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This brief proposes an iterative sliding mode observer (ISMO) for the robust sensorless control of a permanent magnet synchronous motor with variable parameters. In the conventional SMO, a low-pass filter and an additional position compensator for the rotor are used to reduce the chattering coming from the switching by means of a signum function. It is shown that the chattering can be further reduced by using a sigmoid function as the switching function in the observer. This observer is faster in estimating the velocity and position of the rotor than the traditional adaptive SMO, since it does not include the integral operations for the low-pass filter. The proposed ISMO also improves the performance in estimating the motor speed and angle by reducing the estimation error in the back electromotive force by iteratively applying the observer in the sensorless operation. The stability of the proposed SMO is verified by the Lyapunov function in determining the observer gain, and the validity of the observer is demonstrated by simulations and experiments.