Abstract:
A touch sensor includes a touch pad; and a circuit configured to detect radiated energy from the touch pad. An impedance connected in series with the touch pad in the circuit, and the impedance selected to approximately match the impedance of a human finger in proximity to the touch pad. The sensor does not require floating power supplies and ground references, and does not rely upon a receiver in the circuit to detect radiated energy.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to touch sensitive control interfaces, and more particularly, to a touch sensor system for use in such interfaces.  
         [0002]     Due to their convenience and reliability, touch sensitive control interfaces are increasingly being used in lieu of mechanical switches for various products and devices. Touch sensitive control interfaces are used in a wide variety of exemplary applications such as appliances (e.g., stoves and cooktops), industrial devices such as machine controls, cash registers and check out devices, vending machines, and even toys. The associated device may be finger operated by touching predefined areas of the interface, and the device typically includes a controller coupled to the interface to operate mechanical and electrical elements of the device in response to user commands entered through the touch control interface.  
         [0003]     Certain known touch sensors depend upon a radiated signal and a receiver to detect the radiated signals as the users approach the sensors. Other known touch sensors depend on the user&#39;s body to reduce the coupled strength to the receiver, and detect user touches by sensing the amount of the output power that is redirected to the user. Still other touch sensors depend on the body acting as a coupling mechanism that increases the received power, and by sensing the power of signals received, touches can be detected.  
         [0004]     Other types of touch sensors attempt to detect touches by measuring a change in capacitance at the touch interface. The capacitances involved, however, are tiny, and the methods of measuring capacitance tend to be easily affected by noise or even surface contamination.  
         [0005]     U.S. Pat. No. 5,760,715 describes capacitive touch sensors that complete a circuit to earth ground when a user&#39;s finger is adjacent the sensor. The sensors, however, require a power supply that is decoupled from ground, and such a floating power supply and/or virtual ground reference complicates the installation of the sensors in certain devices. The entire system must float to the touch system&#39;s reference point, and consequently some type of signal level conversion must be provided in such systems. Additionally, such sensors require opto-isolators and the like which add to the expense of the sensors.  
         [0006]     It would be desirable to provide a lower cost touch sensor system that may reliably detect touches with a reduced number of components, and while avoiding the installation difficulties of floating power supplies and/or floating ground references.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     According to an exemplary embodiment, a touch sensor comprises a touch pad; and a circuit configured to detect radiated energy from the touch pad. Optionally, an impedance is connected in series with the touch pad in the circuit, and the impedance is selected to approximately match the impedance of a human finger in proximity to the touch pad. Current is transmitted through the impedance when the touch pad is touched by a user, and substantially no current flows through the impedance when the touch pad is not touched by a user.  
         [0008]     In another embodiment, a touch based control system comprises a touch pad, a circuit configured to detect radiated energy from the touch pad without utilizing a receiver in the circuit, and a controller coupled to the circuit and monitoring a detected output from the circuit.  
         [0009]     In still another embodiment, a touch based control system for a device having operative components connected to a device controller is provided. The system comprises a control interface defining at least one touch sensitive area, and a touch sensitive element associated with the touch sensitive area, and the touch sensitive element comprises a touch pad, and a circuit configured to detect radiated energy from the touch pad without utilizing a receiver in the circuit, and an impedance connected in series with the touch pad in the circuit, the impedance selected to approximately match the impedance of a human finger in proximity to the touch pad. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic block diagram of an exemplary touch sensitive control system for a device.  
         [0011]      FIG. 2  is a top plan view of an exemplary control interface for the control system shown in  FIG. 1 .  
         [0012]      FIG. 3  is a circuit schematic of a touch sensor for use with the system shown in  FIG. 1  and the interface shown in  FIG. 2 .  
         [0013]      FIGS. 4   a - 4   d  are scope outputs of the circuit shown in  FIG. 3  under different operation conditions.  
         [0014]      FIG. 5  is a block diagram of the controls for the system shown in  FIG. 1 .  
         [0015]      FIG. 6  is a block diagram of an alternative embodiment of the controls for the system shown in  FIG. 1 .  
         [0016]      FIG. 7  is another circuit schematic of a touch sensor for use with the system shown in  FIG. 1  and the interface shown in  FIG. 2 .  
         [0017]      FIG. 8  is a simulated output response of the circuit shown in  FIG. 7  under different operating conditions.  
         [0018]      FIG. 9  is a circuit schematic of a sensor array according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  is a schematic block diagram of an exemplary touch sensitive control system  100  for a device  102 , which in various embodiments may be an appliance, an industrial machine or any other device in which a touch sensitive control interface is desirable.  
         [0020]     In an exemplary embodiment, the control system  100  includes a controller  104  which may, for example, include a microcomputer or other processor  105  coupled to a user control interface  106  including one or more touch sensitive elements as described further below. An operator may enter control parameters, instructions, or commands and select desired operating algorithms and features of the device  102  via user interface input  106 . In one embodiment a display or indicator  108  is coupled to the controller  104  to display appropriate messages and/or indicators to the operator of the device  102  to confirm user inputs and operation of the device  102 . A memory  110  is also coupled to the controller  104  and stores instructions, calibration constants, and other information as required to satisfactorily complete a selected user instruction or input. Memory  110  may, for example, be a random access memory (RAM). In alternative embodiments, other forms of memory could be used in conjunction with RAM memory, including but not limited to flash memory (FLASH), programmable read only memory (PROM), and electronically erasable programmable read only memory (EEPROM).  
         [0021]     Power to control system  100  is supplied to controller  104  by a power supply  112  configured to be coupled to a power line L. Analog to digital and digital to analog converters (not shown) are coupled to the controller  104  to implement controller inputs and executable instructions to generate controller outputs to operative components  114 ,  116 ,  118  and  120  of the device  102  according to known methods. While four components  114 ,  116 ,  118 , and  120  are illustrated in  FIG. 1 , it is recognized that greater or fewer components may be employed within the scope of the present invention.  
         [0022]     In response to manipulation of the control interface  106 , the controller  104  monitors various operational factors of the device  102  with one or more sensors or transducers  122 , and the controller  104  executes operator selected functions and features according to known methods.  
         [0023]      FIG. 2  is a top plan view of an exemplary control interface  106  for the control system  100  (shown in  FIG. 1 ). The interface  106  includes a panel  130  which defines an interface area  132  for manipulation by a user to enter control commands and instructions for the device  102  (shown in  FIG. 1 ). In different embodiments, the panel  130  may be mounted proximate the operative components  114 - 120  (e.g., dispensing components) of the device  102  (such as in a vending machine), or the panel  130  may be located in a remote location from the components  114 - 120  (such as for moving components of an industrial machine).  
         [0024]     The panel  130  further includes touch sensitive areas  134  arranged in the interface area  132  for user selection and manipulation to enter commands to operate the device  102 . While six touch sensitive areas  134  (corresponding to two rows and three columns of areas illustrated in  FIG. 2 ) are provided in an illustrative embodiment, in alternative embodiments more or less touch sensitive areas  134  may be included in the interface area  106 .  
         [0025]     Associated with each of the touch sensitive areas  134  are touch sensitive elements  136  (shown in phantom in  FIG. 2 ). The elements  136 , and the controller  104  are configured to detect an actual touch, also referred to herein as a touch detection or touch result, at the associated touch sensitive areas  134 . Unlike known switching elements (e.g., membrane switch assemblies), touches are detected electronically, and actual mechanical or electrical switching of a conductive path, and associated reliability issues thereof, is avoided. Moreover, and as explained below, touches are detected without using floating power supplies and/or ground references, and without using a receiver which some known touch sensors employ. Touch sensing may therefore be provided at a lower cost with easier installation than known touch based systems.  
         [0026]     While one control interface  106  is illustrated having one exemplary matrix or array of touch sensitive areas  134 , it is understood that the control system  100  may have more than one control interface  106 , and each control interface  106  may have one or more interface areas  132 . Further, each interface area  132  may include more or less touch sensitive areas  134  corresponding to more or less touch sensitive elements  136  as shown in  FIG. 2 .  
         [0027]      FIG. 3  is a circuit schematic of a touch sensor  150  which may serve as the touch sensitive element  136  in the system  100  shown in  FIG. 1  and the  106  interface shown in  FIG. 2 . Unlike some known touch sensors, the sensor  150  operates without a receiver and without floating power supplies and/or ground references. Rather, as explained below, the sensor  150  indicates a change in radiated power to reliably detect user touches in a cost effective manner, without requiring a receiver in the circuit.  
         [0028]     As shown in  FIG. 3 , the sensor  150  includes a pulse generator  152 , an op amp  154 , a touch pad  156  that in one embodiment is a capacitive element touch sensor element known in the art, and series resistance R 3  connected to a first input of the op amp  154 . A resistor R 2  is connected across the inputs of the op amp  154 , and a biasing resistor R 1  is connected in series with the op amp output. When a user touches the touchpad  156 , a circuit is completed through the touch pad  156  and current flows through R 3  and R 2 . The voltage across the resistor R 2  is amplified and output across the biasing resistor R 1 . By sensing the voltage across the op amp output, touches to the touch pad  156  may be detected in the manner explained below. The sensor  150  may be used as a stand alone circuit, or may be electrically connected to other sensors  150  in an array for a control interface having multiple sensors  150   
         [0029]      FIGS. 4   a - 4   d  are scope outputs of the circuit shown in  FIG. 3  under different operation conditions.  
         [0030]      FIG. 4   a  represents the input signal generated by the pulse generator  152 , and in one embodiment is a square waveform or step input of a predetermined magnitude. The pulse generator  152  produces the input waveform on a periodic basis as shown in  FIG. 4   a.    
         [0031]      FIG. 4   b  represents the output signal of the op-amp, in response to the input pulse signal of the generator  152 , when the touch pad  156  is touched by a user. As seen in  FIGS. 4   a  and  4   b , the output signal slightly lags the input signal, but has a similar periodicity and magnitude such that the output signal may be readily detected by sensing the output signal of the op amp  154 .  
         [0032]      FIG. 4   c  represents the output signal of the op-amp with the series resistance R 3  shorted in the circuit of  FIG. 3  and when the touch pad  156  is touched by a user. In recognition that the sensor  150  shown in  FIG. 3  could be provided without the series resistance R 3 ,  FIG. 4   c  demonstrates that the sensor  150  is nonetheless operable without the series resistance. As seen in  FIGS. 4   a  and  4   c , the output signal slightly lags the input signal, has a similar periodicity to the input signal, but has a magnitude much less than the input signal. While the magnitude of the output signal in  FIG. 4   c  is much less than the magnitude of the output signal shown in  FIG. 4   b  with the series resistance R 3 , the output signal of  FIG. 4   c  may still be readily detected by sensing the output signal of the op amp  154 .  
         [0033]      FIG. 4   d  represents the output of the op-amp  154  when the touch pad  156  is not touched by a user to complete the circuit. It is seen in  FIG. 4   d  that the output of the op amp  154  without the touch pad  156  being touched is different in form than either of the output signals in  FIGS. 4   b  and  4   c  when the touch pad  156  is touched. Thus, by establishing a baseline output signal of, for example,  FIG. 4   d , the output waveforms of  FIGS. 4   b  and  4   c  may be compared to the baseline signal output to determine whether the touch pad  156  has been touched or activated by a user.  
         [0034]     It is believed that the series resistance R 3  acts as a broadband series impedance approximately matching the effective antenna of the human touch. The reduced output levels of  FIG. 4   c  show that this is indeed the case. If the additional series resistance R 3  was just slowing the decay of the voltage across a capacitance, then the voltage divider formed by R 3  and R 2  would have reduced the peak output amplitude instead of the exhibited increase in the peak output amplitude. While R 3  is believed to be desirable due to the increased output amplitude of the op amp  154 , R 3  is optional and may not be included in some embodiments of the invention.  
         [0035]     Referring back to  FIG. 3 , the pulse generator  152  formed by the voltage source and R 4 , representing an approximate internal resistance of the generator  152 , generates a wideband input signal to the touch pad  156 . If the input signal is not radiated from the pad  156 , little or no current will flow through R 2  and the differential voltage input to the op amp  154  will be approximately zero. As a result, the output of the op amp  154  will also be approximately zero for an input pulse.  
         [0036]     However, when an antenna (i.e., the human finger) is connected to the opposite terminal of R 2 , a wideband transmission occurs across the touch pad  156  which forces the equivalent current to flow through R 2 . This current through R 2  then produces a differential voltage at the input of the op amp  154 , and an amplified form of the radiated current will be seen on the output of the op amp  154 . The addition of R 3  allows the effective antenna to be better matched to the source and therefore allows for a higher radiated power level with the resulting higher current. The selection of R 2  and R 3  may be dependent on pad size, isolation dielectric, and op amp characteristics (such as gain-bandwidth product and slew rate). In an exemplary embodiment, R 2  is approximately 250 kΩ, and R 3  is approximately 1000 kΩ, although it is recognized that greater or lesser resistance values could be employed for R 2  and R 3  in other embodiments.  
         [0037]      FIG. 5  is a block diagram of the controls for the system shown in  FIG. 1  including a touch controller  200  in communication with each of the sensors  150  (designated S I  though S 6  in  FIG. 5 ), and operationally connected to the device controller  104 . Like the device controller  104 , the touch controller  200  includes a microcomputer  202  or other processor coupled to the user control interface  106 , and a memory  204  that stores instructions, calibration constants, control algorithms, and other information as required to satisfactorily interface with the device controller  104 . Memory  204  may, for example, be a random access memory (RAM). In alternative embodiments, other forms of memory could be used in conjunction with RAM memory, including but not limited to flash memory (FLASH), programmable read only memory (PROM), and electronically erasable programmable read only memory (EEPROM).  
         [0038]     The touch controller  200  measures the output of the op amp  154  ( FIG. 3 ) of each sensor  150  at the appropriate time, and in one embodiment, the controller  200  sequentially pulses the sensors  150  to provide a full array of touch pads for user touch activation. If, in response to the input pulses, the measured power level output from the respective op amp exceeds a predetermined threshold, a pad touch detection is indicated, and the touch controller  200  signals the device controller  104  accordingly. In further embodiments, the touch controller  200  can perform noise elimination, signal type conversion, and other functions as desired.  
         [0039]     While the touch controller  200  is separately illustrated in  FIG. 5  from the device controller  104 , it is contemplated that the functionality of the touch controller could be integrated into the device controller  104  in other embodiments, and a dedicated touch controller is therefore considered optional to the present invention.  
         [0040]     In a simpler form, as illustrated in  FIG. 6 , a threshold comparator  210  could be placed on the outputs of the sensors  150 , with an output pulse generated by a pulse generator  212  to the device controller  104  for a touch (i.e., signal output exceeding the threshold) and no pulse generated when the signal output is less than the threshold. Thus, in such an embodiment, the signal comparison is made without the aid of a controller.  
         [0041]      FIG. 7  is another circuit schematic of a touch sensor  250  for use with the system  100  ( FIG. 1 ) and the interface  106  ( FIG. 2 ). Like the sensor  150  described above, the sensor  250  operates without a receiver and without floating power supplies and/or ground references. Rather, as explained below, the sensor  250  indicates a change in radiated power to reliably detect user touches in a cost effective manner.  
         [0042]     As shown in  FIG. 7 , the sensor  250  includes an op amp  252 , a touch pad  254 , a resistor R 1  connected across the inputs of the op amp  252 , a load resistance R 2  for output of the op amp  252 , and a biasing resistor R 4  connected to one of the inputs of the op amp  252 . When a user touches the touchpad  254 , a circuit is completed through the touch pad  254  and current flows through R 1  and R 4 . The voltage across the resistor R 1  is amplified and output across the resistor R 2 . By sensing the voltage across the op amp output, touches to the touch pad  254  may be detected in the manner explained below. The sensor  250  may be used as a stand alone circuit, or be electrically connected to other sensors  250  in an array for a control interface having multiple sensors  250 .  
         [0043]     Operationally, the sensor  250  functions much like the sensor  150  previously described, and can be controlled with a dedicated controller similar to the embodiment of  FIG. 5  or without the aid of a controller similar to the embodiment of  FIG. 6 .  
         [0044]      FIG. 8  is a simulated output response of the circuit shown in  FIG. 7 , illustrating a baseline input signal  3  provided by, for example, a pulse generator, an output signal  2  with no touch to the touch pad  254 , and an output signal  1  when the touch pad  254  is touched. As  FIG. 8  demonstrates, the touch signal  1  is easily detected using the sensor  250 .  
         [0045]      FIG. 9  is a circuit schematic of a sensor array  300  according to the present invention including multiple sensors interconnected with one another, and each of the sensors associated with a touch pad of the interface. The sensors are individually constructed and operated according to the examples described above. The exemplary embodiment shown in  FIG. 9  is illustrated without a series impedance for the sensors, although it is recognized that a series impedance for the sensors could be provided in an alternative embodiment.  
         [0046]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.