sensors Article
An Optical Fiber Sensor and Its Application in UAVs for Current Measurements Felipe S. Delgado, João P. Carvalho, Thiago V. N. Coelho * and Alexandre B. Dos Santos Electrical Circuit Department, Federal University of Juiz de Fora, Juiz de Fora 36036-330, Brazil; [email protected] (F.S.D.); [email protected] (J.P.C.); [email protected] (A.B.D.S.) * Correspondence: [email protected]; Tel.: +32-9-9916-1712 Academic Editors: Christophe Caucheteur and Tuan Guo Received: 4 July 2016; Accepted: 6 September 2016; Published: 27 October 2016
Abstract: In this paper, we propose and experimentally investigate an optical sensor based on a novel combination of a long-period fiber grating (LPFG) with a permanent magnet to measure electrical current in unmanned aerial vehicles (UAVs). The proposed device uses a neodymium magnet attached to the grating structure, which suffers from an electromagnetic force produced when the current flows in the wire of the UAV engine. Therefore, it causes deformation on the sensor and thus, different shifts occur in the resonant bands of the transmission spectrum of the LPFG. Finally, the results show that it is possible to monitor electrical current throughout the entire operating range of the UAV engine from 0 A to 10 A in an effective and practical way with good linearity, reliability and response time, which are desirable characteristics in electrical current sensing. Keywords: current sensor; long-period fiber grating; optical fiber sensor; unmanned aerial vehicle
1. Introduction Fiber-optic sensors have acquired growing importance in the field of sensor technologies [1–3]. Compared with conventional devices, optical fiber sensors present many advantages [4,5]. These devices are compact, lightweight, easy to install, inexpensive and insensitive to electromagnetic interference [6], which are key features desired for sensing applications. Thus, optical fiber sensors are extremely versatile in measuring variations in temperature [7,8], strain [9], external refractive index [10], pressure [11], humidity [12] and electrical current in high voltage environments [13]. Optical current sensors using optical fibers have been investigated since the 1960s [14,15]. Since then, several researchers have proposed and demonstrated a number of practical applications using optical fiber current sensors (OFCS) [16–18]. One of the most popular configurations for current measurements is based on the magneto-optic effect, known as the Faraday rotation effect. For example, in [16] the authors reported an OFCS based on the Faraday effect for the measurement of plasma current in tokamaks. In [17] a long-period fiber grating (LPFG) and a Faraday effect sensing element to measure electrical current in transmission lines are described, whereas in [18], the authors have proposed the use of the Faraday rotation effect using a LPFG inscribed on a polarization-maintaining fiber as a sensor demodulator. However, these methods are affected by the fiber birefringence [5] and limited by the small Verdet constant of silica, which requires a very long fiber to enhance the sensitivity [19]. Considering these aspects, we have developed and experimentally investigated a novel OFCS based on a long-period fiber grating combined with a permanent neodymium (Nd2 Fe14 B) magnet attached to the LPFG sensing region. The new device presents a lightweight setup with compact size and simple installation, and provides an indirect measurement of the electrical current of an UAV engine. When the controller activates the drone engines, the battery will supply each one of the four Sensors 2016, 16, 1800; doi:10.3390/s16111800
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engine. When the controller activates the drone engines, the battery will supply each one of the four motors according to the desired flight speed and direction. Thus, electrical current flowing within the wiregenerates generatesa amagnetic magnetic force deflects the permanent magnet and deforms the conductor wire force thatthat deflects the permanent magnet and deforms the LPFG, LPFG, which are effective external deformation sensors [9,20,21]. Therefore, according to the different which are effective external deformation sensors [9,20,21]. Therefore, according to the different shifts shifts in the resonant bands of thetransmitted LPFG transmitted it istopossible to current detect the current in the resonant bands of the LPFG spectrum,spectrum, it is possible detect the demanded demanded by the aircraft engine. by the aircraft engine. ◦ C) with The experimental investigation was carried out under room temperature (27 °C) with the the UAV UAV fixed on the lab bench. Further, Further, the the electrical electrical current current variations variations were were measured measured for for aa single single UAV engine, while all the motors were spinning at the same time. Contrary Contrary to to the the laboratory laboratory environment, environment, complete version version of of the the OFCS OFCSwould wouldbe beembedded embeddedinto intothe thedrone, drone,consisting consistingofofa asimple simplescheme scheme a complete ofof a a LED light source, proposed OFCS, an optical filter photodetectors, the experimental LED light source, thethe proposed OFCS, an optical filter andand photodetectors, sincesince the experimental tests tests were performed a broadband light source and an optical spectrum analyzer the lab. were performed with awith broadband light source and an optical spectrum analyzer in thein lab. 2. Unmanned UnmannedAerial AerialVehicle Vehicle Characteristics Characteristics In order to operate remotely the UAV, UAV, the the user user has has to to send send command command signals to the aircraft, and thus regulate the rotation speed of the four engines engines of of the the UAV. UAV. In other words, the operator regulates the pulse-width modulation (PWM) through a wireless radio-control at 2.4 GHz GHz radio radio frequency frequency (RF), (RF), indicating the desired command. The PWM signals signals do do not not act act directly directly on onthe theUAV UAV engines. engines. Actually, Actually, the flight control unit (FCU), within the aircraft, receives the PWM signals and transmits them to an electronic speed control module (ESC). (ESC). The The ESC ESC module module is is powered powered by by the the battery battery of of the the UAV, UAV, and controls the rotation of the the engine engine for for the the desired desired flight flight conditions. conditions. Figure 1 shows the detailed relationship between the duty cycle of the PWM signals and the electrical byby a single engine of theofUAV, measured by a conventional clamp ammeter. electrical current currentdemanded demanded a single engine the UAV, measured by a conventional clamp The UAV engine demands electrical in current high duty cycles, thus its response not linear ammeter. The UAV engine more demands morecurrent electrical in high duty cycles, thus itsisresponse is throughout the entire operation and range it reaches rotationits speed limitspeed whenlimit the PWM has not linear throughout the entire range operation andits it reaches rotation whensignal the PWM asignal duty has cycle of 73%, which corresponds to 10 A. to 10 A. a duty cycle of 73%, which corresponds
Figure 1. Measured electrical current demanded by the UAV engine under test as a function of Figure 1. Measured electrical current demanded by the UAV engine under test as a function of different different duty cycles of the PWM signals. duty cycles of the PWM signals.
In addition to the 2.4 GHz RF transmission of the pilot signals to the aerial vehicle, more In addition to the 2.4 GHz RF transmission of the pilot signals to the aerial vehicle, more communications technologies are embedded into the UAV system, such as the global positioning communications technologies are embedded into the UAV system, such as the global positioning system (GPS), onboard WiFi signals transferring video signals back to the pilot, and telemetry. There system (GPS), onboard WiFi signals transferring video signals back to the pilot, and telemetry. There is is also noise that may cause electrical magnetic interference (EMI) generated by the motors of the also noise that may cause electrical magnetic interference (EMI) generated by the motors of the aircraft aircraft and ESCs as well. Thus, considering these aspects, the use of an OFCS provides electrical and ESCs as well. Thus, considering these aspects, the use of an OFCS provides electrical current current measurements with reliability, due to its immunity to EMI from the UAV components and measurements with reliability, due to its immunity to EMI from the UAV components and insensitivity insensitivity to entangled frequencies in the same band, which conventional sensors do not provide. to entangled frequencies in the same band, which conventional sensors do not provide.
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3. Theory 3. Theory Long-period gratings (LPFG) are fiber-optic devices that consist of a periodic modulation of the optical fiber properties, which is normally a perturbation the refractive index ofmodulation a fiber section Long-period gratings (LPFG) are fiber-optic devicesofthat consist ofcore a periodic of [22].optical Several techniques have been is reported concerning the fabrication process core of the gratings, such the fiber properties, which normally a perturbation of the refractive index of a fiber as exposure to ultraviolet (UV) have irradiation [22], irradiation by femtosecond pulses in the infrared (IR) section [22]. Several techniques been reported concerning the fabrication process of the gratings, [23], asirradiation CO2 lasers as well asirradiation ion beambyimplantation, etching, mechanical such exposure tobyultraviolet (UV)[24], irradiation [22], femtosecond pulses in the infrared arrangements, and electric arc2 discharges Besides, LPFG usually has aetching, period (Λ) of 100 µm (IR) [23], irradiation by CO lasers [24],[25–28]. as well as ionthe beam implantation, mechanical to 1000 µm andand the electric grating arc promotes coupling between the propagating core mode 01) and(Λ) theof m arrangements, discharges [25–28]. Besides, the LPFG usually has (LP a period different (LPcoupling 0m). The result of the between propagating and 100 µm toco-propagating 1000 µm and thecladding grating modes promotes between thecoupling propagating core mode (LP01 ) and co-propagating modes that can be observed in 0m the). transmission spectrum the LPFG is a high the m different co-propagating cladding modes (LP The result of the couplingofbetween propagating attenuation of the cladding modes, series of attenuation resonance and co-propagating modes that canrepresented be observedby in athe transmission spectrum of thebands LPFGaccording is a high to the following attenuation of theequation cladding[6,29]: modes, represented by a series of attenuation resonance bands according to the following equation [6,29]: 2 (1) 2π m Λ (1) β co − β cl = Λ where and are the propagation constants of the core and the mth-order cladding modes, where β co and βm are the propagation constants of the core and the mth-order cladding modes, respectively, andclΛ is the grating period. The propagation constant can be rewritten as 2πneff/λ, being respectively, and Λ is the grating period. The propagation constant can be rewritten as 2πne f f /λ, being neff and λ the effective refractive index and the light wavelength, respectively. Thus, from Equation (1) the ne f f and λ the effective refractive index and the light wavelength, respectively. Thus, from Equation (1) resonance phase matching condition can be easily determined as [30]: the resonance phase matching condition can be easily determined as [30]: (2) Λ λm = (nco − nm ) Λ (2) cl where λm is the resonance wavelength of the mth-order cladding mode, and nco and are the m effective refractive indices of the fundamental core mode and the mth-order cladding mode, where λm is the resonance wavelength of the mth-order cladding mode, and nco and ncl are the effective respectively. Changes the temperature, strain, index of themode, external medium refractive indices of the in fundamental core mode and refractive the mth-order cladding respectively. surrounding the LPFG sensor can alter the grating period as well as the differential refractive index Changes in the temperature, strain, refractive index of the external medium surrounding the LPFG of the core and cladding modes and thus, lead to variations in the resonance condition due to the sensor can alter the grating period as well as the differential refractive index of the core and cladding rejection wavelengths ) dependence to resonance external parameters it is modes and thus, lead (λ to mvariations in the condition variations due to the[31–33]. rejectionTherefore, wavelengths possible to extract the environmental information from the analysis of the shifts occurring at the λm (λm ) dependence to external parameters variations [31–33]. Therefore, it is possible to extract the parameter. environmental information from the analysis of the shifts occurring at the λm parameter. 4. Experimental 4. Experimental Setup Setup and and Sensing Sensing Principle Principle The experimental experimental setup setup is is shown shown in in Figure Figure 2. 2. It It consists consists of of aa broadband broadband light light source source (BBS), (BBS), an an The optical spectrum spectrum analyzer analyzer (OSA, (OSA, Thorlabs Thorlabs Fourier Fourier Transform Spectrometer, Spectrometer, Thorlabs, Thorlabs, Newton, Newton, NJ, optical USA), the proposed OFCS inside thethe arm of the vehicle, andand the USA), OFCS (LPFG (LPFG++permanent permanentmagnet) magnet)placed placed inside arm of the vehicle, UAV. the UAV.
Figure 2. 2. Experimental Experimental setup setup for for UAV UAVcurrent currentsensing. sensing. Figure
The sensing device is obtained fixing with an epoxy resin a neodymium (Nd2Fe14B) magnet, the LPFG and the flexible holder structure. This configuration, shown in Figure 3, provides unidirectional
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8 is obtained fixing with an epoxy resin a neodymium (Nd2 Fe14 B) magnet,4 of the LPFG and the flexible holderthe structure. configuration, shown the in Figure 3, provides displacements to deform deform LPFG This sensing head. Further, Further, proposed sensorunidirectional compact, displacements to the LPFG sensing head. the proposed sensor isis compact, displacements to deform the LPFG sensing head. Further, the proposed sensor is compact, lightweight lightweight and and easy easy to to install, install, these these characteristics characteristics allow allow itit to to be be easily easily employed employed according according to to any any lightweight and easy to install, these characteristics allow itand to be easily employed according to any displacements UAV features, UAV features, such as conductor wires, ESCs engines position and size. Possible UAV features, such as conductor wires, ESCs and engines position and size. Possible displacements such as conductor wires, ESCs and engines position andgrating size. Possible displacements in directions in other other directions can be be mitigated mitigated using another without the magnet magnet as as other can be be seen in in in directions can using another grating without the can seen can be mitigated using another grating without the magnet as can be seen in literature [34]. literature [34]. [34]. literature
Figure Figure 3. 3. Schematic Schematic representation representation of of the the proposed proposed optical optical fiber fiber current current sensor sensor (OFCS) (OFCS) combining combining aaa Figure 3. Schematic representation of the proposed optical fiber current sensor (OFCS) combining Figure 3. Schematic representation of the proposed optical fiber current sensor (OFCS) combining a LPFG LPFG with with aaa neodymium neodymium permanent permanent magnet. magnet. LPFG with neodymium permanent magnet. LPFG with a neodymium permanent magnet.
An LPFG LPFG sensor sensor was fabricated ininthe the Laboratory ofof Instrumentation and Telemetry (LITel) at An LPFG sensorwas wasfabricated fabricatedin theLaboratory Laboratory Instrumentation and Telemetry (LITel) An of Instrumentation and Telemetry (LITel) at Federal University of Juiz de Fora (UFJF). Figure 4 shows the fabrication scheme of the LPFG sensors. at Federal University of Juiz de Fora (UFJF). 4 shows the fabrication of the LPFG Federal University of Juiz de Fora (UFJF). FigureFigure 4 shows the fabrication scheme scheme of the LPFG sensors. It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges method sensors. It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges method [25,35]. The The fiber was arc-induced using aausing fusion splicersplicer (KL-300T, Jilong,Jilong, Nanjing, China) with method [25,35]. The fiber was arc-induced a fusion (KL-300T, Nanjing, China) [25,35]. fiber was arc-induced using fusion splicer (KL-300T, Jilong, Nanjing, China) with electrical arc discharges discharges of 20 20 bits andand 470470 ms,ms, grating period ofofΛ ΛΛ== =500 500 µm and constant external with electrical arc discharges ofbits 20 bits grating period 500µm µmand andconstant constant external electrical arc of and 470 ms, grating period of tension of 2 g weight [28]. During the fabrication process, an optical spectrum analyzer was used to to tension of 2 g weight [28]. During the fabrication process, an optical spectrum analyzer was used monitor the transmission spectrum characteristics. Further, these fabrication parameters were chosen monitor the the transmission transmission spectrum spectrum characteristics. characteristics. Further, Further, these these fabrication fabrication parameters parameters were were chosen chosen monitor in order to achieve coupling of the highest possible cladding mode near λ 1550 nm, which would = in order to achieve coupling of the highest possible cladding mode near λ = 1550 nm, which would in order coupling possible cladding mode near λ = 1550 nm, which would provide higher sensitivity to external parameters sensing [30,31]. The LPFG sensor microphotograph The LPFG LPFG sensor sensor microphotograph microphotograph provide higher sensitivity to external parameters sensing [30,31]. The after the manufacturing process is shown in Figure 5. There is a periodical diameter reduction and and after the manufacturing process is shown in Figure 5. There is a periodical diameter reduction and after the manufacturing process is shown in Figure There is periodical diameter reduction elongationof which,actually, by the the tapering tapering geometric geometricdeformation deformationafter of the the fiber, fiber, which, actually, were were introduced introduced by after elongation actually, deformation elongation the arc discharges [36]. the arc arc discharges [36]. the
Figure 4. 4. Fabrication Fabrication scheme scheme used used for for fabrication fabrication of of the the LPFGs. LPFGs. Figure Figure 4. Fabrication scheme used for fabrication of the LPFGs.
Figure 5. 5. Microphotograph Microphotograph of of the the side side view view of of the the long-period long-period grating grating sensor sensor produced produced by by arc arc Figure Figure 5. Microphotograph of the side view of the long-period grating sensor produced by arc Figure 5. Microphotograph of the side view of the long-period grating sensor produced by arc discharges. discharges. discharges. discharges.
The present in the sensing region can be attracted or repelledor depending The neodymium neodymiummagnet magnet present inLPFG the LPFG LPFG sensing region can be be attracted attracted repelled The neodymium magnet present in the sensing region can or repelled on the position of its magnetic poles. In the experiment, we have placed the magnet with the depending on on the the position position of of its its magnetic magnetic poles. poles. In In the the experiment, experiment, we we have have placed placed the the magnet magnetLPFG with depending with in order toinprovide aprovide repulsive electromagnetic force, due to the electrical currentcurrent variations in the the LPFG order to a repulsive electromagnetic force, due to the electrical variations the LPFG in order to provide a repulsive electromagnetic force, due to the electrical current variations in the the conductor conductor wire wire of of the the UAV UAV engine. engine. The The force force repels repels the the magnet magnet and and thus, thus, causes causes lateral lateral in deformation on on the the LPFG LPFG sensor, sensor, leading leading to to displacements displacements of of the the resonant resonant attenuation attenuation band band of of the the deformation LPFG. LPFG.
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conductor wire of the UAV engine. The force repels the magnet and thus, causes lateral deformation on the LPFG sensor, leading to displacements of the resonant attenuation band of the LPFG. Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine Sensorsto 2016, 1800 5 of 8fiber according the16,desired PWM control signals. This is the basic principle of the proposed optical according to the desired PWM control signals. This is the basic principle of the proposed optical fiber current sensor. current sensor. Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine
according to the desired PWM control signals. This is the basic principle of the proposed optical fiber 5. Experimental Results 5. Experimental Results current sensor. The response of the current sensor was experimentally investigated by applying PWM duty The response of the current sensor was experimentally investigated by applying PWM duty cycles Results cycles5.inExperimental the range from 45% to 73%, which corresponds to an electrical current of 0–10 A supplied to in the range from 45% to 73%, which corresponds to an electrical current of 0–10 A supplied to a single a single UAV asof shown in Figure 1. We and recorded theby transmission spectrum of Theasmotor, response the current wasmeasured experimentally investigated applying PWM duty UAV motor, shown in Figure 1. Wesensor measured and recorded the transmission spectrum of the LPFG cyclessensor in the range from to 73%, corresponds and to anitelectrical current of 0–10 A supplied the LPFG during the45% process ofwhich the experiment, presented a significant change to in the sensor during the process of the experiment, and it presented a significant change in the transmission, a single UAV as shown in Figure variation, 1. We measured and recorded the6.transmission spectrum of transmission, due motor, to the electrical current as shown in Figure due to the electrical current variation, as shown in Figure 6. the LPFG sensor during the process of the experiment, and it presented a significant change in the transmission, due to the electrical current variation, as shown in Figure 6.
Figure 6. lossloss spectra ofthe thethe LPFG sensor under different electrical current demanded FigureFigure 6. Transmission Transmission loss spectra of LPFG sensor under electrical current demanded by 6. Transmission spectra of LPFG sensor underdifferent different electrical current demanded by UAV engine. thethe UAV engine. by the UAV engine.
Figure 7. Dip wavelength andtransmission transmission loss control signals. Figure 7. Dip wavelength and loss versus versusPWM PWM control signals.
Figure 7. Dip wavelength and1539.36 transmission versus signals. The attenuation band shifted from nm to loss 1544.02 nm,PWM with control a spectral variation shift of
The band shifted 1539.36loss nmshifted to 1544.02 nm, dB with a spectral variation shift of 4.66 attenuation nm. In addition, the dip of thefrom transmission from −16.32 to −11.03 dB, varying 5.29 dB. 4.66 The nm. In addition, the dip of the transmission loss shifted from − 16.32 dB to − 11.03 dB, varying We attenuation can see the detailed response curves of the for thenm, PWM signals in Figure 7, and shift its of band shifted from 1539.36 nmLPFG to 1544.02 with a spectral variation 5.29nm. dB. Weaddition, can see the detailed response curves the LPFG for the PWM signals in Figure 7,5.29 and its corresponding wavelength intensity responses for electrical current in Figures 8 and 9, dB. 4.66 In the dip of theand transmission lossofshifted from −16.32 dB to −11.03 dB, varying respectively. In Figure 7, we observe that the response for PWM duty cycle signals presents the same corresponding wavelength and intensity responses for electrical current in Figures 8 and 9, respectively. We can see the detailed response curves of the LPFG for the PWM signals in Figure 7, and its behavior the electrical current variation, because of their as shown in Figure 1. As In Figure 7, weasobserve that the response for PWM duty cycle signals presents thein same behavior as the corresponding wavelength and intensity responses for correlation, electrical current Figures 8 the and 9, electrical current demanded by the UAV engine increased, the dip wavelength shifted towards higher electrical current variation, their as PWM shownduty in Figure As thepresents electrical current respectively. In Figure 7, webecause observeofthat thecorrelation, response for cycle 1. signals the same wavelengths. By data fitting, the sensitivity of 0.4703 nm/A was achieved with high degree of
behavior as the electrical current variation, because of their correlation, as shown in Figure 1. As the electrical current demanded by the UAV engine increased, the dip wavelength shifted towards higher wavelengths. By data fitting, the sensitivity of 0.4703 nm/A was achieved with high degree of
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demanded by 16, the1800 UAV engine increased, the dip wavelength shifted towards higher wavelengths. Sensors 2016, 6 of 8 By data fitting, the of 0.4703bynm/A was achieved with high of electrical electrical currentsensitivity linearity, confirmed the R2 coefficient derived from the degree fitting process, as seencurrent in electrical linearity, byderived the R2 coefficient the fitting process, as seen linearity, confirmed the R2confirmed coefficient from thederived fitting from process, as seen in Figure 8. in Figure 8. currentby Figure 8.
Figure 8. Experimental data fit of the dip wavelength as a function of electrical current. Figure 8. Experimental data fit of the dip wavelength as a function of electrical current. Figure 8. Experimental data fit of the dip wavelength as a function of electrical current.
Figure 9. Experimental data fit of the transmission loss as a function of the electrical current. Figure 9. Experimental data thetransmission transmission loss of of thethe electrical current. Figure 9. Experimental data fitfitofofthe lossasasa afunction function electrical current. On the other hand, Figure 9 shows the transmission loss decreasing as the electrical current On the other hand,this Figure 9 shows the transmission decreasing electricalfunction current increases. Furthermore, intensity changes behavior can loss be fitted well byas anthe exponential On the other hand, Figure 9 shows the transmission loss decreasing as the electrical current increases. Furthermore, this intensity changes behavior can be fitted well by an exponential function 2 with a R coefficient value of 0.996 over the sensing range. The exponential curve in Figure 9 can be 2 increases. Furthermore, this intensity changes behavior canThe be fitted well by an in exponential with Rcalibrate coefficient of 0.996between over thethe sensing range. curve Figurevariation 9 canfunction be used ato thevalue nonlinearity transmission lossexponential and the electrical current 2 coefficient toOFCS. calibrate the nonlinearity between the transmission loss and the electrical current variation with used aforRthe value of 0.996 over the sensing range. The exponential curve in Figure 9 can be In addition, the behavior of the transmission loss agrees well [23], and is based on the for the OFCS. In addition, the behavior of the transmission loss agrees well [23], and is based on the used periodic to calibrate the nonlinearity between thebytransmission loss and theWhen electrical current variation refractive index modulation caused the external deformation. the LPFG is placed periodic refractive index modulation caused by the external deformation. When thesection LPFG is placed under deformation, it generates a periodic strain field owingloss to the expanded cross and for the OFCS. In addition, the behavior of the transmission agrees well [23], and is area, based on the under deformation, it generates a periodic strain fieldof owing to the fiber expanded cross section area, and thusrefractive affects the index refractive index periodic distribution the optical and produces anLPFG attenuated periodic modulation caused by the external deformation. When the is placed thus dip affects thespectrum, refractive as index periodic distribution the optical fiber and produces an attenuated in the shown in Figure 6. fieldofowing underloss deformation, it generates a periodic strain to the expanded cross section area, and loss dip in the spectrum, as shown in Figure 6.
thus affects the refractive index periodic distribution of the optical fiber and produces an attenuated 6. Conclusions loss dip in the spectrum, as shown in Figure 6. 6. Conclusions We have proposed and experimentally investigated a simple and effective optical fiber sensor
We have proposed and experimentally investigated a simple and effective optical fiber sensor for UAV current monitoring. The novel sensor based on long-period fiber gratings combined with a 6. Conclusions
for UAV current monitoring. Theprovides novel sensor basedway on long-period gratings with a permanent neodymium magnet a practical to measure fiber indirectly the combined electrical current
We have proposed and experimentally investigated simple and effective optical fiber sensor permanent neodymium magnet provides a practical way toa measure indirectly the electrical current for UAV current monitoring. The novel sensor based on long-period fiber gratings combined with a
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permanent neodymium magnet provides a practical way to measure indirectly the electrical current flowing within the conductor wire of the UAV engine. The electrical current sensing has been achieved successfully by measuring the displacements of LPFG spectrum based on wavelength shifts or intensity changes with the entire UAV operation range from 0 A to 10 A, as observed in Figures 8 and 9. The results showed a good wavelength linear sensitivity of 0.4703 nm/A, observed in Figure 8. Besides, using the proposed method, we can also monitor the electrical current in the UAV engine based on intensity changes in the transmitted spectrum, as shown in Figure 9. Finally, we can extend the proposed method to a portable OFCS system and thus, monitor electrical current in every engine of the UAV, since there is no concerns about EMI from the aircraft devices using the proposed OFCS measurement scheme. The portable sensor system would be embedded into the UAV, and would consist of a simple scheme of a LED light source, the proposed OFCS, an optical filter, photodetectors and a simple electronic unit. Acknowledgments: The authors thank for the support provided by Universidade Federal de Juiz de Fora, Capes and FAPEMIG. Duke Energy and ANEEL (Agência Nacional de Energia Elétrica) financially supported this work. Author Contributions: Felipe S. Delgado and João P. Carvalho performed the experiments and Thiago V. N. Coelho and Alexandre B. D. Santos conceived and designed the experiments. All authors wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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Aerssens, M.; Gusarov, A.; Brichard, B.; Massaut, V.; Mégret, P.; Wuilpart, M. Faraday effect based optical fiber current sensor for tokamaks. In Proceedings of the 2011 2nd International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), Ghent, Belgium, 6–9 June 2011; pp. 1–6. Souza, L.S.; Silveira, D.D.; Coelho, T.V.N.; Santos, A.B. Desenvolvimento de um sensor à fibra óptica LPFG utilizado na medição de corrente elétrica. In Proceedings of the 2014 MOMAG, Curitiba, Brazil, 31 August–3 September 2014. Lee, Y.W.; Yoon, I.; Lee, B. A simple fiber-optic current sensor using a long-period fiber grating inscribed on a polarization-maintaining fiber as a sensor demodulator. Sens. Actuators A Phys. 2004, 112, 308–312. [CrossRef] Zhang, R.; Yao, X.S.; Liu, T.; Li, L. The effect of linear birefringence on fiber optic current sensor based on Faraday mirror. In Proceedings of the 2014 SPIE 9274, Advanced Sensor Systems and Applications VI, 92741N, Beijing, China, 9–11 October 2014. Chenda, G.; Xiao, H.; Huang, Y.; Zhou, Z.; Zhang, Y. A novel long-period fiber grating sensor for large strain measurement. In Proceedings of the 2009 Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, San Diego, CA, USA, 8 March 2009. James, S.W.; Tatam, R.P. Optical fiber long-period grating sensors: Characteristics and application. Meas. Sci. Technol. 2003, 14, R49–R61. [CrossRef] Vengsarkar, A.M.; Lemaire, P.J.; Judkins, J.B.; Bhatia, V.; Erdogan, T.; Sipe, J.E. Long-period fiber gratings as band-rejection filters. J. Lightwa. Technol. 1996, 14, 58–65. [CrossRef] Ahmed, F.; Joe, H.E.; Min, B.K.; Jun, M.B.G. Characterization of refractive index change and fabrication of long period gratings in pure silica fiber by femtosecond laser radiation. Opt. Laser Technol. 2015, 74, 119–124. [CrossRef] Coelho, J.M.P.; Silva, C.; Nespereira, M.; Abreu, M.; Rebordão, J. Writing of long period fiber gratings using CO2 laser radiation. Adv. Opt. Fiber Technol. Fundam. Opt. Phenom. Appl. 2015. [CrossRef] Tan, S.Y.; Yong, Y.T.; Lee, S.C.; Rahman, F.A. Review on an arc-induced long-period fiber grating and its sensor applications. J. Electromagn. Waves Appl. 2015, 29, 703–726. [CrossRef] Fujimaki, M.; Ohki, Y.; Brebner, J.; Roorda, S. Fabrication of long-period optical fiber gratings by use of ion implantation. Opt. Lett. 2000, 25, 88–89. [CrossRef] Savin, S.; Digonnet, M.J.F.; Kino, G.S.; Shaw, H.J. Tunable mechanically induced long-period fiber gratings. Opt. Lett. 2000, 25, 710–712. [CrossRef] [PubMed] Bock, W.J.; Chen, J.; Mikulic, P.; Eftimov, T. A novel fiber-optic tapered long-period grating sensor for pressure monitoring. IEEE Trans. Instrum. Meas. 2007, 56, 1176–1180. [CrossRef] Ji, W.B.; Tjin, S.C.; Lin, B.; Ng, C.L. Highly sensitive refractive index sensor based on adiabatically tapered microfiber long period gratings. Sensors 2013, 13, 14055–14063. [CrossRef] [PubMed] Smietana, M.; Bock, W.J.; Mikulic, P.; Chen, J. Increasing sensitivity of arc-induced long-period gratings-pushing the fabrication technique toward its limits. Meas. Sci. Technol. 2011, 22, 015201. [CrossRef] Shu, X.; Zhang, L.; Bennion, I. Sensitivity characteristics of long-period fiber gratings. J. Lightwa. Technol. 2002, 20, 255–256. Wang, S.F.; Chiang, C.C. A notched long-period fiber grating magnetic field sensor based on nanoparticle magnetic fluid. Appl. Sci. 2016, 6, 9. [CrossRef] Tsuda, H.; Urabe, K. Characterization of long-period grating refractive index sensors and their applications. Sensors 2009, 9, 4559–4571. [CrossRef] [PubMed] Antunes, P.; Varum, H.; André, P. Uniaxial fiber Bragg grating accelerometer system with temperature and cross axis insensitivity. Measurement 2011, 44, 55–59. [CrossRef] Rego, G.; Okhotnikov, O.; Dianov, E.; Sulimov, V. High-temperature stability of long-period fiber gratings produced using an electric arc. J. Lightware Technol. 2001, 19, 1574–1579. [CrossRef] Ivanov, O.V.; Rego, G. Origin of coupling to antisymmetric modes in arc-induced long-period fiber gratings. Opt. Express 2007, 15, 13936–13941. [CrossRef] [PubMed] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
An Optical Fiber Sensor and Its Application in UAVs for Current Measurements Felipe S. Delgado, João P. Carvalho, Thiago V. N. Coelho * and Alexandre B. Dos Santos Electrical Circuit Department, Federal University of Juiz de Fora, Juiz de Fora 36036-330, Brazil; [email protected] (F.S.D.); [email protected] (J.P.C.); [email protected] (A.B.D.S.) * Correspondence: [email protected]; Tel.: +32-9-9916-1712 Academic Editors: Christophe Caucheteur and Tuan Guo Received: 4 July 2016; Accepted: 6 September 2016; Published: 27 October 2016
Abstract: In this paper, we propose and experimentally investigate an optical sensor based on a novel combination of a long-period fiber grating (LPFG) with a permanent magnet to measure electrical current in unmanned aerial vehicles (UAVs). The proposed device uses a neodymium magnet attached to the grating structure, which suffers from an electromagnetic force produced when the current flows in the wire of the UAV engine. Therefore, it causes deformation on the sensor and thus, different shifts occur in the resonant bands of the transmission spectrum of the LPFG. Finally, the results show that it is possible to monitor electrical current throughout the entire operating range of the UAV engine from 0 A to 10 A in an effective and practical way with good linearity, reliability and response time, which are desirable characteristics in electrical current sensing. Keywords: current sensor; long-period fiber grating; optical fiber sensor; unmanned aerial vehicle
1. Introduction Fiber-optic sensors have acquired growing importance in the field of sensor technologies [1–3]. Compared with conventional devices, optical fiber sensors present many advantages [4,5]. These devices are compact, lightweight, easy to install, inexpensive and insensitive to electromagnetic interference [6], which are key features desired for sensing applications. Thus, optical fiber sensors are extremely versatile in measuring variations in temperature [7,8], strain [9], external refractive index [10], pressure [11], humidity [12] and electrical current in high voltage environments [13]. Optical current sensors using optical fibers have been investigated since the 1960s [14,15]. Since then, several researchers have proposed and demonstrated a number of practical applications using optical fiber current sensors (OFCS) [16–18]. One of the most popular configurations for current measurements is based on the magneto-optic effect, known as the Faraday rotation effect. For example, in [16] the authors reported an OFCS based on the Faraday effect for the measurement of plasma current in tokamaks. In [17] a long-period fiber grating (LPFG) and a Faraday effect sensing element to measure electrical current in transmission lines are described, whereas in [18], the authors have proposed the use of the Faraday rotation effect using a LPFG inscribed on a polarization-maintaining fiber as a sensor demodulator. However, these methods are affected by the fiber birefringence [5] and limited by the small Verdet constant of silica, which requires a very long fiber to enhance the sensitivity [19]. Considering these aspects, we have developed and experimentally investigated a novel OFCS based on a long-period fiber grating combined with a permanent neodymium (Nd2 Fe14 B) magnet attached to the LPFG sensing region. The new device presents a lightweight setup with compact size and simple installation, and provides an indirect measurement of the electrical current of an UAV engine. When the controller activates the drone engines, the battery will supply each one of the four Sensors 2016, 16, 1800; doi:10.3390/s16111800
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engine. When the controller activates the drone engines, the battery will supply each one of the four motors according to the desired flight speed and direction. Thus, electrical current flowing within the wiregenerates generatesa amagnetic magnetic force deflects the permanent magnet and deforms the conductor wire force thatthat deflects the permanent magnet and deforms the LPFG, LPFG, which are effective external deformation sensors [9,20,21]. Therefore, according to the different which are effective external deformation sensors [9,20,21]. Therefore, according to the different shifts shifts in the resonant bands of thetransmitted LPFG transmitted it istopossible to current detect the current in the resonant bands of the LPFG spectrum,spectrum, it is possible detect the demanded demanded by the aircraft engine. by the aircraft engine. ◦ C) with The experimental investigation was carried out under room temperature (27 °C) with the the UAV UAV fixed on the lab bench. Further, Further, the the electrical electrical current current variations variations were were measured measured for for aa single single UAV engine, while all the motors were spinning at the same time. Contrary Contrary to to the the laboratory laboratory environment, environment, complete version version of of the the OFCS OFCSwould wouldbe beembedded embeddedinto intothe thedrone, drone,consisting consistingofofa asimple simplescheme scheme a complete ofof a a LED light source, proposed OFCS, an optical filter photodetectors, the experimental LED light source, thethe proposed OFCS, an optical filter andand photodetectors, sincesince the experimental tests tests were performed a broadband light source and an optical spectrum analyzer the lab. were performed with awith broadband light source and an optical spectrum analyzer in thein lab. 2. Unmanned UnmannedAerial AerialVehicle Vehicle Characteristics Characteristics In order to operate remotely the UAV, UAV, the the user user has has to to send send command command signals to the aircraft, and thus regulate the rotation speed of the four engines engines of of the the UAV. UAV. In other words, the operator regulates the pulse-width modulation (PWM) through a wireless radio-control at 2.4 GHz GHz radio radio frequency frequency (RF), (RF), indicating the desired command. The PWM signals signals do do not not act act directly directly on onthe theUAV UAV engines. engines. Actually, Actually, the flight control unit (FCU), within the aircraft, receives the PWM signals and transmits them to an electronic speed control module (ESC). (ESC). The The ESC ESC module module is is powered powered by by the the battery battery of of the the UAV, UAV, and controls the rotation of the the engine engine for for the the desired desired flight flight conditions. conditions. Figure 1 shows the detailed relationship between the duty cycle of the PWM signals and the electrical byby a single engine of theofUAV, measured by a conventional clamp ammeter. electrical current currentdemanded demanded a single engine the UAV, measured by a conventional clamp The UAV engine demands electrical in current high duty cycles, thus its response not linear ammeter. The UAV engine more demands morecurrent electrical in high duty cycles, thus itsisresponse is throughout the entire operation and range it reaches rotationits speed limitspeed whenlimit the PWM has not linear throughout the entire range operation andits it reaches rotation whensignal the PWM asignal duty has cycle of 73%, which corresponds to 10 A. to 10 A. a duty cycle of 73%, which corresponds
Figure 1. Measured electrical current demanded by the UAV engine under test as a function of Figure 1. Measured electrical current demanded by the UAV engine under test as a function of different different duty cycles of the PWM signals. duty cycles of the PWM signals.
In addition to the 2.4 GHz RF transmission of the pilot signals to the aerial vehicle, more In addition to the 2.4 GHz RF transmission of the pilot signals to the aerial vehicle, more communications technologies are embedded into the UAV system, such as the global positioning communications technologies are embedded into the UAV system, such as the global positioning system (GPS), onboard WiFi signals transferring video signals back to the pilot, and telemetry. There system (GPS), onboard WiFi signals transferring video signals back to the pilot, and telemetry. There is is also noise that may cause electrical magnetic interference (EMI) generated by the motors of the also noise that may cause electrical magnetic interference (EMI) generated by the motors of the aircraft aircraft and ESCs as well. Thus, considering these aspects, the use of an OFCS provides electrical and ESCs as well. Thus, considering these aspects, the use of an OFCS provides electrical current current measurements with reliability, due to its immunity to EMI from the UAV components and measurements with reliability, due to its immunity to EMI from the UAV components and insensitivity insensitivity to entangled frequencies in the same band, which conventional sensors do not provide. to entangled frequencies in the same band, which conventional sensors do not provide.
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3. Theory 3. Theory Long-period gratings (LPFG) are fiber-optic devices that consist of a periodic modulation of the optical fiber properties, which is normally a perturbation the refractive index ofmodulation a fiber section Long-period gratings (LPFG) are fiber-optic devicesofthat consist ofcore a periodic of [22].optical Several techniques have been is reported concerning the fabrication process core of the gratings, such the fiber properties, which normally a perturbation of the refractive index of a fiber as exposure to ultraviolet (UV) have irradiation [22], irradiation by femtosecond pulses in the infrared (IR) section [22]. Several techniques been reported concerning the fabrication process of the gratings, [23], asirradiation CO2 lasers as well asirradiation ion beambyimplantation, etching, mechanical such exposure tobyultraviolet (UV)[24], irradiation [22], femtosecond pulses in the infrared arrangements, and electric arc2 discharges Besides, LPFG usually has aetching, period (Λ) of 100 µm (IR) [23], irradiation by CO lasers [24],[25–28]. as well as ionthe beam implantation, mechanical to 1000 µm andand the electric grating arc promotes coupling between the propagating core mode 01) and(Λ) theof m arrangements, discharges [25–28]. Besides, the LPFG usually has (LP a period different (LPcoupling 0m). The result of the between propagating and 100 µm toco-propagating 1000 µm and thecladding grating modes promotes between thecoupling propagating core mode (LP01 ) and co-propagating modes that can be observed in 0m the). transmission spectrum the LPFG is a high the m different co-propagating cladding modes (LP The result of the couplingofbetween propagating attenuation of the cladding modes, series of attenuation resonance and co-propagating modes that canrepresented be observedby in athe transmission spectrum of thebands LPFGaccording is a high to the following attenuation of theequation cladding[6,29]: modes, represented by a series of attenuation resonance bands according to the following equation [6,29]: 2 (1) 2π m Λ (1) β co − β cl = Λ where and are the propagation constants of the core and the mth-order cladding modes, where β co and βm are the propagation constants of the core and the mth-order cladding modes, respectively, andclΛ is the grating period. The propagation constant can be rewritten as 2πneff/λ, being respectively, and Λ is the grating period. The propagation constant can be rewritten as 2πne f f /λ, being neff and λ the effective refractive index and the light wavelength, respectively. Thus, from Equation (1) the ne f f and λ the effective refractive index and the light wavelength, respectively. Thus, from Equation (1) resonance phase matching condition can be easily determined as [30]: the resonance phase matching condition can be easily determined as [30]: (2) Λ λm = (nco − nm ) Λ (2) cl where λm is the resonance wavelength of the mth-order cladding mode, and nco and are the m effective refractive indices of the fundamental core mode and the mth-order cladding mode, where λm is the resonance wavelength of the mth-order cladding mode, and nco and ncl are the effective respectively. Changes the temperature, strain, index of themode, external medium refractive indices of the in fundamental core mode and refractive the mth-order cladding respectively. surrounding the LPFG sensor can alter the grating period as well as the differential refractive index Changes in the temperature, strain, refractive index of the external medium surrounding the LPFG of the core and cladding modes and thus, lead to variations in the resonance condition due to the sensor can alter the grating period as well as the differential refractive index of the core and cladding rejection wavelengths ) dependence to resonance external parameters it is modes and thus, lead (λ to mvariations in the condition variations due to the[31–33]. rejectionTherefore, wavelengths possible to extract the environmental information from the analysis of the shifts occurring at the λm (λm ) dependence to external parameters variations [31–33]. Therefore, it is possible to extract the parameter. environmental information from the analysis of the shifts occurring at the λm parameter. 4. Experimental 4. Experimental Setup Setup and and Sensing Sensing Principle Principle The experimental experimental setup setup is is shown shown in in Figure Figure 2. 2. It It consists consists of of aa broadband broadband light light source source (BBS), (BBS), an an The optical spectrum spectrum analyzer analyzer (OSA, (OSA, Thorlabs Thorlabs Fourier Fourier Transform Spectrometer, Spectrometer, Thorlabs, Thorlabs, Newton, Newton, NJ, optical USA), the proposed OFCS inside thethe arm of the vehicle, andand the USA), OFCS (LPFG (LPFG++permanent permanentmagnet) magnet)placed placed inside arm of the vehicle, UAV. the UAV.
Figure 2. 2. Experimental Experimental setup setup for for UAV UAVcurrent currentsensing. sensing. Figure
The sensing device is obtained fixing with an epoxy resin a neodymium (Nd2Fe14B) magnet, the LPFG and the flexible holder structure. This configuration, shown in Figure 3, provides unidirectional
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8 is obtained fixing with an epoxy resin a neodymium (Nd2 Fe14 B) magnet,4 of the LPFG and the flexible holderthe structure. configuration, shown the in Figure 3, provides displacements to deform deform LPFG This sensing head. Further, Further, proposed sensorunidirectional compact, displacements to the LPFG sensing head. the proposed sensor isis compact, displacements to deform the LPFG sensing head. Further, the proposed sensor is compact, lightweight lightweight and and easy easy to to install, install, these these characteristics characteristics allow allow itit to to be be easily easily employed employed according according to to any any lightweight and easy to install, these characteristics allow itand to be easily employed according to any displacements UAV features, UAV features, such as conductor wires, ESCs engines position and size. Possible UAV features, such as conductor wires, ESCs and engines position and size. Possible displacements such as conductor wires, ESCs and engines position andgrating size. Possible displacements in directions in other other directions can be be mitigated mitigated using another without the magnet magnet as as other can be be seen in in in directions can using another grating without the can seen can be mitigated using another grating without the magnet as can be seen in literature [34]. literature [34]. [34]. literature
Figure Figure 3. 3. Schematic Schematic representation representation of of the the proposed proposed optical optical fiber fiber current current sensor sensor (OFCS) (OFCS) combining combining aaa Figure 3. Schematic representation of the proposed optical fiber current sensor (OFCS) combining Figure 3. Schematic representation of the proposed optical fiber current sensor (OFCS) combining a LPFG LPFG with with aaa neodymium neodymium permanent permanent magnet. magnet. LPFG with neodymium permanent magnet. LPFG with a neodymium permanent magnet.
An LPFG LPFG sensor sensor was fabricated ininthe the Laboratory ofof Instrumentation and Telemetry (LITel) at An LPFG sensorwas wasfabricated fabricatedin theLaboratory Laboratory Instrumentation and Telemetry (LITel) An of Instrumentation and Telemetry (LITel) at Federal University of Juiz de Fora (UFJF). Figure 4 shows the fabrication scheme of the LPFG sensors. at Federal University of Juiz de Fora (UFJF). 4 shows the fabrication of the LPFG Federal University of Juiz de Fora (UFJF). FigureFigure 4 shows the fabrication scheme scheme of the LPFG sensors. It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges method sensors. It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges It was manufactured in a single-mode optical fiber (SMF28) using the electric arc discharges method [25,35]. The The fiber was arc-induced using aausing fusion splicersplicer (KL-300T, Jilong,Jilong, Nanjing, China) with method [25,35]. The fiber was arc-induced a fusion (KL-300T, Nanjing, China) [25,35]. fiber was arc-induced using fusion splicer (KL-300T, Jilong, Nanjing, China) with electrical arc discharges discharges of 20 20 bits andand 470470 ms,ms, grating period ofofΛ ΛΛ== =500 500 µm and constant external with electrical arc discharges ofbits 20 bits grating period 500µm µmand andconstant constant external electrical arc of and 470 ms, grating period of tension of 2 g weight [28]. During the fabrication process, an optical spectrum analyzer was used to to tension of 2 g weight [28]. During the fabrication process, an optical spectrum analyzer was used monitor the transmission spectrum characteristics. Further, these fabrication parameters were chosen monitor the the transmission transmission spectrum spectrum characteristics. characteristics. Further, Further, these these fabrication fabrication parameters parameters were were chosen chosen monitor in order to achieve coupling of the highest possible cladding mode near λ 1550 nm, which would = in order to achieve coupling of the highest possible cladding mode near λ = 1550 nm, which would in order coupling possible cladding mode near λ = 1550 nm, which would provide higher sensitivity to external parameters sensing [30,31]. The LPFG sensor microphotograph The LPFG LPFG sensor sensor microphotograph microphotograph provide higher sensitivity to external parameters sensing [30,31]. The after the manufacturing process is shown in Figure 5. There is a periodical diameter reduction and and after the manufacturing process is shown in Figure 5. There is a periodical diameter reduction and after the manufacturing process is shown in Figure There is periodical diameter reduction elongationof which,actually, by the the tapering tapering geometric geometricdeformation deformationafter of the the fiber, fiber, which, actually, were were introduced introduced by after elongation actually, deformation elongation the arc discharges [36]. the arc arc discharges [36]. the
Figure 4. 4. Fabrication Fabrication scheme scheme used used for for fabrication fabrication of of the the LPFGs. LPFGs. Figure Figure 4. Fabrication scheme used for fabrication of the LPFGs.
Figure 5. 5. Microphotograph Microphotograph of of the the side side view view of of the the long-period long-period grating grating sensor sensor produced produced by by arc arc Figure Figure 5. Microphotograph of the side view of the long-period grating sensor produced by arc Figure 5. Microphotograph of the side view of the long-period grating sensor produced by arc discharges. discharges. discharges. discharges.
The present in the sensing region can be attracted or repelledor depending The neodymium neodymiummagnet magnet present inLPFG the LPFG LPFG sensing region can be be attracted attracted repelled The neodymium magnet present in the sensing region can or repelled on the position of its magnetic poles. In the experiment, we have placed the magnet with the depending on on the the position position of of its its magnetic magnetic poles. poles. In In the the experiment, experiment, we we have have placed placed the the magnet magnetLPFG with depending with in order toinprovide aprovide repulsive electromagnetic force, due to the electrical currentcurrent variations in the the LPFG order to a repulsive electromagnetic force, due to the electrical variations the LPFG in order to provide a repulsive electromagnetic force, due to the electrical current variations in the the conductor conductor wire wire of of the the UAV UAV engine. engine. The The force force repels repels the the magnet magnet and and thus, thus, causes causes lateral lateral in deformation on on the the LPFG LPFG sensor, sensor, leading leading to to displacements displacements of of the the resonant resonant attenuation attenuation band band of of the the deformation LPFG. LPFG.
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conductor wire of the UAV engine. The force repels the magnet and thus, causes lateral deformation on the LPFG sensor, leading to displacements of the resonant attenuation band of the LPFG. Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine Sensorsto 2016, 1800 5 of 8fiber according the16,desired PWM control signals. This is the basic principle of the proposed optical according to the desired PWM control signals. This is the basic principle of the proposed optical fiber current sensor. current sensor. Finally, it is possible to measure indirectly the electrical current demanded by the UAV engine
according to the desired PWM control signals. This is the basic principle of the proposed optical fiber 5. Experimental Results 5. Experimental Results current sensor. The response of the current sensor was experimentally investigated by applying PWM duty The response of the current sensor was experimentally investigated by applying PWM duty cycles Results cycles5.inExperimental the range from 45% to 73%, which corresponds to an electrical current of 0–10 A supplied to in the range from 45% to 73%, which corresponds to an electrical current of 0–10 A supplied to a single a single UAV asof shown in Figure 1. We and recorded theby transmission spectrum of Theasmotor, response the current wasmeasured experimentally investigated applying PWM duty UAV motor, shown in Figure 1. Wesensor measured and recorded the transmission spectrum of the LPFG cyclessensor in the range from to 73%, corresponds and to anitelectrical current of 0–10 A supplied the LPFG during the45% process ofwhich the experiment, presented a significant change to in the sensor during the process of the experiment, and it presented a significant change in the transmission, a single UAV as shown in Figure variation, 1. We measured and recorded the6.transmission spectrum of transmission, due motor, to the electrical current as shown in Figure due to the electrical current variation, as shown in Figure 6. the LPFG sensor during the process of the experiment, and it presented a significant change in the transmission, due to the electrical current variation, as shown in Figure 6.
Figure 6. lossloss spectra ofthe thethe LPFG sensor under different electrical current demanded FigureFigure 6. Transmission Transmission loss spectra of LPFG sensor under electrical current demanded by 6. Transmission spectra of LPFG sensor underdifferent different electrical current demanded by UAV engine. thethe UAV engine. by the UAV engine.
Figure 7. Dip wavelength andtransmission transmission loss control signals. Figure 7. Dip wavelength and loss versus versusPWM PWM control signals.
Figure 7. Dip wavelength and1539.36 transmission versus signals. The attenuation band shifted from nm to loss 1544.02 nm,PWM with control a spectral variation shift of
The band shifted 1539.36loss nmshifted to 1544.02 nm, dB with a spectral variation shift of 4.66 attenuation nm. In addition, the dip of thefrom transmission from −16.32 to −11.03 dB, varying 5.29 dB. 4.66 The nm. In addition, the dip of the transmission loss shifted from − 16.32 dB to − 11.03 dB, varying We attenuation can see the detailed response curves of the for thenm, PWM signals in Figure 7, and shift its of band shifted from 1539.36 nmLPFG to 1544.02 with a spectral variation 5.29nm. dB. Weaddition, can see the detailed response curves the LPFG for the PWM signals in Figure 7,5.29 and its corresponding wavelength intensity responses for electrical current in Figures 8 and 9, dB. 4.66 In the dip of theand transmission lossofshifted from −16.32 dB to −11.03 dB, varying respectively. In Figure 7, we observe that the response for PWM duty cycle signals presents the same corresponding wavelength and intensity responses for electrical current in Figures 8 and 9, respectively. We can see the detailed response curves of the LPFG for the PWM signals in Figure 7, and its behavior the electrical current variation, because of their as shown in Figure 1. As In Figure 7, weasobserve that the response for PWM duty cycle signals presents thein same behavior as the corresponding wavelength and intensity responses for correlation, electrical current Figures 8 the and 9, electrical current demanded by the UAV engine increased, the dip wavelength shifted towards higher electrical current variation, their as PWM shownduty in Figure As thepresents electrical current respectively. In Figure 7, webecause observeofthat thecorrelation, response for cycle 1. signals the same wavelengths. By data fitting, the sensitivity of 0.4703 nm/A was achieved with high degree of
behavior as the electrical current variation, because of their correlation, as shown in Figure 1. As the electrical current demanded by the UAV engine increased, the dip wavelength shifted towards higher wavelengths. By data fitting, the sensitivity of 0.4703 nm/A was achieved with high degree of
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demanded by 16, the1800 UAV engine increased, the dip wavelength shifted towards higher wavelengths. Sensors 2016, 6 of 8 By data fitting, the of 0.4703bynm/A was achieved with high of electrical electrical currentsensitivity linearity, confirmed the R2 coefficient derived from the degree fitting process, as seencurrent in electrical linearity, byderived the R2 coefficient the fitting process, as seen linearity, confirmed the R2confirmed coefficient from thederived fitting from process, as seen in Figure 8. in Figure 8. currentby Figure 8.
Figure 8. Experimental data fit of the dip wavelength as a function of electrical current. Figure 8. Experimental data fit of the dip wavelength as a function of electrical current. Figure 8. Experimental data fit of the dip wavelength as a function of electrical current.
Figure 9. Experimental data fit of the transmission loss as a function of the electrical current. Figure 9. Experimental data thetransmission transmission loss of of thethe electrical current. Figure 9. Experimental data fitfitofofthe lossasasa afunction function electrical current. On the other hand, Figure 9 shows the transmission loss decreasing as the electrical current On the other hand,this Figure 9 shows the transmission decreasing electricalfunction current increases. Furthermore, intensity changes behavior can loss be fitted well byas anthe exponential On the other hand, Figure 9 shows the transmission loss decreasing as the electrical current increases. Furthermore, this intensity changes behavior can be fitted well by an exponential function 2 with a R coefficient value of 0.996 over the sensing range. The exponential curve in Figure 9 can be 2 increases. Furthermore, this intensity changes behavior canThe be fitted well by an in exponential with Rcalibrate coefficient of 0.996between over thethe sensing range. curve Figurevariation 9 canfunction be used ato thevalue nonlinearity transmission lossexponential and the electrical current 2 coefficient toOFCS. calibrate the nonlinearity between the transmission loss and the electrical current variation with used aforRthe value of 0.996 over the sensing range. The exponential curve in Figure 9 can be In addition, the behavior of the transmission loss agrees well [23], and is based on the for the OFCS. In addition, the behavior of the transmission loss agrees well [23], and is based on the used periodic to calibrate the nonlinearity between thebytransmission loss and theWhen electrical current variation refractive index modulation caused the external deformation. the LPFG is placed periodic refractive index modulation caused by the external deformation. When thesection LPFG is placed under deformation, it generates a periodic strain field owingloss to the expanded cross and for the OFCS. In addition, the behavior of the transmission agrees well [23], and is area, based on the under deformation, it generates a periodic strain fieldof owing to the fiber expanded cross section area, and thusrefractive affects the index refractive index periodic distribution the optical and produces anLPFG attenuated periodic modulation caused by the external deformation. When the is placed thus dip affects thespectrum, refractive as index periodic distribution the optical fiber and produces an attenuated in the shown in Figure 6. fieldofowing underloss deformation, it generates a periodic strain to the expanded cross section area, and loss dip in the spectrum, as shown in Figure 6.
thus affects the refractive index periodic distribution of the optical fiber and produces an attenuated 6. Conclusions loss dip in the spectrum, as shown in Figure 6. 6. Conclusions We have proposed and experimentally investigated a simple and effective optical fiber sensor
We have proposed and experimentally investigated a simple and effective optical fiber sensor for UAV current monitoring. The novel sensor based on long-period fiber gratings combined with a 6. Conclusions
for UAV current monitoring. Theprovides novel sensor basedway on long-period gratings with a permanent neodymium magnet a practical to measure fiber indirectly the combined electrical current
We have proposed and experimentally investigated simple and effective optical fiber sensor permanent neodymium magnet provides a practical way toa measure indirectly the electrical current for UAV current monitoring. The novel sensor based on long-period fiber gratings combined with a
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permanent neodymium magnet provides a practical way to measure indirectly the electrical current flowing within the conductor wire of the UAV engine. The electrical current sensing has been achieved successfully by measuring the displacements of LPFG spectrum based on wavelength shifts or intensity changes with the entire UAV operation range from 0 A to 10 A, as observed in Figures 8 and 9. The results showed a good wavelength linear sensitivity of 0.4703 nm/A, observed in Figure 8. Besides, using the proposed method, we can also monitor the electrical current in the UAV engine based on intensity changes in the transmitted spectrum, as shown in Figure 9. Finally, we can extend the proposed method to a portable OFCS system and thus, monitor electrical current in every engine of the UAV, since there is no concerns about EMI from the aircraft devices using the proposed OFCS measurement scheme. The portable sensor system would be embedded into the UAV, and would consist of a simple scheme of a LED light source, the proposed OFCS, an optical filter, photodetectors and a simple electronic unit. Acknowledgments: The authors thank for the support provided by Universidade Federal de Juiz de Fora, Capes and FAPEMIG. Duke Energy and ANEEL (Agência Nacional de Energia Elétrica) financially supported this work. Author Contributions: Felipe S. Delgado and João P. Carvalho performed the experiments and Thiago V. N. Coelho and Alexandre B. D. Santos conceived and designed the experiments. All authors wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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