# US 2016/0252544 A1 · Accelerometer control > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2016/0252544 A1 - Document ID: US20160252544A1 - Status: Published application - Kind: A1 - Application number: 15/027,277 - Filed: 2014-09-22 - Published: 2016-09-01 - Earliest claimed: 2013-10-09 - Assignees: Atlantic Inertial Systems Limited - Applicants: Atlantic Inertial Systems Limited - Inventors: Alan Richard Malvern; Kiran Harish - Web page: https://patentgrep.com/patent/US20160252544A1 ## CPC - G01P 15/125: by capacitive pick-up - G01P 15/13: by measuring the force required to restore a proofmass subjected to inertial forces to a null position - G01P 15/131: {with electrostatic counterbalancing means} - G01P 2015/0814: {for translational movement of the mass, e.g. shuttle type} - G01P: MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT (measuring or recording blood flow A61B5/02, A61B8/06; monitoring speed or deceleration of electrically-propelled vehicles B60L3/00; vehicle lighting systems adapted to indicate speed B60Q1/54; determining position or course in navigation, measuring ground distance in geodesy or surveying G01C; combined measuring devices for measuring two or more variables of movement G01C23/00; measuring velocity of sound G01H; measuring velocity of light G01J7/00; measuring direction or velocity of solid objects by reception or emission of radiowaves or other waves and based on propagation effects, e.g. Doppler effect, propagation time, direction of propagation, G01S; measuring speed of nuclear radiation G01T; measuring acceleration of gravity G01V; {measuring or recording the speed of trains B61L23/00; speed indicators incorporated in motor vehicles B60K35/00; measuring frequency or phase G01R; traffic control G08G}) ## Abstract A closed loop method of controlling a capacitive accelerometer uses two servo loops. A Vcrit servo loop uses an output signal (S2) modulated by a sine wave signal (S1). 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. ## Claims ### 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. ### Claim 2 (depends on 1) 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. ### Claim 3 (depends on 1) 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. ### Claim 4 (depends on 3) 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. ### Claim 5 (depends on 1) A closed loop method according to claim 1, further comprising: adding a quadrature signal S3, that is in 90° anti-phase to the modulation signal S1, after providing the error signal. ### Claim 6 (depends on 1) A closed loop method according to claim 1, further comprising: adding a quadrature signal S3, that is in 90° anti-phase to the modulation signal S1, before providing the Vcrit control signal. ### Claim 7 (depends on 5) A closed loop method according to claim 5, comprising: extracting the output signal and subtracting a signal S4 that is in-phase with the modulation signal S1. ### Claim 8 (depends on 7) 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. ### Claim 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. ### Claim 10 (depends on 9) 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. ### Claim 11 (depends on 10) 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. ### Claim 12 (depends on 9) A closed loop control system according to claim 9, further comprising a demodulator arranged to add a quadrature signal S3, that is in 90° anti-phase to the modulation signal S1, after providing the error signal. ### Claim 13 (depends on 9) A closed loop control system according to claim 9, further comprising a demodulator arranged to add a quadrature signal S3, that is in 90° anti-phase to the modulation signal S1, before providing the Vcrit control signal. ### Claim 14 (depends on 9) A closed loop control system according to claim 9, further comprising an output signal extractor arranged to subtract a signal S4 that is in-phase with the modulation signal S1. ### Claim 15 (depends on 14) 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.