Capacitive sensor filtering apparatus, method, and system
A transform is used to transform raw 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;
continuously transforming a rolling sample of the periodically-determined values of capacitance from the time domain to a plurality of data signals in the frequency domain using a transform, each of the data signals in a corresponding frequency bin resulting from the transform and each of the data signals comprising values related to capacitance for corresponding samples of the rolling sample;
analyzing in the frequency domain at least one of the plurality of data signals in the corresponding frequency bin; and
determining whether a touch event has occurred based on the analysis in the frequency domain of the at least one of the plurality of data signals in the corresponding frequency 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 include a last sample, wherein n=2 x and x is an integer greater than or equal to 2, and wherein performance of the transform yields n frequency bins numbered 0 through n−1.
2. 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 frequency bin 0 exceeds a predetermined threshold.
3. The method of claim 1 wherein a touch event is deemed to have occurred when the value of the last sample in a frequency bin 1 is greater than the value of the current sample in the frequency bin 1 .
4. The method of claim 3 wherein a release is deemed to have occurred when the value of the current sample in the frequency bin 1 reaches a minimum.
5. 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 frequency bin 1 and the value of the previous sample in the frequency bin 1 exceeds a predetermined threshold.
6. The method of claim 1 wherein a touch event is deemed to have occurred when the data signal in a frequency bin 1 crosses zero in the positive direction and then crosses zero in the negative direction.
7. 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 frequency bin 1 yields a value that meets or exceeds a predetermined threshold.
8. 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 frequency bin 1 meets or exceeds a predetermined threshold.
9. 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 frequency bin 1 exceeds a system noise level by a predetermined multiplier.
10. The method of claim 9 wherein the multiplier is less than one.
11. The method of claim 9 wherein n is greater than or equal to 4 and the system noise level is determined by summing the absolute values of frequency bins n/2 through n−1.
12. 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 frequency bin 1 meets or exceeds a predetermined threshold that is a function of a system noise level.
13. The method of claim 12 wherein n is greater than or equal to 4 and the system noise level is determined by summing the absolute values of frequency bins n/2 through n−1.
14. The method of claim 1 wherein a touch event is deemed to have occurred when a system noise level increases above a predetermined threshold.
15. The method of claim 14 wherein n is greater than or equal to 4 and the system noise level is determined by summing the absolute values of frequency bins n/2 through n−1.
16. The method of claim 1 wherein the values related to capacitance comprise counts.