Direct thrust force control of a linear permanent magnet synchronous motor

Abstract

This thesis is focused on the direct thrust force control (DTFC) of tubular surface-mount linear permanent magnet synchronous motors (PMSM). The linear PMSMs are typically characterized by a low value of stator inductance and a short pole-pitch. Consequently, linear PMSMs exhibit higher ripple in thrust force under conventional switching table based DTFC which is typically beyond acceptable limits. In order to reduce the ripple and steady-state error in the stator flux and thrust force response of the linear PMSM a novel duty ratio control based control (referred to as DTFC1) is proposed to adapt switching table based DTFC. Another approach to effectively reduce steady-state thrust force ripple is to use Space Vector Pulse Width Modulation (SV-PWM). Five novel DTFC schemes based on SV-PWM are proposed in this thesis and are referred to as PI-DTFC, Optimal-DTFC1, Optimal DTFC2, SM-DTFC1, and SM-DTFC2. In PI-DTFC, the stator flux and thrust force regulation is achieved by two PI controllers. The PI-DTFC scheme is a benchmark for comparison with other SV-PWM based control schemes proposed in the thesis. In PI-DTFC the speed is regulated by using a third PI controller. Optimal-DTFC1 and Optimal DTFC2 are based on a novel 2nd order and a novel 3rd order multiple-input-multiple-output state space models of linear PMSM respectively. The formulation of the optimal control laws in these control schemes are based on a linear quadratic regulator (LQR) based approach with augmented integral action ensuring a zero steady state error. Optimal-DTFC2 provides a combined speed and direct thrust force control of linear PMSM and eliminates the need for the speed PI controller used in the case of Optimal-DTFC1 to close the speed control loop. SM-DTFC1 and SM-DTFC2 are based on the sliding mode control approach with augmented integral action and provide combined speed and direct thrust force control for the linear PMSM. In SM-DTFC1 the sliding surfaces are formulated in terms of the tracking errors in the stator flux and the speed. In SM-DTFC2 integral of the tracking errors in the stator flux and the speed are used. All the proposed control techniques are experimentally validated for their superior control performance in comparison to their respective state of the art techniques. Lastly, a stator flux observer for sensorless speed estimation comprising a linear state observer and an improved sliding mode component is proposed and experimentally validated.