- The robust design of the Pulse Width Modulation (PWM) speed control of DC permanent magnetic motors is studied in this paper using the Six Sigma approach. A first principle model is developed for simulating the PWM motor speed control system. The Six Sigma DMAIC process is followed to reduce the variation in the average motor speed. The main factors of the cause for the average speed variation are identified using Design of Experiment (DOE). The Response Surface Method (RSM) is applied to derive a new design. The new design significantly reduces the variation without tightening the hardware tolerance bands. The entire Six Sigma project is carried out using modelling and simulation. 2008, Inderscience Publishers.