Although sliding mode control systems are qualitatively well known for possessing robust performance, the robustness of an electrical drive system against external disturbances of a system designed in this way, especially a step load application, may vary due to certain factors such as an integral control action and smooth control algorithms which are often incorporated in a practical system. A quantitative analysis of the robustness of a dc drive control system is attempted in this paper. The time domain expressions of the transient speed deviation and its maximum value due to a step load application under sliding mode control have been obtained. A new design approach is developed which effectively enhances the robustness of the system against extemal disturbance. The fast response speed and the robust performance are both achievable without the necessity of compromise. Digital simulations verify the design and results.

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