Disturbance accommodating spacecraft attitude control using thruster

Schmitt trigger logic is combined with DAC(Disturbance Accommodating Control) for the 3-axis spacecraft attitude control. Schmitt trigger control logic has been used for on-off gas jet thruster to reduce high frequency chattering control. However, between the inner and outer deadband, schmitt trigger may also have the faster firing limit cycle than the desired one, which is due to disturbances. This paper is to detect, to identify and to cancel disturbances by DAC. DAC approximate the actual disturbance with a set of designer selected base functions. The base functions are constructed by the linear time invariant system. For a short time interval, disturbances are assumed to be linear combinations of the base function. DAC observer identifies the coefficients of the linear combination of these base functions. The estimated disturbances are canceled by the additional thruster firing which is computed by DAC. Schmitt trigger and DAC observer are combined to improve the pointing accuracy and to save the gas jet usage. Simulation results show that schmitt trigger with DAC has a lower limit cycle frequency and better pointing accuracy with less jet usage.