Spacecraft attitude control using an induction motor actuated reaction wheel with sensorless forced dynamic drive

A new drive control system is presented for reaction wheels enabling an induction motor to replace the usual and far more costly brushless d.c. motor. The drive is sensorless (i.e., no shaft mounted position or velocity sensors are required). The new drive control system competes with the state of the art methods (vector control and direct-torque control) in the following respects. (a) No time varying rotation matrix is needed in the control algorithm through working in the stator-fixed α-β frame. (b) Robust control principles are employed enabling the drive to directly control the angular acceleration of the spacecraft platform on which the reaction wheel is mounted. This feature greatly simplifies the attitude control system design by effectively converting the controlled plant into a pure double integrator. The new drive control system is presented together with an attitude control algorithm with which it may be used. This requires only the output of an attitude sensor, the velocity feedback for attitude stabilisation being obtained from an observer. The attitude control algorithm combines the theories of time optimal control and sliding mode control to achieve near-time-optimal slewing for large step attitude angle reference inputs and precise platform control for constant or slowly varying reference inputs. A special algorithm is employed to estimate the relative angular velocity between the reaction wheel and the spacecraft platform, thereby providing a monitoring facility for wheel momentum unloading