Nonlinear system robust control-application on a stepping motor

Previously very efficient mathematical models for the stepping motor have been developed. The particularity of these models is that they give good results even at high frequencies. Many control algorithms based on the proposed reference models have been developed and tested on stepping motors. The mechanical load is considered constant. The control robustness has never been analysed with this type of control method driving stepping motors. However, the robust control algorithms could permit the use of stepping motors in robotics. In this paper the robust control method for nonlinear systems (stepping motors with variable load) is proposed. The load variations are relatively important and the control robustness is obtained by limiting the global performances. The authors have realised only the first approach to the robust control. Very large nonlinearities of reference model don´t permit classical robust control analysis and synthesis, then the control law is obtained by simulation methods. It is also possible to linearize the reference model and to use standard robust control methods but this approach disables the control at high frequencies. The reference model and the maximal torque control method (optimal control for the stepping motor) are presented, leading to definitions of the stepping motor safety range and safety factor. Then, from the safety range definition, a robustness criterion is obtained. This criterion is based on the stator phases commutation positions. Finally the numerical simulation of the stepping motor control with important mechanical load variations is given