Square law controller for an electrostatic force balanced accelerometer

• US 5,277,053 A
• Filed: 06/29/1992
• Issued: 01/11/1994
• Est. Priority Date: 04/25/1990
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated (Northrop Grumman Corporation)
• Inventors: McLane, Daniel P., Stewart, Robert E.
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US07/908,520
• Time in Patent Office: 561 Days
• Field of Search: 73/517 B, 73/862.61, 73/1 B, 73/1 D, 73/701, 318/648, 364/553, 364/571.01, 364/571.02, 364/571.07
• US Class Current: 73/1.38
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 15/132 with electromagnetic counterbalancing means
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

An electrostatically force balanced accelerometer employs electrostatic, single sided square law forcing after filtering and scaling the pickoff signal to determine a restoring force required to bring the pendulous mass (10) of the accelerometer back to its null position. A signal (44,48) proportional to the square root of the restoring force is applied to an electrostatic square law forcing circuit (50) that applies a restoring force to one side or the other of the pendulum (10). A system output signal (42) is proportional to the calculated restoring force, and thus linearly proportional to the sensed acceleration. The transfer function of the forcing circuit (50) is empirically determined by applying a series of known accelerations and measuring the balancing signal required to restore the pendulous mass (10) to its null position. A feedback signal (44,48) is then generated which has a relation to the pickoff signal that is the inverse of the empirically determined transverse function of the forcing circuit.

Claims

1. A method for driving the sensing member of a closed loop force balanced measuring instrument wherein said sensing member is displaced from a null position in response to an input condition that is to be measured, and wherein a feedback signal is applied to a forcing device that exerts a force on the sensing member tending to decrease displacement of the sensing member from said null position, said forcing device having an unknown transfer function, said forcing device being responsive to said feedback signal to exert a restoring force on said sensing member, said method comprising:

calibrating said instrument to empirically determine said transfer function, said step of calibrating comprising;

applying a series of known accelerations to the instrument,

applying to the instrument forcing device a balancing signal to drive the sensing member to said null position for each of said series of known accelerations, and

measuring the magnitude of each balancing signal and the associated pickoff signal to effectively define and empirically determine the transfer function of the forcing device and instrument scaling,

generating a pickoff signal indicative of an input condition experienced by said sensing member,

processing said pickoff signal to calculate a restoring force representing force required to move said sensing member to said null position and to calculate within the closed loop a feedback signal having a relation to said restoring force that is the inverse of said empirically determined transfer function,

applying said feedback signal to said forcing device, and

generating an output signal based upon said restoring force.

2. A method for actuating the forcing device of a closed loop force balanced instrument having a sensing element that tends to be displaced from a null position upon experiencing a condition to be detected, having a pickoff signal indicative of displacement of said element, and having a forcer with an unknown transfer function, said forcer exerting force on said sensing element in repose to a feedback signal applied to said forcer, said method comprising the steps of:

calibrating said instrument to empirically determine said transfer function of said forcer, said step of calibrating comprising;

applying a series of known accelerations to the instrument,

applying to the instrument forcer a balancing signal to drive the sensing member to said null position for each of said series of known accelerations, and

measuring the magnitude of each balancing signal and the associated pickoff signal to effectively define and empirically determine the transfer function of the forcer and instrument scaling,

processing said pickoff signal within the closed loop to generate a feedback signal having a relation to said pickoff signal that is the inverse of said empirically determined transfer function and to generate an output signal that is a function of said pickoff signal,

applying said feedback signal to said forcer, and

causing said forcer to apply a restoring force to said sensing element.

3. The method of claim 2 wherein said transfer function includes a first order terms, a second order term and third and higher order terms, and wherein said step of calibrating comprises empirically determining said second order term and said third and higher order terms.