Sliding mode control using modified Rodrigues parameters

Spacecraft pointing poses a complex problem involving nonlinear dynamics with either linear and/or nonlinear control laws. The control technique used in this paper is based on sliding mode (variable structure) control. This type of control has been successfully applied for spacecraft pointing and regulation using both Rodrigues (Gibbs vector) quaternion representations. The Rodrigues parameters provide a minimal (i.e., three-dimensional) parameterization. However, a singularity exists for 180-deg rotations which hinders this parameterization for extremely large angle rotations. The sliding mode controller developed here is based on the modified Rodrigues parameters (Marandi and Modi, 1987; Shuster, 1993). The advantages of using this attitude representation include: (1) rotations of up to 360 deg are possible, and (2) the parameters form a minimal parameterization. Simulation results indicate that the new algorithm was able to accurately control the attitude of a spacecraft for large-angle maneuvers. (AIAA)