Abstract:
A touch sensor is provided which detects manual contact by a user to actuate a controlled device. The touch sensor includes a substrate, an RF signal transmitter including a plurality of first conductors, and an RF signal receiver including a plurality of second conductors. The first and second conductors are located in spaced, interdigitated alignment with one another on the substrate, to form a key having a geometry which affords substantial coverage thereof by a human appendage. The RF signal receiver includes an RF signal amplifier electrically coupled to the second conductors. A processor is electrically coupled to the RF signal receiver to detect variations in signal strength corresponding to coverage of the key by a human appendage to selectively actuate the controlled device.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to touch sensors, and more particularly to rigid flat panel touch-sensitive sensors. 
     2. Background Information 
     Touch sensors or touch panels are used in various applications to replace conventional mechanical switches; e.g., kitchen stoves, microwave ovens, and the like. Unlike mechanical switches, these sensors contain no moving parts to break or wear out. Also, touch sensors are easily cleaned due to the lack of openings and cavities which tend to collect dirt and other contaminants. 
     Conventional capacitive touch sensing systems employ a passive form of detection. In such a sensor, a capacitive circuit is driven by a source signal. A key touch, representing a change to the circuit capacitance, results in attenuating the potential, and the resulting voltage level change indicates a key touch. This capacitive circuit is often implemented by the deposition of opposing conductive key pads to opposite sides of a dielectric element. 
     One problem with conventional sensors is that accumulation of foreign deposits on the key pads has the drawback of negating the effect of a touch by the user. Chemicals and abrasion of the exposed key pads may also degrade performance. Also, different dielectric characteristics, such as thickness and dielectric constant, often result in having to change or redesign the size of the key pads to achieve the same capacitance for a different dielectric element in identical applications. 
     Moreover, some known touch panels use a high impedance design which may cause the touch panel to malfunction when water or other liquids are present on the substrate. This presents a problem in areas where liquids are commonly found, such as a kitchen. Since the pads have a higher impedance than the water, the water acts as a conductor for the electric fields created by the touch pads. Thus, the electric fields follow the path of least resistance; i.e., the water. Also, due to the high impedance design, static electricity may cause the touch panel to malfunction. The static electricity is prevented from quickly dissipating because of the high touch pad impedance. 
     Also, many existing touch panels use additional elements, such as transistors, disposed integrally or proximate to each key pad, adding complexity to the touch sensor. 
     Thus, a need exists for an improved touch-sensitive sensor that addresses one or more of the drawbacks of the prior art. 
     SUMMARY 
     According to an embodiment of this invention, a touch sensor detects manual contact by a user to actuate a controlled device. The touch sensor includes a substrate, an RF signal transmitter including a plurality of first conductors, and an RF signal receiver including a plurality of second conductors. The first and second conductors are located in spaced, interdigitated alignment with one another on the substrate, to form a key having a geometry which affords substantial coverage thereof by a human appendage. The RF signal receiver includes an RF signal amplifier electrically coupled to the second conductors. A processor is electrically coupled to the RF signal receiver to detect variations in signal strength corresponding to coverage of the key by a human appendage to selectively actuate the controlled device. 
     In another embodiment of the inventions, a touch sensor is provided to detect user contact to actuate a controlled device. The touch sensor includes a substrate, an RF signal transmitter including a RF signal generator and plurality of first conductors, and an RF signal receiver including a plurality of second conductors. The first and second conductors are disposed in spaced, interdigitated alignment with one another on the substrate, to form a key having a geometry which affords substantial coverage thereof by a human appendage. The RF signal receiver includes an RF signal amplifier electrically coupled to the second conductors. A processor is electrically coupled to the RF signal receiver to detect variations in signal strength corresponding to selective coverage of the key by a human appendage to selectively actuate the controlled device. The RF signal receiver receives a baseline signal when the first and second conductors are free from the coverage, and the RF signal receiver receives an activation signal which is discrete from the baseline signal upon the coverage. An absolute value of the activation signal is less than an absolute value of the baseline signal. 
     In a still further embodiment of the present invention, a method is provided for detecting manual contact by a user to actuate a controlled device. The method includes the steps of providing a substrate, providing an RF signal transmitter including a two or more first conductors, and providing an RF signal receiver including a two or more second conductors coupled to an RF signal amplifier. First and second conductors are located in spaced, interdigitated alignment with one another on the substrate, to form a key having a geometry which affords substantial coverage thereof by a human appendage. A processor is electrically coupled to the RF signal receiver. The processor is used to detect variations in signal strength corresponding to selective coverage of the key by a human appendage to selectively actuate the controlled device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of this invention will be more readily apparent from a reading of the following detailed description of various aspects of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic circuit diagram of a sensor of the present invention; and 
     FIG. 2 is an elevational schematic view of key pad portions of the sensor of FIG. 1, with one key pad being actuated by a user. 
    
    
     DETAILED DESCRIPTION 
     Referring to the figures set forth in the accompanying Drawings, the illustrative embodiments of the present invention will be described in detail hereinbelow. For clarity of exposition, like features shown in the accompanying drawings shall be indicated with like reference numerals and similar features as shown in alternate embodiments in the drawings shall be indicated with similar reference numerals. 
     Referring to FIGS. 1 and 2, the apparatus constructed according to the principles of the present invention is shown. Turning to FIG. 1, the present invention is a capacitive keyboard interface system  10  that advantageously does not require any surface preparation on either side of the keyboard dielectric  30 . The invention thus eliminates any requirement for coatings commonly used on conventional glass keyboards. The invention also advantageously allows other substrates to be used as a keyboard interface and may further operate as a non-contact keyboard. The present invention uses a loosely coupled RF circuit to sense the presence of an external body (such as a finger)  25  (FIG.  2 ). The coupler  16  includes inter-digitated traces (conductors)  24  and  26  on a substrate  28  that is preferably superposed with a non-metallic dielectric  30 . Once a finger  25  is placed proximate the RF Coupler  16 , the coupled signal is reduced since a portion of it is diverted (i.e., ‘grounded’) by the external body  25 . Rather than using conventional ‘field effect’ approaches that use a digital pulse or strobe and then measure the capacitance between electrodes, the present invention advantageously functions as a radio frequency (RF) transmitter/receiver combination. This transmitter effectively broadcasts continuously, and uses a decrease in received signal strength to determine a key press. Advantageously, by requiring a relatively large body to reduce a baseline signal, the present invention substantially eliminates false keypress signals such as typically generated by water on the key pads of high dielectric prior art sensors discussed hereinabove. 
     Specific aspects of an exemplary implementation of the present invention will be discussed in greater detail hereinbelow. 
     Referring to FIG. 2, coupler (key)  16  includes interdigitated conductors  24  and  26  disposed on any suitable substrate  28 . Examples of substrates include a conventional circuit board, polyester sheet, glass, or plastic. Conductors  24  and  26  may conveniently be formed using conventional printed circuit board technology. The non-metallic dielectric  30  is superposed with the substrate  28  between the substrate and a user  25 . Dielectric  30  may be any thin, nonconductive material, such as glass, plastic, fiberglass, etc. In addition, display indicia, such as numerals, may be incorporated (e.g., printed) onto the dielectric  30 . Alternatively, desired indicia may be provided by use of a conventional display  32 , such as a Liquid Crystal Display (LCD), or Light Emitting Diode (LED) display. The display  32  may be located in any convenient location, such as superposed behind the couplers  16  (i.e., on the opposite side of couplers  16  relative to the dielectric  30 ). 
     Although the coupler  16  is shown and described having a discrete substrate  28  and dielectric  30 , the skilled artisan will recognize that a single component  28  may serve as both the substrate onto which the conductors  24  and  26  are disposed, and as the dielectric which may be contacted by a user  25 , without departing from the spirit and scope of the present invention. 
     During operation, in the absence of actuation by a user  25 , RF energy  34  passes between conductors  24  and  26 . When the user  25  touches the front surface of dielectric  30  in superposition with a particular key  16 , a portion of the RF energy, as shown at  34 ′, is drawn to (i.e., effectively ‘grounded’ by) the user  25 . This reduction in signal strength serves as in indication of a keypress. 
     Referring back to FIG. 1, an embodiment of the present invention will be more fully described. As shown, a microprocessor  11  uses an analog multiplexer  12  (e.g., an industry standard 4051 or 74C4051 Mux available from Texas Instruments, Inc. (Dallas, Tex.)) to cycle a signal from an RF signal generator  14  through coupler (‘key’)  16  and into an amplifier  18 . Signal generator  14  may be any conventional oscillator source that generates a relatively high frequency, e.g., 100 KHz to 125 KHz or more. As shown, the signal generator  14  is supplied as an input to a “one of eight” analog Mux  12  to supply the signal to eight keys  16 . The selected output of the Mux  12  is fed to the conductor  24  of the coupler (key)  16 . This conductor  24  effectively serves as a radio antenna or transmitter portion to transmit the Radio Frequency signal, as shown at  34  in FIG. 2, to conductor  26 . Conductor  26  similarly functions as a receiving antenna or receiver portion of the present invention. The amount of signal  34  that is lost through this capacitive transmission/reception is dependent on the geometry of the sensor  16  and its surrounding dielectric. Thus, if a change in dielectric is present, such as provided by the finger of a user  25 , then some energy will be lost through coupling to the finger  25 , such as shown as  34 ′ (FIG.  2 ), and will not be received by conductor  26 , thus dampening or attenuating the signal received by conductor  26 . 
     The mechanism that results in such dampening is well known to those skilled in the art. Briefly described, when an object, such as a finger  25 , comes in contact with the glass (dielectric)  30 , some of the energy is drawn away, such as shown at  34 ′. Any sufficiently large object, such as a human body  25 , has the capability to create its own “ground”. This is why one receives a static shock even though not touching something that is grounded (even if wearing insulative shoes). When the user touches the front surface of the glass, some of the current that is passing between the contacts  24  and  26  is effectively “grounded” through the dielectric to the user. This causes the output current to drop and is detected by the microprocessor. The reactance of the capacitive sensor key  16  also changes, contributing to the reduced signal. 
     The skilled artisan will recognize that amplifier  18  may be any conventional high impedance, high gain amplifier, such as an FET, BiPolar, Op-Amp (Operational Amplifier). Amplifier  18  thus functions similarly to a conventional high gain RF front-end amplifier used in a radio receiver. In the embodiment shown, amplifier  18  includes an NPN transistor  36  and a PNP transistor  38 , that advantageously provide relatively high gain at relatively low cost. 
     As also shown, the transistors are configured as a conventional common emitter amplifier. Resistor  40  on the base of transistor  36  helps keep the transistor&#39;s base from floating due to static charge. Resistor  42  on the collector of transistor  36  limits the current through the transistor  36 . The capacitor  44  between the transistors  36  and  38  nominally eliminates the DC bias on the amplified signal. The two resistors  46  and  48  on the base of the second transistor  38  address the DC biasing of this transistor. The resistor  48  on the emitter of transistor  38  limits the gain and the resistor  50  on the collector limits the current. 
     The amplified signal is fed into a peak detector  20 , which as shown, includes a diode  52 , and a resistor  54  and capacitor  56  pair. The diode  52  is used to convert the AC RF signal into a DC signal. This DC signal is filtered by resistor  54  and capacitor  56 . Any suitable voltage source (not shown) may be used for the amplifier, provided the voltage used does not exceed the specified voltage of the individual components. For example, a source supplying 12 to 15 Volts DC may be used. 
     The resulting DC signal is then fed into an A/D (analog to digital) converter  22  of the microprocessor  11 . The A/D converter converts the incoming signal to a digital value. The microprocessor  11  then uses the incoming steady state digital signal from A/D  22  to set and store a (individual) base line value for each key  16 . This base line value is then compared with subsequent scans. In a preferred embodiment, if the signal generated by a particular key changes slowly, the stored value will track this new value. This advantageously allows for small changes in ambient environment, such as temperature changes and/or accumulation of dirt on the surface of the dielectric  30 , to be compensated. This approach also advantageously permits a different baseline to be stored for each key, to compensate for differences in individual keys, such as geometry and circuit path length. A relatively large negative change in signal level is interpreted by microprocessor  11  as a keypress. Subsequently, if a relatively large positive change is detected, then this is interpreted as a keypress de-activation. As mentioned hereinabove, eight keys  16  are provided in the embodiment shown, using an 8 to 1 Mux  12  (with only three keys  16  shown). However, the skilled artisan will recognize that any desired number of keys  16  may be used, without departing from the spirit and scope of the present invention. 
     In the preceding specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.