Fractional order controller for satellite attitude control system with PWPF modulator

A nonlinear flexible satellite attitude control system with a single degree-of-freedom is considered in the framework of fractional order calculus. A pulse-width pulse-frequency (PWPF) modulated thruster is the main actuator in the system. To avoid interaction of the pulse modulation with the flexible structure modes, a new method - fractional describing function method is proposed to describe the nonlinear dynamics of PWPF actuators. Thus, the plant is considered as a fractional order system and a fractional order [PD] controller is synthesized for the rigid body. The fractional order controller is robust to resonances with high frequencies. Both bandwidth and phase margin of the system with proposed fractional order controller are larger than that of system with traditional lead-phase compensator. The fractional order modeling and design method developed here is practical and simple.