Mechanical design and determination of bandwidth for a two-axis inertial reference unit

Abstract

The inertial reference unit (IRU) serves as a master reference for acquisition, tracking and pointing (ATP) system due to its ability to maintain stability with respect to inertial frame. However, the bandwidth of IRU system is usually limited by the dynamics of mounted inertial sensors and structural resonances. In this paper, magnetohydrodynamics (MHD) angular rate sensor (ARS) with extremely low noise at high frequencies (>3Hz) was combined with a conventional gyro serving as inertial sensing unit of the developed IRU. This sensing unit was verified experimentally to exhibit a nearly perfect transfer function with “unity” gain and “zero” phase error. A central hinge that allows only rotation about the actuating axes was used as the supporting structure. A structural parameter design approach based on analytical stiffness model was developed for the central hinge. The relative errors between theoretical stiffness values, finite element analysis (FEA) simulations and experimental data were found to be within ±13%. A double-T network notch filter with optimal parameters was implemented to suppress the low-frequency mechanical resonance. With the notch filter in the forward path, the open-loop dynamic model of the IRU was measured with the results indicating a completely damped resonant peak and an identification error less than ±0.5 dB in magnitude and ±5° in phase. The developed IRU was ultimately certified to achieve more than ±6 mrad as a drive range and more than 120 Hz as a closed-loop bandwidth under a digital controller designed with frequency shaping technique.