Abstract:
A device for detecting a touch at a surface of a substrate comprises a piezoelectric element positioned at a back surface of the substrate to be touched. A drive circuit is configured to apply a frequency modulated drive signal to the piezoelectric element to detect resonance at the substrate. A first detector arrangement coupled to the piezoelectric element is configured to generate a first output signal indicative of a sensed touch or no touch condition according to detected voltage changes and responsive to the frequency modulated drive signal applied to the piezoelectric element. A second detector arrangement coupled to the piezoelectric element is configured to generate a second output signal indicative of a sensed touch or no touch condition according to a sensed strain on said piezoelectric element. A decision circuit including a processor is configured to identify a touch condition or no touch condition at the surface of the substrate according to the first and second output signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 14/639,964 filed Mar. 5, 2015, which in turn claims priority to, and benefit under 35 U.S.C. §119(e) of, U.S. Provisional Application No. 61/948,312 entitled “Ultrasonic and Strain Dual Mode Sensor For Contact Switch”, filed on Mar. 5, 2014, the entire disclosure of which is hereby incorporated by reference for all purposes as if being set forth in its entirety herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to touch sensitive devices, and more particularly, to an ultrasonic contact sensor or finger pressure sensor. 
       BACKGROUND 
       [0003]    A finger touch switch is an important input device for various types of electronic equipment, including but not limited to computer keyboards, microwave ovens, copier machines, electronic card readers, ATM machines, and the like. Basic sensing mechanisms for identifying the pressed (depressed) point or location typically include metal to metal contact, capacitive or conductive variations of transparent electrodes, magnetic field detection, optical intercept, piezoelectric strain effect, and so on. In order to detect small deformations at the pressed points, one or more thin layers or membranes, or flexible insulators are often utilized. However, such structures often experience wear or fatigue problems, and/or are vulnerable to vandalism. 
         [0004]    Examples of such finger touch switch devices known in the art include reflective mode ultrasonic touch switch devices as described in U.S. Pat. No. 5,673,041, assigned to Measurement Specialties, Inc., and incorporated herein by reference in its entirety. Such devices are based on resonance of a substrate (typically composed of low loss material such as glass or metal), and a piezoelectric element at the back surface of the substrate which detects the resonance. The resonance forms very sharp peaks at integer multiples of half wavelength thickness conditions of the substrate. This resonance is damped by contact at the substrate&#39;s front surface by a finger which is detected by the piezoelectric element. 
         [0005]    Another type of touch switch known in the art (piezo touch switch) is illustrated in U.S. Pat. Nos. 4,896,069 and 6,064,141. This type of touch switch is based on a different mechanism than the ultrasonic touch switch described above. In this case, bending strain of a housing plate (sensor plate) by a pressure force is detected by a piezoelectric element bonded at the back of the plate and used to control switching of electric appliances. Disadvantages of such piezoelectric touch switches include their lack of sensitivity for steady contact force or very low sensitivity for slowly increasing or decreasing force, as well as their undesirably high sensitivity to external vibration or mechanical shock. 
         [0006]    On the other hand, reflective mode ultrasonic touch switches or sensors have little or no sensitivity to external vibration or mechanical shock. Further, ultrasonic touch sensors exhibit sensitivity to steady contact, and when contact force increases beyond a given point or threshold, the switch starts to work (i.e. is activated). However, during operation of ultrasonic touch switches, particularly in applications such as for use in kitchen appliances, surface contamination (whereby a contaminant such as butter, ketchup, oils, greases, or even pure water etc. on the finger adheres to the sensing area) has the same effect as a steady touch of a finger, and can thereby yield a false signal. 
         [0007]    Other disadvantages of the ultrasonic reflective mode touch switch includes its less than desirable (i.e. low) sensitivity to a gloved finger or soft cloth, while for a piezoelectric touch switch, this type of device exhibits essentially the same sensitivity to pressure force, with its sensitivity essentially independent of the material which covers the finger. 
         [0008]    Further shortcomings associated with conventional sensors (ultrasonic or bending strain type) include their difficulty in providing, or the complete absence of, self diagnosis functions. 
         [0009]    Alternative systems and methods are desired. 
       SUMMARY 
       [0010]    An embodiment of the present invention relates to an apparatus that combines the advantageous features of both an ultrasonic touch switch and piezoelectric touch switch utilizing a same sensor element, and wherein two kinds of signals output from both modes are separately detected by filters and amplifiers. In this way, a steady touch force is detected in an ultrasonic mode, and contamination problems associated with the sensor (e.g. due to the presence of contaminating matter on the finger applied to the sensor) is mitigated in a piezoelectric strain mode. Further, the apparatus is configured to detect when the finger is covered by cloth, while also maintaining its insensitivity to external vibration or mechanical shock. 
         [0011]    A system and method for an ultrasonic touch switch combined with a piezoelectric strain mode sensor is disclosed. The sensor structure may be embodied as a substrate with a piezoelectric element at the back surface of the substrate, with the front surface being a touch sensitive surface. Both operational modes are implemented by use of the same sensor element with filtering of the signals so that different advantages of the two modes are combined, while disadvantages of either mode are mitigated. The tolerance of substrate thickness of ultrasonic touch mode is improved by use of a wide range of frequency modulation of the drive signal (FM) by filtering out the fundamental modulation frequency from the output signal whereby only sharp pulses corresponding to abrupt impedance changes of the piezoelectric element are extracted. The amplitude of the sharp pulses decreases with touching of the front surface of the substrate. At the same time the pressure force applied to the front surface causes the substrate to undergo bending displacement and causes piezoelectric element expansion strain to generate a voltage. These two modes are combined and used to control switching of an appliance or equipment. 
         [0012]    In an embodiment, a system and method is disclosed for an ultrasonic touch switch having a substrate with a piezoelectric element at the back surface of the substrate, with the front surface being a touch sensitive surface. The tolerance of the substrate thickness of the ultrasonic touch switch is improved by use of a wide range of frequency deviation of a drive signal and filtering out a fundamental modulation frequency from the output signal such that only sharp pulses corresponding to abrupt impedance changes of the piezoelectric element are extracted. The system filters high frequency components for passage to a decision circuit. The amplitude of the sharp pulses decreases with touching the front surface of the substrate. In this manner, contact by the finger to the surface of the substrate is detected by reduction of the peaks and a switching signal operation is generated to control a switching of an appliance or equipment 
         [0013]    In a practical environment having relatively high levels of vibration and mechanical shock, the design may be modified to reduce the sensitivity of a piezoelectric strain mode, and in the case where a glove (on the hand) is used, the sensitivity of the piezoelectric strain mode is often enhanced. 
         [0014]    According to an aspect of the present invention, ultrasonic impedance variation at resonance is detected by a piezoelectric element at the back surface of the substrate. The resonance takes place in the substrate material when the thickness becomes equal to an integer multiple of half wavelength condition and wherein the impedance of piezoelectric material changes abruptly at the resonance frequency (e.g. at about −5 MHz for 0.5 mm stainless steel). The resonance frequency is determined by the substrate thickness. The thickness has some variation depending on production tolerances of the substrate material. To detect this impedance variation at certain different frequencies for different thickness, the frequency of excitation signal is frequency modulated over a relatively wide range (FM, e.g. over about ˜500 kHz range). An abrupt change of impedance is detected by the amplitude difference of the voltage on the piezoelectric element which converts the FM signal to an amplitude modulated (AM) signal. The modulation frequency of FM is ˜1 kHz for example, and the abrupt change of impedance generates a sharp variation of the AM signal. The system is configured to detect the amplitude of the AM signal and the sharp variation is manifest as a sharp pulse or spike. After removing the modulation frequency component (˜1 kHz), the signal contains only sharp pulses and utilized for detection of touch or non-touch of the substrate surface. In this system, a feature of design is the wide tolerance of the substrate thickness. 
         [0015]    The structure of the ultrasonic touch switch includes the substrate with a piezoelectric element at the back surface and wherein the front surface is adapted to be touched by a finger. The same structure is used as a piezoelectric strain switch in which the pressure force from the front surface slightly bends the substrate and provides planar expansion strain to the piezoelectric sensor. The excitation signal of FM for the ultrasonic mode may be removed by filtering and the signal from only the pressure force may be extracted as an electrical signal, such that the response signal of the piezoelectric strain sensor is not influenced by the excitation voltage of the ultrasonic mode. 
         [0016]    Self diagnosis of the sensor operation may be accomplished using the combination of effects in a single piezoelectric element. Those effects include the ultrasonic touch effect, piezoelectric strain effect, and capacitance check. In the latter case, the capacitance of the piezoelectric element is determined and used for self diagnosis according to its determined capacitance value. In another aspect, thermal energy in the form of heat from the finger impinging upon the contact area influences the temperature of the piezoelectric element, thereby leading to a change of capacitance. Such capacitance change causes generation of a signal similar to the piezoelectric strain effect caused by finger touching. 
     
    
     
       BRIEF DESCRIPTIONS OF DRAWINGS 
         [0017]      FIG. 1  shows a schematic diagram of a dual mode ultrasonic reflective mode and piezoelectric strain switch device according to an embodiment of the invention. 
           [0018]      FIG. 2  illustrates the impedance variation of the piezoelectric element bonded at the back of a substrate, and further shows a frequency modulated signal used to detect impedance variation of the piezoelectric element at the back surface of the substrate according to an embodiment of the invention. 
           [0019]      FIGS. 2 a  and 2 b    illustrate input impedance variations as a function of frequency for PVDF material elements of different thicknesses bonded at the back of a same substrate. 
           [0020]      FIGS. 3 a  and 3 b    illustrate input impedance variations as a function of frequency for ceramic piezoelectric material elements of different thicknesses bonded at the back of a same substrate. 
           [0021]      FIG. 4  shows a more detailed schematic diagram of a portion of the device shown in  FIG. 1  illustrating the capacitive effect of the device according to an embodiment of the invention. 
           [0022]      FIG. 5  illustrates measured data of the DC output from the peak detector of  FIG. 4 , and which exhibits a generally constant value over the frequency variation except for a singular irregular change or spike. 
           [0023]      FIG. 6( a )  shows an oscilloscope waveform of the DC output from the peak detector of  FIG. 4  for 100 Hz modulation and pulse signals for a no finger touch condition according to an embodiment of the invention. 
           [0024]      FIG. 6( b )  shows an oscilloscope waveform of the DC output from the peak detector of  FIG. 4  for 100 Hz modulation and pulse signals for a finger touch condition at the substrate according to an embodiment of the invention. 
           [0025]      FIG. 6( c )  shows oscilloscope waveforms of the DC output from the peak detector of  FIG. 4  for 100 Hz modulation and pulse signals for a no signal condition, and for signals representing a finger depress and finger release condition, according to an embodiment of the invention. 
           [0026]      FIG. 7  shows a schematic diagram of an embodiment of a reflective ultrasonic touch detection circuit according to the invention. 
           [0027]      FIG. 8  shows a schematic diagram of exemplary processing and decision logic associated with the decision circuit of  FIG. 1  for detecting and processing each of the signals A, B, C, D for execution according to embodiments of the present invention. 
           [0028]      FIG. 9  is a table showing exemplary conditions and processing logic based on output signals from the detection system according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0030]      FIG. 1  shows a cross section view of a device  100  configured as a dual mode ultrasonic touch switch combined with piezoelectric strain switch device according to an embodiment of the invention. A piezoelectric element  102 , preferably a piezoelectric polymer material such as polyvinylidene fluoride (PVDF) or polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) in film form, or piezoelectric ceramic (PZT), has electrodes  103 ,  104  disposed on opposing (top and bottom) surfaces of element  102 . A substrate  101  which may be formed, for example, of stainless steel or glass, has first and second opposed parallel surfaces  105  and  106 . On the first surface  105  of the substrate  101  there is a defined touch region, preferably of a size approximately equal to the contact area of an operator&#39;s finger  108 . The top electrode  103  is bonded to the back surface  106  of the substrate  101  opposite from the touch region. By way of example only, the touch region can be defined as part of a computer generated display, wherein the substrate  101 , the piezoelectric element  102  and the electrodes ( 103 ,  104 ) may be relatively transparent, or the touch region can be defined by indicia on the front surface  105 . 
         [0031]    A controller or generator  111  excites piezoelectric element  102  and an ultrasonic wave  131  is launched into substrate  101 . The wave is reflected ( 132 ) at front surface  105  with an absorption by finger skin  108  back to surface  106  and reflected again ( 133 ) to the front direction, as represented by the three vertically oriented directional arrows. The wave propagates back and forth across the substrate. When the thickness is equal to an integer multiple of half wavelength condition of the substrate  101 , resonance of substrate  101  takes place. When a finger is not present on front surface  105  (i.e. where there is nothing touched at front surface  105  so that there is an air boundary for the no finger touch condition), the resonance is strong and gives strong influence to impedance variation to piezoelectric element  102 . Impedance variation by contact of the finger or any other absorbing matter  108  is detected by detector circuit  113  and  115 . 
         [0032]    When an operator&#39;s finger engages the touch region, the finger applies a pressure force and the substrate  101  is pressed downwards. Under these conditions, strain is maximum at the location underneath finger  108 . In response, piezoelectric element  102  expands in the planar direction (represented as horizontal arrow H) and generates a voltage which is processed by circuit  116 . The signal from piezoelectric element  102  may be input to low frequency strain detection circuit  116  for determining piezoelectric strain of low frequency signal components. At the same time the impedance change of piezoelectric layer  102  creates voltage change of electrode  114  with frequency scanning (or modulation) of generator  111  and is fed to peak detector  113  that operates to perform peak detection of signal amplitude variations. The processed signal is applied to downstream detector circuit with AGC  115  (ultrasonic or high frequency signal detection). The signals output from detector circuits  115  and  116  are fed to decision circuit  117  which discriminates between meaningless noise and a finger touch, and provides an output to switch  119  which controls the state of an appliance or equipment device (not shown). 
         [0033]    In one embodiment, in the event decision circuit  117  fails to provide a sufficient output signal to switch  119  by (for example, due to a very small touch force applied to the sensitive area, a low applied force that increases too slowly, a glove or soft cloth over the depressing finger, or contamination of the sensing area, etc.) an indicator  120  is activated to output an alert signal (e.g. a visible illuminator and/or audible warning signal) to the user/system operator. 
         [0034]    The output from peak detector circuit  113  is also applied to capacitive detector circuit  118  which detects the level of capacitance of piezoelectric element  102  for purposes of self diagnosis. The capacitance value is monitored by the high frequency amplitude which is sensed by the DC output value from peak detector  113 . The peak detector output is proportional to the high frequency signal amplitude. The capacitance value itself is detected by circuit  118  and utilized as self diagnosis. When a finger touches the surface  105 , the heat from the finger is transmitted to piezoelectric element  102  which gives variation of capacitance. This change or variation is detected by circuit  118  to be used as self diagnosis, together with ultrasonic signal output from circuit  115 , and piezoelectric strain signal output from circuit  116 . Decision circuit  117  receives and processes these inputs and provides a warning signal through indicator control  120  in the event the inputs are not within threshold or calibrated/expected values. 
         [0035]    The above described functionality may be realized by means of the exemplary circuit shown in  FIGS. 1, 4, and 7 . The frequency modulated (FM) oscillator generates an FM signal and current is fed to piezoelectric element through a series resistor. Because of the effect of the series resistor, the current in piezoelectric element is almost constant while the frequency varies. Impedance  20  of the piezoelectric element is capacitive and decreases with increasing frequency except at the step  21 . The FM signal is converted to an amplitude modulated (AM) signal by the impedance variation of the piezoelectric element. The AM signal has an amplitude variation  22  corresponding to impedance variations  20  of impedance curve  20  (from  FIG. 2 ), of which amplitude variation is detected by the peak detector, and converted to a low frequency signal of which the main component is a 1 kHz signal, but which also includes higher frequency components (in the 5 kHz-25kHz range) as well as some very low frequency components (in the 2 Hz-10 Hz range). The composite signal (CS) is fed to a band-pass amplifier circuit by which low frequency components are removed and only peaks or faster variation signals  24  ( FIG. 2 ) are selected. In the illustrated embodiment, the band pass amplifier includes automatic gain control (AGC) which adjusts the gain steadily to obtain a substantially constant output in the “no-touch” condition so that the threshold setting detects the small quick signal variation for a very slight touch and for which impedance variation  21  is only slightly reduced and reduction of pulse amplitude  24  is smaller. The response time of the gain control may be very slow (e.g. on the order of 5 minutes) so that reduced pulse output by finger touching is not increased by gain control (i.e. should yield a small output). The band-pass amplifier circuit may include additional components such as additional amplification (e.g. 20-40 dB amplifier) and additional bandpass filtering. It is to be understood that the filter-amplifier-filter-AGC sequence is exemplary, and that other such components and sequences may be utilized without departing from the scope of the present invention. Another advantage of the AGC is to decrease variability by making the system sensitivity more consistent or constant, since impedance variation  21  of the piezoelectric element is sensitive to temperature variation, and further due to differences in sensitivity from unit to unit. 
         [0036]      FIG. 2( a )  and  FIG. 2( b )  show simulation results by use of Mason model of the frequency dependent impedance  200  of piezoelectric polymer element  102 .  FIG. 2 a    shows the results for an exemplary 52 um PVDF 9×8 mm 2  material.  FIG. 2 b    shows the results for an exemplary 110 um PVDF 9×8 mm 2  material. Both materials are bonded at back surface  106  of substrate  101  (shown as 0.5 mm stainless steel). The 52 um thick PVDF has its own resonance at 10.4 MHz and the 110 um thick PVDF has resonance at 5.2 MHz. The impedance gradually decreases with increasing frequency but with an abrupt change or spike  201  of impedance of the PVDF material. The abrupt change of impedance becomes less in the case of finger touching. In a simulation curve, the acoustic impedance of the finger is assumed to be 0% (no touching or air), 20% and 40% of human skin impedance 1.5×10 6  kg/m 2 sec. The 20% and 40% values were used due to the grooves of a finger print, which thereby makes partial contact with the target region. It should be noted that the resonance of the substrate and the resonance of the piezoelectric element are distinct resonances. Stainless steel has its own resonance at 5.8 MHz. If the two resonance frequencies (for piezoelectric element and substrate) are close in value to one another, then the effect of touching is small as shown in  FIG. 2 ( b )  and the peak is not significantly influenced by touching. If the two resonance frequencies are not close to one another,  FIG. 2( a ) , the touching effect to the substrate resonance is strongly influenced by the finger, and is detected by piezoelectric element  102 . 
         [0037]      FIGS. 3( a ) and 3( b )  show simulation results of input impedance of ceramic piezoelectric element PZT5A with 300 um and 350 um thicknesses, respectively. The resonance of 300 um PZT is at 6.7 MHz, and that of the 350 um PZT is at 5.4 MHz. The resonance of the substrate 0.5 mm stainless steel is at 5.8 MHz, close to that of 350 um PZT. Abrupt change  301  shown in  FIG. 3( a )  is relatively smaller than the main peak  302  at 7.6 MHz. However, the effect of touching by the finger to the small peak at 6.5 MHz is observed there. On the other hand, for 350 um PZT, the peak at 6.4 MHz is very strong, but the effect of contact by a finger is relatively smaller than 52 um PVDF. Such effect of impedance variation is also used for the same sensor by slightly modified design of the circuit. 
         [0038]    The gradual decrease of impedance of PVDF shown in  FIG. 2( a )  or  2 ( b ) is due to the impedance of piezoelectric element  102  being mostly capacitance C P  (the impedance of a capacitance C P  is 1/ωC P ). Referring now to  FIGS. 1 and 2  in conjunction with  FIG. 4 , the abrupt change of impedance  201  comes from resonance of substrate plate  101  ( FIG. 1 ). When high frequency current is fed from voltage source  400  ( FIG. 4 ) to piezoelectric element  102  (C P ) through coupling capacitor  401  (C C ), the voltage  405  on piezoelectric element  102  is determined only by the capacitance ratio of C C /C P , which does not change with frequency variation even though the impedance  200  varies with frequency, but an irregular change or spike corresponding to the abrupt change  201  remains the same. From such a design, voltage amplitude at  405  encounters sudden change at the substrate resonance  201 . 
         [0039]      FIG. 5  illustrates measured data of the DC output from peak detector  413 , which is mostly constant over the frequency variation of 6-7 MHz except for the irregular change or spike  501 . The irregular change  501  becomes smaller (not shown in  FIG. 5 ) when the front surface  105  is touched by a finger. 
         [0040]    When frequency is scanned or in sinusoidal frequency variation with 100 Hz for example (this represents the FM signal with modulation at 100 Hz), the DC output from the peak detector  413  changes at the spike  501  and forms a pulse with repetition rate of 100 Hz. A positive or negative pulse is formed corresponding to the frequency increase or decrease at every half cycle. These pulses are shown in  FIG. 6( a )  for a no touching condition at the front surface of 0.5 mm stainless steel substrate  105 .  FIG. 6( b )  shows the output when the finger touches on surface  105  and exhibits decreased pulse height. 
         [0041]      FIG. 6( c )  shows two oscilloscope waveforms. The upper trace shows a signal from terminal A when the device is in ultrasonic mode. When finger touched, a strong signal from terminal A is typically detected. However, in the case of the upper trace, depression occurred via pushing of a sharp object (e.g. tip of a driver). No signal results because a small point does not influence resonance of the substrate. The lower trace  604  represents a signal after bandpass filter  716  (pass low range of 10-100 Hz) resulting from the piezoelectric strain effect. Curve  604  illustrates the spikes  604   a  and  604   b , where  604   a  represents a finger depression and  604   b  represents the finger release condition on the substrate, as a function of time. 
         [0042]    The exemplary case of the high frequency is centered at 6.6 MHz and its frequency modulation at 100 Hz and frequency deviation amounts to +−250 kHz, and observed pulse is shown in  FIG. 6 ( a ) . This is the case when nothing is touched at front face  105  of stainless steel. When front face is touched by finger  108 , the pulse decreases as shown in  FIG. 6 ( b ) . 
         [0043]    The above described functionality may be realized by means of the exemplary circuit shown in  FIG. 7 . Referring to  FIG. 7  in conjunction with  FIGS. 1 and 4 , the frequency modulated (FM) oscillator  700  generates an FM signal and current is fed to piezoelectric element  702  (e.g. PVDF film) through a coupling capacitor  701 . The amplitude of FM signal at piezoelectric element  702  exhibits little change over frequency variation, except for the irregular spike illustrated as  21 ,  201  or  501  in  FIGS. 2 and 5 , respectively. This amplitude variation is detected by peak detector  703 , and converted to a pulse signal with a repetition rate the same as the modulation signal. The output of the peak detector is fed to the band pass amplifier  705  with frequency range 5 kHz-25 kHz. The waveform is further reshaped to a filtered (more pure) pulse form by removing base fluctuations or noise components. In the illustrated embodiment, automatic gain control (AGC)  706  is provided after band pass amplifier  705 , which adjusts the gain to obtain a substantially constant pulse height at the “non-touch” condition so that small signal for a very slight touch is detected by exceeding a given predetermined threshold condition. Another advantage of the AGC is to decrease variability by making the system sensitivity more consistent or constant, since impedance variation  201  of piezoelectric element  102  is sensitive to temperature variation, and further due to differences in sensitivity from unit to unit. The response time of the gain control is preferably designed to be very slow (e.g. on the order of minutes) so that reduction of pulse output signal by finger touching is not compensated by the AGC, but should yield an output. It is to be understood that the signal from band pass amplifier circuit  705  in  FIG. 7  is represented as signal A. Further, the FM oscillator block  700  with coupling capacitor  701 , piezoelectric element  702 , peak detector  703  and band pass amplifier circuit  705  corresponds to the generator  111 , piezoelectric element  102 , peak detector  113 , band pass amplifier  115 , and decision circuit  117  shown in  FIG. 1 . 
         [0044]    Referring still to  FIG. 7 , peak detector  739  is configured to detect the peak value of the pulse train from AGC  706 , from which waveforms are shown in  FIG. 6 ( a )  (non touched) and  FIG. 6( b )  (touched). On the condition that the finger does not touch the surface  105 , the output pulse height is relatively large and the large DC value is output from second peak detector  739 . When the finger touches the surface  105 , the output pulse height is low and a small DC value is output from  739 . 
         [0045]    In order to discriminate between finger touching and contamination (e.g. oil, butter, ketchup, water etc.) on surface  105 , time duration detector  740  is used. As the effect of contamination exhibits a signal detection over a much longer time duration than that of a finger push (typically 0.1-0.5 sec), the time duration detector  740  compares the signal as a function of time to determine if the signal represents a finger push or detects contamination on the substrate and yields an output represented as signal A′. 
         [0046]    Circuits  717  and  718  are configured to detect thermal energy (e.g. heat signal) by means of capacitance variation of piezoelectric element  702 . The heat from a finger is relatively quickly transferred to piezoelectric element and the capacitance of piezoelectric element  702  varies due to temperature variation. This quick variation is amplified in circuit  717  (e.g. 40 dB) and passing filter function so that a finger touch may be detected in this manner. This signal is used as self diagnosis function. For example, piezoelectric element  102  may become delaminated from surface  105  but not completely detached or all off (i.e. still contacting  105 ) may yield an output signal even though there is no ultrasonic signal from terminal A. In addition, the problem of lower piezoelectricity of element  102  (e.g. due to excessive heat applied to the piezo material) ultrasonic signal from terminal A decreases (i.e. becomes very small)). However, the heat signal does not decrease by very much. Circuit  718  does not have gain and monitors the capacitance value to detect the signal from peak detector  703 . Circuit  718  has a DC response for checking the steady variation of the signal at  703 . In this way circuit  718  checks for normal functioning of oscillator  700 , coupling capacitor  701 , piezoelectric element  702 , and peak detector  703 . 
         [0047]    By way of non-limiting example, detector circuit  116  shown in  FIG. 1  may be implemented using band pass filters and amplifiers. More specific illustration is provided in  FIG. 7  where there is shown an implementation depicted by circuit  716 . As shown, the composite signal (CS) is fed to low pass circuit  716 . In an exemplary embodiment, circuit  716  has a pass band range of 10-100 Hz so that the high frequency components of the composite signal are removed and only slowly varying signals are selected (e.g. signals in the spectrum range of 10-100 Hz). In the illustrated embodiment, the low pass amplifier circuit  716  includes signal amplification (e.g. 40 dB amplifier) and filtering components. The output from the piezoelectric element  702  due to pressure force from a finger is sensed through this circuit  716  and represented as output signal D.  FIG. 6 ( c )  shows, when the sensing area is pushed by end of sharp tool (such as driver), ultrasonic signal is not observed (upper trace) but piezoelectric strain signal is observed (lower trace). 
         [0048]    Referring still to  FIG. 7  in conjunction with the illustration depicted in  FIG. 1 , the output signals at each of the terminals A and D output from the respective circuits  705  and  716  (corresponding to elements  115  and  116  in  FIG. 1 ) are applied to decision circuit  117  ( FIG. 1 ). Signals A and D may be processed differently according to the requirements of a particular application. By way of non-limiting example only, for the implementation of a general touch pad for office equipment in a quiet room, at least one of signals A or D is fed to the decision circuit  117  which provides a signal to control switch  19 . For applications or equipment which require security such as an ATM or bank related application, both signals of A and D are fed and compared in the decision circuit  117 . In some environments with very strong vibration noise, such as heavy machinery, forklift trucks, or automobiles, signal A is enhanced and signal D is lowered by suitably adjusting the gain of circuits  705  and  716 . For the case where surface contaminants are often contacting the sensing area, such as for use with kitchen appliances or machine tool workshops, signal A is lowered and signal D is enhanced by suitably adjusting the gain of circuits  705  and  716  (e.g.  117   a  of  FIG. 8 ). For some cases, when giving a slight touch to a sensing area, the decision circuit  117  does not trigger an output because the sensed force is too small, and/or too slow in increasing the applied force (pressure), or a soft cloth may cover the finger, or heavy contamination may reside on the sensing face. In such cases, the cause of the non-signal output from decision circuit  117  may be identified from comparison of signals A and D (e.g.  117   b  of  FIG. 8 ), and an alert or warning signal with an identifying reason may be provided via an audible signal and/or illuminating light from indicator controller  120 . 
         [0049]    Decision circuit  117  further includes logic such as one or more AND logic circuits configured to provide output for a condition of existence of both signals A and D, OR logic circuits configured to provide output for a condition of existence of either of signal A or D ( 117   a  of  FIG. 8 ). A programmed microprocessor is configured to determine the use of AND or OR circuits and in what fashion (e.g. both AND/OR mixed or either is enhanced by a certain degree) and make decisions based on the signal inputs ( 117   b  of  FIG. 8 ). By way of non-limiting example, the system may be configured for an application often susceptible to high vibration conditions. Here the system logic may be configured to mitigate false activations (e.g. due to false detections based on signal D mistaking a sensed vibration for a finger touch). This may be mitigated by weighting the signal A output greater than that of signal D, and/or constructing logic that passes signal A output rather than D when discrepancies exist therebetween. 
         [0050]    In another example, in applications where surface contamination is problematic, the circuit may mitigate against false detections output from circuit  705  (signal A) by weighting the signal A output less than that of signal D, and/or constructing logic that passes signal D output rather than A when discrepancies exist therebetween. Other logic configurations may be utilized, based on the particular conditions and environments for use. 
         [0051]    Referring to  FIGS. 8-9  in conjunction with  FIGS. 1 and 7 , there is shown exemplary processing and decision logic for detecting and processing each of the signals A, B, C, D for execution according to embodiments of the present invention. Processing associated with decision circuit  117  of  FIG. 1  is shown schematically in  FIGS. 8-9 . 
         [0052]    Depending on the particular application, failures may result from various different causes as shown in  FIG. 9 . Columns I-VI depict the signal from each of ports or terminals A, A′, B, C, and D in  FIG. 7  and their measured or determined features and conditions. Failure can be identified and used as self diagnosis and a warning signal provided as indicated. An exemplary implementation and processing logic scheme is described below with regard to  FIG. 9 : 
         [0053]    Signal A: Processing is associated with the device in Ultrasonic mode. During finger touch the voltage is reduced (from the no touch condition). 
         [0054]    Signal A′: Processing includes checking the duration of the finger touch condition. If the duration is longer than a predetermined period (e.g. 10 sec), a determination is made that the surface is contaminated 
         [0055]    Signal B: Processing includes a capacitance check of the sensor element which may be performed by determining the voltage level on the sensor element, to detect failure or major temperature change. 
         [0056]    Signal C: Processing includes determining the level of heat signal by determining capacitance change indicative of heat from a finger touch. 
         [0057]    Signal D: Processing includes determining piezoelectric strain signal resulting from finger touch. 
         [0058]    Determined causes of failure of the senor may include the following decisions/determinations: 
         [0059]    Item I: Too much exposure to significant high temperature so that piezoelectricity of film (e.g. PVDF) is lowered by depoling. 
         [0060]    Item II: Too much exposure to significant high temperature so that the length of piezoelectric film (e.g. PVDF) contracts and piezoelectric film is totally or partially separated from the substrate (e.g. stainless steel or metal or glass, or delamination resulting from flawed production process). 
         [0061]    Item III: Lead wire is disconnected by mechanical shock or corrosion by liquid or oxidation or other physical or chemical reason. 
         [0062]    Item IV: Lead wires or electrodes on film are electrically shorted (various reasons). 
         [0063]    Item V: Substrate (e.g. stainless steel or metal layer) is locally deformed at the sensor location. This may be indicative of tampering (e.g. by vandalism or accidental impact) so that strain distribution by touching is influenced (e.g. determined to be outside normal/threshold limits). 
         [0064]    Thus, there has been disclosed a dual mode touch switch, combining ultrasonic touch switch and piezoelectric touch switch modes which detect finger touch at the surface of a substrate and with a piezoelectric element at the back surface. 
         [0065]    When the device is in the ultrasonic mode, frequency deviation of the FM drive signal is wide enough to detect an abrupt change of impedance representing a frequency change so that a large tolerance of thickness of the substrate is ensured. 
         [0066]    The impedance change including the abrupt change may be detected by a circuit that includes a converter for conversion from FM to AM and a peak detector that converts the AM signal to a low frequency signal having sharp pulses that correspond to abrupt impedance changes, and further including a high pass filter that selects only the sharp pulses. 
         [0067]    The amplitudes of the sharp pulses are kept to a substantially constant value by an AGC so as to increase tolerance of sensitivity for different piezoelectric elements. In operation, the amplitudes of the sharp pulses are reduced by finger touch at the surface of the substrate and the detected signal controls switch of the appliance or equipment. 
         [0068]    When the device is operated in the piezoelectric touch switch mode, the signal induced by planar expansion of the piezoelectric element due to pressure from a finger is extracted while operating in ultrasonic mode. 
         [0069]    As shown, the system and method disclosed herein provides that signals from both the ultrasonic mode and the piezoelectric touch mode are used together to determine switch conditions, and/or either one may be enhanced and the other one depressed depending on the application. 
         [0070]    There has also been disclosed a reflective mode ultrasonic switch device which detects finger touch at the surface of a substrate and with a piezoelectric element at the back surface. The tolerance of substrate thickness of this ultrasonic reflective touch switch device is improved by use of a wide range of frequency deviation of drive signal by filtering out the fundamental modulation frequency from the output signal and wherein only sharp pulses corresponding to abrupt impedance changes of the piezoelectric element are extracted and processed to indicate the presence or absence of a finger at the surface. 
         [0071]    While the foregoing invention has been described with reference to the above-described embodiment, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims. Accordingly, the specification and the drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
         [0072]    Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations of variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description.