Calibration of an Inertial Measurement Unit with Skewed Sensor Axes

Haywood Satz

Abstract:	An algorithm was designed to calibrate an Inertial Measurement Unit (IMU) with sensor axes not aligned with the assembly reference frame. Simultaneous constraints of the IMU mechanical design envelope, and sensor performance characteristics, were such as to require sensor input axes to be skewed from the external reference frame, and themselves placed in a non-mutually-orthogonal triad. Sensor bias, scale-factor, and misalignment error models, necessarily, required the projection of specific force and angular rotation rate into the sensor coordinate frame of reference. Those projections were obtained by analytic determination of rotations, and non-orthogonal transformations, which brought the sensor triad into its mutually skewed input-axis placement. There was an objective, while executing the calibration procedure, to monitor the resulting least-squares estimates and verify their credibility by comparison with single-sensor-axis, direct "pencil and paper" calculations. That objective was realized by computing test table gimbal angle commands to orient sensor axes, one-at-a-time, in the locally estimated vertical direction. Those computations required inverse trigonometric formulas, associated with gimballed test platform motions, properly constrained for uniqueness. A computer simulation verified the observability of sensor errors for various sets of chosen gimbal motions, and the determination of IMU calibration coefficients, in the environment of sensor output quantization errors. A brief itemization of the work done here, for the calibration of an IMU with skewed sensors, was the development of : - transformations to model skewed sensor orientation - Least Squares estimates of IMU sensor errors - gimbal angle commands to orient a sensor vertically - single sensor coefficient determinations to compare with Least-Squares estimates.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	725 - 730
Cite this article:	Satz, Haywood, "Calibration of an Inertial Measurement Unit with Skewed Sensor Axes," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 725-730.
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