Abstract:
The present invention offers an object proximity detector and an object position detector. The variation of frequency of an oscillator is used to detect the proximity of an object to the sensor plates. The dependence of frequency on process parameter is minimized by a compensation capacitor. It is not need to calibrate the product during the manufacture. In order to magnify the sensitivity, the sensor plates are placed in the feedback loop of the oscillator, instead of at the input of the oscillator. The independence of the process parameter and increasing of the sensitivity can be achieved by adding the compensation capacitor and place the sensor plates in the feedback loop at the same time. Multiple transmission gates are connected to the input and the output of the oscillator, and the sensor plates are connected to the transmission gates to form an object position detector.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an object proximity detector and an object position detector. In particular, the present invention relates to a position detector with multiple transmission gates connected to the input and output of an oscillator and sensor plates connected to the transmission gates. 
   2. Description of the Related Art 
   Modern proximity detectors have been used to indicate when one object is close to a detector and to measure how far away one object is from the detector. Capacitive sensors and inductive sensors often are used in proximity detectors. Capacitive proximity sensors translate the variation of capacitance to a binary signal, to determine whether that effective capacitance has been exceeded. The variation of capacitance relates to a distance between an object and the sensor plate/plates. There are a variety of well-known ways of measuring capacitance between the sensor plates. One way is to feed an AC signal to an amplifier through sensor plates and to measure the variation of amplitude of AC signal at the output of amplifier. The technology is used in U.S. Pat. No. 5,374,787, U.S. Pat. No. 5,495,077, U.S. Pat. No. 5,841,078, to Robert J. Miller et al., U.S. Pat. No. 5,914,465, U.S. Pat. No. 6,239,389B1, to Timothy P. Allen et al., U.S. Pat. No. 6,028,271, U.S. Pat. No. 6,610,936B2, to David W. Gillespie et al. The system uses this technology consists of a lot of analog circuit, such as amplifier, filter, minimum selector, subtract circuit, sample/hold and A/D converter. The chip size of analog circuit is much bigger than that of digital circuit in an integrated circuit, and is not cost effective. The technology used in U.S. Pat. No. 6,452,514 B1 and U.S. Pat. No. 6,466,036 B1, to Harald Philipp is a charge transfer circuit. In this circuit, an AC voltage source is applied to one plate of the sensor and fed into a signal processor through the other plate of the sensor. The signal processor consists of charge transfer circuit, integrator and voltage measurement circuit. Also a lot of analog circuits used in the above technology. Besides the analog circuits, a lot of high speed analog switches are used. The clock feed-through caused by the parasitic capacitance of analog switches will cause the distortion of the signal. David W. Caldwell et al in U.S. Pat. No. 5,572,205, teaches a touch control system. In this system also apply an AC voltage source to one plate of the sensor and fed to a signal processor through the other plate of the sensor. The signal processor consists of analog circuits, such as peak detector, amplifier and A/D converter. The interference from RF signal will add to the peak detector directly, and caused the error detection of the system. One of the other ways is to connect the sensor plates at the input of an oscillator. The variation of capacitance at the input of oscillator will cause the variation of oscillator frequency. By detection of variation of frequency, the proximity of an object to the sensor plates will be detected. The technology is used in U.S. Pat. No. 6,583,676B2, to Christoph H. Krah et al. The frequency of the oscillator depends on the parameters of process and the power supply voltage. The proximity detectors require frequent calibration to compensate those variations. As described in the patent, the prior art uses two capacitors and a transistor to emulate when the sensor plates of the oscillator is in the proximity or not in the proximity by an object. Because the capacitors and transistor are built in the integrated circuit, the sensitivity of the proximity detector is difficult to be changed and also is difficult to be programmed externally. In order to avoid the frequent calibration of the object proximity sensor, we need to design an oscillator such that the dependence of frequency on process parameters and power supply is reduced to a minimum. The invention is to add addition circuit to the system to compensate the process dependence of oscillator frequency. One of the RC oscillator which is used commonly in the prior art is described in  FIG. 1 . This circuit consists of three inverters,  101 ,  102 ,  103 , a  10 ; resistor  104 , a capacitor  106 , a pair of sensor plates  105  with capacitance C s . The first stage,  101 , is an inverter with Schmitt trigger input. A resistor  104  in the feedback loop of the oscillator is used as the charging/discharging element of the circuit. The frequency of the oscillator is determined by the resistor  104  and the capacitors  105 ,  106 . The waveform of the circuit is shown in  FIG. 2 . Where VTR 2  and VTR 1  are two transfer voltages of the Schmitt trigger input inverter  101 . During the charging cycle of the circuit, when the voltage at the input of the inverter  101  arrives at VTR 2 , the output of the inverter  103  changes state and the circuit starts to a discharge cycle. The voltage at the input of the first inverter  1101  sweeps between VTR 2  and VTR 1 . The period of the oscillator is proportional to R(C s +C) (VTR 2 −VTR 1 )/(V cc −(VTR 2 +VTR 1 )/ 2 )+dt, where dt is the propagation delay of the inverters, and V cc  is the power supply voltage. From the equation, we know, the frequency depends a lot on the transfer voltages VTR 2  and VTR 1 . If the circuit is designed by CMOS process, the voltage gap, VTR 2 −VTR 1 , depends a lot on the threshold voltages of PMOS and NMOS transistors. If the power supply voltage decreases, VTR 2 −VTR 1  will decreases and dt will increase. Because the propagation delay is very small in an integrated circuit, the increasing of dt is not enough to compensate the decreasing of VTR 2 −VTR 1 . 
   OBJECTS OF THE INVENTION 
   It is therefore an object of the invention to provide a position detector with sensitivity independent of the variation of process parameters. 
   It is another object of the invention to provide a position detector with high sensitivity. 
   It is yet another object of the invention to provide a proximity detector with sensitivity independent of the variation of process parameters. 
   It is yet a further object of the invention to provide a proximity detector with high sensitivity. 
   DISCLOSURE OF THE INVENTION 
   A first aspect of the present invention teaches an oscillator circuit of an object proximity detector or an object position detector. In the circuit, an addition capacitor is added. This capacitor is used to add to the voltage swing at the input of Schmitt trigger inverter. This part of voltage gap is proportional to the supply voltage, VCC, but independent of the threshold voltage of the PMOS and the NMOS transistors in the circuit. Two factors cancel each other, and the dependence of oscillator frequency on process parameters and power supply voltage is reduced to a minimum. Besides the independence of process parameters, the sensitivity of the sensor is important also. If dCs is the variation of the capacitance of sensor plates in  FIG. 1 , the variation of period of the oscillator is dT/T=dCs/(C+Cs). If C&gt;&gt;dCs, the sensitivity is low. Another aspect of the present invention teaches a method to increase the sensitivity by connecting the sensor plates to the input and output of the oscillator. The sensitivity will increase by a factor of  2 VCC/(VTR 2 −VTR 1 ). If VCC&gt;&gt;VTR 2 −VTR 1 , the increment of sensitivity is very high. The independence of process parameter and high sensitivity can be maintained at the same in a circuit by combining the advantages of the two circuits. 
   In order to detect the proximity of an object, the object proximity detector consists of an oscillator, a pair of sensor plates, a counter and a microprocessor. During the detection period of the system, a reference count (N 0 ) is always updating. This reference count is defined as the counting number when there is not object in proximity of the sensor. And the reference count is also the maximum count ever measured during the counting process. 
   A predetermined number (N r ) can be input to the microprocessor and used to define the sensitivity. In order to detect the proximity of an object, the counter counts the frequency of oscillator. If the counting number for a definite period is N x , N 0 −N x  can measure the proximity of an object to the sensor. When (N 0 −N x )&gt;N r  is measured, we can determine that an object is in proximity to the sensor. A small N r  means a more sensitive system. The preceding method teaches us how to detect an object in proximity of a sensor. The technology can be expanded and modified to detect an object in proximity of an array of sensors, and to distinguish which sensor in the array is detected. 
   Another preferred embodiment of the present invention teaches an object position detector. In order to design the object position detector, M transmission gates are connected in parallel at the input of the oscillator and N transmission gates in parallel at the output of the oscillator. The output of these transmission gates can be used to form an M×N matrix. A sensor key is formed by a pair of sensor plates, a sensor plate can be connected to one of the M transmission gates and the other sensor plate can be connected to one of the N transmission gates. The control gates of these transmission gates are connected to the outputs of a microprocessor, and are scanned sequentially by the microprocessor. A predetermined number (N r ) can be input to the microprocessor and used to define the sensitivity of each key. The reference count, N 0 , of each key can be updated during the scanning of the key matrix. If (N 0 −N x )&gt;N r  is measured during the scanning of the key matrix, we can determine that an object is in proximity to that key of key matrix. The analog circuits in our invention only consist of an oscillator and two arrays of transmission gates. The circuit of the invention is much simple as compare to the circuit used in the prior art, 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of the invention will be more fully understood with reference to the description of the best embodiment and the drawing wherein: 
       FIG. 1  is a circuit diagram of the oscillator used in proximity detector of the prior art. 
       FIG. 2  is the timing diagram to illustrate the function of circuit in  FIG. 1   
       FIG. 3  is the oscillator circuit of an object proximity detector or an object position detector in according to one embodiment of the present invention. This oscillator will improve the frequency dependence on process variation. 
       FIG. 4  is the timing diagram to illustrate the function of circuit in  FIG. 3   
       FIG. 5  is the oscillator circuit of an object proximity detector or an object position detector in according to one embodiment of the present invention. This oscillator will improve the sensitivity of the system. 
       FIG. 6  is the timing diagram to illustrate the function of circuit in  FIG. 5   
       FIG. 7  is the oscillator circuit of an object proximity detector or an object position detector in according to one embodiment of the present invention, this oscillator will improve the frequency dependence on process variation and the sensitivity of the system. 
       FIG. 8  is the timing diagram to illustrate the function of circuit in  FIG. 7 . 
       FIG. 9  is an object proximity detector in according to one embodiment of the present invention, which includes a pair of sensor plates, a sensor oscillator, a time base oscillator, a counter and a microprocessor. 
       FIG. 10  is an object proximity detector in according to one embodiment of the present invention, which includes a pair of sensor plates, a sensor oscillator, a time base oscillator, a counter, a microprocessor and a power supply regulator. The power supply regulator is used to maintain the stability of the frequency of the oscillators. 
       FIG. 11  is an object position detector in according to one embodiment of the present invention, which includes an array of sensor plates, two arrays of transmission gate, a sensor oscillator, a time base oscillator, a counter and a microprocessor. 
       FIG. 12  is an object position detector in according to one embodiment of the present invention, which includes an array of sensor plates, two arrays of transmission gate, a sensor oscillator, a time base oscillator, a counter, a microprocessor and a power supply regulator. The power supply regulator is used to maintain the stability of the frequency of the oscillators. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The foregoing and other advantages of the invention will be more fully understood with reference to the description of the best embodiment and the drawing as the following description. 
   Proximity sensing technology is useful for applications where an object or a finger is in proximity or touches a sensor plates. And a position sensing technology is useful for application where an object or finger position need to be detected in a sensor array. 
   One embodiment of a proximity sensing circuit or an object position detector of the present invention consists of at least a pair of sensor plates, a sensor oscillator, a time base oscillator, a counter and a microprocessor. In most of application, frequency of the oscillator independent to the variation of process parameters is important. Also high sensitivity is also required.  FIG. 3  is an oscillator circuit of the present invention which can compensate the variation of frequency caused by process parameters. And  FIG. 4  is the timing diagram of voltage at the input of oscillator in  FIG. 3 . As shown in  FIG. 3 , the oscillator consists of three inverters, a first inverter  201 , a second inverter  202  and a third inverter  203  are in cascaded; a capacitor  206  is connected between the input of the inverter  201  and the ground; a pair of sensor plates  205  is also connected between the input of the inverter  201  and the ground; a compensate capacitor  207  is connected between the input of the inverter  201  and the output of the inverter  202 ; a resistor  204  is connected between the input of the inverter  201  and the output of the inverter  203 . The feedback resistor  204  is used to charge and discharge the capacitors  205 ,  206  and  207  at the input of the oscillator. The first inverter  201  is an inverter with Schmitt trigger input. The inverter has two transfer voltages VTR 2  and VTR 1 . During the charging state, the transfer voltage is VTR 2 . During the discharging state, the transfer voltage is VTR 1 . When the voltage at the input of the inverter  201  is increasing to a level of VTR 2 , the output of the inverter  201  will in change state. Waiting for a propagation delay time, the output of inverter  202  will change state. The voltage jump at the output of inverter  202  will propagate through the capacitor  207  to the input of the inverter  201 . The voltage range of the charging and discharging at the input of the oscillator in  FIG. 3  consists of three parts. The first part is VTR 2 −VTR 1 . The second part is that caused by the capacitor  207 , which is  2 V CC (C 2 /(C 1 +C 2 +C s )). The third part is that caused by propagation of the inverters  201 , 202  and  203 . VTR 2 −VTR 1  will decrease if the threshold voltages increase. By the increasing of the internal resistance of the inverter with threshold voltage, the charging or discharging current will decrease if the threshold voltages increase. Thus if the threshold voltage increase, the charging time for VTR 2 −VTR 1  will decrease, the charging time for  2 V CC (C 2 /(C 1 +C 2 +C s )) will increase and propagation delay will increase. By proper choice of the capacitors  205 ,  206  and  207 , the dependence of time period of the oscillator on the process parameters will be reduced to a minimum value. Thus the dependence of frequency of the oscillator on the process parameters will be reduced to a minimum also. And the calibration of the oscillator frequency during manufacture is not necessary. 
   The other circuit of the present invention is shown in  FIG. 5 . And the timing diagram is shown in  FIG. 6 . In this circuit, the sensor  305  is connected between the input of the inverter  301  and the output of the inverter  303 . In this circuit, the transition at the output of the inverter  303  is V CC . The variation of the time period caused by the variation of the sensor capacitance is dT/T=(dCs/(Cs+C 1 ))(( 2 Vcc/(VTR 2 −VTR 1 )). The sensitivity of the circuit in  FIG. 5  is magnified by a factor of  2 V CC /(VTR 2 −VTR 1 ), as compared with the oscillator circuit in  FIG. 1 . 
   In order to improve the dependence of the frequency on the process parameters and the sensitivity of the sensor at the same time, we can combine the advantages of the circuits in  FIG. 3  and in  FIG. 5  together. The circuit with this characteristic is shown in  FIG. 7 . And the timing diagram of the circuit is shown in  FIG. 8 . In this circuit, the sensor capacitor  405  is connected between the output of the inverter  403  and the input of the inverter  401 . The compensating capacitor is connected between the output of the inverter  402  and the input of the inverter  401 . The effect of the capacitor  407  will be cancelled partially by the capacitor  405 . As the charging range at the input of the inverter  401  is concerned, the capacitance of the capacitor  407  must larger than that of the capacitor  405 . By proper choice of capacitance of capacitors  405 ,  406  and  407 , the dependence of frequency on process parameter can be reduced to a minimum. 
   The circuits of the present invention discussed above are oscillators used in object proximity detector or object position detector. An object proximity detector at least consists of a pair of sensor plates, a sensor oscillator, a time-base oscillator, a counter and a microprocessor. The system shown in  FIG. 9  illustrates a proximity detector in according to one embodiment of the present invention. In  FIG. 9  the system consists of a pair of sensor plates  501  connected to a sensor oscillator  502 , a time base oscillator  503 , a counter  504  and a microprocessor  505 . The sensor oscillator  502  with the sensor plates  501  is a circuit which is described in  FIG. 3 ,  FIG. 5  or  FIG. 7 . The time base oscillator  503  provides system clock to the microprocessor  505 . During the detection period of the system, a reference count (N 0 ) is stored in the microprocessor  505  and always updating. This reference count is defined as the counting number when there is not object in proximity of the sensor plates  501 . And the reference count is also the maximum count ever measured in the counting process. A predetermined number (N r ) can be input to the microprocessor  505  and used to define the sensitivity of the object proximity sensor. In order to detect the proximity of an object, the counter  504  counts the frequency of the oscillator. If the counting number for a definite period is N x , N 0 −N x  can measure the proximity of an object to the sensor plates  501 . When (N 0 −N x )&gt;N r  is measured, we can determine that an object is in proximity to the sensor plates  501 . A smaller N r  means a more sensitive system. By input different N r  to the microprocessor  505 , the sensitivity of the object proximity detector can be programmed externally. 
   The frequency of an oscillator will change with the variation of the power supply. In order to improve the stability of an object proximity detector, a system with a power supply regulator  606  is shown in  FIG. 10 . The power supplies of sensor oscillator  602  and time base oscillator  603  are provided by regulator  606 . By this improvement, the system is more stable and higher sensitivity can be obtained. 
   The preceding method teaches us how to detect an object in proximity of a sensor. The technology can be expanded and modified to detect an object in proximity of an array of sensors, and to distinguish which sensor in the array is detected. We call this system as object position detector. 
     FIG. 11  is one embodiment of an object position detector. In this circuit, there are M transmission gates ( 761  to  76 M) connected in parallel at the input  701  of the sensor oscillator  703  and N transmission gates ( 781  to  78 N) connected in parallel at the output  702  of the sensor oscillator  703 . The output of these transmission gates can be used to form an M×N matrix. To form a sensor, we can connect one plate of a sensor plates to one of the M transmission gates ( 731  to  73 M) and connect the other plate of the sensor plates to one of the N transmission gates ( 741  to  74 N). The control gates ( 711  to  71 N and  721  to  72 M) of these transmission gates ( 781  to  78 N and  761  to  76 M) are connected to the outputs ( 711  to  71 N and  721  to  72 M) of a microprocessor  706 , and are scanned sequentially by the microprocessor  706 . A predetermined number (N r ) can be input to the microprocessor  706  and used to define the sensitivity of each key formed by the sensor plates. The reference count, N 0 , of each key can be updated during the scanning of the key matrix. If (N 0 −N x )&gt;N r  is measured during the scanning of the key matrix, we can determine tat an object is in proximity to that key of key matrix. 
   The frequency stability of the oscillators in the object position detector can also be improved by adding a power supply regulator.  FIG. 12  is an object position detector with power supply regulator  807 . The power supply regulator  807  is used to provide the power supply for the sensor oscillator  803  and the time base oscillator  805 . The output voltage of the regulator  807  will not change with the variation of the power supply. The stability of the frequency of the oscillators  803  and  805  is maintained. At this condition, smaller Nr can be input to the microprocessor  806  to get higher sensitivity. 
   Although specific embodiments of the invention have been disclosed, it will be understood by those having skill in the art that minor changes can be made to the form and details of the specific embodiments disclosed herein, without departing from the scope of the invention. The embodiments presented above are for purposes of example only and are not to be taken to limit the scope of the appended claims.