Modified direct thrust control of linear permanent magnet motors with sensorless speed estimation

Abstract

A modified direct thrust control of linear permanent magnet motor with sensorless estimation of mover position and speed is presented. The improved direct thrust control is based on decoupled control of thrust and flux in the stator flux frame using space vector modulation. The thrust regulation loop in the stator flux reference frame is developed using a linearized relation between thrust and load angle. The thrust and flux errors are used as inputs to two PI controllers to generate the orthogonal components of the reference voltage in the stator flux reference frame. These orthogonal components are further converted to stationary frame of reference by using the stator flux vector angle so that required voltage vector can be generated using space vector modulation instead of the conventional switching table. This scheme retains all the inherited advantages of classical direct thrust control. In this research the flux vector angular position and the mover speed is estimated from the orthogonal components of stator flux vector without using any position or speed sensor. The proposed approach is experimentally validated by its application to a prototype linear permanent magnet motor drive system in the laboratory. Practical results prove that excellent thrust and flux response with minimized ripple is achieved by the proposed technique. These practical results also demonstrate the effectiveness of sensorless position and speed estimation. Moreover a constant switching frequency is achieved due to space vector modulation.