ESA GNC Conference Papers Repository

In the context of our collaboration with ESA for the ExoMars mission, CNES is currently developing ARTEMIS, a new rover demonstrator to validate our perception, localization and navigation algorithms on a platform whose design is closer to the future Mars exploration rover than that of our previous demonstrator IARES. Among the new pieces of equipment that ARTEMIS will carry, a new custom AHRS (Attitude and Heading Reference System) will replace the commercial off-the-shelf unit used on IARES in order to have a better insight into all parameters contributing to global localization precision. Unlike most inertial navigation systems described in technical literature, which usually have either three orthogonal gyrometers or none at all, our unit has a single gyrometer on the vertical axis to help in heading determination, along with the usual triaxial accelerometer and magnetometer. Unfortunately, the gyro measures the angular velocity on the horizontal plane of the vehicle reference frame, while the heading is calculated on the horizontal plane of the geographical reference frame, and the relative inclination between both planes is constantly varying with terrain slope as the rover moves. Thus, the use of the gyro measurement to propagate the heading leads to complex, non-linear expressions when hybridizing with magnetic heading if we want to find an exact solution which is accurate not only in its mean value but also in the short term. Furthermore, in order for the raw measurements to be used in precise attitude determination, a specific calibration procedure has to be applied to each sensor. In addition to calibration of gyro bias, 3D calibration is applied to the accelerometers to determine their bias and scale factors, and hard iron and (more unusual) soft iron calibration are applied to the magnetometers to compensate for constant and heading-dependent magnetic deviations due to surrounding structure and equipment. The article presents the environment and the physical characteristics of the inertial measurement unit, introduces the problem of the single axis gyrometer, develops an adapted hybridizing filter step by step and proves its validity through simulation and experimental tests.