Capacitive sensor filtering apparatus, method, and system
A transform is used to transform raw capacitive sensor data from the time domain to the frequency or sequency domain. The transformed data falls into several signal bins. The transformed data in at least one of the signal bins is analyzed to determine whether a touch event or release event has occurred.
1. A method for operating a capacitive sensor, comprising:
providing a sensor electrode;
periodically exciting the sensor electrode and thereby periodically generating an electric field about the sensor electrode and a capacitance between the sensor electrode and a reference potential;
periodically determining in the time domain values of capacitance between the sensor electrode and the reference potential;
transforming a rolling sample of the periodically-determined values of capacitance from the time domain to a plurality of data signals in the sequency domain using a Walsh-Hadamard Transform, each of the data signals in a corresponding sequency bin resulting from the Walsh-Hadamard Transform and each of the data signals comprising values related to capacitance for corresponding samples of the rolling sample;
analyzing in the sequency domain at least one of the plurality of data signals in the corresponding sequency bin; and
determining whether a touch event has occurred based on the analysis of the at least one of the plurality of data signals in the corresponding sequency bin;
wherein the rolling sample is an n-point rolling sample including a current sample and n−1 samples immediately preceding the current sample, wherein the n−1 samples includes a last sample, and wherein n is an integer greater than or equal to 2 corresponding to a size of a Walsh matrix used in performance of the Walsh-Hadamard Transform.
2. The method of claim 1 wherein n equals 8.
3. The method of claim 1 wherein a touch event is deemed to have occurred when the difference between a baseline value and the value of the current sample in a sequency bin 0 exceeds a predetermined threshold.
4. The method of claim 1 wherein a touch event is deemed to have occurred when the value of the last sample in a sequency bin 1 is greater than the value of the current sample in the sequency bin 1.
5. The method of claim 4 wherein a release is deemed to have occurred when the value of the current sample in the sequency bin 1 reaches a minimum.
6. The method of claim 1 wherein a touch event is deemed to have occurred when the difference between the value of the current sample in a sequency bin 1 and the value of the previous sample in the sequency bin 1 exceeds a predetermined threshold.
7. The method of claim 1 wherein n equals at least 8 and wherein a touch event is deemed to have occurred when the data signals in sequency bins 1, 2, 3, 6 and 7 are greater than zero and the data signals in sequency bins 4 and 5 are less than zero.
8. The method of claim 1 wherein a touch event is deemed to have occurred when the data signal in a sequency bin 1 crosses zero in the positive direction and then crosses zero in the negative direction.
9. The method of claim 1 wherein a touch event is deemed to have occurred when integration of successive non-negative transformed values of the samples in a sequency bin 1 yields a value that meets or exceeds a predetermined threshold.
10. The method of claim 1 wherein a release event is deemed to have occurred when the absolute value of the value yielded by integration of successive non-positive transformed values of the samples in a sequency bin 1 meets or exceeds a predetermined threshold.
11. The method of claim 1 wherein a touch event is deemed to have occurred when the sum of the values of the samples in a sequency bin 1 exceeds a system noise level by a predetermined multiplier.
12. The method of claim 11 wherein the multiplier is less than one.
13. The method of claim 11 wherein the system noise level is determined by summing the absolute values of the samples in sequency bins 4, 5, 6, and 7.
14. The method of claim 1 wherein a touch event is deemed to have occurred when the absolute value of the value yielded by integration of successive non-positive transformed values of the samples in a sequency bin 1 meets or exceeds a predetermined threshold that is a function of a system noise level.
15. The method of claim 1 wherein a touch event is deemed to have occurred when a system noise level increases above a predetermined threshold.
16. The method of claim 1 wherein the Walsh-Hadamard transform is a Fast Walsh-Hadamard transform.
17. The method of claim 1 wherein the values related to capacitance comprise counts.