Normalizing capacitive sensing measurements to reduce effects of low ground mass and noise
An input device can be capacitive coupled to earth ground and noise sources which can interfere with the ability of the input device to detect an input object. To mitigate the effects from these capacitive couplings, the input device drives a modulated capacitive sensing signal onto a plurality of sensor electrodes in parallel. The sensor electrodes are coupled to respective analog front ends (AFEs) which generate an output signal for each of the sensor electrodes. The input device combines the output signals for the sensor electrodes to generate a normalization value. For example, the output signals may be digital values which are summed by the input device to generate the normalization value. The input device then divides each of the output signals by the normalization value to result in a normalized signal corresponding to each of the sensor electrodes.
1. An input device, comprising:
a plurality of sensor electrodes arranged in rows and columns in a sensing region of the input device, wherein each row and column comprises at least three sensor electrodes; and
a processing system coupled to the plurality of sensor electrodes, the processing system configured to:
drive a first plurality of capacitive sensing signals in parallel onto a first group of the plurality of sensor electrodes;
generate a first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes in response to driving the first plurality of capacitive sensing signals;
normalize a first capacitive measurement corresponding to one of the plurality of sensor electrodes in the first group using a combination of the first plurality of capacitive measurements; and
determine a location of an input object at least partially based on at least one of:
a comparison of the normalized first capacitive measurement and a detection threshold, wherein the normalized first capacitive measurement is adjusted at least partially based on a number of non-zero capacitive measurements in the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals; and
a comparison of the normalized first capacitive measurement and the detection threshold, wherein the detection threshold is adjusted at least partially based on at least a portion of the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals.
2. The input device of claim 1 , wherein the combination of the first plurality of capacitive measurements is a summation of all of the first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes.
3. The input device of claim 1 , wherein the processing system is configured to:
normalize a second capacitive measurement corresponding to a second one of the plurality of sensor electrodes in the first group using the combination of the first plurality of capacitive measurements; and
determine the location of the input object based on the normalized second capacitive measurement.
4. The input device of claim 3 , wherein determining the location of the input object based on the first and second capacitive measurements comprises:
detecting a change of capacitance based on the normalized first and second capacitive measurements.
5. The input device of claim 1 , wherein the processing system is configured to perform absolute capacitive sensing to generate the first plurality of capacitive measurements for the first group of the plurality of sensor electrodes, and wherein the first plurality of capacitive measurements comprise delta signals derived from comparing output signals generated by analog front ends to baseline measurements.
6. The input device of claim 1 , wherein the plurality of sensor electrodes are arranged in a rectangular array on a common plane.
7. The input device of claim 1 , wherein the processing system is configured to:
drive a second plurality of capacitive sensing signals in parallel onto a second group of the plurality of sensor electrodes, wherein the second group of the plurality of sensor electrodes are in the sensing region;
generate a second plurality of capacitive measurements corresponding to the second group of the plurality of sensor electrodes caused by driving the second plurality of capacitive sensing signals;
normalize a second capacitive measurement corresponding to one of the plurality of sensor electrodes in the second group using a combination of the second plurality of capacitive measurements; and
determine the location of the input object based on the normalized second capacitive measurement.
8. A processing system, comprising:
a sensor module configured to drive a first plurality of capacitive sensing signals in parallel onto a first group of a plurality of sensor electrodes, the plurality of sensor electrodes are arranged in rows and columns, each row and column comprises at least three sensor electrodes;
a determination module configured to:
generate a first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes in response to driving the first plurality of capacitive sensing signals;
normalize a first capacitive measurement corresponding to one of the plurality of sensor electrodes in the first group using a combination of the first plurality of capacitive measurements; and
determine a location of an input object at least partially based on at least one of:
a comparison of the normalized first capacitive measurement and a detection threshold, wherein the normalized first capacitive measurement is adjusted at least partially based on a number of non-zero capacitive measurements in the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals; and
a comparison of the normalized first capacitive measurement and the detection threshold, wherein the detection threshold is adjusted at least partially based on at least a portion of the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals.
9. The processing system of claim 8 , wherein the combination of the first plurality of capacitive measurements is a summation of all of the first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes.
10. The processing system of claim 8 , wherein the determination module is configured to:
normalize a second capacitive measurement corresponding to a second one of the plurality of sensor electrodes in the first group using the combination of the first plurality of capacitive measurements; and
determine the location of the input object based on the normalized second capacitive measurement.
11. The processing system of claim 10 , wherein determining the location of the input object based on the first and second capacitive measurements comprises:
detecting a change of capacitance based on the normalized first and second capacitive measurements.
12. The processing system of claim 8 , wherein the determination module is configured to perform absolute capacitive sensing to generate the first plurality of capacitive measurements for the first group of the plurality of sensor electrodes.
13. The processing system of claim 8 , wherein determining the location of the input object comprises:
determining the location of the input object within a sensing region established by the plurality of sensor electrodes.
14. The processing system of claim 8 , wherein the sensor module is configured to drive a second plurality of capacitive sensing signals in parallel onto a second group of the plurality of sensor electrodes, wherein the second group of the plurality of sensor electrodes are in a sensing region along with the first group of the plurality of sensor electrodes;
wherein the determination module is configured to:
generate a second plurality of capacitive measurements corresponding to the second group of the plurality of sensor electrodes caused by driving the second plurality of capacitive sensing signals;
normalize a second capacitive measurement corresponding to one of the plurality of sensor electrodes in the second group using a combination of the second plurality of capacitive measurements; and
determine the location of the input object based on the normalized second capacitive measurement.
15. A method, comprising:
driving a first plurality of capacitive sensing signals in parallel onto a first group of a plurality of sensor electrodes, wherein the plurality of sensor electrodes are in a sensing region of an input device and are arranged in rows and columns, each row and column having at least three sensor electrodes;
generating a first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes in response to driving the first plurality of capacitive sensing signals;
normalizing a first capacitive measurement corresponding to one of the plurality of sensor electrodes in the first group using a combination of the first plurality of capacitive measurements; and
determining a location of an input object at least partially based on at least one of:
a comparison of the normalized first capacitive measurement and a detection threshold, wherein the normalized first capacitive measurement is adjusted at least partially based on a number of non-zero capacitive measurements in the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals; and
a comparison of the normalized first capacitive measurement and the detection threshold, wherein the detection threshold s adjusted at least partially based on at least a portion of the first plurality of capacitive measurements generated in response to driving the first plurality of capacitive sensing signals.
16. The method of claim 15 , wherein the combination of the first plurality of capacitive measurements is a summation of all of the first plurality of capacitive measurements corresponding to the first group of the plurality of sensor electrodes.
17. The method of claim 15 , further comprising:
normalizing a second capacitive measurement corresponding to a second one of the plurality of sensor electrodes in the first group using the combination of the first plurality of capacitive measurements; and
detecting the location of the input object based on the normalized second capacitive measurement.
18. The method of claim 15 , wherein generating the first plurality of capacitive measurements further comprises:
performing absolute capacitive sensing to generate the first plurality of capacitive measurements for the first group of the plurality of sensor electrodes.
19. The method of claim 15 , wherein the plurality of sensor electrodes are arranged in a rectangular array on a common plane.
20. The method of claim 15 , further comprising:
driving a second plurality of capacitive sensing signals in parallel onto a second group of the plurality of sensor electrodes, wherein the second group of the plurality of sensor electrodes are in the sensing region;
generating a second plurality of capacitive measurements corresponding to the second group of the plurality of sensor electrodes caused by driving the second plurality of capacitive sensing signals;
normalizing a second capacitive measurement corresponding to one of the plurality of sensor electrodes in the second group using a combination of the second plurality of capacitive measurements; and
detecting the location of the input object based on the normalized second capacitive measurement.