Control of permanent magnet synchronous motor with dual-mode position estimation

Optimal torque control of permanent magnet motors requires precise position information of the rotor. Normally, high resolution sensors for position measurement do not meet the robustness requirements for vehicles. In view of robustness and reliability, speed sensorless methods like EMF-based observers are favored for motor control in such applications. Because these observers do not work near zero speed, an additional method is necessary to cover the full speed range. Unfortunately, methods using injection of additional signals do not fulfil the reliability and noise specifications in automotive applications so that speed sensors are still used as standard solution for these electrical drives. In this paper, a novel dual-mode position estimation structure is presented, which uses a robust, low resolution position sensor near zero speed. At normal speed, sensorless EMF-based principles are applied. The proposed method enables a smooth transition between the low speed operation and the fully sensorless operation. The approach is already implemented in a research vehicle with a hybrid propulsion system