Capacitance measurement
Embodiments of the present invention may provide a method of measuring an unknown capacitance of a device. The method may comprise the steps of driving a test signal to a circuit system that includes a current divider formed by the device with unknown capacitance and a reference capacitor; mirroring a current developed in the reference capacitor to a second circuit system that includes a measurement impedance; measuring a voltage within the second circuit system; and deriving a capacitance of the unknown capacitance based on the measured voltage with reference to a capacitance of the reference capacitor and the measurement impedance.
1. A method for measuring a capacitance of a capacitive device, comprising:
applying a first current to a first circuit system comprising a current divider formed by the capacitive device and a reference capacitor;
mirroring a second current developed in the reference capacitor to a second circuit system that includes a measurement impedance;
measuring a voltage generated across the measurement impedance; and
deriving the capacitance of the capacitive device unknown capacitance based on the measured voltage and capacitance value of the reference capacitor and an impedance value of the measurement impedance.
2. The method of claim 1 , further comprising digitizing the measured voltage.
3. The method of claim 1 , further comprising:
applying a fourth current to the reference capacitor such that a difference between the fourth current and the second current is positive for predetermined capacitance values of the capacitive device.
4. The method of claim 1 , wherein the capacitive device is a capacitive actuator, and the method further comprises estimating a position of the capacitive actuator from the derived capacitance of the capacitive device.
5. The method of claim 1 , wherein the capacitive device is a capacitive actuator, and the method further comprises correcting a drive signal of the capacitive actuator based on the derived capacitance of the capacitive device.
6. The method of claim 1 , wherein the capacitive device is a capacitive actuator, and the method further comprises:
driving the capacitive actuator in a driving phase with a drive signal according to a desired position of the capacitive actuator, and measuring during a measurement phase the capacitance of the capacitive actuator; and
correcting the drive signal of the capacitive actuator for a subsequent iteration of the driving and measurement phase based on the derived capacitance of the capacitive device.
7. An integrated circuit for measuring a capacitance of a capacitive device, comprising:
a first current source coupled to an output terminal configured for connection to the capacitive device and supplying a first current;
a current mirror having first and second current paths;
a reference capacitor coupled to the output terminal and the first current path;
a measurement impedance coupled to the second current path; and
an analog-to-digital converter having an input coupled to a second node within the second current path and configured to measure a voltage at the second node.
8. The integrated circuit of claim 7 , further comprising a second current source coupled to a first node between the reference capacitor and the first current path, wherein the second current source is sized to supply a second current that exceeds the first current from the first current source for predetermined capacitance values of the capacitive device.
9. The integrated circuit of claim 7 , wherein transistors of the current mirror have equal sizes.
10. The integrated circuit of claim 7 , wherein transistors of the current mirror have non-equal sizes.
11. The integrated circuit of claim 7 , wherein the capacitive device is a capacitive actuator, the integrated circuit further comprising a controller to drive the first current source iteratively among a drive mode and a measurement mode, wherein:
during the drive mode, the controller drives the first current at a setting determined by a position signal representing a position of the capacitive actuator, and
during the measurement mode, the controller generates a correction signal for a next iteration based on a value output by the analog-to-digital converter commensurate with the voltage measured at the second node.
12. The integrated circuit of claim 11 , wherein, in the drive mode, the controller varies a duration of activation of the first current source.
13. The integrated circuit of claim 11 , wherein the controller dithers onsets of signals output from the first current source from iteration to iteration.
14. A driver and voltage measurement circuit for operating a capacitive actuator, comprising:
a driver providing a drive signal to the capacitive actuator for changing a position of the capacitive actuator; and
a first voltage measurement circuit measuring a voltage representative of a capacitance of the capacitive actuator, the voltage measurement circuit comprising: a first current source coupled to the capacitive actuator and supplying a first current;
a current mirror having first and second current paths;
a reference capacitor coupled to a connection point between the capacitive actuator and the first current path; and
a measurement impedance coupled to the second current path;
wherein the voltage representative of the capacitance of the capacitive actuator is supplied at a second node within the second current path; and
a controller supplying control signals to the driver based on an error signal representing a difference between an actual position of the capacitive actuator derived from the capacitance of the capacitive actuator as determined by the first voltage measurement circuit and a position setting for the capacitive actuator.
15. The driver and voltage measurement circuit of claim 14 , further comprising an analog-to-digital converter for digitizing the voltage measured at the second node and supplying the digitized voltage to the controller.
16. The driver and voltage measurement circuit of claim 14 , further comprising a second current sink configured to discharge both the capacitive actuator and the reference capacitor, wherein the first current source and second current sink are prevented from being enabled at the same time.
17. The driver and voltage measurement circuit of claim 14 , further comprising a second measurement circuit to directly measure a voltage of the capacitive actuator.
18. The driver and voltage measurement circuit of claim 17 , further comprising storage for lookup tables correlating values of the actual position and the directly measured voltage to drive current settings.
19. The driver and voltage measurement circuit of claim 18 , wherein the storage includes sub-tables indexed by values representing gravitational forces acting on the capacitive actuator.
20. The driver and voltage measurement circuit of claim 14 , wherein the controller derives drive current settings based at least in part on values representing gravitational forces acting on the capacitive actuator.