High Force Density Linear Electromagnetic Actuators for Active Suspension

Active suspension is key to improve ride comfort while maintaining good stability of the vehicle under various road conditions. However, active suspensions which employ linear electromagnetic actuators that directly generate thrust force are generally heavy and bulky due to low power density of the current state-of-art linear electromagnetic actuator technologies. This thesis is concerned with research, development and optimisation of high thrust force density linear electromagnetic actuators for active suspensions, as well as with construction and testing of a selected magnetic screw based high force density linear actuator. A number of high force density linear electromagnetic actuator technologies have been investigated. First, two tubular permanent magnet (PM) vernier motors and three dual-stator PM vernier linear motors have been proposed and globally optimised against the target performance and constraints for an active suspension system. The equivalent thermal model and demagnetization assessment are also described and discussed. The outcome is compared with a reference motor under the same volumetric and thermal constraints. It is shown that the advantage of the PM vernier motor is not obvious but the dual-stator PM vernier linear motor exhibit significantly higher force capability with comparable power factor than that of the reference motor. Subsequently, a linear actuator with an integrated magnetic screw and rotary motor is proposed and optimally designed based on the principle of torque-force transmission and magnetic screw thrust force characteristic. The saturation level in the back iron of the rotor is evaluated and the heat transfer due to the losses in the rotary motor to the magnetic screw is also assessed. Further, 3D FE analysis is used to predict magnetic screw based linear actuator (MSLA) performance. It has been shown that the designed MSLA exhibits extremely high thrust force density while meeting the design specifications. The characteristics of unbalanced magnetic force (UMF) in a magnetic screw under static eccentricity have been investigated analytically and by a full 3-D FE model with high-quality meshes. It is shown that the impact of UMF on the mechanical design and the resultant increase in bearing friction are not significant. Finally, the optimised MSLA and dedicated test rig are mechanical designed and constructed. All key mechanical components of the actuator including the motor housing are described in detail. A practical means of employing discrete parallel magnetised magnet array to form helical magnetisation pattern is discussed and measured B-H characteristics of magnets are represented in the FE model. The back EMF, torque characteristic, static thrust force and transient load characteristics are measured and compared with predictions. The test results demonstrate that the linear actuator with the integrated magnetic screw exhibits significantly high force capability than that of traditional linear electromagnetic actuators.