In this chapter has been proposed a robust sensorless cascade control scheme for a Permanent Magnet Synchronous Motor (PMSM) drive. A Discrete Time Variable Structure Control (DTVSC) is considered and the rotor position and speed are obtained through an Adaptive Extended Kalman Filter (AEKF). The performance of the filter is improved by an on line adjustment of the input and measurement noise covariances obtained by a suitably defined estimation algorithm. The proposed solution is experimentally tested on a commercial PMSM drive equipped with a control system based on a floating point Digital Signal Processor (DSP).