METHOD AND APPARATUS FOR TESTING A CAPACITIVE SENSOR

Abstract: Apparatus for testing a capacitive sensor, the apparatus including: means (5, 15, 17, 19) for applying a test input signal to the sensor, the test input signal having a predetermined signal characteristics; and means (7, 18) for extracting a test output signal from the sensor by selecting signals having the predetermined signal characteristics, and blocking signals not having the predetermined signal characteristics.

Description: METHOD AND APPARATUS FOR TESTING A CAPACITIVE SENSOR Field of the Invention 5 The present invention relates to a method and apparatus for testing a capacitive sensor. Background of the Invention In certain applications which employ a capacitive sensor, such as bio- 10 medical applications, the output of the sensor is critical to the operation of a device. Therefore a need exists for an efficient method of testing the operation of the sensor, and at least providing an indication if abnormal operation is detected. I S JP-A-0631 8744 describes a sensor comprising a piezoelectric material affixed on both surfaces with a pair of electrodes. Each electrode is formed in a U-shape with a pair terminals, one at each end. Discontinuities in each electrode can be detected by measuring the resistance between a respective pair of terminals. 20 A problem with the arrangement of JP-A-0631 8744 is that the electrodes must be formed into a special shape. Another problem is that it is not possible to detect discontinuities between the electrodes, for instance due to a breakdown in the piezoelectric material. 2S An object of the invention is to address these problems, or at least to provide the public with a useful alternative. Summary of the Invention In accordance with a first aspect of the present invention there is provided apparatus for testing a capacitive sensor, the apparatus including: means for applying a test input signal to the sensor, the test input signal having a predetermined signal characteristic; and means for monitoring a sensor output signal from the sensor and generating a test output signal which varies in accordance with the presence or absence of the predetermined signal characteristic in the monitored sensor output signal. In accordance with a second aspect of the invention there is provided a method of testing a capacitive sensor, the method including the steps of: applying a test input signal to the sensor, the test input signal having a predetermined signal characteristic; and monitoring a sensor output signal from the sensor and generating a test output signal which varies in accordance with the presence or absence of the predetermined signal characteristic in the monitored sensor output signal. The test input signal can be applied across the sensor without creating unwanted interference with sensing signals which are generated by the sensor during normal operation (and which will not, in general, possess the predetermined signal characteristic) . In one embodiment the test signal lies within a test frequency range, and the means for monitoring blocks signals outside the test frequency range (typically employing a band-limiting filter such as a high-pass, low-pass, comb, notch or band-pass filter) . Typically the test signal is substantially sinusoidal. Typically the apparatus further includes means for extracting a sensing signal from the sensor outout signal by blocking signals outside a sensing frequency range (typically a band-limiting filter such as a high- pass, low-pass, comb, notch or band-pass filter) Typically there is no overlap between the two frequency ranges. In other words the test frequency range lies completely above or completely below the sensing frequency range. In an alternative embodiment the test input signal is encoded with a predetermined code sequence (such as a psuedo-random sequence), and the test output signal is generated by correlating the predetermined code sequence with the sensor output signal. Typically the test input signal is applied to the sensor via an impedance element (eg. a capacitor, resistor, inductor or combination thereof). Preferably the impedance element has an impedance at least 1 0- 1 00 times greater than the impedance of the sensor, at the frequency of the test signal. At its most basic level the apparatus may simply be used to check the presence or absence of the sensor. During normal operation, the sensor will present a known impedance to the test input signal. However if a fault exists the sensor will present a higher or lower impedance to the test signal. This can be detected and used to generate a two-level test output signal (le. a signal with one level during normal operation and another level when the impedance measurement lies outside predetermined performance criteria) . In a preferred embodiment a fault signal is generated when the impedance of the sensor lies outside predetermined performance criteria. In an alternative, more complex system, the test output signal has more than two output values, if for example the capacitive sensor can significantly vary its capacitance value as a part of its normal operation. Typically the apparatus is used to monitor a movement sensor comprising a piezoelectric material which generates sensing signals by movement of the piezoelectric material The invention may be employed in a variety of applications. For instance the sensor may acquire signals from a human or animal subject. One example of such a biomedical system is an infant apnoea monitoring system which employs a capacitive piezoelectric sensor to acquire cardiac, respiratory and/or large motor movement data from an infant during sleep. Another example is an automobile driver monitoring system in which a capacitive piezoelectric sensor mounted in an automobile seat acquires cardiac, respiratory and/or large motor movement signals from a driver of the automobile. Brief Description of the Drawings A number of examples of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of a single-ended piezoelectric sensor system incorporating a sensor testing circuit with the test frequency above the sensor system frequency bands of interest; Figure 2 is a schematic circuit diagram of a differential piezoelectric sensor system incorporating a first sensor testing circuit with the test frequency above the sensor system frequency bands of interest, Figure 3 is a schematic circuit diagram of a differential piezoelectric sensor system incorporating a second sensor testing circuit, Figure 4 is a schematic circuit diagram of a differential piezoelectric sensor system incorporating a third sensor testing circuit, Figure 5 is a schematic circuit diagram of a piezoelectric sensor system incorporating a fourth sensor testing circuit which includes a pseudorandom noise generator; and Figure 6 is a schematic circuit diagram of a linear feedback shift register. Detailed Description of the Preferred Embodiments Referring to figure 1 , a piezoelectric sensor 1 comprises a sheet of polyvinyhdene fluoride (PVDF) film with a pair of electrodes arranged on opposite sides of the film. One electrode is connected to ground and the other electrode is connected to a sensor output line. The PVDF film can be made in quite large sizes. For the purposes of this example, we can assume that a standard A4 sheet size would have a capacitance of somewhere between 1 0 nF and 40 nF, depending on the thickness of the film. Deformation of the film results in the generation of a sensing signal on the sensor output line. The relatively high capacitance of the film means that the sensing signal lies in a relatively low frequency band, below 35 Hz for instance. The sensor output signal on the sensor output line is amplified by an amplifier 2, filtered by a lowpass filter 3 which rolls off at 35 Hz, passed on to electronics 4 (eg. analog-to-digital converter etc), and processed by a microprocessor 9. An oscillator 5 generates an oscillating test input signal at a frequency of 1 0 KHz. The test input signal is applied to the sensor output line via a small capacitor 6 (or a high-valued resistor or resistor/inductor combination) with an impedance 10-1 00 times greater than the impedance of the sensor 1 (at the test frequency) so as not to load the sensor 1 . During normal operation, the relatively low impedance sensor 1 effectively short-circuits the 1 0KHz signal to ground. A bandpass filter 7 is coupled to the output of the amplifier 2 via a capacitor (not labelled). The filter 7 has a bandpass region centred on the 1 0KHz test signal frequency Thus any signals at the 10KHz test frequency are passed onto a diode 8 and the microprocessor 9, and any signals in the 0-10Hz sensing signal frequency band are blocked During normal operation, with the test input signal effectively short- circuited by the sensor 1 , the test signal voltage output by the bandpass filter 7 will lie below a predetermined threshold. However, if a fault is present and the 10KHz test signal is not short circuited by the capacitive sensor, then the test signal voltage level will rise above the threshold. In this case the microprocessor 9 generates a fault detection signal. The microprocessor can simultaneously monitor the detected test signal from diode 8 and process the sensing signal from electronics 4. In the example of Figure 2, a sensor 10 comprises a film sheet and electrodes (not labelled) encased in a grounded electrostatic shield 1 1 . Differential outputs 1 2, 1 3 of the sensor 10 are coupled to positive and negative input terminals of a differential amplifier 1 6. The output of the differential amplifier 1 6 is input to electronic circuitry (not shown) similar to items 3,4,7,8 and 9 shown in Figure 1 . A high frequency oscillator 1 5 is coupled to one output 1 2 of the sensor 1 0 via a capacitor (not labeled) with an impedance at least 10- 1 00 times greater than the impedance of the sensor 1 (at the test frequency) . A high frequency decoupling capacitor 1 4 (with a relatively high impedance at the sensor measurement frequency band, but with a relatively low impedance at the test frequency) is connected between the other differential output 1 3 and ground to complete the shunt to ground. As in the Figure 1 circuit, during normal sensor operation little test signal is present on the output of the differential amplifier 1 6 Figure 3 shows the differential sensing circuit of Figure 2 but with a different sensor testing circuit. In this case the ground shunt capacitor 1 4 is omitted. The test input signal is applied to one differential output of the sensor and a test signal detection circuit 1 8 (comprising a capacitor, bandpass filter and diode) is coupled to the other differential output of the sensor. In the Figure 3 example, in contrast to the Figure 1 and 2 examples, during normal operation the test signal will be passed onto the detection circuit 1 8. Thus the test signal detection circuit 1 8 outputs a signal to the microprocessor (not shown) which generates a fault detection signal when the test signal output by the bandpass filter falls below a predetermined threshold. Figure 4 shows the differential sensing circuit of Figures 2 and 3 but with a third different sensor testing circuit. In this case the test input signal is applied by a differential oscillator drive 1 9 via a pair of capacitors (not labelled) each with an impedance at least 10-1 00 times greater than the sensor 1 . As in the Figure 2 example, during normal operation little test signal is present on the output of the differential amplifier. The testing circuits of Figures 1 -4 all simply illustrate an out-of-band single-frequency detection circuit much higher in frequency than the sensor's target frequency range. It will be appreciated that the same principles apply to a sensor configuration where the test frequency is below the frequency bands of interest from the sensor. Under these circumstances, the sensor frequency bands are isolated instead with highpass filtering. The testing circuits of Figures 1 -4 all simply provide a two-level go/no- go output. However it will be appreciated that the test input signal may also be used to provide a more accurate measurement of the impedance of the capacitive sensor, and hence the sensor integrity. Thus in an alternative example (not shown), the microprocessor 9 monitors the test signal level from the bandpass filter by means of an analogue to digital converter and provides the measurement for display, recording, or other indication. The accurate impedance measurement can be used to determine whether the sensor 1 has been partially damaged, for instance by being cut. Alternatively the multi-level output may be useful in applications in which the capacitance of the sensor is varied as part of the normal operation of the sensor. A further extension of the same principle can utilize instead of single- frequency signals for monitoring of the impedance of the sensor, a band of such frequencies. A pseudo-random sequence is one example that lends itself to simple generation and detection. Furthermore, a pseudo-random sequence can be used not only above or below the sensor frequency bands, but may also be used directly in the sensor's frequency range of interest, as is applied in spread spectrum techniques. Given that the applied Pesudo Random Sequence test signal is known, it may be detected by means of correlation, and thereby separated from the sensor measurement signal. The detected level is then processed as in the previous examples. An example of a system embodying this principle is shown in Figures 5 and 6. Figure 5 is identical to Figure 1 except the oscillator 5 has been replaced with a pseudo-random noise generator 25, the bandpass filter 7 has been replaced by a correlator 26 and the diode 8 has been replaced by an averager 27. The generator 25 may be implemented in the form of a linear feedback shift register shown in Figure 6 (although many different forms of implementation are known in the literature) . A clock signal 20 is supplied to a chain of series-connected f pflops 23. Selected taps of the shift register are summed together with exclusive-OR gates 24 in order to provide a maximal-length sequence of 2 " N - 1 , where N is the number of fhpflops 23 in the shift register. The actual tap points for a maximal length sequence vary with the length of the shift register, but are well known and published in the literature or are relatively easily determined. An output 21 of the generator 25 is fed back to the input 22. As a result, the signal on output 21 is a pseudo-random binary sequence (PRBS) which is uniquely determined by the configuration of tap points for the exclusive-OR gates 24. The pseudo-random binary sequence is also fed to a correlator 26 which generates an output with a DC level (measured by averager 27) which is indicative of the degree of correlation between the output of the sensor and the PRBS. The correlation function implemented by the correlator 26 might, for instance, be carried out by way of a simple phase sensitive detector and low pass filter to both block the sensing signal and recover the test signal. In applications where the sensor measurement signal is of narrow bandwidth, the broad spectral nature of the PRBS may mean that the PRBS may be sufficiently removed from the sensing signal by the LPF 3. Alternatively the PRBS may be completely removed from the sensing signal by correlation and subtraction utilising digital signal processing techniques. Although this invention has been described by way of example it is to be appreciated that improvement and/or modifications may be made thereto without departing from the scope of the invention as defined in the appended claims.

Claims: CLAIMS 1 . Apparatus for testing a capacitive sensor, the apparatus including: means for applying a test input signal to the sensor, the test input signal having a predetermined signal characteristic; and means for monitoring a sensor output signal from the sensor and generating a test output signal which varies in accordance with the presence or absence of the predetermined signal characteristic in the monitored sensor output signal. 2. Apparatus according to claim 1 wherein the test input signal lies within a test frequency range, and the means for monitoring comprises means for blocking signals outside the test frequency range. 3. Apparatus according to any one of the preceding claims further including means for extracting a sensing signal from the sensor output signal by blocking signals outside a sensing frequency range. 4 Apparatus according to claim 2 and 3 wherein the test frequency range lies completely above or completely below the sensing frequency range. 5 Apparatus according to claim 1 wherein the test input signal is encoded with a predetermined code sequence, and the means for monitoring comprises means for correlating the predetermined code sequence with the sensor output signal 6. Apparatus according to claim 5 wherein the code sequence is a pseudo-random code sequence. 7. Apparatus comprising a sensor for generating sensing signals; and apparatus according to any one of the preceding claims for testing the sensor. 8. Apparatus according to claim 7 wherein the sensor is formed to enable the sensor to detect cardiac and/or respiratory signals. 9. A method of testing a capacitive sensor, the method including the steps of: applying a test input signal to the sensor, the test input signal having a predetermined signal characteristic; and monitoring a sensor output signal from the sensor and generating a test output signal which varies in accordance with the presence or absence of the predetermined signal characteristic in the monitored sensor output signal. 1 0. A method according to claim 9 wherein the test input signal lies within a test frequency range, and the test output signal is generated by blocking frequencies in the sensor output signal outside the test frequency range. 1 1 . A method according to claim 9 or 1 0 further including the step of extracting a sensing signal from the sensor output signal by blocking signals outside a sensing frequency range. 1 2. A method according to claim 1 0 and 1 1 wherein the test frequency range lies completely above or completely below the sensing frequency range. 1 3. A method according to claim 9 wherein the test input signal is encoded with a predetermined code sequence, and the test output signal is generated by correlating the predetermined code sequence with the sensor output signal. 1 4. A method according to claim 1 3 wherein the code sequence is a pseudo-random code sequence 1 5. A method of sensing, the method including the steps of providing a sensor which generates sensing signals; and testing the sensor by a method according to any one of claims 9 to 14. 1 6. A method according to claim 1 5 wherein the sensor is formed to enable the sensor to detect cardiac and/or respiratory signals. 1 7. Apparatus according to any one of claims 1 to 8, further comprising means for generating a fault signal when the test output signal lies outside predetermined performance criteria. 1 8. A method according to any one of claims 9 to 1 6 further comprising generating a fault signal when the test output signal lies outside predetermined performance criteria. 1 9. Apparatus according to claim 7 or 8, wherein the sensor is mounted in or on a support (such as a seat or bed) for supporting a human or animal subject. 20. A method according to claim 1 5 or 1 6, the method further including the step of arranging the sensor to monitor a human or animal subject.

Inventors: LITTEK ARNIM HOLGER, POT MICHAEL JOUHANNES, LITTEK, ARNIM, HOLGER, POT, MICHAEL, JOUHANNES
Applicants: LITTEK ARNIM HOLGER, MED DEV LTD, POT MICHAEL JOUHANNES, LITTEK, ARNIM, HOLGER, MED-DEV LIMITED, POT, MICHAEL, JOUHANNES
Application year: 2000
Main classification: G01R35
IPC codes: G01R35