Chapter 12 - Attitude Control: A Case Study

Abstract

A case study is the aim of this chapter: the attitude control design of a hypothetical scientific mission like that of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE). The payload of the mission is a set of six ultrafine accelerometers to be employed for science and for the drag-free control (see Chapters 4 and 11). The main control modes of the GOCE mission are discussed and rearranged on the basis of similar requirements. Each control mode is characterized by its own requirements, sensors, and actuators. The first mode (the Coarse Pointing Mode) is in charge of the spacecraft detumbling after the orbit injection and employs magnetic torquers as actuators and coarse attitude sensors. Magnetic torquers are still the actuators in the successive phase (the Magnetic Fine Pointing Mode [MFPM]), when a fine attitude sensor (a star tracker assembly) becomes available. During the subsequent Propulsion Fine Pointing Mode (PFPM), unlike the GOCE mission, magnetic torquers are replaced by microthrusters; the goal is the attenuation of the angular acceleration within the mission accelerometer range. During this mode, one of the redundant longitudinal mini-thrusters is switched on. The last mode is the drag-free control mode (DFM) in which linear and angular accelerations are attenuated below a threshold dictated by scientific scopes. This is achieved by a hierarchical controller. The inner loop is a wide-band orbital and angular drag-free controller, in charge of contrasting nongravitational perturbing forces (the goal of the orbital control) and all the environmental perturbing torques (the goal of the angular control). To this purpose, the measurements of the six on-board accelerometers are employed. The attitude controller, the outer loop of the hierarchy, which is driven by the star tracker quaternion, accurately aligns the body frame to the orbital frame and cancels the accelerometer bias and drift. The chapter shows that the overall control system can be designed around a mission state predictor that preserves the same structure except for input variables and closed-loop spectra, as they are specific of each control mode. The case study is preceded by preliminary sections on attitude accuracy and the definition of control modes.