Abstract:
A conductive shield that is disposed near the sensor, wherein a signal is driven on the shield that is similar to the signal induced on the sensor, thereby reducing stray capacitances and protecting the sensor from external noise sources, and resulting in a stronger signal reaching the sensor.

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 3858.CIRQ.PR, having Ser. No. 60/908,853 and filed on Mar. 29, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to capacitance sensitive touchpads. Specifically, the invention relates to shielding that can be disposed near the electrodes of the sensor to protect the electrodes from unwanted noise or stray capacitances, thereby increasing the signal to noise ratio and improving performance of the capacitive touchpad. 
         [0004]    2. Description of Related Art 
         [0005]    To understand the capacitive touchpad technology used in the present invention, it is useful to examine one embodiment of touchpad technology that does not use but can be adapted to use the present invention. Specifically, the touchpad technology of CIRQUE® Corporation used for touch and proximity sensing will now be explained. 
         [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 (and proximity sensitive) area  18  of the touchpad. Typically, the touchpad  10  is a rectangular grid of approximately 16 by 12 row and column electrodes, or 8 by 6 electrodes when there are space constraints. Interlaced with these X ( 12 ) and Y ( 14 ) (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 a 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® Corporation touchpad described above uses a grid of X and Y electrodes  12 ,  14  and a separate and single sense electrode  16 , the function of the sense electrode can actually be performed by the X or Y electrodes  12 ,  14  by using multiplexing. Thus, when the X electrodes  12  are being driven with a signal, the Y electrodes  14  can function as sense electrodes. Likewise, the Y electrodes can be driven and the X electrodes then function as the sense electrodes. A touchpad design with or without a dedicated sense electrode can be used. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    In a preferred embodiment, the present invention is a conductive shield that is disposed near the sensor, wherein a signal is driven on the shield that is similar to the signal induced on the sensor, thereby reducing stray capacitances and protecting the sensor from external noise sources, and resulting in a stronger signal reaching the sensor. 
         [0015]    In a first aspect of the invention, a driven shield electrode is disposed adjacent to a sense electrode to at least partially shield the sense electrode from stray capacitances. 
         [0016]    In a second aspect of the invention, the driven shield electrode is disposed adjacent to the sense electrode to at least partially shield the sense electrode from electric noise. 
         [0017]    In a third aspect of the invention, a driven shield substrate is disposed adjacent to a sense electrode to at least partially shield the sense electrode from stray capacitances. 
         [0018]    In a fourth aspect of the invention, the driven shield substrate is disposed adjacent to the sense electrode to at least partially shield the sense electrode from electric noise. 
         [0019]    In a fifth aspect of the invention, a driven shield electrode and a driven shield substrate are provided to at least partially shield the sense electrode from stray capacitances. 
         [0020]    In a sixth aspect of the invention, a driven shield electrode and a driven shield substrate are provided to at least partially shield the sense electrode from electric noise. 
         [0021]    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 
         [0022]      FIG. 1  is a block diagram of a touchpad as taught be the prior art, and which is adapted to function with the present invention. 
           [0023]      FIG. 2  is a perspective view of a portion of a capacitive touchpad sensor as found in the prior art. 
           [0024]      FIG. 3  is a perspective view of a portion of the capacitive touchpad sensor, but now including a driven shield electrode. 
           [0025]      FIG. 4  is a perspective view of a portion of the capacitive touchpad sensor, but now including a driven shield substrate. 
           [0026]      FIG. 5  is a perspective view of a portion of the capacitive touchpad sensor, but now including a driven shield electrode and a driven shield substrate 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    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. 
         [0028]      FIG. 2  is a perspective view of a portion of a capacitive touchpad sensor including a grounded and conductive substrate  30 , a plurality of drive electrodes  32 , a sense electrode  34 , a signal that is coupled to the sense electrode and then buffered  36 , and a means  38  for generating a signal that is emitted from the plurality of drive electrodes  32 . The buffered signal  36  is sent to a measurement circuit of a capacitive touchpad that is not relevant to the present invention. The present invention pertains to a system and method for obtaining a signal on the sense electrode  34  that is protected from both stray capacitances and electrical noise. 
         [0029]    In a portion of a capacitive touchpad  10  as shown in  FIG. 1 , a plurality of drive electrodes  32  emit electrical signals that are measured by the sense electrode  34 . The introduction of a pointing object (not shown) near the drive electrodes  32  and the sense electrode  34 , such as a human finger, has the effect of changing the voltage or charge that is coupled on the sense electrode. To measure the presence or the location of the pointing object on or near to a sensing area as defined by the plurality of drive electrodes  32  and the sense electrode  34 , it is desirable that the pointing object have the maximum effect on the voltage or charge that is coupled on the sense electrode  34  by the signal from the plurality of drive electrodes  32 . 
         [0030]    The capacitive changes that are measured on the sense electrode  34  are very small. Unfortunately, the coupling between the plurality of drive electrodes  32  and the sense electrode  34  is susceptible to many influences that affect the accuracy of the capacitive changes being measured. These influences include variations in manufacturing processes, environmental changes and electrical noise. The present invention is a system and method for improving the accuracy of the capacitive changes being measured. 
         [0031]    Decreasing stray capacitance from the sense electrode  34  to all other conductors will increase the accuracy of the capacitive changes being measured by the touchpad  10 . Furthermore, shielding the sense line  34  from electrical noise sources will also increase the signal-to-noise ratio. 
         [0032]    The present invention provides a driven shield that at least partially surrounds the sense electrode  34 . The shield is driven with a signal that is similar to the signal that is coupled to the sense electrode  34 . 
         [0033]      FIG. 3  illustrates the present invention and comprises a perspective view of a portion of a capacitive touchpad sensor  40  including a grounded conductive substrate  30 , a plurality of drive electrodes  32 , a sense electrode  34 , a signal that is coupled to the sense electrode and then buffered  36 , and a means  38  for generating a signal that is emitted from the plurality of drive electrodes  32 . In addition to these components, a driven shield electrode  42  is disposed between the plurality of drive electrodes  32  and the sense electrode  34 . The driven shield electrodes  42  receive the buffered signal  36  from the sense electrode  34 . 
         [0034]    Stray capacitances can be generated by the drive electrodes  32  when they are not actively being driven. The effect of the driven shield electrode  42  is to reduce the stray capacitances from the drive electrodes  32  or any other conductors that can generate stray capacitances. This effect in turn will enable a stronger signal at the sense electrode  34 . 
         [0035]    The same shielding effect created by the driven shield electrode  42  around the sense electrode  34  also protects the sense electrode from unwanted electrical noise either internal or external to the touchpad  10 . 
         [0036]      FIG. 4  is provided as an alternative embodiment of the present invention. The touchpad  40  comprises a perspective view of a portion of a capacitive touchpad sensor  46  including a plurality of drive electrodes  32 , a sense electrode  34 , a signal that is coupled to the sense electrode and then buffered  36 , and a means  38  for generating a signal that is emitted from the plurality of drive electrodes  32 . The system further comprises a driven shield substrate  48  upon which the drive electrodes  32  and the sense electrode  34  are disposed. The driven shield substrate  48  receives the buffered signal  36  from the sense electrode  34 . 
         [0037]      FIG. 5  illustrates another alternative embodiment of the present invention, and comprises a perspective view of a portion of the capacitive touchpad sensor  40  including a plurality of drive electrodes  32 , a sense electrode  34 , a signal that is coupled to the sense electrode and then buffered  36 , and a means  38  for generating a signal that is emitted from the plurality of drive electrodes  32 . In addition to these components, a driven shield electrode  42  is disposed between the plurality of drive electrodes  32  and the sense electrode  34 , and a driven shield substrate  48  is provided as a means for arranging the drive electrodes  32 , the sense electrode  34  and the driven shield electrodes  42 . The driven shield electrodes  42  and the driven shield substrate  48  receive the buffered signal  36  from the sense electrode  34 . 
         [0038]    It is generally the case that the buffered signal  36  that comes from the sense electrode  34  is going to closely follow the signal that is coupled to the sense electrode from the drive electrodes  32 . However, it should be understood that the signal from the sense electrode can be buffered or amplified. What is important is that the signal be modified so as to enhance protection of the sense electrode  34  as the signal from the drive electrodes  32  is coupled to it. The buffer, amplifier or signal modifier is shown in  FIG. 5  as item  52 . 
         [0039]    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.