator that performs
structural monitoring, monitoring wind turbines [2], wavelength analysis. The 3 port optical circulator is used to
transformer monitoring. The sensed results of the FBG connect between optical source, FBG sensor and the
sensors are analysed using different interrogation interrogator. The figure 1 shows the basic settings of the
techniques. In large scale applications FBG arrays can be interrogation system.
used to measure different parameters [5]. Single optical fiber
with multiple gratings results in a fiber bragg grating array. A spectrometer based interrogator is used in the
Spectroscopy based interrogator is the recent development interrogation system. An optical spectrometer is used to
for numerous microwave photonics applications [6]. Many measure the light properties over a specific portion of the
different interrogators are used which is compatible with electromagnetic spectrum. The spectral lines are produced by
the spectrometer. From the sensor through circulator, the
that of different FBG sensors. But the portable interrogation
spectrometer in the interrogator takes the reflected light. The
system will be of great benefit [7] which uses temperature spectral lines are imaged on the detector photons which are
2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2662

then converted into electrons and are digitized, read out system and the response from the interrogator is received by
through the USB port of the interrogator. The interrogator is the embedded system which is displayed on the screen.
connected to the PC/Laptop which is installed with the
2.2 Transformer oil testing
specific software. The results of the FBG sensor tested and
analyzed with the software installed in the PC. The transformer oil is used as coolant which conducts heat
away from the transformer coil. The core and windings of the
transformer are fully immersed in the transformer oil which
FBG sensor act as an insulating element. The temperature of the
transformer increases at peak periods when there is increase
in demands for electricity generation and distribution
Optical Interrogator networks. This causes failure in the transformer due to its
circulator breakdown. Hence the temperature of the transformer can to
source be monitored by using FBG sensors. Since the transformer oil
acts as coolant in the transformer its temperature should be
monitored by using FBG sensor. The temperature at which
Fig-1: Basic interrogation system the transformer oil produces vapours to occur a flammable
In the interrogation system, the optical source is connected to mixture with air is known as its flash point. In general the
the first port, FBG sensor to the second port and interrogator flash point of the transformer oil is more
to the third port of the circulator. The broad band light than C(> C). The temperature of the transformer oil
travels from first port to the second port of the circulator to should be lower than that of the flash point. Temperature at
illuminate on the FBG sensor. Based on the external which oil just begins to flow under standard test condition is
parameters sensed by the FBG sensor, shift in the Bragg known as pour point of the transformer pour point of the
wavelength occurs and is transmitted from second port to the transformer is important at the places where weather
third port of the circulator where interrogator is connected. condition is ultimately cold.
The figure 2 below shows the reflected light from the FBG
In the transformer oil testing procedure, two fiber bragg
grating needle temperature sensors are used among which
one is place at 60mm from the base level of the kettle and the
other one is placed at 110mm from the base level of the
kettle. The transformer oil is heated up to 70 degrees Celsius
and is poured into the kettle. The oil drainer is attached to the
kettle when it drains the oil, after some time the oil in the
kettle crosses below the upper level sensor and hence sudden
changes in the sensor temperature occurs from 70 degrees
Celsius to 48 degrees Celsius. When the oil drains the lower
level sensor, the temperature of the sensor changes from 70
degrees Celsius to 60 degrees Celsius. Figure 3 shows the
graph of the temperature oil testing result.
Fig-2 Reflected light from FBG sensor
The spectrometer used in the interrogator can be
communicated in three ways:
Dynamic link logic(DLL)
Firmware commands
To communicate with the software, the software
should be installed in the PC. Different softwares can be
designed in accordance with that of the spectrometer and
detector used in the interrogator system using different
programming languages. To communicate using DLLs the
user must write a sort of a program in any programming
language which allows using external libraries. To
communicate using firmware commands one needs any
terminal program, which allows a connection via virtual Fig-3. oil testing result graph
In this testing procedure the interrogator communicates
To replace the software with that of the embedded with the software where the general parameters of the
system, the spectrometer is communicated through firmware spectrometer and the image sensor in the interrogation
commands. These commands are sent through the embedded


system are set by using the software to perform measuring the raspbian operating system. When the SD card is inserted
results which is shown in figure 4. to the raspberry pi, it will boot up the raspberry pi and
different operating system can be downloaded using NOOBS.
In the software, some of the interrogator parameters that
Raspbian operating system should be installed using NOOBS.
to be set are camera setup, interrogation set up, file set up,
buffer set up. The camera set up includes maximum scan rate Python programming is done in the raspberry pi to send the
of the image sensor, exposure time of the detector and the firmware commands to the interrogator through the USB
offset voltage to eliminate the offset error of the image port and to receive the replay from the interrogator that
sensor. should be displayed on the HDMI monitor and the LCD
display. When the interrogator is connected to the raspberry
In the method of replacing the software with that of the
pi the enumeration process will take place. By using the lsusb
embedded system, these parameters are set by using
and sudo dmesg c LINUX commands in the raspberry pi
firmware commands that are sent through the embedded
terminal window, the details of the USB device like product
ID and the vendor ID is obtained.
The USB details is shown in figure 5. The FTDI USART to
USB converter is used in the interrogator which makes USB
port available in the interrogator through which the
commands to be sent to the interrogator.

Fig -4. Interrogator parameters setting by using software.

Fig-5. USB details of the devices connected raspberry pi
3. SELECTTING SUITABLE E EMBEDDED SYSTEM The figure 9 shows that the FTDI USB device converter now
Suitable embedded system should be chosen so that it can attached to ttyUSB0. The ttyUSB0 is the built in USB on the
raspberry pi where tty is the associated terminal with the
send and receive the data from the interrogator. The
interrogator has the 3 main port: power port, parallel I/O current process. The interrogator is connected to the ttyUSB0
port, and the micro USB port. The data communication takes of the raspberry pi and hence in the python program, when
place through the micro USB port. The embedded system is defining the USB device (IMON interrogator) ttyUSB0 should
chosen in such a way that it is capable of detecting the USB be mentioned.
device (IMON interrogator) and performs the enumeration 4.2 Basic settings of the interrogator using firmware
process to initiate the communication with the interrogator. commands
The raspberry pi is the embedded system which is chosen to
communicate with that of the interrogator. The raspberry pi General parameter setting:
system is similar to that of the computer which is capable of 1. *IDN? This command gives the device ID of the
communicating with that of the USB peripheral devices. The spectrometer used in the interrogator.
raspberry pi is having HDMI port where HDMI display should
be connected and it is having the mocro USB port for the 2. *PARAmeter:TINT 10 This command sets the
power supply, Ethernet port to access to the internet. Mouse, integration time of the image sensor used in the
keyboard and other USB devices can be connected through interrogator to 10 ms. The image sensor captures
USB ports. the light spectrum that incident on it and converts
the photons into voltage or current that depends on
4. FBG SENSOR TESTING BY USING EMBEDDED the intensity of the light. Figure 6 shows the setting
SYSTEM of integration time to 10ms.
4.1 Experimental setup
The experimental set up includes the raspberry pi
connected to the interrogator. The FBG needle temperature
sensor and the conventional electric thermocouple sensor are
placed inside the kettle that contains the water at normal
temperature. The SD card installed with the noobs should be
inserted to the raspberry pi. NOOBS is an easy way to install

Fig-8. Scan delay setting

Fig-6. Setting integration time 2. *PARAmeter:FASTScan 10 Fast scan time is the

time taken for next fast scan. This command sets the
3. *PARAmeter:ADCResolution? This command gives fast scan time to 10ms.
the information about the size of the analog to digital
converter used in the interrogator. For example, 16 Settings for measurements: The measurement settings
bit and 32-bit ADC. consist of some commands to perform the measurement of
the FBG temperature sensor.
4. *PARAmeter:PIXEL? -This command gives the
number of pixel information of the image sensor. 1. *PARAmeter:OFFSet 0 This command is used to set
256-pixel image sensor is used in the interrogator. the offset voltage of the image sensor. The offset
voltage is set to 0v by this command. The offset error
5. *PARAmeter:BAUD 115 -This command is used to of the image sensor can be eliminated by using offset
change the baud rate of the interrogator to 115200 voltage.
baud this result is shown in the figure 7. 3 baud rates
are supported by the interrogator (IMON), the 2. *PARAmeter:GAIN? This command is used to get
default baud rate is 921600, and other two baud the gain value. The figure 9 shoes the gain value.
rates are 115200 and 38400.

Fig-9. Gain value

3. *MEAS:TEMPE? This command is used to get the
temperature value measured by the FBG sensor. The
figure 10.
Fig-7: Baud parameter
Time settings: scan time of the detector which is used in the
interrogator need to be set. Some of the commands for time
setting is given below.
1. *PARAmeter:SDELay 10 This command sets the
scan delay of the interrogator 10ms which is shown
in the figure 8. The scan delay is the time difference
between the initiating a measurement and its real
start of measurement.

Fig-10. Temperature value

The measurement result gives the temperature of 31.8
degree Celsius at normal temperature of the needle sensor.
The water in the kettle is heated up in step wise for 5 min and
both the needle sensor and the thermocouple senses the


temperature of the water. The temperature of the sensors is modelled as portable computer system and this makes the
tabulated which is shown in table 1. interrogator module as a portable unit. The system can be
developed to inform the operator when the temperature
Table -1: Tabulated temperature result
exceeds the threshold level.
Sl.N Thermocouple FBG needle sensor
o temperature in degree temperature in REFERENCES
Celsius degree Celsius
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Fig-11: temperature comparison graph JUNE 15, 2015.


The result shows that FBG interrogator can be
communicated with the embedded system, this replaces the
software communication with the FBG interrogator. By
communicating interrogator with the raspberry pi, the
interrogator system can be designed as a single module
without using computer and hence it reduces the cost. This
interrogation module is flexible and hence with the low cost
the results of the FBG can be tested.
5.1 Future work:
The interrogation is communicating with the raspberry pi
and the results are displayed on the HDMI screen. Instead of
using HDMI screen, TFT LCD can be used in order to make the
module as simple as possible. Key board and mouse are
connected to the interrogator. The keyboard and the mouse
can be replaced with human machine interface(HMI). Hence
using TFT LCD as display and using HMI raspberry pi can be