Sensors & Transducers - International Frequency Sensor Association

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Sensors & Transducers, Vol. 148, Issue 1, January 2013, pp. 1-1020 pF. The output obtained from this circuit is apulse width modulated signal where the duty cycle isproportional to the change in capacitance. Theprototype circuit provides a sensitivity of 246 mV/pF.The output PWM signal was low pass filtered, levelshifted and amplified. The circuit is very useful formeasurement of capacitance of the range of tens ofpF or smaller. The output obtained is a linearresponse with high sensitivity. The circuit usesCMOS electronics which can be easily implementedas an ASIC in chip. The measurement techniquerequires only a small number of components and isrelatively simple.Fig. 3. Schematic Diagram for Capacitance Measurementbased on RC phase Delay.the microcontroller is set as output to provide a highlogic, and hence charging the unknown capacitance‘C’ between the port and ground to a voltage V oh(output high voltage) through the internal pull upresistance ‘R p ’ of the microcontroller. Secondly thesame port pin is set to high impedance input mode,and the embedded timer starts timing the process asthe capacitance discharges through ‘R’ (a resistanceR>>R p , included in parallel with ‘C’). As soon as thevoltage drops to the threshold voltage V th of theembedded Schmitt trigger buffer, the timer stops. V th ,V oh , R are considered constant and hence themeasured discharge time ‘t’ is utilized to calculatethe value of the sensor’s unknown capacitance.In order to minimize the effect of straycapacitance, a method of three signal calibration hasbeen adopted. This technique requires themeasurement of (a) offset measurement (b) sensormeasurement and (c) reference measurement. Theabsolute error obtained by applying this interfacecircuit is below 4 % FSR for 1 pF < C s (sensorcapacitance) < 10 pF, when C r =10 pF (referencecapacitance), and below 1.5 % for 10 pF < C s< 100 pF, when C r =100 pF.2.4. Micro Controller Interface for LowValue Capacitance SensorsF. Reverter et al., has proposed a very simple andlow cost microcontroller interface circuit [4] for lowvalue capacitive sensors. The basic principle drivingthe measurement is that the microcontroller measuresthe time needed to discharge an unknown capacitancethrough a known resistance ‘R’, and this measuredtime ‘t’ is utilized for determining the unknowncapacitance. Fig. 4 shows a schematic diagram of thecircuit. The technique used in the article toimplement the same is that first a digital port pin of2.5. Measuring Capacitance using OscillatorFrank M. L. van der Goes et al., describes a newcapacitive sensor interface based on a novel type ofoscillator [5], whose frequency is insensitive to lowand high frequency interfering signals by theapplication of a third order high pass filter. The fullyintegrated 0.7 μm CMOS circuit shows an inaccuracyof less than 100 aF (Attofarad) with respect to a 2 pFreference capacitor over a -30 ºC to 70 ºCtemperature range.Fig. 4. Schematic of Measuring Capacitance with Microcontroller based Interfacing.4
Sensors & Transducers, Vol. 148, Issue 1, January 2013, pp. 1-10Fig. 5 shows a schematic diagram of the circuit.The basic system is a first order system with a thirdorder filter to suppress low frequency interference. Ithas low sensitivity to noise and the locking effects tothe microcontroller have been reduced by applicationof dithertechniques. The two port measurementsystem consists of a network of sensor capacitancecursed with two parasitic capacitors. The only way toeliminate the effects of the two parasitic capacitors isto maintain a specific potential difference across thetwo terminals of the system and to transmit a specificcurrent at the input. A microcontroller is used tomeasure the oscillator period. The entire system iscontinuously auto-calibrated for additive andmultiplicative errors using a reference capacitor inplace of sensor’s capacitance. The microcontroller isused to measure three time periods namely referencetime, offset time and measured time for the specificsensor’s capacitance. These periods are measured bythe microcontroller, yielding the digital numberswhich in turn are used to calculate the finalcapacitance value.Fig. 5. Schematic Diagram for Measuring Capacitance using Oscillator.The output is independent of the gain and offsettime as the calibration is done continuously and alsoslow variations in gain and offset time do not affectthe output. The measurement range is 0 to 2 pF andthe sampling time obtained is 330 ns. The result hasnegligible influence of parasitic capacitance of up to300 pF. The system shows very low sensitivity to 1/fnoise, thus enabling the use of low cost CMOSprocess.2.6. Measuring Capacitance basedon Phase AngleAshkan Ashrafi et al., provides a technique ofcapacitance-to-phase angle conversion for measuringvery small capacitance change [6] (Fig.6). There is alinear relationship between the output phase andinitial capacitance change. The main features of thistechnique are extremely high stray immunity andvery high resolution.A sinusoidal wave is generated using an oscillatorand then this wave is used for generating twobalanced signals with 180º phase difference. Phase ofone of the signals is shifted using R and C. Toovercome the nonlinear problem a conventionalquadrature phase sensitive detector (PSD) is used.PSD provides the cotangent of the output phase. Byusing the switching multipliers, DC output isobtained which is then fed into ADC. The outputfrom ADC is given to the microcomputer. Thereference voltage for ADC is provided by passing itthrough DAC so that the variation of temperaturechanges on reference voltage is very small. Formaximum highest possible resolution the resolutionof ADC is adjusted. The prototype was developed totest the circuit. Testing results show that it canmeasure minimum displacement of 7.710 -12 m. Thenonlinearity is less than 0.5 %. A linear and highresolution output is obtained by this scheme. Thisscheme can be used to measure very smalldisplacement in various applications.Fig. 6. Schematic Diagram based on Capacitance to Phase Angle Converter for Measuring Capacitance.5
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