Patent Publication Number: US-2011068810-A1

Title: Sensing method and driving circuit of capacitive touch screen

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is based on a U.S. provisional patent application No. 61/166,700 filed Apr. 3, 2009. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a capacitive touch screen, and more particularly to a sensing method of a capacitive touch screen, and also to a driving method of a capacitive touch screen. 
     BACKGROUND OF THE INVENTION 
     Touch screens have been widely applied to a variety of portable electronic devices due to the features of easy manipulation and matured development. Among the commercially available touch screens, resistive touch sensors and capacitive touch sensors are currently the most popular to be used in touch screens for manipulation detection. Capacitive touch sensors are particularly popular and commercially talent in the art for being capable of supporting multi-touch techniques. 
     A capacitive touch sensor principally detects a change in capacitance resulting from electrostatic interaction between an electrode and a part of a human body approaching or touching the electrode, e.g. a finger. For implementing such detection means, a variety of capacitive touch sensor solutions are developed to acquire precise capacitive changes. 
     Please refer to  FIG. 1  which schematically illustrates a capacitive touch sensing circuit according to prior art. As shown, the sensing circuit includes a capacitive switch set  10 , a sigma-delta modulator  11 , a modulator bitstream filter  13 , a clock generator  14  and a firmware  15 . The clock generator  14  generates a clock signal which is referred to for on/off control of switches Sw 1  and Sw 2  included in the capacitive switch set  10 . The capacitive switch set  10  further includes a sensing capacitor Cs. When the switch Sw 1  is in an open-circuit state and the switch Sw 2  is in a conducting state, the sensing capacitor Cs charges the integrating capacitor Cint of the sigma-delta modulator  11 . An output voltage of a comparator  111  included in the sigma-delta modulator  11  is switched to a high level as soon as the integrating capacitor Cint is charged to a level of the reference voltage signal Vref, wherein the time required for charging the integrating capacitor Cint up to the reference level is linearly dependent on the capacitance of the sensing capacitor Cs. Furthermore, the output voltage of the comparator  11  is latched by a latch  112  and used as a gating signal to control the enabling of a counter  130  included in the modulator bitstream filter  13 . Therefore, the capacitance of the sensing capacitor Cs will be able to be estimated by a decision logic unit  150  included in the firmware  15  as it correlates to the counted value outputted by the counter  130 . 
     The above mentioned prior art has a number of disadvantages. For example, charging of the integrating capacitor Cint involves many charge/discharge cycles of the sensing capacitor which consumes power and time. In addition, one integrating capacitor Cint is required for each sensing circuit. A parallel architecture using such a technique would therefore require many integrating capacitors which either requires a great deal of area in the chip or many external components. If a sequential architecture is adopted for measuring the capacitance of many sensors, noise would be an issue and sufficient filtering and shielding needs to be implemented. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides a sensing method of a capacitive touch screen with reduced noise. 
     The present invention also provides a driving circuit of a capacitive touch screen, capable of implementing a sensing method of a capacitive touch screen to reduce noise. 
     In an aspect of the present invention, a sensing method of a capacitive touch screen, which includes a plurality of sensing capacitors, comprises steps of: selecting at least one of the plurality of sensing capacitors into a reference capacitor unit; calculating capacitance differences between the reference capacitor unit and other sensing capacitors; and locating a touched position on the capacitive touch screen according to the capacitance differences. 
     In another aspect of the present invention, a driving circuit of a capacitive touch screen for implementing differential capacitance measurement, which includes a reference capacitor unit with a reference capacitance and a plurality of sensing capacitors, comprises: a reference signal generator coupled to the reference capacitor unit and generating a pair of complementary reference voltage signals according to the reference capacitance; a plurality of sensing circuits coupled to the plurality of sensing capacitors and the reference signal generator, and receiving the pair of complementary reference voltage signals for measuring capacitance differences between the reference capacitor unit and the plurality of sensing capacitors; and a positioning circuit coupled to the sensing circuits for locating a touched position on the capacitive touch screen according to the measured capacitance differences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a functional block diagram schematically illustrating a capacitive touch sensing circuit according to prior art; 
         FIG. 2A˜FIG .  2 C are schematic diagrams illustrating an example of a touch screen layout where the present invention can be applied to, wherein  FIG. 2A  illustrates the use of a central sensing capacitor as the single reference capacitor;  FIG. 2B  illustrates the use of an external capacitor as the single reference capacitor; and  FIG. 2C  illustrates the use of multiple references. 
         FIG. 3  is a functional block diagram schematically illustrating a driving circuit of a capacitive touch screen for implementing differential capacitance measurement according to an embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating an example of differential capacitance measuring means for use with the driving circuit of  FIG. 3 ; and 
         FIG. 5  is a functional block diagram schematically illustrating a driving circuit of a capacitive touch screen for implementing differential capacitance measurement according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to  FIG. 2A , which schematically illustrates an example of a touch screen layout where the present invention can be applied to. In this example, the touch screen  2  is disposed with 90 separate sensing capacitors  201 ˜ 290 , in spite the number of the sensing capacitors can be selected depending on practical needs. According to the present invention, one of the sensing capacitors  201 ˜ 290  is selected into a reference capacitor unit to be a reference sensor and the capacitance of the reference sensor is used as a reference capacitance. Then differences between the reference capacitance and each of the other capacitances are calculated. By comparing the differences, the touched position by the user can be identified. 
     Principally, any of the sensing capacitors can be used as the reference. In an embodiment of the present invention, the central one  20   n  is selected as the reference sensor and subjected to a subtracting operation with the other sensing capacitors  201 ˜ 290 . Alternatively, different reference sensors can be chosen in rotation for an averaging effect. 
     In another embodiment, an external capacitor  200  can be selected into a reference capacitor unit as a reference sensor, as illustrated in  FIG. 2B , and differences between the external reference capacitance  200  and each of the sensing capacitances  201 ˜ 290  in the panel are calculated. By comparing the differences, the touched position by the user can be identified. 
     In a further embodiment, differential measurements are confined to a smaller area, and multiple sensing capacitors are selected into a reference capacitor unit. The sensing capacitances  201 ˜ 290  are divided into groups and multiple references Refl˜Refm are used in different groups for respective subtracting operations, as illustrated in  FIG. 2C . By comparing the differences, the touched position by the user can be identified. 
     In still another embodiment, all the sensing capacitors are selected into a reference capacitor unit, and the average capacitance of all sensing capacitors  201 ˜ 290  are used as the reference capacitance to be compared with the sensing capacitances  201 ˜ 290 . Differences between the reference capacitance and each of the sensing capacitances  201 ˜ 290  are calculated. By comparing the differences, the touched position by the user can be identified. 
     By the differential method according to the present invention, changes in capacitance of one sensor relative to another are detected. The differential method of touch sensing may lend itself to parallel measurement of all sensors. This reduces the problem of noise because the noise is correlated. Speed of touch detection can be increased because potentially less filtering will be required. Also, because a measurement can be done with a single charge/discharge cycle of each sensor compared with multiple cycles as used by other techniques power consumption can be reduced. Furthermore, in conventional touch screens the detection circuit(s) of the sensors often need to be calibrated to allow for varying measurement conditions. Due to the present method using differential techniques the problem of calibration is simplified because many changes in measurement conditions are the same for all sensors. 
     Hereinafter, a driving circuit of a capacitive touch screen for implementing the above-described differential capacitance measurements according to an embodiment of the present invention is illustrated with reference  FIG. 3 . The driving circuit includes a reference signal generator  30   n  and a plurality of identical sensing circuits  301 ˜ 390 . The reference signal generator  30   n  is coupled to the reference sensing capacitor  20   n  as shown in  FIG. 2A  while the sensing circuits  301 ˜ 390  are coupled to the sensing capacitors  201 ˜ 290 , respectively. The reference signal generator  30   n  generates a pair of complementary reference voltage signals Vrefp and Vrefn according to the reference capacitance for driving the differential capacitance measurements with the sensing circuits  301 ˜ 390 . An example of the differential capacitance measurement is illustrated in  FIG. 4 , in which the coupling of the sensing circuit  301  to the reference signal generator  30   n  is shown, and may refer to Prakash &amp; Abshire, “A Fully Differential Rail-to-Rail Capacitance Measurement Circuit for Integrated Cell Sensing”, IEEE SENSORS 2007 Conference, p. 1444-1447, which is incorporated herein for reference. 
     The capacitance differences between the reference sensing capacitor  20   n  and each of the sensing capacitors  201 ˜ 290  are thus realized as analog output voltages V 01 ˜V 90  excluding Vn corresponding to the reference signal generator  30   n . With the operational timing control by a control logic unit  60  coupled to the reference signal generator  30   n  and the sensing circuits  301 ˜ 390 , the output voltages V 01 ˜V 90  are converted into digital data by corresponding analog-to-digital converters  401 ˜ 490  which serve as a positioning circuit. The digital data are then inputted to a decode and interface logic unit  50 , to be processed, thereby realizing the touched position. 
     It is to be noted that the embodiment illustrated with reference to  FIG. 3  is exemplified to be used with the reference setting illustrated in  FIG. 2A . Similar circuitry can also be applied to other reference settings to accomplish differential capacitance measurement, which is understood by those skilled in the art. For example, an additional reference capacitor is provided in the embodiment of  FIG. 2C , and then an additional reference signal generator is included in the driving circuit. 
       FIG. 5  illustrates a driving circuit of a capacitive touch screen for implementing differential capacitance measurements according to another embodiment of the present invention. In this embodiment, less analog-to-digital converters are used by grouping the sensing circuits. For example, the sensing circuits  301 ˜ 390  are divided into three groups so that only three analog-to-digital converters  81 ˜ 83  are required. For such implementation, the analog output voltages V 01 ˜V 90  outputted by the sensing circuits  301 ˜ 390 , as described with reference to  FIG. 3 , are sampled and held for a specified period of time by corresponding sampling and holding units  601 ˜ 690 , and then sequentially selected through multiplexers  71 ˜ 73 . It is advantageous in simplification of circuitry. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.