Abstract:
A piezoelectric and/or electret sensing device includes a piezoelectric and/or electret transducer for producing a measurement current in response to mechanical stimulus, and a control and evaluation circuit connected to the transducer. The control and evaluation circuit includes a transimpedance amplifier having a first and a second input, the transducer being operatively connected between the first input and a reference node, and an electrical waveform generator for generating an electrical waveform, the electrical waveform generator being operatively connected between the second input and the reference node.

Description:
TECHNICAL FIELD 
     The present invention generally relates to piezoelectric or electret sensors, which may e.g. be used for detecting vibration or displacement. More specifically, the present invention relates to diagnostics of such sensors. 
     BACKGROUND ART 
     Piezoelectric or electret transducers are well known as such. They are typically used to convert a mechanical stimulus (such as vibration, movement, strain etc.) into an electrical signal. 
     US patent application 2010/0295563 A1 discloses an occupant detection system with an electrode arrangement for a seat of an automotive vehicle, wherein the electrode arrangement includes a first electrode, a second electrode and an electric insulator layer sandwiched between the first and second electrodes. When the electrode arrangement is in place in the seat, the first electrode forms with vehicle ground a first capacitor having a first capacitance influenceable by an occupying item in the detection region through interaction of the occupying item with the electric field, the first electrode forms with the second electrode a second capacitor having a second capacitance and the second electrode forms a first capacitor plate of a third capacitor having a third capacitance. The second plate of the third capacitor may be formed by conductive surfaces at vehicle ground potential or a third electrode formed by further conductive surface in the vehicle seat, e.g. a seat heater, the seat pan or a third electrode arranged behind the second electrode (as seen from the first electrode). The fluctuations of at least one of the first, second and third capacitances are measured and the frequency spectrum of the measured fluctuations is analyzed in order to obtain an indicator of the seat occupancy state. According to a preferred embodiment of the invention of US 2010/0295563 A1, the electric insulator layer between the first and the second electrode comprises an electret layer and/or a piezoelectric layer. In this configuration, the evaluation circuit determines, among others, a current induced in the first and/or second electrode by the electret or piezoelectric layer (in response to pressure being applied to the electret or piezoelectric layer) and derives the fluctuations of the second capacitance from the induced current. 
     U.S. Pat. No. 6,531,884 B1 relates to a diagnostics device for testing a piezoelectric sensor. The diagnostics device includes an AC source that applies an AC signal to the piezoelectric sensor at two or more different frequencies. Measurement circuitry coupled to the piezoelectric sensor measures a response of the sensor to the applied AC signal and provides a measured output related to a sensor resistance and a sensor capacitance of the piezoelectric sensor. Diagnostic circuitry provides a diagnostic output as a function of the measured output. As a disadvantage of the disclosed system, the measurement mode and the diagnostics mode must be carried out one after the other. 
     BRIEF SUMMARY 
     An improved piezoelectric and/or electret sensing device with self-diagnostics capability is herein provided. 
     A piezoelectric and/or electret sensing device comprises a piezoelectric and/or electret transducer for producing a measurement current in response to mechanical stimulus, and a control and evaluation circuit connected to the transducer. According to the invention, the control and evaluation circuit comprises a transimpedance amplifier having a first input and a second input, the transducer being operatively connected between the first input and a reference node (e.g. a node at ground potential), and an electrical waveform generator for generating an electrical waveform, the electrical waveform generator being operatively connected between the second input and the reference node. The transimpedance amplifier is configured to control the first input such that the difference in electric potential between the first input and the second input be cancelled and to output an output voltage, with respect to the electric potential at the reference node, indicative of electrical current flowing across the transducer. The control and evaluation circuit is configured to identify a first component of the output voltage indicative of the measurement current and a second component of the output voltage indicative of the current caused by the generation of the waveform. The control and evaluation circuit is, furthermore, configured to detect a short circuit and/or a broken wire based upon the second component. Such short circuit or broken wire may e.g. be detected by comparing the second component with a respective threshold. 
     As will be appreciated, the piezoelectric and/or electret transducer and the waveform generator (which may e.g. comprise an oscillator and/or a pseudo-random noise generator) are connected electrically in series between the first input and the second input of the transimpedance amplifier. The waveform generator produces its waveform in a portion of the frequency spectrum that is not occupied by the measurement signal output by the transducer, i.e. the electrical waveform is spectrally separated from the measurement current. In that way, the first component of the output voltage and the second component thereof can be easily separated by the control and evaluation circuit. 
     Preferably, the control and evaluation circuit comprises a processing chain for identifying the first component, the processing chain for identifying the first component comprising a filter operatively connected to the transimpedance amplifier, the filter being configured so as to suppress the second component. 
     The control and evaluation circuit may comprise a processing chain for identifying the second component. The processing chain for identifying the second component may comprise a synchronous rectifier, synchronized on the waveform, and a low-pass filter, for identifying the second component of the output voltage. Specifically, the processing chain for identifying the second component could comprise a mixer, operatively connected with the transimpedance amplifier and the electrical waveform generator for multiplying the output voltage with the waveform, and a low-pass filter. 
     Preferably, the processing chain for identifying the second component comprises an IQ demodulator for identifying the second component as a complex voltage comprising an in-phase part and a quadrature-phase part. In this case, the control and evaluation circuit may be configured to detect the short circuit and/or broken wire by monitoring whether the complex voltage remains within a predefined area of the complex plane. 
     The electrical waveform generator may comprise an AC signal source, such as, e.g. an oscillator. 
     According to a preferred variant of the invention, the electrical waveform generator is configured to generate a pseudo-random noise waveform, e.g. a sine carrier wave modulated by a binary pseudo-random noise code. 
     According to a particularly preferred variant of the invention, the control and evaluation circuit comprises an analog-to-digital converter operatively connected with the transimpedance amplifier for converting the output voltage into a digital signal and a processor configured to identify the first component and the second component by analyzing the digital signal. The processor could e.g. comprise an application-specific integrated circuit (ASIC), a digital signal processor (DSP), a field-programmable gate array (FPGA), or a microcontroller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein: 
         FIG. 1  is a block schematic diagram of a piezoelectric sensing device according to a first preferred embodiment of the invention; 
         FIG. 2  is a block schematic diagram of a piezoelectric sensing device according to a second preferred embodiment of the invention; 
         FIG. 3  is a block schematic diagram of a piezoelectric sensing device according to a third preferred embodiment of the invention; 
         FIG. 4  is a block schematic diagram of a piezoelectric sensing device according to a fourth preferred embodiment of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a piezoelectric sensing device  10  according to a first preferred embodiment of the invention. The piezoelectric sensing device  10  comprises a piezoelectric transducer  12  that produces a current (referred to herein as the “measurement current”) in response to a mechanical stimulus (vibration, movement, strain or the like). The transducer  12  is connected between ground and the first input of a transimpedance amplifier  14 . The transimpedance amplifier  14  (depicted as including an operational amplifier  16  and a feedback network with a capacitor  18  and a resistor  20 ) converts the current from the transducer  12  into a corresponding voltage on the output terminal of the transimpedance amplifier  14 . The second input (reference input) of the transimpedance amplifier  14  is connected to a diagnostics signal source  22 , which applies a variable voltage signal (electrical waveform) to the second input. The transimpedance amplifier  14  attempts to maintain a zero voltage difference between its first and second input by driving a current across the feedback network, i.e. across the transducer  12 . The output voltage of the transimpedance amplifier is thus modulated in consequence. The total current flowing across the transducer  12  thus corresponds to the sum of the diagnostics current (caused by the voltage modulation at the second input) and the measurement current (i.e. the electrical response to a mechanical stimulus). The total current can be calculated as the ratio between the output voltage of the transimpedance amplifier and the impedance of the transducer  12  and its wiring. 
     The modulation imposed by the diagnostics signal source  22  is chosen such that it does not significantly overlap in frequency with the measurement signal. In practice, this may best be achieved by selecting the frequency band containing the modulation well above the upper bound of the frequency band containing the measurement signal. For instance, the frequency spectrum of the measurement signal could range from a few Hz up to some kHz (e.g. from 1 Hz to 1 kHz). A possible frequency range for the diagnostics signal could then be from 10 kHz to 100 KHz (or even higher). 
     The output of the transimpedance amplifier  14  is routed to a first and a second processing chain. The first chain extracts the component of the output voltage that is caused by the transducer  12 , whereas the second chain extracts the component of the output voltage that results from the modulation of the voltage at the second input. 
     For extracting the first component of the output voltage (indicative of the measurement current), the output terminal of the transimpedance amplifier is connected to the filter  24 , which removes all frequencies of the modulation band. The filter output is connected to a first ADC (analog-to-digital converter) input of a microcontroller  26 , which evaluates the measurement signal (e.g. detects the presence of a human when the piezo transducer is arranged in a floor covering or a seat). 
     The output of the transimpedance amplifier is also routed to a multiplying mixer (frequency mixer)  28  which is driven at its LO (local oscillator) input with the modulation signal generated by the diagnostics signal source  22 . The output of the multiplying mixer  28  is routed to low-pass filter  30 , and from there a second ADC input of the microcontroller  26 . The multiplying mixer  28  and the low-pass filter  30  co-operate as a synchronous rectifier. The voltage at the output of filter  30  is therefore indicative of the conductance of the transducer  12  and its wiring. The microcontroller  26  monitors the voltage at the output of filter  30  by comparing it with a first threshold to detect a short circuit, and a second threshold to detect an open circuit (broken wire). In other words, if the monitored voltage leaves a predefined range of values, the microcontroller  26  detects a short circuit or a circuit interruption. A corresponding warning signal may then be issued and the evaluation of the measurement signal be suspended. 
       FIG. 2  shows a piezoelectric sensing device  10  according to a second preferred embodiment of the invention. The device of  FIG. 2  differs from the device of  FIG. 1  only in that it is arranged and configured for full IQ-demodulation of the diagnostics signal. Low-pass filter  30  outputs the in-phase (I-) part (denoted v x ) of the diagnostics signal. A second synchronous rectifier (composed of multiplying mixer  28 ′ and low-pass filter  30 ′) is provided that outputs the quadrature-phase (Q-) part (denoted v y ) of the diagnostics signal. To this end the mixer  28 ′ is driven at its LO input with a 90-degree-phase-shifted copy of the modulation voltage, which is provided by phase shifter  32 . The complex voltage v x +j v y  is indicative of the complex impedance of the transducer  12  and its wiring. The microcontroller  26  may thus monitor whether the complex voltage remains within a predefined area of the complex plane and output a warning signal if it detects abnormal impedance. 
       FIG. 3  shows a piezoelectric sensing device  10  according to a third preferred embodiment of the invention. The device of  FIG. 3  is similar to the one shown in  FIG. 2 . Only the differences will thus be discussed hereinafter. Pseudo-random noise code generator  36  (e.g. a linear feedback shift register) produces a binary pseudo-random noise waveform, which is multiplied in multiplying mixer  34  with a sine carrier wave output by oscillator  22 ′. (Oscillator  22 ′, mixer  34  and pseudo-random noise code generator  36  form together an electrical waveform generator.) The spread-spectrum signal output by the multiplying mixer  34  serves as the reference voltage input to the transimpedance amplifier  14 . 
     The multiplying mixer  28  multiplies the output voltage of the transimpedance amplifier  14  with the original spread-spectrum signal. The low-pass filter  30  thus outputs the in-phase part (v x ) of the output voltage of the transimpedance amplifier. The cutoff frequency of filters  30 ,  30 ′ is chosen substantially lower than the bit rate (also called chip rate) of the PRN signal. The multiplying mixer  28 ′ multiplies the output voltage of the transimpedance amplifier  14  with a copy of the spread-spectrum signal, in which the carrier wave has been shifted by 90 degrees. The low-pass filter  30 ′ thus outputs the quadrature-phase part (v y ) of the output voltage of the transimpedance amplifier. The microcontroller  26  monitors the complex voltage v x +j v y  (indicative of the impedance of the transducer  12  and its wiring) and outputs a warning signal if it detects an abnormal impedance. 
     The use of a pseudo-random noise code on the diagnostics signal is especially useful if interference with other electronic devices shall be avoided. Interfering signals are indeed cancelled in the low-pass filters  30 ,  30 ′ unless they present high cross-correlation with the pseudo-random noise code, which is very unlikely. Furthermore, interference signals coming from the piezo or electret transducer itself are suppressed. This allows the measurement frequency spectrum and the diagnostics frequency spectrum to overlap without affecting the self-diagnostic capabilities of the device. 
     A fourth preferred embodiment of the invention is illustrated in  FIG. 4 . According to this embodiment, the transimpedance amplifier  14  and the diagnostic signal source  22  are connected to an ADC (analog-to-digital converter)  38 . The ADC  38  samples the voltage output by the transimpedance amplifier  14  at a sampling rate sufficiently high to allow the microcontroller  26  to extract the IQ information at the frequency of the diagnostic signal. The ADC  38  may be clocked by the diagnostic signal source. The microcontroller  26  then performs signal-processing corresponding to the embodiments of  FIG. 1, 2 or 3  on the digital signal. 
     While specific embodiments have been described in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.