# Patent

## Title
Driven shield for capacitive touchpads

## 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.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This document claims priority to and incorporates by reference all of the subject matter included in the provisional patent application, having Ser. No. 60/908,853 and filed on Mar. 29, 2007.


BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of Related Art
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.
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.
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.
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.
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.
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.
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.
The process above is repeated for the Y or column electrodes 14 using a P, N generator 24
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
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.
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.
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.
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.
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.
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.
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.
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
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.
FIG. 2 is a perspective view of a portion of a capacitive touchpad sensor as found in the prior art.
FIG. 3 is a perspective view of a portion of the capacitive touchpad sensor, but now including a driven shield electrode.
FIG. 4 is a perspective view of a portion of the capacitive touchpad sensor, but now including a driven shield substrate.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

## Claims

1. A method for shielding a portion of a capacitive sensor used in a touchpad, said method comprising:
1) providing two drive electrodes for generating a signal and one sense electrode disposed between the two drive electrodes for measuring a portion of the signal that is coupled to the one sense electrode; and
2) providing two driven shield electrodes, one driven shield electrode on each side of and between the one sense electrode and the two drive electrodes, the two driven shield electrodes being electrically coupled to the one sense electrode for receiving a modified signal from the one sense electrode, the two driven shield electrodes at least partially shielding the one sense electrode from undesired electrical and capacitive signals, including stray capacitive signals from the two drive electrodes.

2. The method as defined in claim 1 wherein the method further comprises providing a buffer between the one sense electrode and the two driven shield electrodes to generate the modified signal and to thereby increase a signal to noise ratio on the at least one sense electrode by reinforcing the signal on the at least one sense electrode.

3. The method as defined in claim 1 wherein the method further comprises providing an amplifier between the one sense electrode and the two driven shield electrodes to generate the modified signal and to thereby increase a signal to noise ratio on the one sense electrode.

4. The method as defined in claim 2 wherein the method further comprises:
1) providing a driven shield substrate, wherein the two drive electrodes and the one sense electrode are attached to the driven shield substrate;
2) electrically coupling the driven shield substrate to the one sense electrode to thereby receive the modified signal from the one sense electrode, and wherein the driven shield substrate is positioned so as to at least partially shield the one sense electrode from undesired electrical and capacitive signals.

5. The method as defined in claim 4 wherein the method further comprises providing a buffer between the one sense electrode and the two driven shield substrates to generate the modified signal and to thereby increase a signal to noise ratio on the one sense electrode.

6. The method as defined in claim 4 wherein the method further comprises providing an amplifier between the one sense electrode and the driven shield substrate to generate the modified signal and to thereby increase a signal to noise ratio on the one sense electrode.

7. The method as defined in claim 5 wherein the method further comprises reducing stray capacitance between the two drive electrodes and the one sense electrode to thereby increase the strength of the portion of the signal that is coupled to the one sense electrode.

8. The method as defined in claim 7 wherein the method further comprises shielding the one sense electrode from electrical noise to thereby increase the strength of the portion of the signal that is coupled to the one sense electrode.

9. A system for shielding a capacitive sensor used in a touchpad, said system comprised of:
two drive electrodes and means for generating a signal thereon;
one sense electrode disposed between the two drive electrodes and means for measuring a portion of the signal that is coupled to the one sense electrode; and
two driven shield electrodes, one disposed on each side of the one sense electrode and between the one sense electrode and the two drive electrodes, the two driven shield electrodes being electrically coupled to the one sense electrode for receiving a modified signal from the one sense electrode, and wherein the two driven shield electrodes are at least partially shielding the one sense electrode from undesired electrical and capacitive signals, including stray capacitive signals from the two drive electrodes.

10. The system as defined in claim 9 wherein the system is further comprised of a buffer disposed between the one sense electrode and the two driven shield electrodes to thereby generate the modified signal.

11. The system as defined in claim 9 wherein the system is further comprised of an amplifier disposed between the one sense electrode and the two driven shield electrodes to thereby generate the modified signal.

12. The system as defined in claim 10 wherein the system is further comprised of a driven shield substrate, wherein the two drive electrodes and the one sense electrode are attached to but electrically isolated from the driven shield substrate, wherein the driven shield substrate is electrically coupled to the one sense electrode to thereby receive the modified signal from the one sense electrode, and wherein the driven shield substrate is positioned so as to at least partially shield the one sense electrode from undesired electrical and capacitive signals.

13. The system as defined in claim 12 wherein the system is further comprised of a buffer disposed between the one sense electrode and the driven shield substrate to thereby generate the modified signal.

14. The system as defined in claim 12 wherein the system is further comprised of an amplifier disposed between the one sense electrode and the driven shield substrate to thereby generate the modified signal.

15. The system as defined in claim 9 wherein the system is further comprised of a plurality of drive electrodes.

16. The system as defined in claim 15 wherein the system is further comprised of a plurality of sense electrodes.

17. A system for shielding a capacitive sensor used in a touchpad, said system comprised of:
two drive electrodes and means for generating a signal thereon;
one sense electrode disposed between the two drive electrodes and means for measuring a portion of the signal that is coupled to the one sense electrode;
two driven shield electrodes;
a driven shield substrate, wherein the two driven shield electrodes and the driven shield substrate are electrically coupled to the one sense electrode for receiving a modified signal from the one sense electrode, wherein the two drive electrodes, the one sense electrode and the two driven shield electrodes are coupled to the driven shield substrate, and wherein the two driven shield electrodes and the driven shield substrate are disposed adjacent to the one sense electrode to thereby at least partially shield the one sense electrode from undesired electrical and capacitive signals including stray capacitive signals from the two drive electrodes.

18. The system as defined in claim 17 wherein the system is further comprised of a buffer disposed between the one sense electrode on one side, and the two driven shield electrodes and the driver shield substrate on the other side, to thereby generate the modified signal.

19. The system as defined in claim 17 wherein the system is further comprised of an amplifier disposed between the one sense electrode on one side, and the two driven shield electrodes and the driven shield substrate on the other side, to thereby generate the modified signal.

20. A system for shielding a capacitive sensor used in a touchpad, said system comprised of:
two drive electrodes and means for generating a signal thereon;
one sense electrode disposed between the two drive electrodes and means for measuring a portion of the signal that is coupled to the one sense electrode; and
a driven shield substrate, wherein the one sense electrode and the two drive electrodes are attached to the driven shield substrate, and wherein the driven shield substrate is electrically coupled to the one sense electrode for receiving a modified signal from the one sense electrode to thereby at least partially shield the one sense electrode from undesired electrical and capacitive signals including stray capacitive signals from the two drive electrodes.

21. The system as defined in claim 20 wherein the system is further comprised of two driven shield electrodes, one disposed on each side of the one sense electrode and between the one sense electrode and the two drive electrodes, the two driven shield electrodes being electrically coupled to the one sense electrode for receiving a modified signal from the one sense electrode, and wherein the two driven shield electrodes are at least partially shielding the one sense electrode from undesired electrical and capacitive signals, including stray capacitive signals from the two drive electrodes.