Patent Publication Number: US-2009231286-A1

Title: Active touch panel voltage compensation circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates to an active touch panel voltage compensation circuit, particularly to a circuit for the uniform voltage distribution on an electrode layer of a touch panel. 
     BACKGROUND OF THE INVENTION 
     With the rapid development of LCD panels and the touch control technology, touch panels have been extensively used in mobile phones, information products and various exhibitions. Among them, the resistive type and capacitive type are the most widely touch panels. In a capacitive touch panel, an electrode layer is electroplated on a glass substrate, and a protection layer is formed on the electrode layer. From four corners, voltages are applied to the electrode layer to create an equipotential figure of the voltage distribution on the electrode layer. An active area is defined from the equipotential figure, and the touch panel can precisely detect the contact position inside the active area. The active area is usually a rectangle defined by the central points of the inward-concaved equipotential lines. The reason why the equipotential lines are concaved inward may be that the impedance of the panel induces the attenuation of voltage, or that the output variation of the power source (applying voltage to the electrode layer) induces the variation of the equipotential lines, or that another environmental factor induces the variation of the equipotential lines. Refer to  FIG. 1  a diagram schematically showing the distribution of equipotential lines on a conventional touch panel. A power source applies voltage from several voltage-applying points to the electrode layer and makes the voltage distributed on the electrode layer. The voltage attenuates slightly from the voltage-applying points to the center of the electrode layer. The equipotential lines  13  define a rectangular area as the active area of the touch panel  1 . As there is a given spacing between two voltage-applying points, voltage attenuates in between the voltage-applying points, which causes the equipotential lines  13  to recede toward the center of the touch panel  1 . The equipotential line  13  is inward concaved to form an arc shape. The more far away from the voltage-applying points a section of the equipotential line  13 , the more obvious the deflection. Thus, the active area of the touch panel  1  can only be a rectangle within the centers of the inward concaved equipotential lines  13 . If the equipotential line  13  is concaved inward too much, the width of the non-touch control edge will be too great. Too great a non-touch control edge increases the cost and impairs the area reduction of the touch panel. Some prior arts modify the electrode layout to improve the inward-concaved equipotential lines  13 . For example, a R.O.C. patent No. 1236627 disclosed “Linear Electrodes of Touch Panel”, wherein several straight segments, several protruding tips and several protruding rods, which extend toward the center, are used to improve the distribution of the equipotential lines and straighten the equipotential lines. However, the inward extension of the straight segments, protruding tips and protruding rods contrarily generates an inactive area in the perimeter of the touch panel. A U.S. Pat. No. 7,075,522 disclosed a “Touch Panel Structure for Increasing Active Area”, which also modifies the electrode layout to improve the distribution of the equipotential lines. Although the abovementioned prior arts can improve the linearity of the equipotential lines, they still have room to improve. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to provide a compensation circuit to regulate the voltage applied to the electrode layer of a touch panel to optimize the distribution of the equipotential lines and increase the active area of the touch panel, which can overcome the problem of the inward-concaved equipotential lines that the prior arts cannot effectively solve. 
     The present invention discloses an active touch panel voltage compensation circuit, which is to improve the voltage distribution along the edges of the electrode layer of a touch panel, and which comprises: a power supply unit, a differential amplifier unit, and a processing unit. The power supply unit applies an initial voltage to a plurality of voltage-applying points of the electrode layer. The differential amplifier unit sets a calibration level and is coupled to a plurality of detection points corresponding to the plurality of voltage-applying points to obtain a plurality of detected voltages. The detected voltages are compared with the calibration level to determine a plurality of deviation values corresponding to the plurality of detection points. The processing unit works out the compensation voltages for the voltage-applying points according to the deviation values, generates a plurality of gain-modulation signals according to the compensation voltages, and sends the gain-modulation signals to the power supply unit. The power supply unit regulates the initial voltage to the plurality of compensation voltages so that the voltages detected in the detection points can reach the calibration level. Thereby, the voltage attenuation between two voltage-applying points of the touch panel can be corrected. The circuit of the present invention can automatically regulate the voltages of the plurality of voltage-applying points to make the equipotential lines more close to the border of the electrode layer. 
     Therefore, the present invention can enlarge the active area defined by the equipotential lines and improve the accuracy of the touch panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the distribution of the equipotential lines on a conventional touch panel. 
         FIG. 2  is a block diagram schematically showing a circuit according to the present invention. 
         FIG. 3  is a diagram schematically showing the distribution of the equipotential lines on the touch panel with the circuit of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, the technical contents of the present invention will be described in detail in cooperation with the drawings. 
     The present invention is an active touch panel voltage compensation circuit, which is to improve the voltage distribution of a touch panel. Refer to  FIG. 2  a block diagram schematically showing a circuit according to the present invention. There is a plurality of voltage-applying points in a touch panel  1 , and there are four voltage-applying points  11   a ,  11   b ,  11   c  and lid in this embodiment. The circuit of the present invention comprises: a power supply unit, a differential amplifier unit  3  setting a calibration level, and a processing unit  5  calculating the output of the power supply unit. The power supply unit further comprises a plurality of voltage gain circuits  2   a ,  2   b ,  2   c  and  2   d , which respectively control the voltages output to the plurality of voltage-applying points  11   a ,  11   b ,  11   c  and lid. The voltage-applying points  11   a ,  11   b ,  11   c  and lid provide an initial voltage to start the touch panel  1 . A plurality of detection points  12   a ,  12   b ,  12   c  and  12   d  is respectively arranged besides the corresponding voltage-applying points  11   a ,  11   b ,  11   c  and lid. The differential amplifier unit  3  is coupled to the plurality of detection points  12   a ,  12   b ,  12   c  and  12   d  to obtain a plurality of detected voltages and compares the detected voltages with the calibration level to obtain a plurality of deviation values. An AC/DC converter  4  is arranged in between the differential amplifier unit  3  and the processing unit  5  to convert the deviation values into digital signals. The processing unit  5  works out the compensation voltages for the voltage-applying points  11   a ,  11   b ,  11   c  and  11   d  according to the deviation values and generates a plurality of gain-modulation signals according to the compensation voltages. A DC/AC converter  6  converts the gain-modulation signals into analog signals and sends the analog signals to the voltage gain circuits  2   a ,  2   b ,  2   c  and  2   d  to make the voltage gain circuits  2   a ,  2   b ,  2   c  and  2   d  regulate the initial voltage to the plurality of compensation voltages to adjust the voltages applied to the voltage-applying points  11   a ,  11   b ,  11   c  and lid so that the voltages detected in the detection points  12   a ,  12   b ,  12   c  and  12   d  can reach the calibration level. Once the voltages detected in the detection points  12   a ,  12   b ,  12   c  and  12   d  can reach the calibration level, the voltage attenuation-induced deviations neighboring the voltage-applying points  11   a ,  11   b ,  11   c  and lid are corrected to near ideal values so that equipotential lines  13  (shown in  FIG. 3 ) of the touch panel  1  can get closer to the border of the electrode layer of the touch panel  1  lest the equipotential lines  13  be too much inward concaved and the active area of the touch panel  1  be decreased. Refer to  FIG. 3  a diagram schematically showing the distribution of the equipotential lines of the touch panel with the circuit of the present invention. Via comparing  FIG. 3  with  FIG. 1 , it is found that the present invention can effectively improve the distribution of the equipotential lines  13  of the touch panel  1 , make the equipotential lines  13  get closer to the border of the electrode layer of the touch panel  1 , and thus increase the active area of the touch panel  1 . Thereby, the accuracy of touch control can also be promoted. The present invention uses feedback signals to automatically detect deviations and calculate the compensation voltages to correct the deviations no matter whether the deviations result from the output variation of the power supply or the impedance variation of the touch panel  1  or another factor. Thus, the present invention can save maintenance personnel a lot of examination and correction time. 
     The abovementioned deviation values may be proportional to or inversely proportional to the compensation voltages. The differential amplifier unit  3  may further comprise a differential amplifier and a multiplex switch. The multiplex switch sequentially conducts the detected voltages to the differential amplifier so that the differential amplifier can sequentially generate the plurality of deviation values. Alternatively, the differential amplifier unit  3  comprises a plurality of differential amplifiers corresponding to the plurality of detection points  12   a ,  12   b ,  12   c  and  12 , and the differential amplifiers simultaneously generate the plurality of deviation values. The processing unit  5  receiving the plurality of deviation values is a multiplex processor. 
     The preferred embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation made by the persons skilled in the art according to the spirit of the present invention is to be also included within the scope of the present invention, which is based on the claims stated below. 
     From the above discussion, the present invention is proved to have improvements over the prior arts. Therefore, the present invention indeed possesses novelty and non-obviousness and meets the condition for a patent. Thus, the Inventors file the application for a patent. It will be appreciated if the patent is fast approved.