Capacitive Sensing

• US 20100219846A1
• Filed: 03/02/2009
• Published: 09/02/2010
• Est. Priority Date: 03/02/2009
• Status: Active Grant

• Current Assignee: Neodrón Limited (Atlantic IP Services Limited)
• Original Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Inventors: Dubery, John Stanley
• Granted Patent: US 8,237,456 B2
• Time in Patent Office: Days
• US Class Current: 324/686
• CPC Class Codes: G01D 1/00 Measuring arrangements giving results other than momentary value of variable, of general application G01D3/00 takes precedence; in tariff metering apparatus G01D4/00; transducers not specially adapted for a specific variable G01D5/00
• CPC Class Codes: G01D 15/00 Component parts of recorders for measuring arrangements not specially adapted for a specific variable
• CPC Class Codes: G01D 21/00 Measuring or testing not otherwise provided for
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 29/0878 Sensors; antennas; probes; detectors wave guide measuring sections G01R1/24
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

A multi-channel capacitive sensor for measuring the capacitances of a plurality of sense electrodes to a system reference potential. The sensor comprises a sample capacitor having a first terminal and a second terminal, a first diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a first sense electrode, and a second diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a second sense electrode. The sample capacitor and diodes are coupled to a control circuit, e.g. implemented in a microcontroller. The control circuit is operable to apply a drive signal, e.g. a series of voltage pulses, to the first terminal of the sample capacitor while simultaneously applying a bias signal to the second terminal of one or other of the diodes to prevent the diode from conducting the drive signal. Thus charge transfer techniques can be used to measure multiple capacitances while sharing a common sample capacitor. This helps reduce inter-channel drift. Further measurement channels may be added by providing further diodes and corresponding sense electrodes. With three or more channels the scheme requires only one additional control circuit connection per additional channel.

Claims

1. A multi-channel capacitive sensor for measuring the capacitances of a plurality of sense electrodes to a system reference potential, the sensor comprising:

a sample capacitor having a first terminal and a second terminal;

a first diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a first sense electrode;

a second diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a second sense electrode; and

a control circuit operable to apply a drive signal to the first terminal of the sample capacitor and to apply a bias signal to the second terminal of the first and/or second diodes so as to selectively prevent the first and/or second diodes from conducting the drive signal.

2. The multi-channel capacitive sensor of claim 1, wherein the first terminals of the diodes are anodes and second terminals of the diodes are cathodes and the drive signal comprises a positive-going signal relative to the system reference potential.

3. The multi-channel capacitive sensor of claim 1, wherein the first terminals of the diodes are cathodes and second terminals of the diodes are anodes and the drive signal comprises a negative-going signal relative to the system reference potential.

4. The multi-channel capacitive sensor of claim 1, wherein the control circuit is further operable to temporarily couple the first and/or second sense electrodes to the system reference potential so as to selectively remove charge coupled to the first and/or second sense electrodes by the drive signal.

5. The multi-channel capacitive sensor of claim 1, wherein the control circuit is further operable to temporarily apply a common reset signal to both terminals of the sample capacitor to remove charge accumulated on the sample capacitor by the drive signal.

6. The multi-channel capacitive sensor of claim 1, wherein the control circuit is further operable to measure a signal indicative of an amount of charge coupled to the sample capacitor by the drive signal.

7. The multi-channel capacitive sensor of claim 6, wherein the control circuit is operable to couple the second terminal of the sample capacitor to the system reference potential while measuring the signal indicative of an amount of charge coupled to the sample capacitor by the drive signal.

8. The multi-channel capacitive sensor of claim 1, wherein the drive signal comprises one or more voltage transitions.

9. The multi-channel capacitive sensor of claim 8, wherein the control circuit is operable to couple the second terminal of the sample capacitor to the system reference potential after each voltage transition.

10. The multi-channel capacitive sensor of claim 8, wherein the signal indicative of an amount of charge coupled to the sample capacitor by the drive signal is a measure of the voltage on the sample capacitor after the controller has applied a predetermined number of voltage transitions.

11. The multi-channel capacitive sensor of claim 8, wherein the signal indicative of an amount of charge coupled to the sample capacitor by the drive signal is a count of the number of voltage transitions applied by the control circuit before a measure of the voltage on the sample capacitor exceeds a measurement threshold.

12. The multi-channel capacitive sensor of claim 1, wherein the control circuit is further operable to compare a measurement of the capacitance of the first sense electrode with a measurement of the capacitance of the second sense electrode.

13. The multi-channel capacitive sensor of claim 1, wherein the control circuit is further operable to provide an output signal indicative of a measurement of the capacitance of the first and/or second sense electrode and/or a signal indicative of a comparison of a measurement of the capacitance of the first sense electrode with a measurement of the capacitance of the second sense electrode.

14. The multi-channel capacitive sensor of claim 1, further comprising a third diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a third sense electrode, wherein the control circuit is further operable to apply a bias signal to the second terminal of the third diode so as to selectively prevent the third diode from conducting the drive signal.

15. The multi-channel capacitive sensor of claim 1, further comprising at least one further diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a one of a corresponding at least one further sense electrode wherein the control circuit is further operable to apply a bias signal to one or more of the at least one further diode so as to selectively prevent the at least one further diode from conducting the drive signal.

16. The multi-channel capacitive sensor of claim 1, wherein the control circuit comprises a microcontroller.

17. A device comprising a multi-channel sensor according to claim 1.

18. The device of claim 17, wherein the device comprises a receptacle for housing a variable quantity of a material, and the multi-channel sensor is arranged so that the capacitance of the first sense electrode depends on a level of the material in the receptacle and the capacitance of the second sense electrode does not depend on a level of the material in the receptacle, whereby a measurement of the capacitance of the second sense electrode provides a reference measurement for comparison with a measurement of the capacitance of the first sense electrode.

19. A method of measuring the capacitances of a plurality of sense electrodes to a system reference potential, the method comprising:

providing a sample capacitor having a first terminal and a second terminal;

providing a first diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a first sense electrode;

providing a second diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a second sense electrode;

applying a drive signal to the first terminal of the sample capacitor;

and applying a bias signal to the second terminal of the first and/or second diodes so as to selectively prevent the first and/or second diodes from conducting the drive signal.