Abstract:
A touch panel having a substantially even coating of a conductive material on a non-conductive substrate and then covering the conductive material with a dielectric material, wherein a novel current measuring circuit reduces the effect of stray capacitance on the accuracy of a current measurement so that the relative X and Y position of an object on the touch panel can be determined using simple ratio equations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This document claims priority to, and incorporates by reference all of the subject matter included in the provisional patent application docket number 4455.CIRQ.PR, having Ser. No. 61/116,592 and filed on Nov. 20, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to touchpad technology. More specifically, the present invention is a new method of determining the position of a pointing object on a surface capacitance touch panel. 
         [0004]    2. Description of Related Art 
         [0005]    A well-known touchpad technology uses a surface capacitance touch panel  10  as shown in  FIG. 1 . Such a touch panel  10  is a solid sheet of a conductive material  16  disposed on an insulating substrate  18  such as glass, with sensors  12  disposed at the corners. The traditional method of measuring the position of a pointing object  14  or the “touch position” on the surface capacitance touch panel  10  is to apply an AC signal on all four corners of the touch panel&#39;s conductive layer  16 . The conductive layer  16  can be made, for example, of Indium Tin Oxide (ITO). 
         [0006]    To create the touch panel  10 , the surface of the glass substrate  18  is flooded or covered with a substantially even layer of a resistive ITO material which forms a sheet resistance. A dielectric is then applied to cover the ITO conductive material. 
         [0007]    After applying the AC signal to the conductive ITO material  16 , the next step is to triangulate the touch position using the current flowing through each corner. It is common to apply either a sine wave or a square wave. 
         [0008]    If an object such as a finger  14  comes in contact with the surface of the touch panel  10 , a capacitor is formed between the ITO surface  16  and the finger tip  14 . The capacitance value is very small, typically in the order of 50 pF. The amount of charge or current that has to be measured going into each corner  12  of the panel is therefore very small. Because the current is so small, the system is very susceptible to stray capacitance. Thus, the accuracy of touch panels  10  is often an issue. 
         [0009]    Accordingly, what is needed is a new method of triangulating the position of the object on the touch panel surface that increases the accuracy of measurements and decreases susceptibility to stray capacitance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to provide a touch panel that uses a new method to determine a position of an object touching the surface thereof. 
         [0011]    It is another object to provide a new method of measuring current that is less susceptible to stray capacitance. 
         [0012]    In a first embodiment, the present invention is a touch panel having a substantially even coating of a conductive material on a non-conductive substrate and then covering the conductive material with a dielectric material, wherein a novel current measuring circuit reduces the effect of stray capacitance on the accuracy of a current measurement so that the relative position of an object on the touch panel can be determined using simple ratio equations. 
         [0013]    These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of a touch panel as found in the prior art. 
           [0015]      FIG. 2  is a perspective view of a touch panel  10  that is made in accordance with principles of the prior art. 
           [0016]      FIG. 3  is a perspective view of a touch panel  10  that is made in accordance with the principles of the present invention. 
           [0017]      FIG. 4  is a circuit diagram showing how a sensitive current measuring circuit comprised of a capacitor and a current measuring sensor is applied to the touch panel. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
         [0019]      FIG. 2  shows the surface of a touch panel  10  as found in the prior art. The lines  20  are indicative of the voltage gradient that is produced across the surface of the touch panel when a voltage is applied at two corners of the surface. For example, the voltage is applied at corners  22  and  24  resulting in the constant voltage gradient shown. There is significant distortion of the voltage gradient lines  20  which is common to many touch panels  10 . 
         [0020]      FIG. 3  is a perspective view of a touch panel  10  that is made in accordance with the principles of the present invention. A new and novel approach to determining the position of an object on the touch panel is to charge a large capacitor and then apply this “flying capacitor” to the touch panel  10 . In the flying capacitor method of the present invention, this method measures the instantaneous and total current induced in a contact on a surface of the touch panel  10  when a constant voltage gradient is produced across the surface in a single axis. 
         [0021]    Linearity of a voltage gradient can improve accuracy of the touch panel. Therefore, in a first step, it is desirable but not essential that a lower resistance material be added around the edges of the touch panel  10  on the surface. The voltage gradient lines  20  become closer and more linear from a top edge  26  to a bottom edge  28 . 
         [0022]      FIG. 4  is a circuit diagram showing how a sensitive current measuring circuit comprised of a capacitor and a current measuring sensor is applied to the touch panel  10  in a first embodiment of the present invention. Any charge that is taken from the touch panel  10  is measured with the current measuring circuit. 
         [0023]    In this embodiment, four measurements X 1 , X 2 , X 3  and X 4  must be taken in order to determine the location of a pointing object  50  (located arbitrarily on the touch panel  10 ) on the surface of the touch panel  10 . Therefore, the first step is to electrically couple a positive node of the flying capacitor  30  to a first side  40  of the touch panel  10  while the negative node is electrically coupled to an opposite second side  42  of the touch panel along with a sensor or current measuring circuit  44 . The current measuring circuit  44  can be an ammeter. 
         [0024]    The voltage gradient is formed across the surface of the touch panel  10  from the first side  40  to the second side  42 , and to the sensor circuit  44 . A finger or other pointing object  50  touching the surface of the touch panel  10  at any given point will cause a drain on the current that is being measured by the sensor circuit  44 . The drain in current to the sensor circuit  44  is a function of the distance of the finger from the first and second sides  40 ,  42  of the touch panel  10 . The first measurement X 1  is thus the current leaving the touch panel  10  at the second side  44 . 
         [0025]    Assume that the first side  40  is arbitrarily a left side of the touch panel  10  as shown in  FIG. 4 . The second side  42  would therefore correspond to the right side of the touch panel  10 . The first and second sides  40 ,  42  are arbitrarily selected and can be switched with no change in the method of the present invention. 
         [0026]    The second current measurement X 2  is taken by switching the positive and negative nodes of the flying capacitor  30  between the first and second sides  40 ,  42  of the touch panel  10 . The current measuring circuit  44  is also moved when the circuit is reversed to take current measurement X 2 . 
         [0027]    A position of the pointing object  50  can be determined as a ratio of current measurements X 1  and X 2 . The position of the pointing object  50  is a value that is easily assigned to be between zero and one, and is determined using equation 1: 
         [0000]        X=X 1/( X 1+ x 2) 
         [0028]    Two similar measurements are taken using the top  26  and bottom  28  or third and fourth sides of the touch panel  10 . The positive node of the flying capacitor  30  can be coupled to the top edge  26  or the bottom edge  28  first. The decision regarding which edge to connect to the positive node first is arbitrary. The result is current measurements Y 1  and Y 2 . A Y position ratio is then obtained using equation 2: 
         [0000]      Y+Y1/(Y1+Y2) 
         [0029]    The strength of the present invention as described above is that the flying capacitor  30  is used to create the high current required to produce the constant voltage gradient on the surface of the touch panel  10  and thus enable direct measurement of the current leaving the surface though contacts on the surface. The current induced in the low resistance material is much larger than the current induced in the pointing object on the surface. Having a large current to measure increases the accuracy of the system and reduces the effect that stray capacitances can have on the measurements. 
         [0030]    It should be understood that the charge on the flying capacitor  30  is rapidly being refreshed in order to maintain the voltage gradient across the touch panel  10 . The process of disconnecting the flying capacitor  30  from the touch panel  10 , refreshing the charge, and then reconnecting the flying capacitor to the touch panel  10  is well known to those skilled in the art and is not an aspect of the present invention. 
         [0031]    It is also possible to determine a Z position of the pointing object relative to the surface of the touch panel  10 . The Z location of the pointing object is determined using equation 3: 
         [0000]        Z =( X 1+ X 2+ Y 1+ Y 2)/4 
         [0032]    The advantage of the embodiment of the present invention described above is that a voltage gradient is formed across the touch panel  10  using a relatively crude yet simple current measuring circuit  44 . Nevertheless, a measurement of the current going to the pointing object is very precisely measured because there is no other path for the current to follow other than between the positive and negative nodes of the flying capacitor  30  and the pointing object  50 . 
         [0033]    It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.