Method for compensating for differences in capacitance between multiple capacitive sensors

• US 8,072,230 B1
• Filed: 02/06/2009
• Issued: 12/06/2011
• Est. Priority Date: 04/01/2005
• Status: Expired due to Fees

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Seguine, Dennis
• Primary Examiner(s): NATALINI, JEFF WILLIAM
• Assistant Examiner(s): Zhu, John
• Application Number:
  US12/367,279
• Time in Patent Office: 1,033 Days
• Field of Search: 324/669, 324/665, 324/672
• US Class Current: 324/672
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G01R 35/00 Testing or calibrating of apparatus covered by the other groups of this subclass
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

Disclosed is method for compensating for variation in the capacitance between multiple capacitive sensors. Prior to sensing operations, baseline capacitance values can be acquired for all sensors. A correction factor can be calculated based on such baseline values. During sensing operations (run-time), variations in capacitance from baseline values can be modified by appropriate correction factors. Sensitivity between sensors can thus be made more uniform.

Claims

1. A method for compensating for differences in capacitance between each of a plurality of capacitive sensors that have a baseline capacitance absent an input event, the method comprising:

establishing a baseline capacitance value for each capacitive sensor;

generating a correction factor for each capacitive sensor, the correction factor based on a relative difference in the baseline capacitance values;

acquiring run-time capacitance values by exposing each capacitive sensor to input events; and

comparing a run-time capacitance value of each capacitive sensor to the baseline capacitance value of each capacitive sensor to generate a compensated capacitance value for each capacitive sensor.

2. The method of claim 1, wherein:

establishing the baseline capacitance value for each capacitive sensor includes repeatedly charging and discharging the capacitance of each capacitive sensor over a predetermined time period to generate a count value for each capacitive sensor.

3. The method of claim 1, wherein:

establishing the baseline capacitance value for each capacitive sensor includes sequentially coupling each capacitive sensor to a common node charged by a constant current source.

4. The method of claim 1, wherein:

establishing the baseline capacitance value for each capacitive sensor includes generating a baseline count value for each capacitive sensor; and

generating the correction factor for each capacitive sensor includes;

determining a maximum baseline count value from among all the baseline count values.

5. The method of claim 4, wherein:

acquiring the run-time capacitance values includes generating a run-time count value for each capacitive sensor.

6. The method of claim 1, further comprising applying the correction factor to each capacitive sensor, wherein:

establishing the baseline capacitance value for each capacitive sensor includes establishing baseline count values for each capacitive sensor;

acquiring the run-time capacitance values includes establishing run-time count values for each capacitive sensor; and

generating the correction factor for each capacitive sensor includes

determining a maximum baseline count value from among all the baseline count values,

dividing the maximum baseline count value by the baseline count value of the capacitive sensor, wherein the applying the correction factor to each capacitive sensor includes;

subtracting the run-time count from the baseline count of each capacitive sensor, and

multiplying the difference by the correction factor for the capacitive sensor.

7. The method of claim 1, further including:

comparing each compensated capacitance value to a predetermined threshold value to determine if the corresponding capacitive sensor has experienced an input event.

8. The method of claim 1, wherein:

acquiring the run-time capacitance values includes acquiring at least two input values for each capacitive sensor; and

calculating a centroid value from the input values of the capacitive sensors.

9. The method of claim 8, further including:

determining a position of a switch by comparing the centroid value to a predetermined position value.

10. The method of claim 1, wherein:

establishing the baseline capacitance value for each capacitive sensor includes generating a baseline count value for each capacitive sensor; and

generating the correction factor for each capacitive sensor includes determining an average baseline count value from all the baseline count values.

11. A method comprising:

providing a current to each of a plurality of capacitive sensors;

generating a baseline count value for each of the plurality of capacitive sensors and a subsequent run-time count value for each of the plurality of capacitive sensors, each count value corresponding to a rate at which a corresponding capacitive switch charges;

generating a compensation value for each of the plurality of capacitive sensors based on a relationship between a maximum baseline count value and the baseline count value of each of the plurality of capacitive sensors;

storing the compensation value corresponding to each of the plurality of capacitive sensors; and

modifying a difference between the baseline count value and the run-time count value for each of the plurality of capacitive sensors by the compensation value for the sensors.

12. The method of claim 11 further comprising dividing the maximum baseline count value by the baseline count value of each of the plurality of capacitive sensors to generate the compensation value for each of the plurality of capacitive sensors and modifying the difference between the baseline count value and the run-time count value for each of the plurality of capacitive sensors by the compensation value to generate a modified sense value.

13. A method comprising:

controlling each of a plurality of input switches by a different input control signal, each of the plurality of input switches having a signal path coupled between a corresponding capacitance source to a common node, each capacitance source having a substantially constant value in an initial mode and subject to potential variation in a run-time mode;

coupling a common current source to the common node;

comparing capacitance values corresponding to each capacitance source to a reference value in the initial mode; and

subsequently comparing capacitance values corresponding to each capacitance source to the reference value in the run-time mode.

14. The method of claim 13 further comprising enabling a common node set switch, coupled between the common node and a predetermined voltage node.

15. The method of claim 14 wherein the common current source is coupled between a high power supply node and the common node;

and the common node set switch is coupled between the common node and a low power supply node.

16. The method of claim 13 including generating a count value corresponding to each capacitance source based on subsequently comparing the capacitance values.

17. The method of claim 16 further comprising generating count values corresponding to each capacitance source in the initial mode and generating at least one correction factor from at least one of the initial mode count values that compensates for differences between the count values of different capacitance sources generated in the initial mode.