Detection of and compensation for stray capacitance in capacitive touch sensors

• US 7,567,240 B2
• Filed: 05/31/2005
• Issued: 07/28/2009
• Est. Priority Date: 05/31/2005
• Status: Expired due to Fees

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Peterson, Richard A. Jr., Lessard, Bruce A., Bulat, Bulent, Taylor, Gordon F.
• Primary Examiner(s): Mengistu; Amare
• Assistant Examiner(s): Rainey; Robert R
• Application Number:
  US11/141,228
• Publication Number:
  US 20060267953A1
• Time in Patent Office: 1,519 Days
• Field of Search: 345/173, 345/156, 178/18.01, 178/18.05, 178/18.06
• 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 level of a detected signal exceeds a threshold value above a baseline signal level generated from a detected signal;

in response to the step of determining, using the test touch position to determine whether the detected signal is due to an impending touch input or stray capacitance; and

in response to the step of using the test touch position, controlling updates to the baseline signal level.

2. The method of claim 1, further comprising suspending updates to a baseline signal level when it is determined that the detected signal is due to an impending touch input and disabling suspension of updates to the baseline signal level when it is determined that the detected signal is due to stray capacitance.

3. The method of claim 1, further comprising

developing a baseline signal level for the touch sensor system over a first period of time

developing a profile for the detected signal over a second period of time;

determining a rate of change in the level of the detected signal;

using the rate of change to determine whether the detected signal is due to an impending touch input;

using the rate of change to determine whether the detected signal is due to a stray capacitance

updating the baseline signal level based on the level of the detected signal when it is determined that the detected signal is due to stray capacitance; and

independently tracking signals determined to be due to stray capacitance.

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 determined to be due to an impending touch input and using a second time constant different from the first to filter signals determined to be due to stray capacitance, wherein stray capacitance is not caused by a touch input.

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 that the detected signal is due to stray capacitance, and wherein stray capacitance is not caused by a touch input.

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 based on the touch sensor size in the direction of the determined distance.

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, and wherein the detected signal is determining to be due to stray capacitance if the level of the detected signal does not subsequently exceed the touch threshold.

17. The method of claim 16, wherein the touch threshold is dynamically adjusted based on whether the detected signal is determined to be due to an impending touch input or stray capacitance.

18. The method of claim 1, further comprising using a rate of change of the level of the detected signal to determine whether the detected signal is due to an impending touch input or 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 relative to a baseline signal level generated from the detected signal;

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

in response to the step of determining, using the rate of change and the test touch position to determine whether the detected signal is due to an impending touch input or due to stray capacitance; and

in response to the step of using the rate of change and the test touch position, controlling updates to the baseline signal level.

20. The method of claim 19, wherein when the rate of change exceeds a predetermined value, the detected signal is due to stray capacitance.

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 an impending touch input to the resistive layer based on

analyzing the signals against one or more features characteristic of stray capacitances, including detection of a level of a detected signal being greater than a threshold value that is defined relative to a baseline signal level generated from a detected signal and a related detection of a test touch position; and

in response to the analysis, controlling updates to the baseline signal level.

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.