Adaptive Sensor Bias Estimation in Nine Degree of Freedom Inertial Measurement Units: Theory and Preliminary Evaluation

Abstract

Nine degrees of freedom (DOF) inertial measurement units (IMUs) comprised of three-axis magnetometers, three-axis accelerometers, and three-axis angular rate sensors are commonly used in attitude and heading reference systems (AHRSs). Two classes of AHRSs exist: systems that estimate true-North heading and systems that estimate magnetic-North heading. True-North heading AHRSs require high-end angular rate sensors which are sensitive enough to dynamically estimate Earth-rate (typically fiber-optic and ring-laser gyros), while magnetic-North AHRSs employ gyros that are not sensitive enough to dynamically estimate Earth-rate (i.e. all MEMS gyros). Thus, magnetic-North AHRSs employ magnetometers for estimating heading. This paper will focus on this class of magnetic-North AHRSs. These systems fuse IMU measurements to generate estimates of the instrument's roll, pitch, and magnetic heading. However, their accuracy is limited by sensor measurement bias that is unknown a priori. Hence, accurate sensor bias estimation and compensation is essential for true attitude estimation. This paper reports a novel adaptive sensor bias observer for sensor measurement biases in 9-DOF IMUs. The algorithm requires smaller angular movements of the instrument than other reported sensor bias calibration methods, does not require a priori knowledge of local fields like the local magnetic field or the local gravity vector, and does not require knowledge of the attitude of the instrument. Stability proofs, preliminary simulations, and a full-scale vehicle experimental evaluation are reported.