Abstract:
A system and method for balancing the capacitive charge on touch sensor electrodes so that every two adjacent routes have the same capacitance as any other adjacent two routes, wherein routing electrodes are spaced further and further apart, or graduated, as they get longer, to thereby balance the capacitance on the touch sensor electrodes without having to add or subtract an offset from each touch sensor electrode.

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 5101.SMII.PR, having Ser. No. 61/589,735, and filed Jan. 23, 2012. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    1. Field Of the Invention 
         [0003]    This invention relates generally to touch sensors including touchpads and touchscreens. More specifically, the present invention is a method of balancing capacitance on the electrodes to improve performance. 
         [0004]    2. Description of Related Art 
         [0005]    There are several designs for capacitance sensitive touch sensors. It is useful to examine the underlying technology to better understand how any capacitance sensitive touch sensor can be modified to work with the present invention. 
         [0006]    The CIRQUE® Corporation touchpad is a mutual capacitance-sensing device and an example is illustrated as a block diagram in  FIG. 1 . In this touchpad  10 , a grid of X ( 12 ) and Y ( 14 ) electrodes and a sense electrode  16  is used to define the touch-sensitive area  18  of the touchpad. Typically, the touchpad  10  is a rectangular grid of approximately 16 by 12 electrodes, or 8 by 6 electrodes when there are space constraints. Interlaced with these X ( 12 ) and Y ( 14 ) (or row and column) electrodes is a single sense electrode  16 . All position measurements are made through the sense electrode  16 . 
         [0007]    The CIRQUE® Corporation touchpad  10  measures an imbalance in electrical charge on the sense line  16 . When no pointing object is on or in proximity to the touchpad  10 , the touchpad circuitry  20  is in a balanced state, and there is no charge imbalance on the sense line  16 . When a pointing object creates imbalance because of capacitive coupling when the object approaches or touches a touch surface (the sensing area  18  of the touchpad  10 ), a change in capacitance occurs on the electrodes  12 ,  14 . What is measured is the change in capacitance, but not the absolute capacitance value on the electrodes  12 ,  14 . The touchpad  10  determines the change in capacitance by measuring the amount of charge that must be injected onto the sense line  16  to reestablish or regain balance of charge on the sense line. 
         [0008]    The system above is utilized to determine the position of a finger on or in proximity to a touchpad  10  as follows. This example describes row electrodes  12 , and is repeated in the same manner for the column electrodes  14 . The values obtained from the row and column electrode measurements determine an intersection which is the centroid of the pointing object on or in proximity to the touchpad  10 . 
         [0009]    In the first step, a first set of row electrodes  12  are driven with a first signal from P, N generator  22 , and a different but adjacent second set of row electrodes are driven with a second signal from the P, N generator. The touchpad circuitry  20  obtains a value from the sense line  16  using a mutual capacitance measuring device  26  that indicates which row electrode is closest to the pointing object. However, the touchpad circuitry  20  under the control of some microcontroller  28  cannot yet determine on which side of the row electrode the pointing object is located, nor can the touchpad circuitry  20  determine just how far the pointing object is located away from the electrode. Thus, the system shifts by one electrode the group of electrodes  12  to be driven. In other words, the electrode on one side of the group is added, while the electrode on the opposite side of the group is no longer driven. The new group is then driven by the P, N generator  22  and a second measurement of the sense line  16  is taken. 
         [0010]    From these two measurements, it is possible to determine on which side of the row electrode the pointing object is located, and how far away. Pointing object position determination is then performed by using an equation that compares the magnitude of the two signals measured. 
         [0011]    The sensitivity or resolution of the CIRQUE® Corporation touchpad is much higher than the 16 by 12 grid of row and column electrodes implies. The resolution is typically on the order of 960 counts per inch, or greater. The exact resolution is determined by the sensitivity of the components, the spacing between the electrodes  12 ,  14  on the same rows and columns, and other factors that are not material to the present invention. 
         [0012]    The process above is repeated for the Y or column electrodes  14  using a P, N generator  24   
         [0013]    Although the CIRQUE® touchpad described above uses a grid of X and Y electrodes  12 ,  14  and a separate and single sense electrode  16 , the sense electrode can actually be the X or Y electrodes  12 ,  14  by using multiplexing. 
         [0014]    With this background in touch sensor technology, it is possible to analyze the improvement provided by the present invention. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    In a preferred embodiment, the present invention is a system and method for balancing the capacitive charge on touch sensor electrodes so that every two adjacent routes have the same capacitance as any other adjacent two routes, wherein routing electrodes are spaced further and further apart, or graduated, as they get longer, to thereby balance the capacitance on the touch sensor electrodes without having to add or subtract an offset. 
         [0016]    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 
         [0017]      FIG. 1  is a block diagram of the components of a capacitance-sensitive touchpad as made by CIRQUE® Corporation and which can be operated in accordance with the principles of the present invention. 
           [0018]      FIG. 2  is a diagram showing implementation of a first embodiment of the present invention that shows modified paths for drive and sense electrodes on an X and Y touch sensor electrode grid. 
           [0019]      FIG. 3  is a schematic diagram that illustrates a first embodiment wherein the capacitance between any two adjacent electrodes is altered by varying spacing between electrodes to affect the amount of charge and capacitance on the electrodes. 
           [0020]      FIG. 4  is a schematic diagram that illustrates another embodiment wherein the spacing between the routing traces is altered to affect the amount of charge and capacitance on the electrodes. 
           [0021]      FIG. 5  is a schematic diagram that illustrates another embodiment wherein one or more routing traces include an overshoot or stub that may affect the amount of charge and capacitance on the electrodes. 
           [0022]      FIG. 6  is a schematic diagram that illustrates another embodiment wherein one or more steps may be inserted along the length of the routing traces to affect the amount of charge and capacitance on the electrodes. 
           [0023]      FIG. 7  is a schematic diagram that illustrates another embodiment wherein the thickness of the parallel routing traces  32  is adjusted to affect the amount of charge and capacitance on the electrodes. 
           [0024]      FIG. 8  is a schematic diagram that illustrates another embodiment wherein a plurality of interleaved branches are disposed on at least some of the routing traces to affect the amount of charge and capacitance on the electrodes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    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. 
         [0026]    It should also be understood that the term “touch sensor” may be used interchangeable with the terms touchpad, touchscreen, trackpad, touch panel, touch input device and touch sensitive device throughout this document. 
         [0027]    Traditional projected mutual capacitance touch sensors may have significant offset from one side of the touch sensor to the other due to variations in the routing lengths and spacing between the electrodes. In other words, there can be differences in the amount of charge on sense and drive electrodes when they are supposed to be balanced, simply because of variations in length from one electrode to the next. To account for these differences in charge and capacitance, a touch sensor circuit can able to provide an offset that will compensate for this natural charge imbalance. In other words, a small amount of charge may be introduced onto various electrodes in order to achieve a balanced charge on each electrode. 
         [0028]    Creating offsets for each of the electrodes may introduce complexity into circuit design and may require more circuitry and power. Therefore it would be advantageous to provide a circuit design that inherently compensates for a substantial amount of the differences in electrical charge that may naturally occur on the electrodes. It should also be understood that the electrodes referred to in this document may be drive or sense electrodes as those terms are understood by those skilled in the art. 
         [0029]    In a multi-layer touch sensor, all the electrodes on one layer may be drive electrodes and all the electrodes on another layer may be sense electrodes. The functions of the electrodes may also be changed. For example, in some embodiments, the function of the electrodes in each layer may be switched between drive and sense functions. 
         [0030]      FIG. 2  is a top schematic view of electrodes as they may be disposed on a substrate in the prior art. In this figure, the electrodes  30  may be considered to be equidistantly spaced apart. The actual number and size of the electrodes  30  is not relevant and may vary without affecting the principles of the present invention. Coupled to each of the electrodes  30  is a routing trace  32 . The routing traces  32  connect each of the electrodes  30  to touch sensor circuitry  34 . The touch sensor circuitry  34  may include driving or sensing circuits depending on if the electrodes  30  are functioning as drive or sense electrodes. The actual path or shape of the routing traces  32  is not limited to the example shown. The figure is for illustration purposes only in order to teach the concepts of the present invention. 
         [0031]    There are several features of the routing traces  32  that will be noted. First, the routing traces  32  may be comprised of a plurality of straight lengths. In this embodiment, the routing traces  32  may make turns, but the routing traces are straight between the turns. 
         [0032]    Second, the routing traces  32  have no overshoots or stubs. An overshoot or stub is a relatively small continuation of a trace that is essentially a dead end. Thus an overshoot will result in a turn or branch of the routing trace  32  being made before reaching the end of the routing trace. 
         [0033]    Finally, the spacing between parallel lengths of the routing traces  32  is substantially equidistant. In other words, when the routing traces  32  are running parallel to each other for some distance, the spacing between each of the routing traces  32  may be the same. Any variations that may appear in the figure may be ignored. 
         [0034]      FIG. 3  illustrates a first embodiment of the present invention in that the capacitance between any two adjacent electrodes  30  and routing traces  32  may be made to be similar to any other two adjacent electrodes and routing traces by varying spacing between electrodes. Accordingly, a first embodiment of the present invention is to vary the spacing  36  between the routing traces  32  along at least one portion of a path that connects the touch sensor circuitry  34  to the electrodes  30 . The portion of the path where adjustments are made to spacing may be referred to as a shared path. The shared path is where the routing traces  32  are substantially parallel to each other. 
         [0035]    In this embodiment, the spacing  36  between routing traces  32  that travel a longer path between the touch sensor circuitry  34  and the electrodes  30  may be made greater than the spacing between routing traces that travel a shorter path. The spacing  36  between the routing traces  32  may be described as graduated as the spacing increases. Thus, where there is a shorter shared path of the routing traces  32 , the spacing between them may be smaller than the spacing between routing traces where the shared path is longer. The increase in spacing from the smallest space between routing traces  32  and the largest space between routing traces may be gradual. 
         [0036]    It should be understood that while the routing traces are shown as following straight paths, the routing traces may follow a path that is curvilinear or any other desired shape. It should also be understood that the electrodes  30  and the routing traces  32  are both electrodes. The routing traces  32  are simply the portion of the electrode that is not being used as part of an XY grid of a touch sensor. It should also be understood that the electrodes  30  shown are forming at least a portion of the column electrodes of an XY grid of a touch sensor. Another layer of electrodes on a different layer of the touch sensor would form the orthogonal layer of electrodes that are the row electrodes. The principles of the present invention apply to both the column and row electrodes of the touch sensor. 
         [0037]      FIG. 4  illustrates another embodiment of the present invention. In this embodiment, the spacing  36  between the routing traces  32  that have a shorter path between the touch sensor circuitry  34  and the electrodes  30  is greater than the spacing between routing traces that travel a longer path. The result of this change would be to increase the imbalance on the electrodes  30 . Thus, it is an aspect of the invention to be able to change the charge and thus the capacitance on the routing traces  32 , whether it is to increase the imbalance of capacitance as shown in  FIG. 4 , or to decrease the imbalance as shown in  FIG. 3 . 
         [0038]      FIG. 5  illustrates another embodiment of the present invention. In this embodiment, one or more routing traces  32  include an overshoot  38  or stub that may affect the amount of charge on the electrodes  30 , and thus the capacitance. The length of the overshoot  38  may be adjusted for each of the routing traces  32  in order to make fine adjustments to the amount of charge on each of the electrodes  30 . 
         [0039]    An overshoot  38  may actually be placed at more than one location along a single routing trace  32 . Therefore, there may be one or more overshoots  38  disposed on a routing trace  32 . However, it is observed that an overshoot  38  may have a greater effect on the charge and capacitance of the electrodes  30  if the overshoot is parallel to a different routing trace  32 . Accordingly, the overshoots  38  are most effective when placed on parallel segments of the routing traces  32 . 
         [0040]      FIG. 6  illustrates another embodiment of the present invention. In this embodiment, one or more steps  40  may be inserted along the length of the routing traces  32 . Some routing traces  32  may include a step  40 , while others may not in the same design. Furthermore, the steps  40  may be varied in shape and direction of a path. The ultimate purpose of the steps  40  is to change the amount of charge and thus capacitance on the routing traces  32  in order to achieve a balance in capacitance among some or all of the routing traces  32 . 
         [0041]    It may be an object of the present invention to enable every two adjacent routing traces  32  to have a capacitance that is substantially similar to the capacitance of any other two adjacent routing traces. Likewise, it may be an object of the present invention to enable every two adjacent electrodes  30  to have a capacitance that is substantially similar to the capacitance of any other two adjacent electrodes. The term “substantially similar” may mean that the capacitance of the routing traces  32  or the electrodes  30  is such that no offset needs to be applied. The term may also mean that some relatively small offsets may need to be applied. 
         [0042]    A simple method of calculating the relative capacitance Cr between routing traces  32  (which is also applicable to electrodes  30 ) is with the following formula: 
         [0000]        Cr =(length of shared route)/(distance between shared route) 
         [0043]    Another aspect of the present invention is related to the concept of balance. There are two types of balance that are relevant to the present invention. The first type of balance may be directed to the balancing of adjacent electrodes when performing mutual capacitance. Mutual capacitance may be defined as the capacitance of two adjacent electrodes relative to each other. In other words, it is the capacitance between them. In contrast, self-capacitance is the capacitance between a single electrode and a reference which is typically going to be ground. When using mutual capacitance for a touch sensor, an object such as a finger that disrupts the mutual capacitance between electrodes causes a decrease in capacitance. In contrast, when using self-capacitance for a touch sensor, an object such as a finger that disrupts the self-capacitance of an electrode to ground causes an increase in capacitance. 
         [0044]    The second type of balance may be directed to undesirable parasitic capacitances that may occur between electrodes. The principles of the present invention may be more effective in controlling an imbalance caused by parasitic capacitance versus mutual capacitance. 
         [0045]      FIG. 7  is an illustration of another embodiment of the present invention. In this embodiment, the thickness of the parallel routing traces  32  is adjusted, as well as the spacing  36  between them. It may also be possible to only adjust the thickness of the routing traces  32  while making the spacing between them uniform. Changing the thickness of the shorter routing traces  32  and making them thicker than the longer routing traces may reduce parasitic capacitance. 
         [0046]      FIG. 8  is an illustration of another embodiment of the present invention. In this embodiment, a plurality of interleaved branches  42  are attached to at least some of the routing traces  32 . The number of interleaving routing traces  32  may be adjusted to achieve the desired balance in capacitance on the electrodes  30 . Some routing traces  32  may not have any interleaved branches  42 , while other routing traces may have multiple interleaved branches. The space between the interleaved branches  42  may be adjusted to achieve the desired capacitance. The length of the interleaved branches  42  may also be varied. 
         [0047]    It should be understood that the concepts of using graduated spacing, overshoots, steps, thickness and interleaved branches may be used separately or in any combination with each other in order to achieve the desired balance in capacitance. 
         [0048]    In an alternative embodiment of the present invention, the principles of the present invention may be applied to a single-layer touch sensor as well as the multi-layer touch sensor already described. In the single layer touch sensor, the drive and sense electrodes  30  are placed adjacent to each other on the same substrate. 
         [0049]    In another alternative embodiment, the features of graduated spacing  36 , the use of overshoots  38  and steps  40  may be combined in any combination in order to achieve the desired results of changing the amount of charge and thus capacitance on the routing traces  32  in order to achieve a balance in capacitance without having to use offsets. 
         [0050]    In another alternative embodiment, adjustments are made to the routing traces  32  that are not along a shared path. The adjustments may include, but should not be considered to be limited to, spacing between routing traces and overshoots. 
         [0051]    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.