System and Method for Mitigating Errors in Electrostatic Force Balanced Instrument

• US 20080295597A1
• Filed: 05/30/2007
• Published: 12/04/2008
• Est. Priority Date: 05/30/2007
• Status: Active Grant

• Current Assignee: Litton Systems Incorporated
• Original Assignee: Litton Systems Incorporated
• Inventors: Stewart, Robert E., Griffith, Robert
• Granted Patent: US 7,614,300 B2
• Time in Patent Office: Days
• US Class Current: 73/514.18
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

System and method for mitigating errors in electrostatic force balanced instrument is provided. The system and method mitigate errors in measurement readings caused by charge buildup in force balanced instruments that employ charge pulses to generate an electrostatic force to null an inertial proof mass disposed between opposing electrodes. The system and method mitigate charge buildup by applying charge pulses to each opposing electrode of a sensing element for a given charge cycle time period in a normal polarity configuration followed by charge pulses to each opposing electrode of the sensing element for a second given charge cycle time period in a reverse polarity conjuration.

Claims

1. A force balanced instrument system comprising:

a sensing element having an inertial proof mass disposed between a first electrode plate and a second electrode plate;

a switching system switchable between providing charge pulses to the sensing element in a normal polarity configuration and providing charge pulses to the sensing element in a reverse polarity configuration; and

a control logic device that controls the switching of the switch system to provide charge pulses to the sensing element in the normal polarity configuration for a first charge cycle time period and to provide charge pulses to the sensing element in the reverse polarity configuration for a second charge cycle time period.

2. The system of claim 1, further comprising a single reference voltage that provides charge pulses to the sensing element in both the normal polarity configuration and reverse polarity configuration.

3. The system of claim 1, wherein the control logic device in the reverse polarity configuration configures the switching system to alternately couple a selected plate of the first and second electrode plates to one of ground and virtual ground, alternately couple a non-selected plate of the first and second electrode plates to the proof mass, and alternately provide charge pulses to the non-selected plate and coupled proof mass.

4. The system of claim 3, wherein the control logic device in the normal polarity configuration configures the switching system to alternately couple a non-selected plate of the first and second electrode plates to one of ground and virtual ground, to couple the proof mass to one of ground and virtual ground and to alternately couple a selected plate of the first and second electrode plates to receive a charge pulse.

5. The system of claim 1, further comprising an operational amplifier that receives a charge pulse from a first portion of the switching system and provides the charge pulse to a second portion of the switching system that selects between providing the charge pulse to one of the top electrode plate and bottom electrode plate.

6. The system of claim 1, wherein the first electrode plate charges to a voltage indicative of the displacement of the first electrode plate to the proof mass in response to a charge pulse and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the second electrode plate in response to a charge pulse in the normal polarity configuration, and the first electrode plate charges to a voltage indicative of the displacement of the second electrode plate to the proof mass and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the first electrode plate in response to a charge pulse in the reverse polarity configuration.

7. The system of claim 1, further comprising:

a first sample and hold device;

a second sample and hold device;

a difference amplifier that provides a difference voltage based on a first voltage sampled by the first sample and hold device and a second voltage sampled by the second sample and hold device; and

wherein the first sample and hold device samples the voltage on the first electrode plate and the second sample and hold device samples the voltage on the second electrode plate during application of charge pulses.

8. The system of claim 1, the normal polarity configuration providing charge pulses alternately to the first and second electrode plates and the reverse polarity configuration providing charge pulses alternately to the first electrode plate and coupled proof mass and the second electrode plate and coupled proof mass.

9. A force balanced instrument having a sensing element having an inertial proof mass disposed between a first electrode plate and a second electrode plate, the instrument comprising;

means for providing charge pulses;

means for switching between applying charge pulses to a sensing element in a normal polarity configuration and applying charge pulses to a sensing element in a reverse polarity configuration; and

means for controlling a charge cycle time period for applying charge pulses to the sensing element in a normal polarity configuration and controlling a charge cycle time period for applying charge pulses to the sensing element in a reverse polarity configuration.

10. The instrument of claim 9, further comprising:

a first means for sampling a first voltage on the first electrode plate of the sensing element induced by a charge pulse;

a second means for sampling a second voltage on the second electrode plate of the sensing element induced by a charge pulse;

means for generating a difference voltage associated with the difference between the first voltage and the second voltage; and

means for controlling an amount of time a charge is retained on the first electrode plate and the amount of time a charge is retained on the second electrode plate based on the difference voltage.

11. The instrument of claim 9, wherein the means for controlling in the reverse polarity configuration configures the means for switching to alternately couple a selected plate of the first and second electrode plates to one of ground and virtual ground, to alternately couple a non-selected plate of the first and second plates to the proof mass, and alternately provide charge pulses to the non-selected plate and coupled proof mass.

12. The instrument of claim 9, wherein the means for controlling in the normal polarity configuration configures the means for switching to alternately couple a non-selected plate of the first and second electrode plates to one of ground and virtual ground, to couple the proof mass to one of ground and virtual ground and to alternately couple a selected plate of the first and second electrode plates to receive a charge pulse.

13. The instrument of claim 9, wherein the means for controlling configures the means for switching to alternate between applying a charge pulse to the first and second electrode plates in the normal polarity configuration and alternate between applying a charge pulse to the first electrode plate and coupled proof mass and applying a charge pulse to the second electrode plate and coupled proof mass in the reverse polarity configuration.

14. The instrument of claim 13, wherein the first electrode plate charges to a voltage indicative of the displacement of the first electrode plate to the proof mass in response to a charge pulse and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the second electrode plate in response to a charge pulse in the normal polarity configuration, and the first electrode plate charges to a voltage indicative of the displacement of the second electrode plate to the proof mass and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the first electrode plate in response to a charge pulse in the reverse polarity configuration.

15. The instrument of claim 9, wherein the means for providing charge pulses comprises a single reference voltage that provides charge pulses to the sensing element in both the normal polarity configuration and the reverse polarity configuration.

16. A method for mitigating error in a force balanced instrument that employs a sensing element having an inertial proof mass disposed between a first electrode plate and a second electrode plate, the method comprising:

switching the sensing element to one of a normal polarity configuration and a reverse polarity configuration;

applying a charge pulse alternately to the first electrode plate and the second electrode plate for a first charge cycle time period;

determining a first difference voltage between voltages induced by the charge pulses on the first electrode plate and the second electrode plate for each charge cycle sequence of the first charge cycle time period;

aggregating the first difference voltage over time to set a duty cycle associated with the charge cycle sequence of the first charge cycle time period;

switching to the other of the normal polarity configuration and the reverse polarity configuration;

applying a charge pulse alternately to the first electrode plate and the second electrode plate for a second charge cycle time period;

determining a second difference voltage between voltages induced by the charge pulses on the first electrode plate and the second electrode plate for each charge cycle sequence of the second charge cycle time period; and

aggregating the second difference voltage over time to set a duty cycle associated with the charge cycle sequence of the second charge cycle time period.

17. The method of claim 16, wherein the switching the sensing element to the reverse polarity configuration comprises alternately coupling a selected plate of the first and second electrode plates to one of ground and virtual ground and alternately coupling a non-selected plate of the first and second plates to the proof mass, such that charge pulses are applied to the non-selected plate and coupled proof mass.

18. The method of claim 16, wherein the switching the sensing element to the normal polarity configuration comprises alternately coupling a non-selected plate of the first and second electrode plates to one of ground and virtual ground, such that charge pulses are applied to the selected plate.

19. The method of claim 16, wherein the first electrode plate charges to a voltage indicative of the displacement of the first electrode plate to the proof mass in response to a charge pulse and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the second electrode plate in response to a charge pulse in the normal polarity configuration, and the first electrode plate charges to a voltage indicative of the displacement of the second electrode plate to the proof mass and the second electrode plate charges to a voltage indicative of the displacement of the proof mass to the first electrode plate in response to a charge pulse in the reverse polarity configuration.

20. The method of claim 16, wherein the charge pulses are provided by a same single reference voltage for both the normal polarity configuration and the reverse polarity configuration.