Test Evaluation of a High Performance Pulse Rebalanced Gyro for Strapdown IMU Applications

Abstract

Recent mission requirements in the strapdown Inertial Measurement Unit (IMU) field have emphasized the need for inertial sensors which combine the capability of providing precision attitude reference with long mission life. The single-axis hydrodynamic-spin-bearing gyroscope operated with pulse-rebalance servo electronics is a prime candidate sensor for these missions. This paper provides a summary of recent test results and analytical evaluation for a high-performance instrument of this type. The sensor parameters and performance test results summarized in this paper are based on an extensive evaluation program of the 64 PM RIG gas-bearing gyro. The 64 PM RIG design base is the 64 AC RIG (25 IRIG), which was initially developed by MIT, with the primary difference between the two instruments being the use of a permanent magnet DC torquer (rather than an AC torquer) in the 64 PM RIG. The 64 AC RIG was originally developed and produced in large quantities for an inertially-stabilized platform used in a missile application and has since been used as an inertial sensor on several spacecraft programs (OSO-7 and ATS-6). A single-degree-of-freedom gyro, the 64 PM RIG utilizes a hydrodynamic gas bearing, a microsyn pickoff, a magnetic suspension, and a permanent-magnet torquer, and is fully floated to minimize error torques about its sensitive axes. Sensor drift data is summarized which demonstrates the bias variability over a 15 hour period to be less than 0.001 deg/hr. The specific strapdown IMU performance areas which are addressed include sensor scale factor linearity and stability, the noise output of the sensor as characterized by its power spectral density, the attitude reference error buildup as a function of the interval between bias updates, and performance of the sensor in the dynamic environment typically encountered during vehicle maneuvers. The pulse-rebalance technique implemented is binary (forced-limit-cycle), and uses quantized pulse-width modulation of the gyro torquing signal. A dual-scale design of the pulse-rebalanced sensor has been implemented and evaluated. The pulse-rebalanced sensor has a demonstrated capability of maintaining an attitude reference within 1.0 arc-second for time periods exceeding 30 minutes. Test data demonstrating scale factor non-linearity of less than 40 PPM and scale factor stability below a PPM/day is presented. The 64 PM RIG sensor is being incorporated into two strapdown IMUs which will provide the primary attitude reference for the International Ultraviolet Explorer (IUE) and the High Energy Astronomy Observatory (HEAO) spacecrafts. The six-sensor IUE/IMU and the modular two-sensor HEAO/IMU are discussed from the packaging (size, weight, power) and functional viewpoints. Included is a brief description of the hybridized pulse-rebalance servo electronics used for the two programs.