Fibre-optic voltage sensor using an optical lever.pdf

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Fibre-op t ic vo l tage sensor u s ing an o pt ica l leverI?Dinev

lndexing terms: Fibre-optic sensors, Optical lever

Abstract: A new type of linear voltage sensor,able to measure constant and alternating voltagein a wide frequency and amplitude range, ispresented. The sensor is based on a bendingpiezoelectric-motor element with an optical fibreattached to it. The deflection of the fibre is sensedby an optical lever and a linear positionphotosensor. The sensor has a dynamic range ofover three decades (71 dB), linearity error lessthan 0.1% and variable upper amplitude andfrequency range. The simple and flexible designallows the proposed sensor to have almost anydesired response and sensitivity. It is used mainlyfor monitoring constant, low- and medium-frequency voltages in power-utility applications.

1 IntroductionWith each year the application of the fibre-optic sen-sors expands into new areas. One imminent and veryimportant field is voltage monitoring in the power-util-ity applications. Several fibre-optic voltage-sensor con-figurations have recently been studied. Several of thesesensors are interferometer based, using a piezoactiveplastics coating, bounded to the glass fibre. The appliedvoltage generates strains in the piezoactive plasticsmaterial which results in an optical phase shift in oneof the Mach-Zender arms [l]. Other groups are associ-ated with the Pockels effect [I] or optical amplitudemodulation [2] .

This paper describes a simple voltage sensor which isable to measure constant and alternating voltage in awide frequency and amplitude range. The sensor isbased on the bending of the piezoelectric-motor ele-ment (bilinear), sensed by an optical fibre and opticallever, described in [3]. The sensor has a flexible andsimple design, and works mainly with low- andmedium-frequency voltages with a dynamic range overtliree decades.2 Sensor designThe bending piezoelectric-motor element consists oftwo bound-together piezo plates which are polarised inopposite directions. If this piezo element is supportedas a cantilever beam, the applied voltage causes oneQ IEE, 1997IEE Proceedings online no 19971115P,iper received 11th June 1996The author is with the Florida Atlantic University, Department of Electri-cal Engineering, 777 Glades Road, PO Box 3091, Boca Raton FL 33431,U SA

layer to expand while the other contracts. The netresult is a bending displacement of the free cantileverend which is much greater than the length deformationof either of the two layers. The free-end displacementX ( t ) as a function of the applied voltage V(t) for abimorph mode, series operation, is given in [4] to be

where L, t and d31are the cantilever length, thicknessand piezo constant, respectively. It can be seen fromthe above equation that the bilinear piezo element hasa linear response.

photosensor

V

amplifier- 1

V

I

Fig.1 Schematic diugram o u voltage sensor utilising an optical leverBased on the above results, a new type of voltage

sensor utilising a bilinear piezo element, able to meas-ure direct (DC) or alternating (AC) voltage, is shownin Fig. 1. One end of the rectangular profile (38" x18") piezo element (model 10.201, Piezo SystemsInc.) is supported by a wall, forming a cantilever beamwith length L = 25" The end of a standard multi-mode 50/125/200pm optical fibre is attached to the freeend of the sensing element, as shown in Fig. 1. Theother fibre end is illuminated by a light-emitting diode(LED). The fibre end, attached (glued) to the cantilev-er's free end, is placed in the focal point of a microlenscf= 3.9mm, d =2.6") forming in this way an opticallever similar to that described in [3]. A linear-positionphotosensor mounted at a distance S = 100" fromthe lens provides a displacement advantage A = S/f =25. It was shown in [3] that, provided that the fibredeflection is within the microlens paraxial region(*380pn), the lever has a linear response, so fromeqn. 1 the sensor described above will have a linearresponse and the output voltage U ( t )will be

where G is the circuit conversion gain (G =-2mV/p).To improve the sensor performance, the sensor'selectronic is shielded.

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3 Ex p e r i m e n t a l r e s u l t sTo test the static performance of this voltage sensor,the bilinear piezo element, wired for the bimorphmode, series operation, is connected to a linear powersupply. The sensor's input voltage V is varied in therange 0-180V a set of measurements for the sensor'soutput voltage U is taken. The polarity of the powersupply is then reversed and another set of measure-ments for V and U is taken over the same voltagerange. The sensor's response-calibration curve is shownin Fig. 2. As can be seen, the sensor has a linearresponse in the interval (-18OV to 180V) with a lOOmVresolution and a position-linearity error not exceeding0.1%. The sensor has been measured to have a dynamicrange of 3600 (71dB). By varying the parametersf andS, the sensor's sensitivity can be changed. A sensitivityof 20mV (in +40V range) is measured if a microlenswithf= 1.8" is used (S=200"). The fundamentalupper limit of the sensor depends on the maximum rec-ommended voltage for the bilinear piezo element,which for the model used is *180V. By using a high-voltage divider, the sensor's upper limit can bechanged. In an experiment performed with a 1O:ldivider the sensor's upper limit was increased tok1800V with a sensitivity of about 1V.

-1 ' 1 ' 1 ' 1 ' , , I , , , , [

50 applied voltage,V100 150 200

-15Fig.2 Sensor's response-calibration curveIt has to be emphasised that the measured values arenot the global sensor limits. Since the high-voltagedivider ratio and the optical-lever-displacement advan-

tage A can vary, the sensor's scope is extremely wide,but the dynamic range for a given set of parametersstill remains around 70dB.

The photosensor has very low position temperaturedrift (< 0.1 w / K ) over the sensing area which, asdescribed in [3], makes the optical lever very stable asthe temperature varies. When the temperature waschanged from 25C to 50"C, a 75mV drift in the out-put signal was found, mainly due to the fluctuations ofthe piezo-element parameters.

A digital storage oscilloscope and a functionalgenerator were used to test the dynamic performance ofthe sensor. A square-wave voltage V(t) with anamplitude +20V and frequency Y =20Hz was suppliedto the sensor and channel 1 of the oscilloscope. Thesensor's output U ( t )was monitored on channel 2. Firstthe experiment was performed without damping of thepiezo element, but a substantial ringing was observedin the output signal U ( t ) . The main reason for this isthat the generator provides the square-wave pulses witha riseifall time less than 5ns, which causes a shock254

excitation of the piezo element, and creates ringing.Since the resonant frequency of the element used hasbeen measured to be 320Hz, an electric damping bymeans of a lowpass filter is used to increase the rise/falltime of the excitation pulse (oil damping is notapplicable, since the both layers of the piezo elementare conductive). The picture obtained is shown inFig. 3.It can be seen from Fig. 3 that some ringing stillexists, but further damping may result in a reduction ofthe sensor's frequency response.

Fig.3Y =20H2, T =20.0 msidivision0 V indicates the signal ground levela Channel 1: V(t) ,10Vidivisionb Channel 2 U(t) , 1 Vidivision

Sensor 's response to a square-wave excitation

Fig.4Y = 120H2, T =5.0 msidivision0 V indicates th e signal ground levela Channel 1: V(t),10 Vidivisionb Channel 2: U( t ) , 1 Vidivision

Senso r's response to a sinusoidal excita tion

The experiment described above was repeated with asinusoidal waveform V(t) (amplitude kl0V and fre-quency Y = 120Hz) supplied to the sensor and to chan-nel 1 of the oscilloscope (no damping was applied inthis case). The sensor's output U ( t ) was monitored onchannel 2 and the picture obtained is shown in Fig. 4.The sensor has been measured to have a negligiblephase lag. It can be seen from Fig. 4 that the ringing isnot present in the output U(t), since a smooth inputvoltage V(t) s applied. The sensor shows similar behav-iour when the voltage frequency varies in the 0-360Hzrange. Actually, by varying the cantilever length L orthe width of the piezo element, the sensor's natural fre-quency can be increased to several kilohertz. However,if the sensor works close to its resonant frequency anew calibration curve must be taken since the fre-quency response is no longer flat in this region.

Since this is a cantilever-based system, it is expectedto be sensitive to external vibrations. To test this, thesensor was placed on a workbench near to a working

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source of vibrations. A constant voltage V(t)with anamplitude of 50V was applied to the sensor and theoutput U ( t ) was monitored on the oscilloscope. Noamplitude variations greater than 50mV were observedin . the output signal. For reference, the vibrometerdescribed in [3], placed at the same position reads 2.1 Vvibration amplitude.4 Conclus ionsThis paper describes a new type of voltage sensorwhich is able to measure constant and alternating volt-ages over a wide frequency and amplitude range. Thesensor is based on a bending piezoelectric motorelement, with an optical fibre attached to it, the deflec-tion of which is sensed by an optical lever and linear-

position photosensor. The sensor has a dynamic rangeof over three decades (71dB), linearity error less than0.1%, temperature stability over a 25 K-range and vari-able upper amplitude and frequency range. The simpleand flexible design allows the proposed sensor to havealmost any desired response and sensitivity. It is usedmainly for monitoring constant, low- and medium-fre-quency voltages in power utility applications.

ReferencesKROHN, D.A.: Fiber optic sensors: fundamentals and applica-tions (Research Triangle Park, USA, 1994), 2nd ed., pp. 192-199ADOLFSSON, M. , and BROGARDH, T.: Optical fibre measur-ing devices. US Patent 4 547 729, 1985DINEV, P.: A two-dimensional fiber-optical vibration sensor,Meus. Sei. Technol.,1995,6, (9), pp. 1395-1398Piezo Systems Inc.: Product catalogue, 1990, p. 29

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Fibre-optic voltage sensor using an optical lever.pdf