PULSED CAPACITANCE MEASURING CIRCUITS AND METHODS

• US 20090167325A1
• Filed: 02/07/2008
• Published: 07/02/2009
• Est. Priority Date: 12/28/2007
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Geaghan, Bernard O.
• Granted Patent: US 7,830,157 B2
• Time in Patent Office: Days
• US Class Current: 324/660
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G06F 2203/04107 Shielding in digitiser, i.e. guard or shielding arrangements, mostly for capacitive touchscreens, e.g. driven shields, driven grounds
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• 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
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes
• CPC Class Codes: H03K 17/962 Capacitive touch switches

Abstract

Capacitance measuring circuits and methods apply electrical charge pulses to ramp voltage signals across a voltage threshold, and use the number of pulses to determine the capacitance. Capacitances at multiple locations can be measured by storing a pulse counter value in a register associated with each voltage signal channel as that voltage signal crosses the threshold. Effects of electrode resistance on the capacitance measurements can be mitigated by using charge pulses to ramp the voltage signals and waiting for signal quiescence between pulses.

Claims

1. For use in a device that measures capacitances at each of a plurality of locations by applying an electrical charge to ramp respective voltage signals associated with each of the locations, a method comprising:

incrementally ramping the respective voltage signals using a series of electrical charge pulses to charge each of the capacitances until each of the ramped voltage signals crosses a threshold voltage level, and for each pulse;

incrementing a pulse count value and storing the pulse count value in registers associated with any of the voltage signals determined to have crossed the threshold voltage level after waiting for quiescence in the voltage signals; and

determining the capacitances using the count values stored in the associated registers.

2. The method of claim 1, further comprising repeating the ramping step over multiple ramping cycles.

3. The method of claim 2, wherein the multiple ramping cycles alternate between ramping up the voltage signals across the threshold voltage level and ramping down the voltage signals across the threshold voltage level.

4. The method of claim 1, wherein prior to the step of incrementally ramping the voltage signals, ramping the voltage signals from a low reference voltage level to a high reference voltage level at or near the threshold voltage level.

5. The method of claim 4, further comprising storing in a common register a number of main clock cycles determined to occur during ramping from the low reference voltage level to the high reference voltage level.

6. The method of claim 5, wherein determining the capacitances comprises using the count values stored in the associated registers in combination with the number of main clock cycles stored in the common register.

7. The method of claim 4, wherein a constant current is applied to ramp the voltage signals from the low reference voltage level to the high reference voltage level.

8. The method of claim 4, wherein a pulsed current is applied to ramp the voltage signals from the low reference voltage level to the high reference voltage level.

9. The method of claim 1, wherein each of the plurality of locations is associated with one of a plurality of touch sensor electrodes.

10. For use in a device that measures capacitance of an electrode by applying an electrical charge to the electrode through an electrode resistance to thereby ramp a voltage signal, a method comprising:

accumulating clock cycle counts during an initial ramping of the voltage signal from a first reference level to a second reference level;

incrementally ramping the voltage signal using a series of electrical charge pulses to charge the capacitance until the voltage signal crosses a threshold voltage level, for each pulse incrementing a pulse count value and waiting for quiescence in the voltage signals before initiating another pulse; and

determining the capacitance using the accumulated clock cycle counts and the pulse count value.

11. The method of claim 10, wherein the electrical charge pulses have a pulse duration that is less than an RC time constant associated with the capacitance and the electrode resistance.

12. The method of claim 10, wherein waiting for quiescence in the voltage signals comprises waiting for a duration that is longer than an RC time constant associated with the capacitance and the electrode resistance.

13. The method of claim 10, further comprising performing the recited steps in each of a plurality of measurement channels, each measurement channel associated with a different capacitance measurement location.

14. The method of claim 10, further comprising repeating the step of incrementally ramping the voltage signal over multiple ramping cycles.

15. The method of claim 14, wherein the multiple ramping cycles alternate between ramping up the voltage signal across the threshold voltage level and ramping down the voltage signal across the threshold voltage level.

16. A device that measures capacitances at each of a plurality of locations by applying an electrical charge to ramp respective voltage signals associated with each of the locations, the device comprising:

a ramp control circuit to incrementally ramp the voltage signals using a series of electrical charge pulses, thereby charging the respective capacitances;

a pulse counter to increment a pulse count value after each electrical charge pulse; and

a plurality of registers, each register associated with one of the voltage signals to store the pulse count value when the associated voltage signal is determined to have crossed the threshold voltage level after waiting for voltage signal quiescence.

17. For use in a device that measures capacitance of an electrode by applying an electrical charge to the electrode through an electrode resistance, a method of mitigating effects of the electrode resistance on the capacitance measurement, the method comprising:

using a series of pulses to charge the capacitance and ramp a resulting voltage signal to a threshold voltage level;

waiting between each pulse for quiescence in the voltage signal voltage; and

determining the capacitance using the number of pulses needed for the voltage signal to reach the threshold voltage level.