Abstract:
An electronic proximity switch employs two oscillators coupled through two electrical paths, one associated with an antenna serving as a proximity switch sensor. Absorption by an object near the antenna changes the coupling between the oscillators producing a dramatic phase shift that may be detected and used for switching purposes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to electronic proximity switches, and in particular to an electronic proximity switch which is highly resistant to mechanical and electrical noise and to temperature changes.  
         [0002]     Electronic proximity switches may detect the presence of an object, for example, a human hand near a switch plate, to provide an electrical signal that may be used to switch a circuit or activate a mechanism. Unlike a typical mechanical switch, an electronic proximity switch does not require moving parts, such as a button or switch operator, that may wear or break. Because an electronic proximity switch may activate at a distance, it can be more readily sealed against environmental contamination and protected from damage. Further, activation at a distance allows the switch to be activated rapidly without careful hand placement or the need for one&#39;s fingers to be free to press a button or the like.  
         [0003]     A significant problem with electronic proximity switches is their susceptibility to unintended triggering. This can occur in electronic proximity switches that are used outdoors, such as with automotive door lock applications, where there is a risk that rain or debris carried in the wind could activate the switch. Further, the sensitive electronics of the electronic proximity switch often can be triggered by electromagnetic interference, for example, from electrical appliances, radio transmitters, or lightning discharge. The circuitry of electronic proximity detectors, which must measure small changes in electrical fields, can be sensitive to drift in the value of electronic components of the circuitry caused by aging or changes in temperature. Steps to avoid these problems can increase the complexity and cost of the electronic proximity switch, rendering it impractical for many consumer applications.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is perspective view of a faceplate of an automotive lock that may use the proximity switch of the present invention;  
         [0005]      FIG. 2  is a schematic diagram of the principle elements of the proximity switch of  FIG. 1 ; and  
         [0006]      FIG. 3  is a set of graphs of electrical voltage as a function of time at various points in the schematic diagram of  FIG. 2  for the two cases, the first when the electronic proximity switch senses the presence of a hand or the like, and the second when the electronic proximity switch does not sense the presence of a hand or the like. 
     
    
     SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a simple electronic proximity switch that is highly resistant to false triggering and to changes in component values over time or with temperature.  
         [0008]     Generally, the invention transmits an oscillator signal along two paths and then compares the relative strength of the signal along those paths. One path passes through an antenna that may couple with a sensed object to reduce the strength of the signal along that path. The use of two paths neutralizes the effect of any change in the signal strength from the oscillator that would affect the signal in both paths equally, allowing the circuit to be highly sensitive to changes caused by the sensed object.  
         [0009]     Preferably, the comparison of the signals from the two paths is done with a second oscillator that is electrically identical to the first oscillator, so that component drift and aging affects both oscillators similarly, allowing the oscillators to track closely. The second oscillator may compare the signals of the two signal paths by giving the signals of each path different phase shifts. The second oscillator identifies the dominant signal by its greater amplitude and oscillates sympathetically with this dominant signal—the phase indicating the relative strength of the signals on the two paths. The second oscillator provides highly selective frequency and phase discrimination, rejecting external noise signals.  
         [0010]     Specifically, the present invention provides an electronic proximity switch having an oscillator providing an electrical signal along a first and second transmission path between the oscillator and a signal strength comparator. The first transmission path communicates with a sensor antenna for a free space transmission of the electrical signal in a sensor region and the second transmission path is shielded from the sensor region. A signal strength comparator provides a switch output determined by a relative strength of the electrical signal received over the first and second transmission paths.  
         [0011]     Thus, it is one object of at least one embodiment of the invention to provide a simple and robust circuit that is relatively insensitive to changes in the absolute strength of the oscillator signal.  
         [0012]     At least one of the first and second transmission paths may include a phase shift element and the comparator may be a second oscillator coupling to both the first and second transmission paths to adopt a phase of the combined electrical signals so that the switch may detect the phase of the second oscillator to determine a dominant signal path.  
         [0013]     It is thus another object of at least one embodiment of the invention to provide a highly frequency selective method of comparing signals on the two paths.  
         [0014]     The relative phase shift between the first and second transmission paths is 180 degrees.  
         [0015]     Thus, it is another object of at least one embodiment of the invention to provide a “snap action” in the shifting of the frequency of the second oscillator such as provides clean switching. The second oscillator will remain oscillating at one phase until the relative strength of the signals along the first and second transmission paths changes, and then will abruptly shift phase.  
         [0016]     The first and second oscillators may have identical components and may be tuned to a same frequency.  
         [0017]     Thus, it is another object of at least one embodiment of the invention to provide a simple method of negating the effects of component tolerances and thermal drift and aging through the use of identical oscillator and detector circuits.  
         [0018]     The phase shift element in the second transmission path may be a capacitor providing 180 degrees of phase shift.  
         [0019]     It is thus another object of at least one embodiment of the invention to provide a simple method of coupling the two oscillators to provide a default state.  
         [0020]     The phase shift element in the first transmission path may be an electrical inverter and a capacitor formed by the antenna.  
         [0021]     It is thus another object of at least one embodiment of the invention to provide a secondary coupling between the oscillators that allows signals along the two signal paths to be distinguished by phase shift.  
         [0022]     The signal strength comparator further receives the signal from the first oscillator to determine a phase of the second oscillator  
         [0023]     It is thus another object of at least one embodiment of the invention to provide a phase sensitive comparison that further reduces the effects of external electromagnetic interference.  
         [0024]     The antenna structure may be at least two electrodes electrically insulated from each other, one attached to the first oscillator, and one attached to the second oscillator.  
         [0025]     It is another object of at least one embodiment of the invention to provide a simple antenna structure that can be placed on a variety of surfaces.  
         [0026]     The two electrodes may be attached to an automotive door handle.  
         [0027]     It is another object of at least one embodiment of the invention to provide a switch suitable for activating the courtesy lights or the like of an automobile.  
         [0028]     The electronic proximity switch may include a timing element requiring a given relative strength between the electrical signals of the two transmission paths of a predetermined duration before the switch output is activated.  
         [0029]     It is yet another object of at least one embodiment of the invention to provide a switch that is resistant to short-term interference, for example, rain drops or debris in the air.  
         [0030]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     Referring now to  FIG. 1 , the present invention provides a electronic proximity switch  10  that may, for example, form part of a keyless entry system on an automobile or the like.  
         [0032]     In this embodiment, the proximity switch  10  provides an antenna  12  formed as part of a handle escutcheon on an automobile door  14 . The antenna  12  provides a first conductive plate  18  insulated from, but adjacent to, a second conductive plate  20 , both of which may be visible or covered beneath a plastic face of the escutcheon. A person&#39;s hand  22  approaching the antenna  12  disrupts an electrical field  15  extending between the conductive plate  18  and the second conductive plate  20 . Disruption of the electrical field is sensed to produce an electrical signal that may be used to actuate a door release mechanism or the like.  
         [0033]     Referring now to  FIG. 2 , the conductive plate  18  and second conductive plate  20  are part of a first conductive path extending between oscillator  24  and oscillator  30  and further including inverter  26 , amplifier  28  and summing node  45 . The oscillator  24  operates to produce a megahertz signal, for example, a square wave or sine wave output, preferably at about 1.3 MHz. The output from the oscillator  24  passes through inverter  26  to change the phase of the output by 180 degrees, and then through amplifier  28  providing sufficient power to produce a sufficiently strong electrical field  15  between conductive plate  18  and conductive plate  20 . The signal  42  from conductive plate  20  is received by a first input of a summing node  45  and then by the input of an oscillator  30 .  
         [0034]     A second conductive path between oscillator  24  and oscillator  30  is provided by a coupling capacitor  32  that receives the output of oscillator  24  and provides a signal  40  to a second input of the summing node  45  to be summed with the signal  42  from the plate  20  and provided to the input of oscillator  30 . The coupling capacitor  32  is sized so that the relative phase shift between the first transmission path to the summing node  45  and the second transmission path to the summing node is substantially 180 degrees.  
         [0035]     Referring now also to  FIG. 3 , the two transmission paths between the oscillator  24  and oscillator  30  are initially set up by adjustment of the plates  18  and  20 , or the introduction of an attenuating element, such as a resistor (not shown) so that the signal  42  passing through conductive plate  18  and second conductive plate  20  in the first path is much stronger than the signal  40  passing through capacitor  32  in the second path so that an output  44  of the oscillator  30  is in phase with signal  42 . For this reason, the output signal from the summing node  45 , as it passes to the input of the oscillator  30 , will be 180 degrees out of phase with the signal  40 —the phase indicating the relative strength of the signals on the two paths.  
         [0036]     Electrically, the relative strength of the signals on the two paths is determined by a signal strength comparator  29  formed of oscillator  30 , phase detector  33 , low pass filter  34 , integrator/charge pump  36  and output amplifier  38 , as will now be described.  
         [0037]     Oscillator  30  is preferably electrically identical to oscillator  24 , having the same operating frequency as oscillator  24  and using the same design and component values. In this way, the natural frequency of the oscillators  30  and  24  will tend to be the same with changes in temperature and as components age. The input of the oscillator  30  may be any point in the oscillator circuit where a signal equal in frequency to that of the oscillators  24  and  30  and provided to that input will tend to shift the phase of the oscillator  30  to the phase of the signal by way of sympathetic oscillation. It will be recognized that oscillator  30  acts as a narrow band filter, and thus is largely immune to spurious noise outside of the frequency of oscillator  24  to provide a high degree of noise rejection.  
         [0038]     Signal  40  and the output oscillator  30  are then compared by a phase detector  33 , for example, a multiplier circuit, or for a square wave signal  40 , an exclusive NOR gate whose output tends to a high state when the signal  40  and the output oscillator  30  are in-phase and a low state when signal  40  and the output oscillator  30  are out of phase. In this case, when the person&#39;s hand  22  is not present, the signal  40  and the output oscillator  30  will be out of phase and the signal  46  will be generally low.  
         [0039]     Small phase errors, electrical noise, or interference like rain will cause minor high state excursions in signal  46  whose output is filtered by a low pass filter  34  to produce signal  48  approximating a rolling time average of signal  46  and being generally in a low state. This signal  48  is provided to a charge pump  36  which produces a falling integrated output  50  that may be provided to an output amplifier  38 , for example a Schmidt trigger, to provide a low state output signal  52  indicating that a person&#39;s hand  22  is not close to the conductive plates  18  and  20 . These stages tend to reject noise that is not in phase or  180  out of phase with signal  40  and to reject the momentary phase errors described above.  
         [0040]     When a person&#39;s hand  22  is brought near the conductive plate  18 , the coupling between the conductive plate  18  and the second conductive plate  20  is weakened causing the coupling through capacitor  32  to provide the dominant signal causing the signal  40  and the output  44 ′ of oscillator  30  to be in phase. The relative coupling between the oscillators  24  and  30  along the first and second transmission paths can be used to adjust the distance at which the hand  22  triggers this in phase condition.  
         [0041]     Again, signal  40  and the output oscillator  30  are compared by a phase detector  33  which will produce a generally high state signal  46 ′, which is provided to the low pass filter  34  to produce signal  48 ′ and then to the charge pump  36  to produce a rising integrated output  50 ′ that may be provided to an output amplifier  38  to provide a high state output signal  52 ′ indicating that a person&#39;s hand  22  is close to the conductive plates  18  and  20 . This signal  52 ′ may be used to trigger a switch or a mechanical lock or a courtesy light or the like.  
         [0042]     The present invention is not limited for use in automobiles, but may be used in appliances, such as washing machines and the like. By changing the relative phase between signals  42  and  40  to less than 180 degrees, the output of the summing node  45  will vary continuously with changes in relative signal&#39;s strength. This continuous output can be used to provide distance detection.  
         [0043]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but includes 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.