A novel approach to robust motion control of electrical drives with model order uncertainty

Abstract

A novel approach to the control of plants with model order uncertainty as well as parametric errors and external disturbances is presented, which yields a specified settling time of the step response with zero overshoot. The method is applied to a motion control system employing a permanent magnet synchronous motor. A single controller is designed to cater for mechanical loads that may exhibit significant vibration modes. The order of the complete controlled system (i.e., the plant) will therefore depend on the number of significant vibration modes. The controller is of the cascade structure, comprising an inner drive speed control loop and an outer position control loop. The main contribution of the paper is a completely new robust control strategy for plants with model order uncertainty, which is used in the outer position control loop. Its foundations lie in sliding mode control, but the set of output derivatives fed back extend to a maximum order depending on the maximum likely rank of the plant, rather than its known rank. In cases where the maximum order of output derivative exceeds the plant rank, in theory, virtual states are created that raise the order of the closed-loop system while retaining the extreme robustness properties of sliding mode control. Algebraic loops (caused by zero or negative rank of the open-loop system) are avoided by embodying filtering with a relatively short time constant in the output derivative approximations. The speed control loop is also new. Although it is based on the forced dynamic vector control principle, already developed by the author and co-researchers for drives with current fed inverters, for the first time, a version for voltage fed inverters is presented with a view to future implementation of space vector modulation to improve the smoothness of the stator current waveforms. The new forced dynamic control law requires an estimate of the load torque and its first derivative and a special observer is presented for this purpose. An initial evaluation of the method is made by considering three plants with different orders and ranks, the first being the unloaded drive, the second being the drive controlling the motor rotor angle with a mass-spring load attached and the third being the drive controlling the load mass angle of the same attached mass-spring load. The simulations indicate that the control system does indeed yield robustness including plant order uncertainty and further investigations, both theoretical and experimental, are recommended.