Capacitive sensing
A multi-channel capacitive sensor comprises a sample capacitor having first and second terminals, 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. The control circuit is operable to apply a drive signal 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.
1. A multi-channel capacitive sensor comprising:
a sample capacitor coupled to a first sense electrode and to a second sense electrode, the first sense electrode associated with a first sense capacitor and a first diode and the second sense electrode associated with a second sense capacitor and a second diode, the first sense capacitor being distinct from the second sense capacitor, the sample capacitor configured for use in measuring a capacitance of the first sense capacitor and in measuring a capacitance of the second sense capacitor; and
a control circuit programmed to:
apply, at a first time via a drive channel, a first drive signal to the sample capacitor;
apply, while allowing the first drive signal received from the sample capacitor to be conducted by the first diode to the first sense electrode associated with the first sense capacitor, a first signal to the second sense capacitor to prevent the first drive signal from being conducted by the second diode to the second sense electrode associated with the second sense capacitor;
apply, at a second time via the drive channel, a second drive signal to the sample capacitor of the multi-channel capacitive sensor; and
apply, while allowing the second drive signal received from the sample capacitor to be conducted by the second diode to the second sense electrode associated with the second sense capacitor, a second signal to the first sense capacitor to prevent the second drive signal from being conducted by the first diode to the first sense electrode associated with the first sense capacitor.
2. The multi-channel capacitive sensor of claim 1 ,
wherein:
the sample capacitor comprises a first terminal and a second terminal;
the first diode comprises a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to the first sense electrode; and
the second diode comprises a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to the second sense electrode.
3. The multi-channel capacitive sensor of claim 2 , wherein:
applying the first signal to prevent the first drive signal from being conducted to the second sense electrode comprises applying a first bias signal to the second terminal of the second diode to prevent the second diode from conducting the first drive signal to the second sense electrode; and
applying the second signal to prevent the second drive signal from being conducted to the first sense electrode comprises applying a second bias signal to the second terminal of the first diode to prevent the first diode from conducting the second drive signal to the first sense electrode.
4. The multi-channel capacitive sensor of claim 1 , wherein the control circuit is further operable to:
temporarily couple the first sense electrode to a system reference potential so as to selectively remove charge coupled to the first sense electrode by the first drive signal; and
temporarily couple the second sense electrode to a system reference potential so as to selectively remove charge coupled to the second sense electrode by the second 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 the first and second terminals of the sample capacitor to remove charge accumulated on the sample capacitor by one or more of the first and second drive signals.
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 one or more of the first and second drive signals.
7. The multi-channel capacitive sensor of claim 6 , wherein:
the sample capacitor comprises a first terminal and a second terminal; and
the control circuit is operable to couple the second terminal of the sample capacitor to a system reference potential while measuring the signal indicative of an amount of charge coupled to the sample capacitor by one or more of the first and second drive signals.
8. The multi-channel capacitive sensor of claim 6 , wherein one or more of the first and second drive signals each comprises one or more voltage transitions.
9. The multi-channel capacitive sensor of claim 8 , wherein:
the sample capacitor comprises a first terminal and a second terminal; and
the control circuit is operable to couple the second terminal of the sample capacitor to a 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 one or more of the first and second drive signals 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 one or more of the first and second drive signals 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 at least one of the first and second sense electrode and 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 2 , wherein:
the multi-channel capacitive sensor further comprises 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; and
the control circuit is further operable to apply a third signal to the second terminal of the third diode to prevent the third diode from conducting the drive signal to the third sense electrode.
15. The multi-channel capacitive sensor of claim 1 , wherein the control circuit comprises a microcontroller.
16. A device comprising:
one or more processors; and
one or more memory units coupled to the one or more processors, the one or more memory units collectively storing logic configured to, when executed by the one or more processors, cause the one or more processors to perform operations comprising:
applying, at a first time via a drive channel, a first drive signal to a sample capacitor of a multi-channel capacitive sensor, the sample capacitor coupled to a first sense electrode and to a second sense electrode, the first sense electrode associated with a first sense capacitor and a first diode and the second sense electrode associated with a second sense capacitor and a second diode, the first sense capacitor being distinct from the second sense capacitor, the sample capacitor configured for use in measuring a capacitance of the first sense capacitor and in measuring a capacitance of the second sense capacitor;
applying, while allowing the first drive signal received from the sample capacitor to be conducted by the first diode to the first sense electrode associated with the first sense capacitor, a first signal to the second sense capacitor to prevent the first drive signal from being conducted by the second diode to the second sense electrode associated with the second sense capacitor;
applying, at a second time via the drive channel, a second drive signal to the sample capacitor of the multi-channel capacitive sensor; and
applying, while allowing the second drive signal received from the sample capacitor to be conducted by the second diode to the second sense electrode associated with the second sense capacitor, a second signal to the first sense capacitor to prevent the second drive signal from being conducted by the first diode to the first sense electrode associated with the first sense capacitor.
17. The device of claim 16 , wherein:
the device comprises a receptacle for housing a variable quantity of a material; and
the multi-channel capacitive sensor is arranged so that a capacitance of the first sense electrode depends on a level of the material in the receptacle and a 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.
18. The device of claim 16 , wherein the circuitry is operable to perform operations comprising measuring a signal indicative of an amount of charge coupled to the sample capacitor by the drive signal.
19. A method comprising:
applying, at a first time via a drive channel, a first drive signal to a sample capacitor of a multi-channel capacitive sensor, the sample capacitor coupled to a first sense electrode and to a second sense electrode, the first sense electrode associated with a first sense capacitor and a first diode and the second sense electrode associated with a second sense capacitor and a second diode, the first sense capacitor being distinct from the second sense capacitor, the sample capacitor configured for use in measuring a capacitance of the first sense capacitor and in measuring a capacitance of the second sense capacitor;
applying, while allowing the first drive signal received from the sample capacitor to be conducted by the first diode to the first sense electrode associated with the first sense capacitor, a first signal to the second sense capacitor to prevent the first drive signal from being conducted by the second diode to the second sense electrode associated with the second sense capacitor;
applying, at a second time via the drive channel, a second drive signal to the sample capacitor of the multi-channel capacitive sensor; and
applying, while allowing the second drive signal received from the sample capacitor to be conducted by the second diode to the second sense electrode associated with the second sense capacitor, a second signal to the first sense capacitor to prevent the second drive signal from being conducted by the first diode to the first sense electrode associated with the first sense capacitor.
20. The method of claim 19 , further comprising measuring a signal indicative of an amount of charge coupled to the sample capacitor by the drive signal.