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ISSN (ONLINE): 2454-9762 ISSN (PRINT): 2454-9762 Available online at www.ijarmate.com

International Journal of Advanced Research in Management, Architecture, Technology and Engineering (IJARMATE) Vol. 3, Issue 5, May 2017

A Plastic Optical Fiber Accelerometer Suitable for Marine Applications Aravinth Kumar D 1, Aravind Kumar B2, Mosesraj E3, Rajkumar A4, Sathish Kumar K5

UG Scholar, Department of Marine Engineering, PSN College of Engineering and Technology, Melathediyur, India Abstract — In this manuscript, an optical plastic fiber based accelerometer is described, robust and sensitive enough to be used in marine applications, either in ships propeller shaft vibration monitoring, in costal piers or waterfronts. In either case, the output signal can be used for a structural health monitoring scheme. Index Terms— Accelerometer; Plastic Optical Fiber; Vibration Monitoring; Structural Health Monitoring.

I. INTRODUCTION Nowadays, accelerometers, key sensors for technologies, capable to detect shock, tilt, vibration, motion and position, are used in vehicles engineering, navigation, biological and medical applications, industry and machine vibration, structural monitoring and inclination, seismic activity and volcano logy, consumer electronics, image stabilization, among others. Due to the severe conditions of structures or elements in marine environment, a robust and sensitive accelerometer may be required for this particular utilization. This document describes the implementation of an optical plastic fiber based accelerometer, robust, insensitive to watering splashes and humidity and sensitive enough to be used in marine applications, either in ships propeller shaft vibration monitoring, in costal piers or waterfronts. In any case, the output signal can be used for the determination of the demands imposed to the structure or to be part of a structural health monitoring scheme. In hazardous environments, such as energy production facilities (e.g. dams, nuclear power plants or wet environments), the use of optical sensors, namely optical fiber sensors, provides great advantages over conventional electro-mechanic technologies, such as immunity to electromagnetic interference, electric isolation, small size, reduced aesthetic impact, resistance to corrosion and high signal/noise ratio. In most of the proposed and implemented optical fiber based solutions, the sensor element is a fiber Bragg grating (FBG), used to monitor an inertial mass movement, directly related with the external acceleration. The stiffness of the mass’ support and the optical fiber Young modulus are key parameters on the sensor characteristics, such as sensitivity and resonant frequency. The use of FBGs (provides immunity

to the optical power source fluctuations due to the wavelength-encoded nature of the output signal), high signal/noise ratio allows the multiplexing of a large sensors number within the same optical fiber. However, FBG based sensors rely on the stretching/ compression of a very sensitive and fragile silica fiber, which makes the sensor less robust for several harsh applications, such as in marine ones. As well, for the employment of optical FBGs accelerometers, the requirement of wavelength encoded interrogation systems is still a weakness, imposing high implementation costs. The polymeric optical fiber (POF) technology has been improved largely due to the use in the optical telecommunications field, as it was in the past for the case of the Silica-based fibers. POF based sensors, for a wide variety of applications, fulfills also the current needs of an updated and economical viable solution for applications in harsh environments. Usually, POF fibers are large diameter Poly (methyl methacrylate) fibers (core diameter ≈ 1 mm), potentially less expensive than Silica optical fibers, providing higher robustness and flexibility. POF´s elastic limit is 10%, compared to 1% of the Silica fibers, tolerating a strain breakage higher than 30%. Some POF intensity based accelerometers composed of a mechanical cantilever have already been proposed. Those sensors were designed to sense the acceleration proportional to the optical losses associated with the POF fixed to the cantilever. The sensitivity of the sensor was improved engraving groves in the POF bending region. In this case, considering the harsh environmental conditions of marine conditions, a robust intensity encoded optical accelerometer was presented based on POFs, combined the optical technology advantages with a low cost interrogation technique. The system makes use of simpler connectors with high numerical aperture that can be coupled with standard low cost LEDs (Light Emitting Diodes) sources and photo detectors, providing a simpler and lower cost solution to monitor small structures. The proposed sensor can be used as part of a low cost sensing system to monitor boats or marine structures health, wave levels, vibration control in waterfronts, and so on.

ISSN (ONLINE): 2454-9762 ISSN (PRINT): 2454-9762 Available online at www.ijarmate.com

International Journal of Advanced Research in Management, Architecture, Technology and Engineering (IJARMATE) Vol. 3, Issue 5, May 2017

This work is organized as follows: after an introduction reporting the importance of such sensors and advantages of the optical technology, the proposed sensor is introduced and described in the next section. The following section shows the experimental characterization of the accelerometer, allowing assessing its sensitivity and resonant frequency. Finally, some conclusions have been drawn in the final section. II. ACCELEROMETER DESCRIPTION AND WORKING PRINCIPLE The accelerometer main working principle is based on the misalignment between two large core POF fibers, as exemplified in Fig. 1. The sensor structure is based on an inertial mass, supported by an L-shaped Aluminum cantilever beam, connected to the support through a steel leaf spring. The sensor performance and the minimization of the cross-axis sensitivity is mostly due to the leaf spring dimensions (mainly thickness and square shape) and those are directly related with the accelerometer dynamic range, sensitivity and resonant frequency. The POF fibers were firstly aligned with external triaxial translation stages to maximize the transmitted optical power. Afterwards, the fiber on the left of Fig. 1, was moved 0.5 mm down in the vertical direction to induce an initial misalignment, and finally, both fibers were permanently fixed with epoxy resin to the inertial mass and to the structure support. The gap between the two POF fibers is approximately 0.5 nm.

III. EXPERIMENTAL CHARACTERIZATION AND TESTING The optical signal from a LED (IFE93, Industrial Fiber Optics) was applied, with an emitting power of 115 µW, injected in one POF. The optical signal receiver collecting the signal from the output POF was a photodiode (IFD91, Industrial Fiber Optics), which was followed by an electrical transimpedance amplification stage, providing at the output an amplified electrical signal. The data acquisition was completed with a 12 bit ADC (USB6008, National Instruments), with a sampling rate of 2 kHz and then processed with a Lab view® application. To obtain the system (POF sensor and electronic amplification stage) sensitivity to acceleration, the proposed accelerometer was fixed in a metallic breadboard and exposed to a random acceleration within the sensitive direction during 34 s. Then they obtained signal was directly compared with the signal from a solitary calibrated electronic triaxial accelerometer (34201A, Summit), shown in Fig. 1. The data collected during the 34 s test, measured with these two accelerometers, are shown in Fig. 2, from which a sensitivity of 0.675±0.001 V/G was estimated for the POF system.

FIG. 1 PHOTOGRAFY OF THE POF SENSOR WITH INDICATION OF THE MAIN COMPONENTS AND SOLIDARY CALIBRATED ELECTRONIC ACCELEROMETRE FOR SIGNAL COMPARISON

Exposed to external acceleration, the inertial mass moves, which will impose an additional misalignment between the two POF fibers, resulting in the modification on the transmitted optical power. If the inertial mass moves up, a further misalignment is induced and the transmitted optical power decreases, if the inertial mass moves down, the POFs came more aligned and more optical power is transmitted to the photo detector.

FIG. 2 EXPERIMENTAL DATA COLECTED WITH THE ELECTRONIC CALIBRATED ACCELEROMETER AND SIMULTANEOUSLY WITH THE POF SENSOR, WHEN BOTH ARE EXPOSED TO THE SAME EXTERNAL ACCELERATION. THE BOTTOM FIGURE SHOWS BOTH SIGNALS IN THE RANGE 10-20 SECONDS, FOR BETTER COMPARISON

ISSN (ONLINE): 2454-9762 ISSN (PRINT): 2454-9762 Available online at www.ijarmate.com

International Journal of Advanced Research in Management, Architecture, Technology and Engineering (IJARMATE) Vol. 3, Issue 5, May 2017

For this particular test, random low frequency acceleration was applied in the metallic breadboard. The obtained data has confirmed the good performance of the POF sensor against the response of a calibrated commercial sensor, with a normalized root mean squared deviation between both signals of 20.46 mG measured along the test duration. Due to the use of low cost LEDs, photo detectors and electronic amplifiers, the noise variance is considerable; however, this value could be reduced. In particular, it was believed that by using a 14 bit ADC and low noise electronic amplifier, 10% of this noise level could be achieved, which is enough for many engineering applications including marine ones. The resonant frequency of the sensor is a parameter of highly importance. In order to estimate its value, a vertical load was applied to the inertial mass and suddenly removed, as shown in Fig. 3 (being this procedure repeated six times), which will result in the mass oscillation at the natural frequency of the sensor. The sensor response has been measured to obtain its natural frequency value by the application of a Fast Fourier Transform (FFT) on the accelerogram time domain data.

The frequency spectrum of the sensor response is shown in Fig. 4 from which, by peak picking, it is possible to estimate the natural frequency of the POF accelerometer, yielding a value of 32.75±0.05 Hz. The POF accelerometer natural frequency value is adequate to monitor several low natural frequency engineering structures which have Eigen frequencies below 20 Hz. However, the natural frequency of the sensor can be improved and custom-made to the application for which the sensor is meant to be used, mainly through the adjustment of the steel leaf spring thickness. As a test, to demonstrate if the POF system was able to detect impulses on a real situation, the electronic calibrated accelerometer and the POF based one were fixed to a heavy metallic steel plate (≈ 5 kg) placed over the measuring point at the center of a rectangular reinforced concrete 6 m span slab, located at the second floor of the Physics Department building at University of Aveiro, Portugal. In this way it can be assumed that the steel plate and the building, at the measuring point, have the same acceleration and displacement components. Fig. 5 shows the acceleration evolutions recorded with the two types of accelerometers in a 50 s time period, during which five mechanical impulses were applied by the synchronized vertical impulsion of two persons at a point proximally 2 m away from the measuring position.

FIG. 3 a) POF SYSTEM OUTPUT SIGNAL MEASURED ALONG TIME, APPLYING A VERTICAL LOAD AND AFTER REMOVING IT (6 IMPULSES); b) DETAIL OF ONE PROCESS

FIG. 5 ACCELERATION HISTORIES RECORDED DURING THE TEST ON A RC SLAB WITH BOTH ACCELEROMETERS

The results presented in Fig. 5 demonstrate the good performance of the POF based accelerometer against a calibrated electronic accelerometer, capable to detect impulses with acceleration values lower than 25 mG.

FIG. 4 FREQUENCY SPRECTRUM OBTAINED BY APPLYING A FFT TO THE TEMPORAL RESPONSE OF FIG. 3

In most vessels, the movement is made through the propeller forces. Frequently, the propeller rotates at high speeds through a shaft supported by roller bearings. If an

ISSN (ONLINE): 2454-9762 ISSN (PRINT): 2454-9762 Available online at www.ijarmate.com

International Journal of Advanced Research in Management, Architecture, Technology and Engineering (IJARMATE) Vol. 3, Issue 5, May 2017

abnormal roller bearing or propeller is working in the wrong conditions, the vibration change measured close to the shaft can be noticed by an accelerometer. The proposed POF accelerometer is robust and sensitive enough to be used in such applications. IV. CONCLUSION A low cost and robust accelerometer based on polymeric optical fiber has been proposed for marine applications. The accelerometer exhibits a sensitivity of 0.675±0.001 V/G and a resonant frequency of 32.75±0.05 Hz, which makes it suitable for marine and some SHM applications. REFERENCES [1]

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AUTHORS BIOGRAPHY Aravinth Kumar D was born in 1996. He is currently a UG Scholar in PSN college of Engineering and Technology Tirunelveli, in the Department of Marine Engineering. His research interests include Based on Plastic Optical Fiber Accelerometer Suitable for Marine Applications.

Aravind Kumar B was born in 1995. He is currently a UG Scholar in PSN college of Engineering and Technology Tirunelveli, in the Department of Marine Engineering. His research interests include Based on Plastic Optical Fiber Accelerometer Suitable for Marine Applications.

Mosesraj E was born in 1995. He is currently a UG Scholar in PSN college of Engineering and Technology Tirunelveli, in the Department of Marine Engineering. His research interests include Based on Plastic Optical Fiber Accelerometer Suitable for Marine Applications.

Raj Kumar A was born in 1995. He is currently a UG Scholar in PSN college of Engineering and Technology Tirunelveli, in the Department of Marine Engineering. His research interests include Based on Plastic Optical Fiber Accelerometer Suitable for Marine Applications.

Sathish Kumar K was born in 1996. He is currently a UG Scholar in PSN college of Engineering and Technology Tirunelveli, in the Department of Marine Engineering. His research interests include Based on Plastic Optical Fiber Accelerometer Suitable for Marine Applications.