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 capacitive sensor apparatus comprising:
a sensor electrode; and
a control circuit electrically connected to the sensor electrode, the control circuit configured to:
periodically excite the sensor electrode and thereby periodically generate an electric field about the sensor electrode and a capacitance between the sensor electrode and a reference potential;
periodically determine in the time domain values of capacitance between the sensor electrode and the reference potential;
continuously transform 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 Fourier Transform, each of the data signals in a corresponding frequency bin resulting from the Fourier Transform and each of the data signals comprising values related to capacitance for corresponding samples of the rolling sample;
analyze in the frequency domain at least one of the plurality of data signals in the corresponding frequency bin; and
determine whether a touch event has occurred based on the analysis 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 Fourier Transform yields n frequency bins numbered 0 through n−1.
2. The apparatus 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 apparatus 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 apparatus 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 apparatus 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 apparatus 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 apparatus 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 apparatus 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 apparatus 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 apparatus of claim 9 wherein the multiplier is less than one.
11. The apparatus 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 apparatus 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 apparatus 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 apparatus of claim 1 wherein a touch event is deemed to have occurred when a system noise level increases above a predetermined threshold.
15. The apparatus 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 apparatus of claim 1 wherein the Fourier transform is a Fast Fourier transform.
17. The apparatus of claim 1 wherein the values related to capacitance comprise counts.