Touch detection techniques for capacitive touch sense systems

• US 8,248,084 B2
• Filed: 03/14/2011
• Issued: 08/21/2012
• Est. Priority Date: 03/31/2006
• Status: Expired due to Fees

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Bokma, Louis W., Page, Andrew C., Seguine, Dennis R.
• Primary Examiner(s): NATALINI, JEFF WILLIAM
• Application Number:
  US13/047,620
• Publication Number:
  US 20120043976A1
• Time in Patent Office: 526 Days
• Field of Search: 324/658, 345/173
• US Class Current: 324/658
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input
• CPC Class Codes: H03K 2217/96066 Thumbwheel, potentiometer, scrollbar or slider simulation by touch switch
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe

Abstract

A technique for recognizing and rejecting false activation events related to a capacitance sense interface includes measuring a capacitance value of a capacitance sense element. The measured capacitance value is analyzed to determine a baseline capacitance value for the capacitance sensor. The capacitance sense interface monitors a rate of change of the measured capacitance values and rejects an activation of the capacitance sense element as a non-touch event when the rate of change of the measured capacitance values have a magnitude greater than a threshold level, indicative of a maximum rate of change of a touch event.

Claims

1. A method, comprising:

measuring capacitance values of a capacitance sense element;

analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element;

monitoring a rate of change of the measured capacitance values; and

rejecting an activation of the capacitance sense element as a non-touch event if conditions indicative of the non-touch event are sensed, wherein the conditions indicative of the non-touch event comprise the rate of change of the measured capacitance values having a magnitude greater than a threshold level, wherein the threshold level is indicative of a maximum rate of change of a touch event.

2. The method of claim 1, wherein the threshold level is a positive threshold and the maximum rate of change is a positive, maximum rate of change of the touch event.

3. The method of claim 1, wherein the threshold level is a negative threshold, and the maximum rate of change is a negative, maximum rate of change of the touch event.

4. The method of claim 1, wherein rejecting the actuation comprises masking all actuations of the capacitance sense element for a period after sensing the conditions indicative of the non-touch event.

5. The method of claim 1, further comprising determining if the non-touch event is an electrostatic discharge (ESD) event, wherein the determining comprises:

detecting that the non-touch event has conditions indicative of an ESD event, wherein the conditions indicative of the ESD event include at least one of the following;

the rate of change of the measured capacitance values turning positive and having a first magnitude greater than a positive threshold,

the rate of change of the measured capacitance values turning negative having a second magnitude greater than a negative threshold, and

the measured capacitance values crossing an ESD threshold below the baseline capacitance value.

6. The method of claim 5, wherein the rejecting the actuation comprises masking all actuations of the capacitance sense element for a period after sensing the conditions indicative of the ESD event.

7. The method of claim 1, further comprising:

analyzing the measured capacitance values to determine that the capacitance sense element has been activated when the measured capacitance values cross an activation threshold;

and

analyzing the measured capacitance values to determine that the capacitance sense element has been deactivated when the measured capacitance values cross a deactivation threshold, wherein the deactivation threshold is less than the activation threshold.

8. The method of claim 7, wherein analyzing the measured capacitance value to determine the baseline capacitance value further comprises holding the baseline capacitance value substantially steady during activation events of the capacitance sense element.

9. The method of claim 1, further comprising updating the baseline capacitance value based at least in part upon a weighted moving average of the measured capacitance values.

10. The method of claim 1, further comprising:

analyzing the measured capacitance values to determine whether the measured capacitance values drop below the baseline capacitance value by a threshold value for a predetermined period; and

updating the baseline capacitance value based on the measured capacitance values measured after the measured capacitance values dropped below the negative value. Zone Name;

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11. An apparatus, comprising:

a capacitance sense interface including a capacitance sense element having a baseline capacitance value associated with the capacitance sense element;

a capacitance measurement circuit coupled to the capacitance sense interface to measure capacitance values of the capacitance sense element and output signal indicative of the measured capacitance values; and

event logic coupled to analyze the signals to monitor a rate of change of the measured capacitance values and to reject an activation of the capacitance sense element as a non-touch event if conditions indicative of the non-touch event are sensed, wherein the conditions indicative of the non-touch event comprise the rate of change of the measured capacitance values having a magnitude greater than a threshold level, wherein the threshold level is indicative of a maximum rate of change of a touch event.

12. The apparatus of claim 11, wherein the event logic is further configured to determine if the non-touch event is an electrostatic discharge (ESD) event when at least one of the following conditions are detected:

the rate of change of the measured capacitance values turning positive and having a first magnitude greater than a positive threshold, the rate of change of the measured capacitance values turning negative having a second

magnitude greater than a negative threshold, and

the measured capacitance values crossing an ESD threshold below the baseline capacitance value.

13. The apparatus of claim 11, further comprising hysteresis logic coupled to analyze the signals to determine whether the capacitance sense element has been activated based on whether the measured capacitance values cross an activation threshold value and to determine whether the capacitance sensor has been deactivated based on whether the measured capacitance values cross a deactivation threshold value, wherein the deactivation threshold is lower than the activation threshold value.

14. The apparatus of claim 11, further comprising baseline logic coupled to analyze the signals to determine whether the baseline capacitance value associated with the capacitance sense element has drifted and to update the baseline capacitance value.