Accelerometer control

Patent metadata

Field	Value
Publication number	US-2016252544-A1
Application number	US-201415027277-A
Country	US
Kind code	A1
Filing date	Sep 22, 2014
Priority date	Oct 9, 2013
Publication date	Sep 1, 2016
Grant date	—

Abstract

A closed loop method of controlling a capacitive accelerometer uses two servo loops. A Vcrit servo loop uses an output signal (S 2 ) modulated by a sine wave signal (S 1 ). The Vcrit control signal adjusts the magnitude of the PWM drive signals applied to the fixed capacitor electrodes of the accelerometer, thereby optimising open loop gain.

First claim

1 . A closed loop method of controlling a capacitive accelerometer comprising a proof mass moveable relative to first and second fixed capacitor electrodes, the method comprising:
applying in-phase and anti-phase pulse width modulation (PWM) drive signals to the first and second fixed capacitor electrodes with an adjustable mark/space ratio; operating in closed loop so that mechanical inertial forces are balanced by electrostatic forces to maintain the operating point of the proof mass at a null position; detecting an output signal from the accelerometer representing a displacement of the proof mass from the null position to provide a error signal; using the error signal so as to vary the mark/space ratio of the PWM drive signals so that the accelerometer output signal is proportional to acceleration; adding a sine wave modulation at a frequency fto the output signal from the proof mass before providing the error signal; further detecting a phase shift resulting from the sine wave modulation at the frequency f so as to recognise a critical drive signal magnitude Vcrit representing the null position; and providing a Vcrit control signal so as to adjust the magnitude of the PWM drive signals applied to the first and second fixed capacitor electrodes, or applying a separate, adjustable drive signal to at least one further fixed capacitor electrode, to lock to Vcrit and thereby optimise open loop gain.

2 . A closed loop method according to claim 1 , comprising:
directly adjusting the magnitude of the PWM drive signals applied to the first and second fixed capacitor electrodes to lock to Vcrit.

3 . A closed loop method according to claim 1 , comprising:
adjusting the magnitude of the drive signal applied to at least one further fixed capacitor electrode to lock to Vcrit.

4 . A closed loop method according to claim 3 , wherein the at least one further fixed capacitor electrode is independent of the first and second fixed capacitor electrodes.

5 . A closed loop method according to claim 1 , further comprising:
adding a quadrature signal S 3 , that is in 90° anti-phase to the modulation signal S 1 , after providing the error signal.

6 . A closed loop method according to claim 1 , further comprising:
adding a quadrature signal S 3 , that is in 90 ° anti-phase to the modulation signal S 1 , before providing the Vcrit control signal.

7 . A closed loop method according to claim 5 , comprising:
extracting the output signal and subtracting a signal S 4 that is in-phase with the modulation signal S 1 .

8 . A closed loop method according to claim 7 , further comprising:
providing the extracted output signal as a measurement of the acceleration acting on the proof mass.

9 . An accelerometer closed loop control system comprising:
a capacitive accelerometer comprising a proof mass moveable relative to first and second fixed capacitor electrodes; a pulse width modulation (PWM) generator arranged to apply in-phase and anti-phase PWM drive signals to the first and second fixed capacitor electrodes with an adjustable mark/space ratio; a PWM servo operating in closed loop so that mechanical inertial forces are balanced by electrostatic forces to maintain the operating point of the proof mass at a null position; an output signal detector arranged to detect an output signal from the accelerometer representing a displacement of the proof mass from the null position to provide an error signal, wherein the PWM servo uses the error signal to vary the mark/space ratio of the PWM drive signals so that the accelerometer output signal is proportional to acceleration; a modulator arranged to add a sine wave modulation at a frequencyfto the output signal before providing the error signal to the PWM servo; and a Vcrit servo arranged to: (i) detect a phase shift resulting from the sine wave modulation at the frequency f so as to recognise a critical drive signal magnitude Vcrit representing the null position; and (ii) provide a Vcrit control signal to adjust the magnitude of the PWM drive signals applied to the first and second fixed capacitor electrodes, or to apply a separate drive signal to a further fixed capacitor electrode, to lock to Vcrit and thereby optimise open loop gain.

10 . A closed loop control system according to claim 9 , wherein the Vcrit servo is arranged to adjust the magnitude of the drive signal(s) applied to at least one further fixed capacitor electrode.

11 . A closed loop control system according to claim 10 , wherein:
each of the first and second fixed capacitor electrodes comprises a plurality of laterally spaced, fixed capacitive electrode fingers arranged to interdigitate with a set of laterally spaced, moveable capacitive electrode fingers extending from the proof mass; and the at least one further fixed capacitor electrode comprises a plurality of laterally spaced, fixed capacitive electrode fingers that are not interdigitated with the moveable capacitive electrode fingers.

12 . A closed loop control system according to claim 9 , further comprising a demodulator arranged to add a quadrature signal S 3 , that is in 90° anti-phase to the modulation signal S 1 , after providing the error signal.

13 . A closed loop control system according to claim 9 , further comprising a demodulator arranged to add a quadrature signal S 3 , that is in 90° anti-phase to the modulation signal S 1 , before providing the Vcrit control signal.

14 . A closed loop control system according to claim 9 , further comprising an output signal extractor arranged to subtract a signal S 4 that is in-phase with the modulation signal S 1 .

15 . A closed loop control system according to claim 14 , wherein the output signal extractor provides a measurement of the acceleration acting on the proof mass.

Assignees

• Atlantic Inertial Systems Ltd

Inventors

• Malvern Alan Richard
• Harish Kiran

Classifications

• G01P15/125 Primary

  by capacitive pick-up · CPC title

• G01P2015/0814

  for translational movement of the mass, e.g. shuttle type · CPC title

• G01P15/131

  with electrostatic counterbalancing means · CPC title

• G01P15/13

  by measuring the force required to restore a proofmass subjected to inertial forces to a null position · CPC title