Atlantic Inertial Systems, Inc.

John Keith Sheard

Nicholas Mark Faulkner

Matthew Williamson

Christopher M Gregory

A vibrating structure gyroscope comprises a resonant structure arranged to vibrate under stimulation from a primary drive electrode. A drive system is arranged to vibrate the vibrating structure at a resonance frequency. An automatic gain control unit varies an amplitude of a primary drive signal (P). A controller operates the gyroscope such that in a first mode of operation, the automatic gain control unit varies an amplitude of the drive signal (P) between an operating range defined by upper and lower bounds and in a second mode operation, in which the automatic gain control unit sets the amplitude of the drive signal (P) to a predetermined level outside of the operating range. In the second mode of operation an amplitude of a primary sense signal (P) is measured after a predetermined time period to determine an oscillation cycle count during said predetermined time period.

Gyroscope

A method of demodulating a MEMS sensor pickoff signal from a vibrating resonator of said sensor, the method comprising: sampling the pickoff signal with an asynchronous ADC at a sampling rate of at least 50 times the resonant frequency of the resonator to generate a stream of samples; generating a first value by combining samples from said stream of samples according to a selected operation, said operation being selected in dependence on a synchronous clock signal that is synchronous to the resonant frequency of the resonator, said synchronous clock signal having a frequency at least twice the resonant frequency of the resonator; and counting the number of samples contributing to the first value. The increased sampling rate of the pickoff signal allows a much higher number of samples to be taken into account, thereby reducing noise. However, the ADC asynchronously from the resonator of the MEMS sensor.

Signal processing

A vibrating structure gyroscope includes a permanent magnet, a structure arranged in a magnetic field of the permanent magnet and arranged to vibrate under stimulation from at least one primary drive electrode and a drive system that includes: one primary drive electrode arranged at least one primary sense electrode arranged to sense motion in the vibrating structure and a drive control loop controlling the primary drive electrode dependent on the primary sense electrode. The structure also includes a compensation unit arranged to receive a signal from the drive system representative of a gain in the drive control loop and arranged to output a scale factor correction based on that signal. As the magnet degrades (e.g. naturally over time as the material ages), the magnetic field weakens. To compensate for this, the primary drive control loop will automatically increase the gain.

An inertial measurement system for a spinning projectile includes: a first, roll gyro to be oriented substantially parallel to the spin axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; calculate a roll angle error; provide the roll angle error as an input to a Kalman filter that outputs a roll angle correction and a roll rate scale factor correction; and apply the calculated roll angle correction and roll rate scale factor correction to the output of the roll gyro.

Inertial navigation system

An inertial measurement system comprising: a first, roll gyro with an axis oriented substantially parallel to the spin axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros; operate a Kalman filter that receives a plurality of measurement inputs including at least roll angle, pitch angle and yaw angle and that outputs at least a roll angle error; initialise the Kalman filter with a roll angle error uncertainty representative of a substantially unknown roll angle; generate at least one pseudo-measurement from stored expected flight data; provide said pseudo-measurement(s) to the corresponding measurement input of the Kalman filter; and apply the roll angle error from the Kalman filter as a correction to the roll angle.

An inertial measurement system for a spinning projectile comprising: first (roll), second and third gyros with axes arranged such that they define a three dimensional coordinate system; at least a first linear accelerometer; a controller, arranged to: compute a current projectile attitude comprising a roll angle, a pitch angle and a yaw angle; compute a current velocity vector from the accelerometer; combine a magnitude of said velocity vector with an expected direction for said vector to form a pseudo-velocity vector; provide the velocity vector and the pseudo-velocity vector to a Kalman filter that outputs a roll gyro scale factor error calculated as a function of the difference between the velocity vector and the pseudo-velocity vector; and apply the roll gyro scale factor error from the Kalman filter as a correction to the output of the roll gyro.

An inertial measurement system () for a longitudinal projectile, comprising a first, roll gyro to be oriented substantially parallel to the longitudinal axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro such that they define a three dimensional coordinate system. The system further comprises a controller (), arranged: &#x2014;to compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; &#x2014;for at least two time points, to compare the computed pitch and yaw angles with expected values for the pitch and yaw angles; &#x2014;for each of said at least two time points, to calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; &#x2014;to calculate a roll angle error difference between said at least two time points; &#x2014;to calculate the total roll angle subtended between said at least two time points; &#x2014;to calculate a roll angle scale factor error based on said computed roll angle error difference and said total subtended roll angle and apply the calculated roll angle scale factor error to the output of the roll gyro.

Inertial navigation system with compensation of roll scale factor error

A method of compensating for signal error is described, comprising: receiving a first signal from a first sensor, said first signal indicative of a movement characteristic; applying an error compensation to said first signal to produce an output signal; applying a variable gain factor to said error compensation; receiving a second signal from a second sensor indicative of said movement characteristic; wherein said error compensation is calculated using the difference between said output signal and said second signal, and said variable gain factor is calculated using said first signal.

Multiple sensor integration

An inertial measurement system for a longitudinal projectile comprising: a first, roll gyro to be oriented substantially parallel to the longitudinal axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro such that they define a three dimensional coordinate system; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; compare the computed pitch and yaw angles with expected values for the pitch and yaw angles; calculate a roll angle error and a roll scale factor error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; and apply the calculated roll angle error and roll scale factor error to the output of the roll gyro.

A device is described herein for determining azimuth comprising a MEMS inertial measurement unit (IMU), a GPS system comprising a GPS antenna and receiver, and a processor configured to receive data from said IMU and from said GPS system, said processor being configured to process said IMU data and said GPS data to derive a true north reference based on said IMU data and said GPS data. A method for determining azimuth is also described herein.

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Plymouth, United Kingdom

John Keith Sheard