Detection of and compensation for stray capacitance in capacitive touch sensors

• US 20060267953A1
• Filed: 05/31/2005
• Published: 11/30/2006
• Est. Priority Date: 05/31/2005
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Lessard, Bruce A., Bulat, Bulent, Taylor, Gordon F., Peterson, Richard A. Jr.
• Granted Patent: US 7,567,240 B2
• Time in Patent Office: Days
• US Class Current: 345/173
• CPC Class Codes: G06F 3/04186 Touch location disambiguation
• CPC Class Codes: G06F 3/0444 using a single conductive element covering the whole sensing surface, e.g. by sensing the electrical current flowing at the corners
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe
• CPC Class Codes: H03K 2217/96071 characterised by the detection principle
• CPC Class Codes: H03K 2217/96073 Amplitude comparison

Abstract

The present invention provides systems and methods for detecting stray capacitance in capacitive touch sensors. The existence of stray capacitance can lead to errors in touch detection and touch position determination. Such errors can be avoided or corrected when the stray capacitance is detected. Detecting stray capacitance includes analyzing signals for features characteristic of stray capacitance noise events. Such features can include spatial features such as the location of a test touch position determined from signals caused by stray capacitance, as well as temporal features such as the rate of change of the detected signals.

Claims

1. A method of detecting stray capacitance in a touch sensor system that includes a capacitive touch sensor having a resistive layer covering an active area, electronics coupled to a plurality of locations on the resistive layer for determining touch position based on currents flowing through the plurality of locations due to capacitive coupling of a touch implement with the resistive layer, the method comprising:

determining a test touch position when a detected signal level exceeds a threshold value; and

using the test touch position to determine existence or absence of stray capacitance.

2. The method of claim 1, further comprising suspending updates to a baseline signal level when the absence of stray capacitance is determined and disabling suspension of updates to the baseline signal level when the existence of stray capacitance is determined.

3. The method of claim 1, further comprising independently tracking signals due to stray capacitance when the existence of stray capacitance is determined.

4. The method of claim 3, wherein the signals due to stray capacitance are used to correct the baseline signal level.

5. The method of claim 3, wherein the signals due to stray capacitance are used to adjust one or more signal thresholds.

6. The method of claim 1, further comprising using a first time constant to filter signals when the absence of stray capacitance is determined and using a second time constant different from the first when the existence of stray capacitance is determined.

7. The method of claim 6, wherein the first time constant is longer than the second time constant.

8. The method of claim 1, wherein a test touch position within a determined distance from a defined point on the capacitive touch sensor indicates existence of stray capacitance.

9. The method of claim 8, wherein the determined distance is preset.

10. The method of claim 8, wherein the determined distance is based on criteria including touch sensor size.

11. The method of claim 8, wherein the determined distance is within +/−

5% of the defined point.

12. The method of claim 8, wherein the determined distance is refined based on a recorded history of stray capacitance events.

13. The method of claim 8, wherein the defined point is the center of the touch sensor.

14. The method of claim 8, wherein the defined point is preset.

15. The method of claim 8, wherein the defined point is developed from a recorded history of stray capacitance events.

16. The method of claim 1, wherein the threshold value is lower than a touch threshold.

17. The method of claim 16, wherein the touch threshold is dynamically adjusted based on the presence or absence of stray capacitance.

18. The method of claim 1, further comprising using a rate of change of signal level to confirm the existence or absence of stray capacitance.

19. A method of detecting stray capacitance in a touch sensor system that includes a capacitive touch sensor having a resistive layer covering an active area, electronics coupled to a plurality of locations on the resistive layer for determining touch position based on currents flowing through the plurality of locations due to capacitive coupling of a touch implement with the resistive layer, the method comprising:

detecting a signal on the resistive layer;

developing a profile for the signal for a period of time that includes a rise in signal level above a threshold;

determining a rate of change in the signal profile during the rise; and

using the rate of change to determine existence or absence of stray capacitance.

20. The method of claim 19, wherein when the rate of change exceeds a predetermined value, the existence of stray capacitance is indicated.

21. A capacitive touch screen system comprising:

a capacitive touch sensor comprising a resistive layer; and

electronics coupled to the resistive layer at a plurality of locations on the resistive layer for detecting signals caused by capacitive couplings to the resistive layer, the electronics configured to determine a location of a touch implement capacitively coupled to the resistive layer based on currents flowing through the plurality of locations due to the capacitive coupling, and where the electronics are further configured to distinguish signals caused by coupling of stray capacitances to the resistive layer from signals caused by coupling of touch implements to the resistive layer based on analyzing the signals against one or more features characteristic of stray capacitances.

22. The touch sensor system of claim 21, further comprising an electronic display disposed in close proximity to the capacitive touch sensor and viewable therethrough.

23. The touch sensor system of claim 21, wherein the one or more feature characteristic of stray capacitances include proximity of a test touch position to a determined location indicative of stray capacitances, the test touch position calculated when the signals exceed a threshold value.

24. The touch sensor system of claim 21, wherein the one or more feature characteristic of stray capacitances include a rate of change of the signals when the signals exceed a threshold value.