PIEZOELECTRIC SENSING SYSTEMS

Abstract: A piezoelectric sensing system is disclosed wherein a vibrating piezoelectric sensing element (50) is driven by a periodic drive signal at or close to the resonant frequency of the sensing element, and wherein a return signal from the sensing element is monitored to detect a shift in the resonant frequency.

Description: PIEΞZOELECTRIC SENSING SYSTB1S This invention relates to a piezoelectric sensing system wherein a variable characteristic of a piezoelectric sensor is monitored to detect variations in a characteristic of a medium surrounding the sensor. It is known to provide a fluid level detector in which the change in resonance frequency of a piezoelectric element when the element is immersed in a fluid is detected. Such an arrangement is disclosed, for example, in EP-A-0123189. In this known detector, the piezoelectric element is driven by an external drive oscillation circuit and resonance is detected by monitoring the phase difference between the • -voltage and current- in. the drive circuit. At resonance, the voltage and current will be in phase. There is a need for a more flexible and less costly system for sensing fluid levels. According to the present invention, there is provided a piezoelectric sensing system wherein a vibrating piezoelectric element is driven by a periodic drive signal at or close to the resonant frequency of the sensing element, and wherein a return signal from the sensing element is monitored to detect a shift in the resonant frequency. Preferably, the drive signal is applied intermittently to the sensing element, and the return signal is monitored in the intervening intervals between the drive signals using time division multiplexing. The drive signal is preferably applied from an I/O port of a microcontroller, and the port is then held at a predetermined constant voltage, or logic level, for a short delay period before monitoring the return signal. This prevents the return signal swamping any variations in the signal from different sensing elements. Preferably the drive signal is initially scanned through a range of frequencies and the return signal is monitored to detect the frequency closest to the resonant frequency of the sensing element. The sensing system is of wide application and can be used whenever variations in the characteristic of a particular medium that affect the resonant frequency of the sensing element are to be detected. The system is particularly suited to the detection of fluid levels, for example the level of the oil in an oil storage tank. By way of example only, some embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is plan view of a multi-point fluid level detector; Fig. 2 is a detail of one of the sensing elements in the detector of Fig. 1;- - Fig. 3 is perspective view of the housing for the monitoring circuit of the detector; Fig. 4 is a circuit diagram of the circuit for driving and monitoring one sensing element of the detector; Fig. 5 is a waveform diagram illustrating an initial phase in the operation of the circuit of Fig. 4; Fig. 6 is a waveform diagram illustrating a subsequent phase in the operation of the circuit of Fig. 4; Fig. 7 is a circuit diagram of an alterative circuit for driving a piezoelectric sensing element and detecting the resonance frequency of the element in a fluid level detector; Fig. 8 is a second alternative circuit; Fig. 9 is a third alternative circuit: and Fig. 10 is a waveform diagram illustrating some of the waveforms in the circuit of Fig. 9. Figs. 1-3 illustrate a fluid level detector consisting of successive piezoelectric sensing elements 50 spaced apart along an elongate probe. A single sensing element 50 is shown in more detail in Fig. 2. In use, the probe is positioned within a container, and the level of a fluid in the container is detected by sensing the number of sensing elements 50 immersed in the fluid. The probe shown in Figs. 1-3 is more fully described and separately claimed in our copending application based on British Patent Application 9408189.0 and being filed concurrently herewith. To sense whether or not a sensing element is immersed in the fluid, each element is connected to a monitoring circuit on a printed circuit "" board ' 51 contained " within a housing 52 as shown in Fig. 3. The monitoring circuit is shown schematically in Fig. 4 and includes a microcontroller 60. for example an EPROM-based 8-bit CMOS microcontroller, such as the single chip PIC16C5X Series produced by Microchip Technology Inc. The microcontroller 60 includes an I/O circuit 70 capable of operating as an input (switch SI open) and an output (switch SI closed). Fig. 4 shows one such I/O port 71 connected to a piezoelectric sensing element 50 through an external resistor 63. Each sensing element 50 is connected to a respective I/O port 71 of the microcontroller 60. The piezoelectric sensing element 50 consists of a piezo ceramic disc 64 bonded to a larger diameter backing plate 65. The backing plate is earthed, and a square wave voltage synthesised in the microcontroller 60 is applied to the disc. This produces vibrations of the plate 65 at the frequency of the drive signal. The sensing element will have a characteris ic resonant frequency dependent on the size, mass and mounting of the element, and this resonant frequency will vary according to the viscosity of the medium surrounding the element. Accordingly, there will be a significant shift in the resonant frequency when the element becomes immersed in, or emerges from, the fluid in the container as the level of the fluid rises or falls. The shift in resonant frequency is detected by monitoring the characteristics of the return signal generated by the sensing element 50 when excited by a drive signal from the microcontroller 60. The drive signal 66 (Fig. 5) is initially swept through a range of frequencies with the I/O port 71 operating firstly as an output port for each sampled frequency and then, in the intervening periods, as an input port to receive the return signal 67 generated by the sensing element. During this scanning phase of the process, the drive and return signals have the form shown in Fig. 5. The purpose of the scanning phase is to establish a point close the resonant frequency of the sensing element. This is achieved by the microcontroller 60 counting the number of cycles of the return signal 67 which are received before the signal decays below the threshold level of the input gate., and detecting the peak number of cycles. Having determined this approximate resonant point, the microcontroller 60 continues by feeding bursts of a drive signal 66 to the resonating element at a frequency close to the detected resonance point, measuring the period of the return signal from each burst, and adjusting the frequency of the drive signal on each occasion so that it lies as close as possible to the measured period. Since the frequency of the drive signal is synthesised in the microcontroller using a digital processor, the microcontroller is only able to generate an output frequency in discrete steps of, for example, 4 microseconds corresponding to frequency steps of 50-lOOHz dependent on clock speed, software and the type of microcontroller. Accordingly, several bursts of the drive signal are necessary before the output settles itself at a particular frequency or hunts back and forth between two frequencies either side of the resonant frequency. During this tracking phase of the operation, the signals have the form shown in Fig. 6. By measuring the period of the return signal 67, the microcontroller is able to detect a shift in the resonance frequency when the sensing element becomes immersed in, or emerges from, the fluid in the container. If the return signal 67 was fed back into the I/O circuit 70 immediately the drive signal 66 was removed, the peak to peak magnitude of the signal could be around twice the drive voltage and this could mask the variation in the peak return signals at resonance and just off-resonance. Accordingly, during the tracking phase shown in Fig. 6, each return signal is damped by holding the output 71 at a constant value (logic 0) for a few cycles before admitting the return signal. In this manner, some of the energy in the resonating sensing element is discharged before the return signal is detected. The sensing system thus operates on the basis of time division multiplexing using a single input/output pin 71 of the microcontroller 60. The microcontroller includes a crystal oscillator providing a reference clock signal, .and the drive signal 66 is synthesised by a software program responsive to the period of the return signal 67. In addition, the return signal 67 must be biased to ensure that it crosses the logic thresholds of the input gate to the microcontroller. This could be achieved by a resistor network. However, in the present example, it is achieved as a by-product of the DC average component so that a DC bias of half the drive voltage will be achieved. The bias will be finely controlled by a combination of duty cycle of the drive signal and the duration and polarity of the hold phase. The microcontroller 60 includes a pair of parasitic diodes Dl, D2 associated with the input circuit 70. At the end of the drive and hold phases, the drive is removed and becomes high impedance. At this point, the resonating plate 65 of the sensing element 50, when excited near its resonant frequency, will continue to oscillate, distorting the ceramic disc 64 and thus producing a voltage. When the drive signal is of a frequency near to the resonance of the piezo ceramic assembly (including the mounting legs 72, electrical contacts and most significantly the surrounding medium) and sufficient cycles have been applied to achieve peak distortion, the peak voltage of the returned signal 67 will be greater than the supply voltage Vcc to the drive circuit. Accordingly, the diodes Dl, D2 will conduct when the positive half cycle exceeds Vcc (+ 0.6v) and when the negative half cycle is below GND (-0.6v>. During these periods the current drawn will tend to charge or discharge the bias circuit to bring the average of the generated return waveform to midway between the supply voltage rails of the drive circuit, thus ensuring that the generated voltage fully achieves both logic states, i.e. it crosses the logic thresholds of the input gate, at least for the early cycles of the decaying voltage. The drive phase of Fig. 5 is a series of drive cycles, the number being sufficient to achieve maximum energy levels for a resonator of given dimensions and low enough to ensure noise levels do not become significant. During this period, energy is built up in the resonating plate provided the drive signal is close to the resonant frequency of the assembly. Upon removal of the drive signal (at ti) the resonator plate 65 will continue to vibrate or ring, as a drum would once struck, producing a voltage that will be clipped by the parasitic diodes Dl, D2 and current limited by the external series resistor 63. This continuing vibration will decay in a predictable time governed by the value of the series resistor, the quality of the mounting system of the resonator, and the mass and mechanical damping created by the conductors 80 (Fig. 2) that carry the signals to and from the piezo ceramic disc 64. In addition, the combination of the value of the resistor 63 and the final drive level of the signal 66 before the drive is removed, will set the centre of the resultant waveform. Selecting the resistor 63 such that the centre voltage is close to the logic threshold of the microcontroller (typically 1.5v - 2.5v) will increase sensitivity at the expense of noise immunity. The circuit has a number of significant advantages, including low cost drive and detect circuits, a large feedback signal (created by the large dimensions of the ceramic used for both drive and feedback) and a closed loop resonator principally under software control, enabling complex filtering, phase shift, drive frequency synthesis, signal damping and increased drive cycles to compensate for degradation of the resonator caused by ageing or build up of deposits. Referring next to the embodiment of Fig. 7, a piezoelectric sensing element 10 is driven by a self-drive oscillator circuit 14. The sensing element 10 consists of a piezo ceramic disc 11 bonded to a resonating plate 12, the disc 11 including a voltage-generating island 13 which feeds back a signal to the drive circuit 14 when the plate 12 is vibrating at the resonant frequency. Since the resonant frequency will depend on the medium in which the sensing element is immersed, the change in resonance frequency of the sensing element can be used to detect the presence or absence of a fluid at a particular level in a container. In Fig. 7 this change in resonance frequency is detected by comparing the drive voltage being fed to the sensing element with a reference voltage. At resonance, the voltage and current supplied to the sensing element are in phasej the impedance of the sensing element is at its minimum so that the drive current is at a maximum, and the voltage from the feedback element 13 is at its maximum. Accordingly, if the sensing element is initially resonating in air and then becomes immersed in a fluid, such as oil, the resulting change in phase and/or amplitude of the drive current can be detected. Referring to Fig. 7 in more detail, the drive circuit is self- oscillating and consists of a transistor TRl and resistors Rl, R2 and R3. The feed back voltage from the sensing element sustains the oscillations and is fed to the junction between resistors Rl and R3. The phase relationship of the feed back voltage will be leading for a drive frequency lower than the resonant frequency and lagging for a drive frequency above resonance. Near resonance, there occur very large shifts in phase for small changes in frequency. A timed sampler 16 samples the output of the comparator 15 for a known period and counts the number of transitions (2 transitions per cycle) to determine the frequency of the assembly. Fig. 8 illustrates an alternative arrangement in which a microcontroller chip 20 having a crystal controlled clock source SI providing a reference frequency monitors the feed back loop to a drive circuit 31 and applies a frequency threshold to detect the absence or presence of a fluid medium around the piezoelectric sensing element 30. A potential problem with such a circuit is that of over-resonance. It has been observed that a self-oscillating drive circuit shows a tendency to stabilise at multiple odd harmonic frequencies. This is particularly true when the piezo disc of the sensing element is mounted vertically and is half submerged. In the circuit of Fig. 8, this problem is overcome by providing a low pass filter in the feed back circuit. The low pass filter is formed by the capacitor Cl and the resistor R5. Another potential problem of self-oscillating drive circuits is that of start-up. The approach adopted in Fig. 8 is to "kick start" the oscillator at or near the two expected resonant states. The P. Q on the microcontroller 20 is a tristate connection initially providing synthesized frequency output which can "drive" the transistor stage TR2 into oscillation. Thus, to determine whether the sensing element 30 is or is not immersed in a fluid, the drive circuit 31 is first driven near to the lower of the two resonant frequencies for, say, 200 cycles and then P AQ is set to high impedance so that the circuit will free run. The frequency drift of the circuit is then observed and, if the sensing element is in a wet condition, the circuit will stabilise at the lower of the two resonant frequencies. On the other hand, if the sensing element is in a dry condition, the circuit will drift to the higher resonant frequency. The circuit is then "kick started" at a frequency near the higher of the two resonant frequencies and again the frequency drift is observed. If the circuit in each case fails to stabilise at one of the two resonant frequencies, this indicates a fault condition. In this example, the frequency is detected in the microcontroller 20 by counting the transitions on the feedback signal. Referring next to the alternative circuit of Fig. 9 and to the waveform diagrams of Fig. 10, a microcontroller 41 again having a crystal controlled clock source SI provides a synthesised square wave frequency signal V, to a drive circuit 42 for driving a piezoelectric element 40 at its resonant frequency. A sinusoidal feed back signal V- from the piezoelectric element is supplied to a feed back circuit which includes a peak detector transistor TR3 and resistors R8, R9 and RIO. The base/emitter junction of TR3 coupled with resistors R8, R9 provide a bias and threshold reference of approximately 0.6 volts for the peak detector circuit. The output V β from the peak detector has a pulse width proportional to the peak voltage. Accordingly, to detect the presence or absence of a fluid around the piezoelectric element, the output of the frequency synthesizer is swept from below the lower of the two resonant frequencies to above the higher of the two resonant frequencies and the mark space ratio of Vp is monitored by an appropriate circuit in the microcontroller. 41 at incremental frequencies within the swept range. When the mark space ratio is 1:1 (corresponding to a 50% duty cycle) this indicates that the synthesizer is at the resonant frequency. The value of the resonant frequency will then indicate whether the piezoelectric element is in or out of the fluid.

Claims: CLAIMS 1. A piezoelectric sensing system wherein a vibrating piezoelectric sensing element is driven by a periodic drive signal at or close to the resonant frequency of the sensing element, and wherein a return signal from the sensing element is monitored to detect a shift in the resonant frequency. 2. A sensing system according to claim 1 wherein the period of the return signal is detected and the detected period controls the frequency of the drive signal applied to the sensing element. "3. A sensing system- according to claim 1 or claim 2 wherein the drive signal is removed when monitoring the return signal. 4. A sensing system according to claim 3 wherein the drive signal is applied intermittently to the sensing element, and wherein the return signal is monitored in the intervening intervals between the drive signals using time division multiplexing. 5. A sensing system according to claim 3 or claim 4, in which the monitoring of the return signal is delayed for a predetermined interval after removal of the drive signal. 6. A sensing system according to claim 5, in which the drive signal is supplied from an I/O port of a microcontroller, and the return signal is received at the said port. 7. A sensing system according to claim 6, in which the I/O port is held at a predetermined voltage during the delay interval and prior to monitoring the return signal. 8. A sensing system according to any one of the preceding claims wherein the drive signal is initially scanned through a range of frequencies including the expected resonant frequency of the sensing element. 9. A sensing system according to claim 8, in which the return signal is monitored during the said scanning of frequencies to detect the drive signal closest to the resonant frequency. 10. A sensing system according to claim 9, in which the return signal is monitored during the scanning of frequencies by detecting the number of received cycles having an amplitude exceeding a predetermined threshold level. 11. A sensing system according to any one of the preceding claims, wherein the drive signal is applied successively to a plurality of the sensing elements spaced apart from one another, and the return signals from each sensing element are monitored to detect any variation in a characteristic of a medium surrounding the elements. 12. A sensing system according to claim 11, in which the return signals are monitored to detect the presence or absence of a fluid at the level of the respective sensing elements. 13. The use of a sensing system according to any one of the preceding claims for sensing the level of a fluid. 14. A sensing system substantially as herein described with reference to Figs. 1 - 6 of the accompanying drawings. 15. A sensing system substantially as herein described with reference to Fig. 7, Fig. 8 or Fig. 9 of the accompanying drawings.

Inventors: CASSELDEN IAN, CASSELDEN, IAN
Applicants: AR MET PLASTICS LIMITED, CASSELDEN IAN, AR-MET PLASTICS LIMITED, CASSELDEN, IAN
Application year: 1995
Main classification: G01F23
IPC codes: G01F23, G01N29