Abstract:
A differential touch sensor apparatus for detecting the presence of an object such as a human appendage, the apparatus having a first electrode, a second electrode positioned proximate to the first electrode, a differential circuit connected to the first and second electrodes, and a pulse or other signal source connected to provide electrical signals that generate an electric field between the first and second electrodes. Introduction of an object near the first electrode affects the electric field between the first and second electrodes, thereby affecting the voltage difference between them. A differential circuit provides an output signal responsive to the difference in voltage between the first and second electrodes. In an alternative embodiment, a strobe electrode is provided proximate to both said first and second electrodes and the pulses or other signals are provided to the strobe electrode to induce an electric field between the strobe electrode and each of the first and second electrodes.

Description:
This application claims the benefit of U.S. Provisional Application 60/032,318, filed Dec. 10, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a touch panel system, and more particularly, to touch sensors attached to one side of a substrate for detecting user contact of the opposite side of the substrate. 
     BACKGROUND OF THE INVENTION 
     Touch panels are used in various applications to replace conventional mechanical switches: e.g., kitchen stove, microwave ovens, and the like. Unlike mechanical switches, touch panels contain no moving parts to break or wear out. Mechanical switches used with a substrate require some type of opening through the substrate for mounting the switch. These openings, as well as openings in the switch itself, allow dirt, water and other contaminants to pass through the substrate or become trapped within the switch. Certain environments contain a large number of contaminants which can pass through substrate openings, causing electrical shorting or damage to the components behind the substrate. However, touch panels can be formed on a continuous substrate sheet without any openings in the substrate. Also, touch panels are easily cleaned due to the lack of openings and cavities which collect dirt and other contaminants. 
     Existing touch panel designs provide touch pad electrodes attached to both sides of the substrate; i.e., on both the“front” surface of the substrate and the “back” surface of the substrate. Typically, a tin antimony oxide (TAO) electrode is attached to the front surface of the substrate and additional electrodes are attached to the back surface. The touch pad is activated when a user contacts the TAO electrode. Such a design exposes the TAO electrode to damage by scratching, cleaning solvents, and abrasive cleaning pads. Furthermore, the TAO electrode adds cost and complexity to the touch panel. 
     Known touch, panels often use a high impedance design which may cause the touch panel to malfunction when contaminants such as 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 can cause the touch panel to malfunction. The static electricity is prevented from quickly dissipating because of the high touch pad impedance. 
     Existing touch panel designs also suffer from problems associated with crosstalk between adjacent touch pads. The crosstalk occurs when the electric field created by one touch pad interferes with the field created by an adjacent touch pad, resulting in an erroneous activation such as activating the wrong touch pad or activating two pads simultaneously. 
     Known touch panel designs provide individual pads which are passive. No active components are located in close proximity to the touch pads. Instead, lead lines connect each passive touch pad to the active detection circuitry. The touch pad lead lines have different lengths depending on the location of the touch pad with respect to the detection circuitry. Also, the lead lines have different shapes depending on the routing path of the line. The differences in lead line length and shape cause the signal level on each line to be attenuated to a different level. For example, a long lead line with many corners may attenuate the detection signal significantly more than a short lead line with few corners. Therefore, the signal received by the detection circuitry varies considerably from one pad to the next. Consequently, the detection circuitry must be designed to compensate for large differences in signal level. 
     Many existing touch panels use a grounding mechanism, such as a grounding ring, in close proximity to each touch pad. These grounding mechanisms represent additional elements which must be positioned and attached near each touch pad, thereby adding complexity to the touch panel. Furthermore, certain grounding mechanisms require a different configuration for each individual touch pad to minimize the difference in signal levels presented to the detection circuitry. Therefore, additional design time is required to design the various grounding mechanisms. 
     The use of conventional touch panels or touch sensors in stoves, microwave ovens, and the like, places such touch sensors in an environment where they can potentially come into frequent contact with conductive liquids or contaminants. The presence of a conductive liquid on any touch sensor could create a false output thereby causing the control circuit to initiate an output action where none was intended. Such liquids, when in the form of a large puddle or drops, can actually span two or more individual touch sensors. This again leads to the potential for false input signals. 
     Recent improvements in touch panel design include techniques which lower the input and output impedance of the touch sensor itself, thereby making the sensors highly immune to contaminants and false activations due to external noise sources. U.S. Pat. No. 5,594,222 describes such a technique. Even though this approach has several advantages over the prior art, there are some attributes that may limit its application. For instance, the resulting sensor may be inherently sensitive to temperature variations. As long as the temperature variations at the output are small relative to legitimate signal changes and are small relative to signal variations due to transistor variations, then a single transistor or other amplifying device will be quite satisfactory. However, in applications where there is little dynamic range to allow for compensation by software and where temperature changes are significant relative to legitimate signal changes, another approach would be useful to eliminate or greatly reduce the effects of temperature. Also, even though the low impedance approach of this technique can differentiate between contaminants with some finite amount of impedance and a human touch with some finite amount of impedance, this technique may not be enough to inherently differentiate extremely low levels of impedance. Such examples of this situation would exist when a sensor (i.e., both the inner and outer electrode) is covered with a large amount of contaminants, greatly reducing the impedance of the inner pad. Another example would be where a conductive material such as a metal pan covers an entire singular sensor. 
     Thus, it would be desirable to provide a touch panel which prevents false signal generation in the presence of highly conductive materials, relatively substantial temperature changes, and other effects common to both the inner and outer electrode and associated circuitry. 
     SUMMARY OF INVENTION 
     The present invention greatly improves, if not completely solves, the above mentioned problems by providing for a comparison between two electrodes that make up a touch sensor. The inventive touch sensor has one or more first electrodes and one or more second electrodes coupled to a circuit means for measuring the difference in electrical potential between the first and second electrodes. The first and second electrodes typically would be placed on the same surface of the substrate, opposite the side of the substrate that would be used as the touch surface. The first electrode is spaced proximate to the second electrode such that a comparison can be made between the voltage on the first electrode and the second electrode when affected by a touch input. The differential measuring circuit will provide for the rejection of common mode signals such as temperature, electrical noise, power supply variations, and other inputs that would tend to affect both electrodes equally. 
     The inventive touch pad can be used in place of existing touch pads or to replace conventional switches. The touch pad is activated when a user contacts or approaches near to the substrate with a human appendage, such as a fingertip. The touch pad can be used, for example, to turn a device on or off, adjust temperature, set a clock or timer, or any other function performed by a conventional switch. In addition to improving and solving problems associated with existing touch pad designs, the present invention also is useful in applications which presently use membrane switches. The touch pad of the present invention is well suited for use in environments where temperature variations are extreme, where substantial amounts of contaminants are present or where metal objects may be placed on or over the touch pad. 
     In the preferred form, a strobe electrode is connected through a first resistor to a first electrode and through a second resistor to a second electrode. An electric field is generated at each electrode in response to a strobe signal being applied to the strobe electrode. An electric potential is developed at each electrode. Two transistors are arranged in a differential measuring circuit which is connected to the first and second electrodes for measuring the difference in voltage between the first and second electrodes. A sense line is attached to the output of the differential measuring circuit, which in the preferred embodiment carries a detection signal to a peak detector circuit. The output of the differential measuring circuit is altered when the substrate is touched by a user. 
     In the preferred form, two matched transistors are configured as a differential pair, each located in close proximity to the touch pad. The transistors work together to amplify the differential input signal, to buffer the touch pad from the effects of strobe and sense traces, and to reduce the output impedance of the touch pad. Also, by using matched transistors, the output of the differential circuit will change little with temperature variations. 
     The inner and outer electrodes are connected to separate inputs to the differential circuit such that when a first electrode is affected more by the induced electric field than the second electrode, the differential circuit will provide a higher output voltage level. Also, in the preferred embodiment the circuit will generate a lower output when the second electrode is affected more than the first electrode by the electric field. When both electrodes generate equal or similar responses, the output of the differential circuit will change little. These conditions will be created, for example, when a fingertip substantially covers the first electrode but not the second electrode. This will generate a higher output signal. Another condition is created when contaminates substantially cover the second (outer) electrode but not the first (inner) electrode. This will generate a lower level output signal. Another condition would be when a metal pan covers both of the first and second electrodes. Given this condition, in the preferred embodiment, the response of the two electrodes will be substantially equal and, therefore, the output of the differential measuring circuit will change little from the previous no-touch condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features, advantages and other uses of the present invention will become more apparent by referring to the following detailed description and drawings in which: 
     FIG. 1 illustrates an inventive touch pad as viewed from the back surface of the substrate; 
     FIG. 2 is a cross-sectional view generally taken along line  2 — 2  in FIG. 1; 
     FIG. 3 is a cross-sectional view, similar to FIG. 2, but showing an alternate mounting of the active components to the substrate; 
     FIG. 4 is an electrical schematic representation of the touch pad shown in FIGS. 1 and 2; 
     FIGS. 5A,  5 B,  5 C and  5 D are waveforms of the sense output under various input stimuli; 
     FIG. 6 illustrates the strobe signal waveform; and 
     FIG. 7 is a view, similar to FIG. 1, but of an alternate embodiment of the inventive touch pad. 
     Similar indicia numbers in FIGS. 1,  2 ,  3 ,  4 , and  7  indicate similar elements. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a single touch pad  13  is shown attached to a dielectric substrate  10 . It should be understood that many, if not most, applications will include multiple touch pads and related circuitry on the substrate. 
     Substrate  10  can be manufactured from any type of dielectric material, such as glass, ceramic, plastic or similar materials. In the preferred embodiment, substrate  10  is manufactured from glass and has a uniform thickness of approximately 3 mm. The thickness of substrate  10  varies with the particular application such that a thicker substrate is used where additional strength is required. If substrate  10  is manufactured from glass, typical substrates can be as thin as approximately 1.1 mm and as thick as approximately 5 mm. If substrate  10  is manufactured from plastic, the substrate can be less than 1 mm thick, similar to the material used in plastic membrane switches. 
     Substrate  10  has a front surface  12  and an opposite back surface  14 . A user activates the touch pad  13  by touching front surface  12  of substrate  10 , providing the necessary stimuli. 
     The touch pad  13  includes a first, conductive or inner electrode pad  16  and a second, conductive or outer electrode  18  which substantially surrounds the first electrode. A space is located between first electrode  16  and second electrode  18 . Preferably, first electrode  16  has dimensions such that the electrode may be covered by a user&#39;s fingertip or other human appendage when the front surface is touched. 
     In the preferred embodiment, first electrode  16  is square and second electrode  18  has a square shape which conforms to the shape of the first electrode  16 . However, it will be understood that various geometric shapes may also be used for the first electrode  16  including, but not limited to, rectangles, trapezoids, circles, ellipses, triangles, hexagons, and octagons. Regardless of the shape of first electrode  16 , second electrode  18  at least partially surrounds the first electrode  16  in a spaced apart relationship. 
     It may be recognized that even though the pad geometry in FIG. 1 is one way to arrange the electrode structure, there are many other shapes and sizes that would work also, depending on the application and size of the appendage. One example could be an arrangement where a hand might be the appendage of interest instead of a finger. In this case, the spacing between the two electrodes could be spaced farther apart and the two electrodes would be much larger. 
     Similarly, it may be recognized that even though the pad geometry in FIGS. 1 and 7 each show specific ways to arrange the electrode structure, there are many other shapes and sizes that would work here also, depending on the application and size of the appendage. One example would be where the two electrodes are spaced farther apart and the two electrodes are larger. 
     Preferably, first electrode  16  is a solid conductor. However, first electrode  16  may also have a plurality of apertures or may have a mesh or grid pattern. 
     In the preferred embodiment, a third electrode, strobe electrode  22  is provided, as Shown in FIG.  1 . The strobe electrode  22  is a thin conductor formed on the substrate  10 . The strobe electrode  22  is spaced across from the second electrode  18 . Preferably, the strobe electrode  22  is spaced from both sides of the second electrode  18  as shown in FIG.  1 . The strobe electrode  22  is also adjacent the first electrode  16 . In this manner, one portion of the strobe electrode  22  is spaced between the second electrode  18  and the first electrode  16  such that the single strobe electrode  22  acts as a strobe line for both the first electrode  16  and the second electrode  18 , as seen in FIG.  1 . 
     As shown in FIG. 1 the strobe electrode or line  22  is connected to a voltage source  60 . 
     Strobe line  22  carries a strobe signal such as, for example, a square wave in the preferred embodiment, (shown in FIG. 6) from a source  60 . In the square preferred embodiment, the wave oscillates between 0 and +5 volts at a frequency between 25 khz and 50 khz. Alternatively, the strobe signal may have a frequency less than 25 khz or greater than 50 khz, depending on the detection circuitry used. Furthermore, the strobe signal may oscillate between 0 and +3 volts, 0 and +12 volts, 0 and +24 volts, −5 volts and +5 volts, or any other voltage range, depending on the voltage readily available from the device being controlled. 
     Preferably, the strobe signal has a rise time of approximately 7 nsec. However, rise times up to 110 nsec or even larger may also be used. Faster rise times, such as 7 nsec, provide lower input impedances and may be preferred. The strobe signal creates an electric field at the touch pad, as described hereinafter. 
     The strobe signal has a sharp rising edge (shown in FIG. 6) which creates a difference in the electrical potential between the strobe line  22  and each of second electrode  18  and first electrode  16 . This difference in potential between electrodes  15 ,  18  and  22  creates an arc-shaped electric field between the electrodes, as shown by the dashed lines in FIG.  2 . The electric field extends past front surface  12  and through substrate  10 . Although not shown in FIG. 2, the electric field between electrodes  16 ,  18  and  22  follows a similar arc-shaped path away from the back surface  14  of the substrate  10 . This path is almost a mirror image of the dashed lines shown in FIG. 2, extending downwardly rather than upwardly. 
     As shown in FIG. 2, the electric fields created are in opposition to one another. For example, the field paths shown in FIG. 2 originate from strobe electrode  22 , at opposite sides of the first electrode  16 , and from strobe electrode  22  to second electrode  18 . 
     Referring again to FIG. 1, a sense or output line  24  is attached to substrate  10  connected to the output of differential circuit  32 , which is described hereinafter. Sense line  24  carries a detection or operate signal from the touch pad  13  to activate suitable detection or control circuitry as described in detail in my U.S. Pat. No. 5,594,222 which issued Jan. 14, 1997, the contents of which is incorporated herein. 
     As shown in FIGS.  1 , 2  and  4 , surface mount components are electrically connected to the touch pad  13 . The surface mount components include resistor  28  connected between the strobe electrode  22  and the second electrode  18 , and resistor  30  connected between the first electrode  16  and the strobe electrode  22 . The resistors  28  and  30  may have a value of 2.2 K ohms, as shown in the preferred embodiment, thereby providing a relatively low discharge input impedance for the touch pad  13 . 
     The differential circuit denoted generally by reference number  32  is also connected to the electrodes  16 , 18  and  22 . The differential circuit  32  includes two transistors Q 1  and Q 2  arranged in a differential pair with the emitters of both transistors Q 1  and Q 2  connected to strobe electrode  22  through resistor  34 . 
     The base of transistor Q 1  is connected at second electrode  18  to resistor  28 , with its collector connected to ground. The base of transistor Q 2  is connected to resistor  30  via first electrode  18 . The collector of transistor Q 2  is connected to the sense line  24  and to ground through resistor  48 . 
     Preferably, each transistor Q 1  and Q 2  is a PNP transistor, such as transistor model number MPS3906. Alternately, a NPN transistor, MOSFET, or any other active, triggerable electrical component may be used in place of a PNP transistor. 
     FIG. 4 also schematically illustrates stray, parasitic and other capacitance coupling between the various electrodes  16 , 18  and  22 . Capacitor  37  represents capacitive coupling between the strobe electrode  22  and the second electrode  18 . Capacitor  33  represents capacitive coupling between the strobe electrode  22  and the first electrode  16 . Capacitor  35  represents first electrode field disturbance (i.e., modeled as capacitive coupling between the first electrode  16  and the ground). Capacitor  36  represents stray sense line capacitance. Capacitor  38  represents second electrode  18  field disturbance. Capacitor  40  represents stray strobe line capacitance. Resistor  29  represents the resistance of strobe electrode  22 . Resistor  30  in the present embodiment serves to bias transistor Q 2  on during the leading edge of the strobe pulses and forms a discharge path for capacitors  33  and  35 . Similarly resistor  28  forms a discharge path for capacitor  37  and  38  and biases transistor Q 1  on during the leading edge of the strobe pulses. 
     The differential circuit  32  operates in such a way that transistors Q 1  and Q 2  act as a differential pair. Common emitter resistor  34  serves to generate negative feedback which will generate the differential action of the sensor circuit. If the base of transistor Q 2  is biased higher than the base of Q 1 , more current will flow through the collector of Q 2  thereby generating an increase of voltage across resistor  48 . If the base of transistor Q 1  is biased higher than the base of transistor Q 2 , then the majority of the emitter current will flow through the collector of transistor Q 1  thereby leaving less current to flow through the collector of transistor Q 2  generating a decrease of voltage across resistor  48 . If the bias applied to the base of transistor Q 1  is increased and the bias applied to the base of transistor Q 2  is also increased to a voltage equal to the bias on the base on the base of transistor Q 1 , then the differential circuit is balanced, and there is no appreciable increase in the collector current of Q 2  and the voltage change across resistor  48  will be small, if any. 
     The differential circuit  32  provides several advantages with respect to the operation of the touch sensor  13 . This operation can be seen in FIGS. 5A-5D, which depict output voltage on sense line  24  in response to various stimuli or lack of stimuli applied to the first and second electrodes  16  and  18 . As shown in FIG. 5A where there is no first or second electrode stimuli, the signal  220  on strobes line  22  will rise from 0 volts to a maximum of approximately 5.0 volts. Although there is a relatively small output voltage  240  on the sense line  24 , essentially due to the slight difference in the biasing of the transistors Q 1  and Q 2 , the output voltage on sense line  24  is at a minimal steady state amount. 
     As shown in FIG. 5B, with a stimulus applied to the first electrode  16  (i.e., a finger tip placed on front surface  12  in the area of first electrode  16 ), and no stimulus to the second electrode  18 , the output voltage on sense line  24  rises to a maximum of over 3.0 V, which is appreciably greater than the steady state amount, and then falls off exponentially. In FIG. 5C, a stimulus applied only to the second electrode  18  results in a voltage on sense line  24  which is less than the steady state voltage. Finally, as shown in FIG. 5D, when stimuli are applied to both first and second electrodes  16  and  18 , the output voltage is close to the steady state voltage. 
     The differential circuit  32  acts to generate output proportional to a difference between the stimuli applied to first and second electrodes  16  and  18 . Thus, the output  24  is substantially more sensitive to a difference in stimuli applied to first and second electrodes  16  and  18  then to the magnitude of the stimuli. If substantial amounts of contaminants or conductive materials are placed over both the first and second electrodes  16  and  18 , there will be various responses from the touch sensor  10  depending on the nature of the contaminates, with higher conductivity contaminants tending to generate lowered responses. Such a substantial amount of contaminate need only be as large as the enclosed area of the second electrode  18 . This arrangement makes the touch sensor  10  highly immune to false triggering due to substantial contamination or conductive material at a localized area while allowing responses to small differences between the first and second electrodes. 
     Further, differential circuit  32  minimizes drift due to temperature changes in the active components since the bias of both transistors Q 1  and Q 2  will change together such that the current through resistor  48  will not change substantially. Finally, changes relating to power supply, input signals, component drift electrical noise, etc., common to both of electrodes  16  and  18  and transistors Q 1  and Q 2  will tend not to affect the output of differential circuit  32 . 
     In addition to differential circuit  32 , other methods may be used to process the differential signal associated with the first and second electrodes  16  and  18 . Current differencing techniques and mirrors typically used in Norton amplifiers, MOS type transistors, and voltage input operational amplifiers are examples of the types of circuits that could be used. 
     With reference to an alternative embodiment shown in FIG. 3, electrodes  16 , 18 , and  22 , and sense line  24  are attached to a flexible carrier  25  manufactured from a polyester material such as Consolidated Graphics No. HS-500, Type 561, Level 2, 0.005 inches thick. Electrodes  16 , 18 , and  22 , and sense line  24  are formed using a conductive silver ink, such as Acheson No. 427 SS, 0.5 mills thick. The active components Q 1  and Q 2  are then attached to the electrodes and lines. A dielectric layer  27  is placed over the electrodes and lines to protect the conducting surfaces. Preferably the dielectric  27  is Acheson No. ML25089, 1.5 mills thick. The flexible carrier  25  is then bonded to substrate  10  using an adhesive  29  such as 3M No. 457. The flexible carrier  25  can be curved and twisted to conform to the shape of substrate  10 . 
     Alternatively, with reference to FIG. 2, electrodes  16 ,  18 , and  22 , and sense line  24  can be attached directly to substrate  10 . The active components are then attached to electrodes  16 , 18  and  22 , and to sense line  24 . 
     In operation, the touch pad  13  is activated when a user applies stimuli by contacting or approaching substrate  10 . The touch pad  13  will sense contact by a fingertip or other appendage which causes a sufficient disruption of the electric field potential between electrodes  16  and  18 . 
     The base current of transistors Q 1  and Q 2  is determined by the equation l b =C(dV/dT) where l b  is the base current, C is the capacitance of the touch pad field, and dV/dT is the change in voltage with respect to time. The change in voltage with respect to time is created by the change in voltage level of the oscillating strobe signal. When a user contacts the touch pad  13  formed by electrodes  16 ,  18  and  22 , the field capacitance of capacitor  33  is reduced while the field capacitance of capacitor  35  is increased. Due to the relative close proximity of electrode  18  on back surface  14  to the user contact on front surface  12  in the preferred embodiment, there will be an increase of field capacitance on capacitor  38  also, though not as great as the field capacitance of capacitor  35 . 
     In the preferred embodiment, transistor Q 2  amplifies and buffers the detection signal in close proximity to the touch pad  13 . This reduces the difference in signal level between touch pads caused by different lead lengths and lead routing paths. By providing a more uniform detection signal level, greater amplification is possible while maintaining the signal level between, for example, 0 and +5 volts. 
     In the embodiment shown in FIG. 7, the strobe electrode  22  is eliminated. The bases of transistors Q 1  and Q 2  are still connected to the second and first electrodes  18  and  16 , respectively. The strobe signal is applied directly to the bases of Q 1  and Q 2  through resistors  50  and  52 . Q 1  is biased on by the resistor  50  and the field capacitance developed by electrode  18 . In a similar manner, Q 2  is biased on by the resistor  52  and the field capacitance developed by the electrode  16 . A field potential difference generated by the transient voltages applied to electrodes  16  and  18  is developed. The potential difference will cause the bias on Q 1  and the bias on Q 2  to differ proportionally with the field potential difference associated with electrodes  18  and  16 . This embodiment provides less isolation between first and second electrodes  16  and  18  as compared to the circuit of FIG.  1 . Even with less isolation, there are many applications where the level of performance provided by this embodiment is adequate. The benefits derived from the insensitivity of the differential circuit arrangement to common mode influences, such as the effects associated with the application environment, etc., mentioned above, is preserved in this alternative embodiment of FIG.  7 . 
     While only two embodiments of the present invention have been shown, it will be obvious to those skilled in the art that numerous modifications may be made without departing from the spirit of the claims appended hereto.