Capacitance measuring circuit and method

• US 8,207,944 B2
• Filed: 12/19/2006
• Issued: 06/26/2012
• Est. Priority Date: 12/19/2006
• Status: Expired due to Fees

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Geaghan, Bernard O.
• Primary Examiner(s): Patel, Nitin
• Assistant Examiner(s): HARRIS, DOROTHY H
• Application Number:
  US11/612,790
• Publication Number:
  US 20080142281A1
• Time in Patent Office: 2,016 Days
• Field of Search: 345/173
• US Class Current: 345/173
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: G06F 3/0444 using a single conductive element covering the whole sensing surface, e.g. by sensing the electrical current flowing at the corners

Abstract

Disclosed are systems and methods of measuring a plate capacitance, which include accumulating a first signal representative of charge over a plurality of switch-controlled plate charging cycles and accumulating a second signal representative of charge over another plurality of switch-controlled plate discharging cycles. The accumulated first and second signals can then be used to determine the capacitance on the plate. Such systems and methods can be useful in capacitive touch sensing devices such as capacitive buttons and capacitive touch panels.

Claims

1. A method for measuring a capacitance on a plate comprising:

accumulating a first signal representative of charge over a plurality of switch-controlled charging cycles during which fixed frequency positive voltages are applied to the plate;

accumulating a second signal representative of charge over a plurality of switch-controlled discharging cycles during which fixed frequency negative voltages are applied to the plate; and

in separate signal paths, processing the first and second signals and, in response, using the accumulated first and second signals, to determine the capacitance.

2. The method of claim 1, further comprising alternating the charging cycles and discharging cycles, isolating the signals in the signal paths, respectively, and processing the first and second signals nonlinearly by developing the accumulated first and second signals exponentially through circuitry that includes at least one resistive path.

3. The method of claim 1, further comprising using one or more sample integrators to accumulate the first and second signals.

4. The method of claim 3, wherein the first and second signals are accumulated by separate sample integrators.

5. The method of claim 4, further comprising repeating the accumulating steps multiple times, and wherein the separate sample integrators used to accumulate each of the first and second signals are alternated each time the accumulating steps are performed.

6. The method of claim 3, wherein the one or more sample integrators are one or more capacitors.

7. The method of claim 3, wherein the first and second signals are charges.

8. The method of claim 3, wherein the first and second signals are accumulated by the same sample integrator.

9. The method of claim 1, wherein the fixed frequency voltages applied to the plate result in a voltage waveform that is symmetric around a predetermined voltage over a period of time, independent of the capacitance being measured.

10. The method of claim 1, wherein each accumulating step includes a fixed number of cycles.

11. The method of claim 1, wherein each accumulating step is completed upon reaching a predetermined threshold of signal accumulation.

12. The method of claim 1, wherein the charging cycles and discharging cycles are controlled using the same switch.

13. The method of claim 1, wherein the charging cycles and discharging cycles are controlled using different switches.

14. The method of claim 1, wherein the plate comprises a conductive layer covering at least a portion of a touch sensitive area in a touch sensor.

15. The method of claim 1, wherein the plate comprises one of a plurality of elongated conductors disposed over a touch sensitive area in a touch sensor.

16. The method of claim 1, wherein accumulating the first signal occurs on a first capacitor and accumulating the second signal occurs on a second capacitor and further including

accumulating, on the second capacitor, a third signal representative of a charge over a plurality of switch-controlled charging cycles during which fixed frequency positive voltages are applied to the plate;

accumulating, on the first capacitor, a fourth signal representative of a charge over a plurality of switch-controlled charging cycles during which fixed frequency negative voltages are applied to the plate; and

using the accumulated third and fourth signals to determine the capacitance.

17. The method of claim 1, wherein processing the first and second signals includes processing the first and second signals nonlinearly by developing the accumulated first and second signals exponentially by use of resistive circuitry.

18. The method of claim 1, further including processing the first and second signals nonlinearly by processing via signal compensation to account for the signal paths.

19. A method for measuring a capacitance on a plate comprising:

accumulating a first signal representative of charge over a plurality of switch-controlled charging cycles during which fixed frequency positive voltages are applied to the plate;

accumulating a second signal representative of charge over a plurality of switch-controlled discharging cycles during which fixed frequency negative voltages are applied to the plate;

using the accumulated first and second signals to determine the capacitance; and

completing the step of accumulating the first signal prior to beginning the step of accumulating the second signal.

20. A device for measuring capacitance on a plate comprising:

two or more switches having open and closed states and disposed so that a first configuration of open and closed states of the two or more switches applies fixed frequency positive voltage pulses to the plate, and a second different configuration of open and closed states of the two or more switches applies a fixed frequency negative voltage pulses to the plate;

control circuitry disposed to control the two or more switches;

one or more signal accumulators disposed to accumulate and process using a first active circuit, a first signal representative of the capacitance to be measured during charging of the plate under the first configuration of open and closed states, and to accumulate and process using a second active circuit, a second signal representative of the capacitance to be measured during discharging of the plate under the second configuration of open and closed states; and

measuring circuitry, responsive to the first and second active circuits, to determine the capacitance to be measured using the accumulated first signal and the accumulated second signal.

21. The device of claim 20, wherein the two or more switches comprise at least a charge switch and a discharge switch, the first configuration including opening the charge switch and opening the discharge switch, and the second configuration including opening the charge switch and closing the discharge switch.

22. The device of claim 20, wherein the one or more signal accumulators comprise one or more capacitors.

23. The device of claim 20, wherein the plate is an electrode in a capacitive touch sensing device.

24. A capacitive touch panel device comprising:

at least one resistive element disposed in an active area of the touch panel;

a plurality of pre-determined capacitance measurement points, each located on the at least one resistive element such that a touch input in the active area produces a capacitance on the at least one resistive element that can be proportionately measured at each of the plurality of pre-determined capacitance measurement points to determine the touch input location; and

a plurality of capacitive measuring devices, each one of the plurality of capacitive measuring devices electrically connected to a different one of the plurality of pre-determined capacitance measurement points, wherein the capacitive measuring devices are each configured and arranged to measure a signal representative of capacitance at the respective capacitance measurement point under both charging and discharging conditions.

25. The capacitive touch sensor of claim 24, wherein the at least one resistive element is a single resistive layer that is continuous across the active area.

26. The capacitive touch sensor of claim 24, wherein the at least one resistive element comprises an electrically contiguous, segmented resistive layer that substantially covers the active area.

27. The capacitive touch sensor of claim 24, wherein the at least one resistive element comprises a plurality of individual sensing elements that together substantially cover the active area.

28. The capacitive touch sensor of claim 24, wherein the capacitive measuring devices are each configured and arranged, to sense active-circuit outputs respectively for charging and discharging conditions, to measure a signal representative of capacitance at the respective capacitance measurement point under both the charging and discharging conditions.

29. The capacitive touch sensor of claim 24, wherein the at least one resistive element includes one of the following:

a single resistive layer that is continuous across the active area;

an electrically contiguous, segmented resistive layer that substantially covers the active area; and

a plurality of individual sensing elements that together substantially cover the active area.