Three-dof orientation measurement based on optical sensor for reaction sphere

Reaction spheres, serving as the core unit to provide three-axis attitude control, are promising alternatives to conventional momentum exchange devices for the Attitude Determination and Control System (ADCS) due to its three-degree-offreedom (three-DOF) rotation with an integrated device. The measurement of spherical rotor orientation is critical to the close-loop control for reaction sphere actuator as well as the angular momentum management via the ADCS. Nowadays, the techniques for simultaneous measurement of multiple DOF motions have already been demonstrated in various fields including industry, machinery and automation. Conventional measuring methods based on the single-axis encoder or mechanical linkages are restricted to measure complex motions, especially three-DOF motions owing to the mechanical friction and complicated structure, while the optical sensor could capture the images of rotor surface without contact and obtain the spatial rotational speed through image processing with relatively simple architecture and lower cost. In this paper, a novel speed measuring system is proposed on the basis of optical sensor for the rotor orientation of reaction sphere actuator, whose spherical rotor can be accelerated about any desired axis. Firstly, the operational principles of the optical sensor and the reaction sphere are introduced, and the mean resolution of optical sensor affected by different materials and patterns of rotor surface is studied to determine the prior surface. Afterwards, the design concept and theory of measuring system composed of three optical sensors for the three-DOF motion are presented along with the analytical model of input-output parameters. Finally, the design feasibility of sensor system for measuring the rotational rotor orientation is validated experimentally using prototype system which can achieve the rotational speed 1500 r/min and the effect of system parameters on the resolution, average error, responsivity and measuring range is analyzed by comparing the theoretical results and experimental results. The simulation results show that the optical sensor system can meet the requirements of accuracy and measuring range, and the results are expected to serve as a basis for the parameter optimization which can greatly improve the sensor performance of the reaction sphere.