Research On Operational Mechanism Of A Rotary-linear Permanent Magnet Actuator

With a demand for a wider diversity of different motions and higher performance in the motion control area, electric machines or actuators of multi degree of freedom(Do F) and their direct drive systems are drawing more and more attention. As an important variety of two-Do F machines, rotary-linear machines have been given considerable research efforts in the past decades. This paper proposes a novel rotary-linear permanent magnet actuator, which features a spiral stator slot configuration. It is capable of producing torque and thrust, and as a result, helical motion by the excitation of a single stator. By axially cascading two stators of different slot directions, it can produce rotary and linear motions as well. This actuator has the advantages of a compact structure, high efficiency and high torque and thrust density. It has a good prospect in applications such as robotics, machining, EV/HEVs, PCB printing and so forth.First, the stator and rotor configuration of this actuator is introduced and the expressions for phase back-EMFs are deduced analytically. These expressions reveal the relative frequency and amplitude of phase EMF under rotary, linear and helical motions, as well as the phase unbalance caused by linear motions. The conventional transformation theorem between reference frames is modified to adapt the actuator’s unique stator and rotor structure and different motion modes. The transformed flux and voltage equations are put forward and the mechanism of torque and thrust production are given in two point of views, electromechanical energy conversion and the delivery of electromagnetic power. The ratio of torque and thrust is given in terms of structural parameters.Then, taking the excitation of a single stator and its helical motion as the research subject, the model for electromagnetic field simulation is established using Mag Net. The simulation results for field distribution and phase back-EMFs are obtained. The radial component of the air gap flux density distribution is extracted and its harmonics are analyzed by 2D Fourier transform. The curves of torque and thrust versus time, as well as those of time averaged torque and thrust versus current are given. The torque and thrust constants for magnetically linear range are calculated.After that, the theory of torque and thrust production by two axially cascaded stators is explained and the entire structure of the proposed rotary-linear actuator is introduced. The frequency and phase sequence of the phase currents of the two stators are explained for rotary or linear motions of forward and reverse directions. The different motions are simulated and the efficiency of these motions is calculated.Finally, the axial forces under both no-load and load conditions are compared to find the causes of the relatively large thrust pulsation. To obtain the minimum peak-to-peak thrust pulsation, the pulsation due to end effect need to be depressed. This is achieved by translating the axial force of one stator to find the distance between two stators that results in the minimum thrust pulsation. With this method, the thrust pulsation is reduced by about 60%.