BIAS REDUCTION IN FORCE REBALANCED ACCELEROMETERS

• US 20140165691A1
• Filed: 12/19/2012
• Published: 06/19/2014
• Est. Priority Date: 12/19/2012
• Status: Active Grant

• Current Assignee: Northrop Grumman Guidance And Electronics Company, Inc.
• Original Assignee: Robert E. Stewart
• Inventors: STEWART, ROBERT E.
• Granted Patent: US 9,341,646 B2
• Time in Patent Office: Days
• US Class Current: 73/1.38
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

A system is provided for the continuous reduction, in real time, of bias in a force rebalanced accelerometers having a proof mass coupled to an accelerometer housing by a flexure suspension. The system comprises a closed loop, force rebalance servo that provides control voltage to the proof mass to null an electrical pickoff signal that indicates the motion of the proof mass with respect to the accelerometer housing, wherein a time varying disturbance signal is injected into the force rebalance servo that results in the generation of a time varying voltage in the output of the force rebalance servo that corresponds to a magnitude of the net positive spring of the combined flexure suspension and electrostatic springs acting on the proofmass. The system also comprises a negative electrostatic spring servo that applies a negative electrostatic spring DC voltage to each of a pair of negative electrostatic forcer electrodes.

Claims

1. A system for reducing bias in an accelerometer having a proof mass coupled to an accelerometer housing by a flexure suspension and a pair of negative electrostatic forcer electrodes spaced apart from the proofmass on opposing sides, the system comprising:

a force rebalance servo that provides a control signal to the proof mass in response to an electrical pickoff signal that indicates the motion of the proof mass relative to the accelerometer housing, wherein a time varying disturbance signal is injected into the force rebalance servo that results in the generation of a time varying signal in an output of the force rebalance servo that corresponds to a magnitude of a net positive spring force of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass as it moves in response to the time varying disturbance signal; and

a negative electrostatic spring servo that applies a negative electrostatic spring DC voltage to the pair of negative electrostatic forcer electrodes to cancel the net positive spring force of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass in response to the nulling of the time varying signal in the output of the force rebalance servo.

2. The system of claim 1, wherein the pickoff signal is a phase difference signal generated at the proof mass in response to capacitance changes caused by the motion of the proof mass with respect to the accelerometer housing and interaction with a first AC excitation signal applied to a first capacitive pickoff electrode and a second AC excitation signal applied to a second capacitive pickoff electrode, wherein the first capacitive pickoff electrode and the second capacitive pickoff electrode are disposed at opposite sides of the proof mass and the first AC excitation signal is 180°

out of phase with the second AC excitation signal.

3. The system of claim 2, wherein a first DC voltage is applied to a first electrostatic forcer electrode and a second DC voltage is applied to a second electrostatic forcer electrode, wherein the first electrostatic forcer electrode and the second electrostatic forcer electrode are disposed at opposite sides of the proofmass and the first DC voltage is of opposite polarity than the second DC voltage, such that the control signal output of the force rebalance servo applied to the proofmass interacts with the first DC voltage and second DC voltage to generate an electrostatic force to move the proofmass to the pickoff electrical null.

4. The system of claim 2, wherein a first DC voltage is applied to the first capacitive pickoff electrode and a second DC voltage is applied to the second capacitive pickoff electrode, wherein the first DC voltage is of opposite polarity than the second DC voltage, such that the control signal output of the force rebalance servo applied to the proofmass interacts with the first DC voltage and second DC voltage to generate an electrostatic force to move the proofmass to the pickoff electrical null.

5. The system of claim 2, wherein the force rebalance servo comprises a gain amplifier that amplifies the pickoff signal and a demodulator that demodulates the amplified pickoff with respect to the first AC excitation signal into an error voltage.

6. The system of claim 5, wherein the force rebalance servo comprises a servo compensation amplifier that integrates the error voltage to provide a control voltage to be applied to force rebalance the proofmass to the electrical pickoff null.

7. The system of claim 6, wherein a time varying disturbance signal is injected into a summing node with the error voltage to be provided to the servo compensation amplifier.

8. The system of claim 1, wherein the negative electrostatic spring servo comprises a gain amplifier that amplifies the capacitance coupled output of the force rebalance servo and a demodulator that demodulates the force rebalance servo signal with respect to the time varying disturbance signal.

9. The system of claim 8, further comprising a servo compensation amplifier that integrates the demodulated force rebalance signal to provide the negative electrostatic spring DC voltage.

10. An accelerometer system comprising:

a proof mass coupled to an accelerometer housing by a flexure suspension and a pair of negative electrostatic forcer electrodes spaced apart from the proof mass on opposing sides;

a force rebalance servo that provides a control signal to the proof mass in response to an electrical pickoff signal that indicates motion of the proof mass relative to the accelerometer housing, wherein a time varying disturbance signal is injected into a summing node of the force rebalance servo that results in the generation of a time varying signal in the output of the force rebalance servo that corresponds to a magnitude of the net positive spring force of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass as it moves in response to the disturbance signal; and

a negative electrostatic spring servo that applies a negative electrostatic spring DC voltage to each of the pair of negative electrostatic forcer electrodes to cancel the net positive spring of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass in response to the nulling of the time varying signal in the output of the force rebalance servo.

11. The system of claim 10, wherein the pickoff signal is a phase difference signal generated at the proof mass in response to a capacitance change caused by the motion of the proof mass with respect to the accelerometer housing and interaction with a first AC excitation signal applied to a first capacitive pickoff electrode and a second AC excitation signal applied to a second capacitive pickoff electrode, wherein the first capacitive pickoff electrode and the second capacitive pickoff electrode are disposed at opposite sides of the proof mass and the first AC excitation signal is 180°

out of phase with the second AC excitation signal.

12. The system of claim 11, wherein a first DC voltage is applied to a first electrostatic forcer electrode and a second DC voltage is applied to a second electrostatic forcer electrode, wherein the first electrostatic forcer electrode and the second electrostatic forcer electrode are disposed at opposite sides of the proof mass and a first DC voltage is of opposite polarity than the second DC voltage, such that the control signal output of the force rebalance servo applied to the proof mass interacts with the first DC voltage and second DC voltage to move the proof mass to the pickoff electrical null.

13. The system of claim 11, wherein a first DC voltage is applied to the first capacitive pickoff electrode and a second DC voltage is applied to the second capacitive pickoff electrode, wherein the first DC voltage is of opposite polarity than the second DC voltage, such that the control signal output of the force rebalance servo applied to the proof mass interacts with the first DC voltage and second DC voltage to generate an electrostatic force to move the proof mass to the pickoff electrical null.

14. The system of claim 12, wherein the force rebalance servo comprises:

a gain amplifier that amplifies the pickoff signal;

a demodulator that demodulates the amplified pickoff signal into an error voltage based on the first AC excitation signal; and

a servo compensation amplifier that integrates the error voltage to provide a control voltage to be applied to the proof mass, wherein a time varying disturbance signal is injected into the summing node with the error voltage to be provided to the servo compensation amplifier.

15. The system of claim 10, wherein the negative electrostatic spring servo comprises:

a gain amplifier that amplifies the capacitance coupled output signal of the force rebalance servo;

a demodulator that demodulates the amplified force rebalance signal with respect to the time varying disturbance signal; and

a servo compensation amplifier that integrates the amplified demodulated force rebalance signal to provide the negative electrostatic spring DC voltage.

16. A method for reducing bias in an accelerometer having a proof mass coupled to an accelerometer housing by a flexure suspension and a pair of negative electrostatic forcer electrodes spaced apart from the proofmass on opposing sides, the method comprising:

injecting a time varying disturbance signal into a force rebalance servo that results in the generation of an output voltage that corresponds to a magnitude of a net positive spring force of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass as it moves in response to the disturbance signal; and

applying a negative electrostatic spring DC voltage to each of a pair of negative electrostatic forcer electrodes to cancel the net positive spring of the combined flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass in response to the nulling of the time varying signal in the output of the force rebalance servo.

17. The method of claim 16, further comprising:

applying a first AC excitation signal to a first capacitive pickoff electrode;

applying a second AC excitation signal to a second capacitive pickoff electrode, wherein the first capacitive pickoff electrode and the second capacitive pickoff electrode are disposed at opposite sides of the proof mass and the first AC excitation signal is 180°

out of phase with the second AC excitation signal;

capturing and demodulating a phase difference pickoff signal generated at the proof mass in response to a capacitance change due to the motion of the proof mass with respect to the accelerometer housing and interaction with the first AC excitation signal and the second AC excitation signal to generate an error voltage; and

integrating the error voltage to generate the control voltage that corresponds to the force rebalance signal.

18. The method of claim 17, further comprising:

applying a first DC voltage to a first electrostatic forcer electrode; and

applying a second DC voltage to a second electrostatic forcer electrode, wherein the first electrostatic forcer electrode and the second electrostatic forcer electrode are disposed at opposite sides of the proof mass and the first DC voltage is of opposite polarity than the second DC voltage, such that the control voltage applied to the proof mass interacts with the first DC voltage and second DC voltage to generate an electrostatic force to move the proof mass to the pickoff electrical null.

19. The method of claim 17, further comprising:

applying a first DC voltage to the first capacitive pickoff electrode;

applying a second DC voltage to the second capacitive pickoff electrode, such that the control voltage applied to the proof mass interacts with the first DC voltage and second DC voltage to generate an electrostatic force to move the proof mass to the pickoff electrical null.

20. The method of claim 16, further comprising;

demodulating the force rebalance signal with respect to the time varying disturbance signal;

integrating the demodulated force rebalance signal to provide a negative electrostatic spring DC voltage; and

applying the negative electrostatic spring DC voltage to each of the pair of negative electrostatic forcer electrodes to cancel the net positive spring of the flexure suspension and electrostatic spring associated with bias voltages acting on the proof mass in response to nulling of the time varying signal in the output of the force rebalance servo.