Abstract:
A capacitive touch switch suitable for use in areas with high environmental and electrical contamination employs a complex excitation signal that is demodulated to better distinguish it from environmental effects. Improved discrimination allows these sensed signals to be offset against a signal developed by a reference electrode to allow the system to operate robustly with contamination on the electrodes that would otherwise trigger or obscure pulses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of US provisional application 60/913,112 filed Apr. 20, 2007, and PCT Application No. PCT/US2008/060812, filed Apr. 18, 2008, the disclosures of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to capacitive touch switches and in particular to a capacitive touch switch having improved immunity against false triggering. 
         [0003]    Touch switches, which sense a change in the electrical environment around the touch point, for example, when a finger is moved over a stationary electrode near the touch point, provide a number of advantages over conventional mechanical switches with movable contacts. Because touch switches have no moving parts, they are particularly well-suited to applications where there is mechanical shock and long life is required. Further, because touch switches do not require a movable operator, they may be easily sealed from environmental contaminants such as dirt or water. 
         [0004]    One type of capacitive touch switch senses a change in capacitive coupling between an antenna electrode and a sense electrode. This change may be, for example, a decrease in coupling between the antenna and sense electrode caused by the diversion of electrical energy into a capacitive coupling to the user&#39;s hand. By sensing capacitive effects only, it is possible to cover the sensing electrodes with an insulating protective layer. 
         [0005]    For capacitive touch switches of reasonable size, the measured capacitive coupling is relatively small and thus sensitive circuitry must be used to detect the “touch”. Such sensitive circuitry is prone to false triggering caused by electromagnetic interference from other electrical devices. In addition, the small changes in capacitive coupling caused by a finger touch, can often be overwhelmed by larger environmental capacitive changes, for example, those caused by environmental contaminants such as dirt, water, or ice, preventing detection of the finger touch. 
         [0006]    While the mechanical advantages of capacitive touch switches recommend them for automotive use where they would resist mechanical shock and environmental contamination, the problems of false triggering and signal saturation have prevented their widespread adoption. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a touch switch providing separate sense and reference electrodes that may be used to separately measure capacitive coupling with respect to a common antenna electrode. The reference electrode is displaced from a touch point of the switch and thus provides a measure of environmental contamination that may be used to adjust a switching threshold of the sense electrode (located near the touch point) making the switch far more resistant to contamination of the touch surface with dirt, water, ice and the like. 
         [0008]    Specifically then, the present invention provides a capacitive touch switch having an electrically insulating touch surface providing a touch area for activation of the touch switch. Behind the touch surface are positioned a sense electrode at the touch area, an antenna electrode proximate to the sense electrode, and a reference electrode proximate to the antenna electrode but removed from the touch area relative to the sense electrode. A detector circuit monitors the signal flowing between the antenna electrode and each of the sense electrode and reference electrode to provide a switch output based on a comparison of the capacitive coupling between the antenna electrode and the sense electrode compared to a threshold that is a function of the capacitive coupling between the antenna electrode and the reference electrode. 
         [0009]    It is thus an object of one embodiment of the invention to compensate for changes in the environment of the switch allowing more robust discrimination under a range of circumstances. 
         [0010]    The antenna electrode may form a ring around the sense electrode and the reference electrode is outside of the ring. In addition the reference electrode may form a ring around the antenna electrode. 
         [0011]    It is thus an object of one embodiment of the invention to provide comparable capacitive coupling between the antenna electrode and each of the sense electrode and reference electrode while displacing the reference electrode from the touch point. 
         [0012]    Alternatively, the reference electrode may be broken in an access direction likely to be proximate to a user&#39;s hand during use of the touch switch. 
         [0013]    It is thus an object of one embodiment of the invention to permit the use of the capacitive touch switch in applications where a user&#39;s fingers must reach around a handle or the like to access the touch point. 
         [0014]    The signal from the reference electrode may be averaged over a time window greater than the time window of an average applied to the signal for the sense electrode. 
         [0015]    It is thus an object of one embodiment of the invention to further tailor the reference electrode to sensing slowly changing environmental conditions. 
         [0016]    The capacitive switch may further include a peak detector system detecting a peak and trough of a signal received at the reference and sense electrodes to provide a difference between their heights. 
         [0017]    Thus it is an object of one embodiment of the invention to effectively increase the sensitivity of the switch by evaluating the difference between highest and lowest received pulse signals rather than simply height of the received signal. 
         [0018]    The detector circuit may include a modulator producing an irregular excitation signal communicated to the antenna electrode, wherein the irregular excitation signal consists of a series of pulses varying in at least one of magnitude and spacing, and a demodulator communicating with the sense electrode to receive the irregular excitation signal as capacitively coupled from the antenna electrode, the demodulator discriminating between the irregular excitation signal and other electrical signals to trigger the switch output. 
         [0019]    Thus it is one object of one embodiment of the invention to provide improved discrimination against regular electronic interference. 
         [0020]    The demodulator may operate synchronously with the modulator. 
         [0021]    Thus it is an object of one embodiment of the invention to provide a simple means of discriminating against phase and frequency shifted interference. 
         [0022]    The irregular excitation signals may consist of a series of pulses varying in both magnitude and spacing. 
         [0023]    Thus it is an object of one embodiment of the invention to provide multiple dimensions of signal irregularity to discriminate the switch signal from environmental electrical signals. 
         [0024]    The demodulators may correlate a received signal with the irregular excitation signal to provide a de-correlation. 
         [0025]    It is thus an object of one embodiment of the invention to provide a multidimensional comparison of the received signal to the modulated signal. 
         [0026]    The switch may include a timer element requiring detection of a demodulated signal by the demodulator for a predetermined time period before producing an output switch signal. 
         [0027]    It is thus an object of one embodiment of the invention to further guard against false activation caused by short-term interference. 
         [0028]    The pulses may have a range of dominant frequencies between substantially 100 Hz and 5 kHz. 
         [0029]    It is thus an object of one embodiment of the invention to employ a frequency range allowing practical circuit design while avoiding interference in this frequency range. 
         [0030]    The touch switch may further include a monitoring circuit monitoring asymmetry of the signal from the second electrode with respect to ground. 
         [0031]    It is thus an object of one embodiment of the invention to provide a method of detecting damage or contamination to the switch. 
         [0032]    The modulators may be implemented at least in part by a single microprocessor. 
         [0033]    It is thus an object of one embodiment of the invention to provide a practical and cost-effective method of complex signal processing with a touch switch. 
         [0034]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  is a simplified, exploded view of a touch switch of the present invention showing a sense electrode encircled by an antenna electrode in turn surrounded by a reference electrode, all communicating with a microprocessor; 
           [0036]      FIG. 2  is an elevational cross-section through a center of the electrode assembly of  FIG. 1  showing a finger touch at a touch point and showing capacitive couplings affecting triggering of the present invention; 
           [0037]      FIG. 3  is a figure similar to that of  FIG. 2  showing capacitive couplings associated with environmental dirt, water, or ice; 
           [0038]      FIG. 4  is a functional block diagram of the operation of the present invention as implemented in the microprocessor including peak detection, modulation and demodulation, averaging, and threshold; 
           [0039]      FIG. 5  is a set of three vertically aligned graphs showing a pulse sequence delivered to the antenna electrode of  FIG. 1 , the signal received by the sense electrode, and a reconstructed signal used by the demodulator of  FIG. 4 ; 
           [0040]      FIG. 6  is a set of two vertically aligned graphs, the first showing a change in signals from the sense electrode and reference electrode in different environmental conditions and in a touch and no-touch state positioned above a graph showing a switch signal; 
           [0041]      FIG. 7  is a fragmentary detail of the peak capture circuitry of  FIG. 4  used for detection of electrode malfunction; 
           [0042]      FIG. 8  is an elevational view of an alternative electrode arrangement for use in a switch position on a blind side of a latch; and 
           [0043]      FIG. 9  is a figure similar to that of  FIGS. 2 and 3  showing the addition of front surface grooves to promote proper finger placement. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0044]    Referring now to  FIG. 1 , a touch switch  10  of the present invention provides electrode assembly  12  consisting of a touch surface  14  providing an insulating protective layer covering a set of metal electrodes including a sense electrode  16 , an antenna electrode  18 , and a reference electrode  20  supported on a substrate  22  that may be rigid or flexible. 
         [0045]    In the preferred embodiment, the sense electrode  16  may be spaced from and surrounded by an annular antenna electrode  18  which in turn may be spaced from and surrounded by an annular reference electrode  20 . 
         [0046]    A microprocessor  24  provides a digital to analog output  26  received by a power amplifier  28  to provide a set of voltage pulses to the antenna electrode  18  according to a stored program as will be described. The power amplifier  28  may, for example, be a simple transistor circuit of the type well known in the art. 
         [0047]    Signals from the sense electrode  16  and reference electrode  20  are received by buffer amplifiers  30  and  32  respectively and provided to analog to digital inputs of the microprocessor  24  which may monitor voltages at these electrodes. The microprocessor  24  may further provide a digital output  34  providing a switch output for use in controlling other electrical devices in a manner of standard mechanical switches. 
         [0048]    Referring now to  FIG. 2 , as will be described in more detail below, the signal applied by power amplifier  28  to the antenna electrode  18  may be capacitively coupled to the sense electrode  16  as shown by coupling capacitances  36   a  resulting generally from the proximity of the conductors of sense electrode  16  and pulse antenna electrode  18 . Similar coupling capacitances  36   b  communicate between the antenna electrode  18  and the reference electrode  20 . As a result of these capacitances  36   a  and  36   b,  a certain amount of electrical power from the antenna electrode  18  is coupled to the sense electrode  16  and the reference electrode  20 . 
         [0049]    As will be described in greater detail below, the touch switch  10  couples a pulsed signal from an antenna electrode  18  as a transmitter to the sense electrode  16  as a receiver. The electrodes  16  and  18 , being in proximity with each other, form a capacitor having a substantially constant capacitance so that a capacitive coupled signal is then introduced from the antenna electrode to the sense electrode. A “switch” signal will be derived from the signal that is at the sense electrode according to subsequent processing circuitry. Generally the switch signal is the composite of four factors; (1) the signal induced by the antenna electrode  18 , (2) the effects of unwanted stray or parasitic capacitance, (3) the effects of the processing circuitry, and (4) the capacitance of an object as it approaches the sense electrode (such as a finger). The effect of the fourth factor (i.e. a user&#39;s finger) causes a coupling to ground through the user&#39;s body at the sense electrode. This “grounding” effect on the sense electrode causes a measurable loss in signal energy and can be used to determine a switch state. 
         [0050]    Referring still to  FIG. 2 , a touch from a finger  40  or the like at a touch area  42  centered on the sense electrode  16  will produce coupling capacitance  36   c  between the finger  40  and either or both of the antenna electrode  18  and the sense electrode  16 , essentially drawing power away from the sense electrode  16  decreasing the signal strength at the sense electrode  16 . This effect will disproportionately decrease the signal at the sense electrode  16  compared to the reference electrode  20 , the latter of which is further removed from the touch area  42  than the sense electrode  16 . As will be described in more detail below, this difference in signals caused by the touch of finger  40  will be detected and used to trigger the touch switch  10 . 
         [0051]    Referring now to  FIG. 3 , the presence of contaminants on the touch surface  14 , for example raindrops  46 , will also cause capacitive coupling  36   c,  but this capacitive coupling will equally affect the signal strength at sense electrode  16  and reference electrode  20  providing a method of distinguishing between a touch at touch area  42  and such environmental contamination. 
         [0052]    Referring now to  FIG. 4 , in processing the signals from the sense electrode  16  and reference electrode  20 , the microprocessor  24  may implement a detection circuit  65  through an internal software program, a modulator  50  providing a pulse sequence  52  to power amplifier  28  and thus to antenna electrode  18 . Referring also to  FIG. 5 , this pulse sequence  52  preferably produces an irregular excitation signal meaning that it consists of a series of pulses  56   a  and  56   b  (unipolar in this example) where pulses  56   a  have a lesser magnitude than pulses  56   b  and thus are irregular in amplitude, and where pulses  56   a  are spaced closer to the succeeding pulse  56   b  than to the preceding pulse  56   b  and thus are irregular in time. Generally the repetition rate of each pulse  56   a  and pulse  56   b  will be approximately 5 kHz. The irregular excitation signal is intended to be readily distinguishable from common environmental electrical noise, for example, 60-cycle line power interference or the ignition signal of an automotive engine. 
         [0053]    Referring again to  FIG. 4 , the pulse sequence  52  will be received at the sense electrode  16  and reference electrode  20  as a series of spikes  58   a  and  58   b  (shown in  FIG. 5 ) representing generally the derivative of pulse  56   a  and spikes  58   c  and  58   d  representing generally the derivative of pulse  56   b.  As such, spike  58   a  is positive going and aligned approximately with the leading edge of pulse  56   a  and spike  58   b  is negative going and aligned approximately with the trailing edge of pulse  56   a.  During normal operation of the touch switch  10 , the magnitude of spikes  58   a  and spikes  58   b  are generally proportional to the height of pulse  56   a.    
         [0054]    Similarly, spike  58   c  corresponds to the leading edge of pulse  56   b  and spike  58   d  corresponds to that falling edge of pulse  56   b.  The magnitude of spikes  58   c  and  58   d  are larger comporting with the greater amplitude of pulse  56   b  compared to pulse  56   a.    
         [0055]    These spikes  58  are received by the buffer amplifiers  30  and  32  and converted by the analog-to-digital converter of the microprocessor  24  to digital signals for digital processing. These digital signals are then received at excursion detectors  70  and  72  respectively, each implemented in software as will be the case with the following described elements within the microprocessor  24 . 
         [0056]    The excursion detectors  70  and  72  consist of a peak grabber  74  and a trough grabber  76  respectively controlled by the modulator  50  to sample and hold a peak magnitude  77  of spikes  58   a  (or spikes  58   c ) and a trough magnitude  78  of spikes  58   b  (or spike  58   d ). The values of the peak magnitude  77  for each spike (e.g.  58   a ) has subtracted from it the trough magnitude  78  of the corresponding spike (e.g.  58   b ), as indicated by summing blocks  80  to essentially double the detected amplitude of received signal from the sense electrode  16  and reference electrode  20 . The output  82  of the summing blocks  80  provides a measure of the relative amplitude of the underlying pulses  56  received at each of the sense electrode  16  and reference electrode  20 . 
         [0057]    The synchronous detection of the peak magnitude  77  and trough magnitude  78  triggered by the modulator  50  provides additional resistance to the effects of electrical interference that occur outside of the time periods of pulses  56   a  and  56   b.    
         [0058]    Referring still to  FIG. 4 , the values of the output  82  for a cycle of pulses  56  (shown in  FIG. 5 ), indicated by a signal from the modulator  50 , are collected by demodulators  84  and  86 , the former associated with the sense electrode  16  and the latter with the reference electrode  20 . These outputs  82  are used to create a demodulated signal  87  essentially mirroring pulse sequence  52  attenuated by the degree of capacitive coupling and possibly corrupted by noise. Note that absent noise or change in attenuation, the demodulated signal  87  provides pulses  83   a  and  83   b  having different amplitudes and different spacing corresponding generally to pulses  56   a  and  56   b  of pulse sequence  52 . Electrical interference, indicated schematically as  90 , may cause a change in the height of spikes  58   a ′ and  58   b ′ causing a corresponding change in the height of pulses  83   a ′ and  83   b ′ corresponding to these spikes  58   a ′ and  58   b ′. More generally, electrical interference  90  may add false pulses  83  or completely eliminate pulses  83 . 
         [0059]    The demodulators  84  and  86  may, for example, demodulate the signal  87  by cross correlation between the signal  87  and the pulse sequence  52  thereby being sensitive both to pulses  83  that are too high or too low or shifted in phase. The demodulators  84  and  86  provide match signals  94  and  96  respectively whose output values indicate the degree of correlation. 
         [0060]    These match signals  94 ,  96  are received by window averagers  101  and  102  respectively which average these demodulated values for different time windows, with the window averager  102  having a longer averaging window. This difference between time windows has the effect of smoothing the changes in the output of modulator  86  to better track relatively slowly changing environmental conditions while allowing rapid response from the demodulator  84  attracting relatively quick detection of finger touches and the like. 
         [0061]    Referring now to  FIG. 6 , the match signal  94 ′ may have two states indicated by touch match signal  94 ′ a  when a finger is at the touch position and no-touch match signal  94 ′ b  when no finger is at the touch position. The strengths of these signals will vary depending on the environmental conditions around the touch switch  10 , varying between dry, wet, and dynamic wet, the latter condition indicating blowing or streaming water. Generally the detection circuit  65  must establish a threshold between the touch match signal  94 ′ a  and no-touch match signal  94 ′ b  that may distinguish touches  110   a,    110   b,  and  110   c  occurring in each of the different states of dry, wet, and dynamic wet. A single threshold  112  will generally be inadequate because of the substantial rise in the no-touch match signal  94 ′ b  under dynamic wet conditions. Accordingly the present invention tracks the signal  96 ′ to produce a multistep threshold  114  being a function of signal  96 ′ and rising as one moves into the dynamic wet state. In this case, for example, the threshold may be switched based on a rise of signal  96 ′ above a predetermined threshold  116 . 
         [0062]    The signal  96 ′ thus provides a running baseline against which to compare the match signal  94 ′ that adapts to changes in the environment of the touch switch  10 . 
         [0063]    As indicated in  FIG. 4 , match signal  94 ′ is compared to a threshold produced by threshold function  97  accepting as an argument signal  96 ′. The result of this comparison provided by summing block  118  provides a signal  120  which may be received by a time threshold block  124  filtering out very short touches  110   a,  for example, to provide additional immunity with respect to electrical noise. 
         [0064]    Referring now to  FIGS. 5 and 7 , environmental interference such as electrical interference  90  or surface contamination may produce asymmetry in spikes  58 ′ a  and  58 ′ b  as shown in  FIG. 5 . This asymmetry may be detected by means of a comparator  140  receiving the output of the peak grabber  74  and trough grabber  76  and adding the two together. When the spikes  58  are symmetric, this output of comparator  140  is zero indicating proper operation. When they are asymmetric this output has magnitude which can be used to trigger an error signal indicating a problem with the touch switch  10 . 
         [0065]    Referring now to  FIG. 8 , the touch surface  14  may, for example, be positioned on the inside of a pocket  130  formed, for example, by a latch handle requiring the user&#39;s fingers  40  to cross over one side of the touch surface  14  in order to reach the touch area of the sense electrode  16 . In this case reference electrode  20  may be broken and removed from the area over which the fingers  40  cross so as to preserve its quality as measuring environmental rather than touch changes. 
         [0066]    Referring now to  FIG. 9 , a front side of the touch surface  14  may have the touch area  42  circumscribed by a tactile feature  150  such as a groove positioned over the reference electrode  20  to cause a user to place their fingers (not shown) toward the sense electrode  16  and away from the reference electrode  20 . This feature  150  is particularly helpful when the touch surface  14  is positioned, for example, as described with respect to  FIG. 8 , in a manner so as to be out of sight. 
         [0067]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.