Research on flux-modulated direct-drive motors for robotic manipulators

Abstract

With the development of automation fields such as industrial robots and electrified transportation, direct-drive motors that enable multi-degree-of-freedom motions are attracting much attention. Taking rotary-linear motion as an example, the original transmission is accomplished by two rotary motors with ball screws. This non-direct-drive motion mechanism has low integration and low energy conversion efficiency. The motor that can replace this mechanism needs to have two main characteristics. The first is that a single motor can realize rotary and linear movements in any position over the full stroke. The second is that the motor needs to have high torque density and force density. Existing rotary-linear motor solutions generally have the trouble of low torque density and force density. Some motor schemes also have drawbacks in terms of short stroke, mutual influence of magnetic fields in both directions, difficulty in control, and inability to decouple motion in both directions. Therefore, this thesis aims to explore a novel rotary-linear machine. By adopting the structure of dual-PM multiple direction flux modulation, the torque density and force density are significantly improved. Meanwhile, the drawbacks mentioned above are avoided by some design optimizations. And some control algorithms are proposed to further improve the motor torque and force performance. Firstly, the basic scheme used for flux modulation is determined in terms of winding configuration. Through multi-objective optimization and comparison of schemes, the most suitable winding approach for the flux modulation motor is selected first. This step is to prepare for the design of the rotary-linear machine later. Then, a novel direct-drive rotary-linear machine (RLM) that is suitable for two-degree-of-freedom (2-DOF) kinematic devices has been proposed. This highly integrated machine, with its dual-PM flux modulation structure, possesses high torque and force densities. Instead of the commonly adopted non-direct-drive structure of dual rotary motors combined with the ball screw, the proposed RLM will produce rotary linear motion directly. The design also mitigates the mutual interference of magnetic fields in two different directions. The innovative structure enables the motor to rotate, move linearly or performs helical movement throughout the whole stroke. In this chapter, the configuration of the RLM is introduced first. Then, the consideration of electromagnetic design and the operation principle are explained. Next, the electromagnetic performance of the dual-PM RLM in different states is evaluated and compared with the existing RLM. Finally, a prototype has been produced as the test sample. The experimental results verify the validity of the proposed dual-PM RLM design for 2-DOF motion. After that, the problem of cogging torque enlargement introduced by dual-PM flux modulation motors during machining is researched. Because of the low magnetic reluctance of this motor, the torque is very sensitive to dimensional tolerance. The torque ripple in a synchronous motor may result in noise and vibration. However, the main contributor to torque ripple is the cogging torque. This research analyzes the cogging torque in several different scenarios on the precise model and model with partial random mechanical tolerances. Through derivation calculation and simulation analysis, it is found that the impact of mechanical tolerance on cogging torque can be predicted. It is proven that the impact of mechanical tolerances on cogging torque is significant through analysis. The torque harmonics brought by tolerances is a major part of cogging torque, and its impact is inevitable in the motor entity. In order to reduce the torque ripples, this chapter proposes a harmonic current injection (HCI) method. HCI, as a feedforward compensation of the current loop, generates torque harmonics purposefully to counteract torque ripple. A dual-PM Motor with a split-tooth structure is simulated and tested in this chapter to demonstrate the analysis of torque fluctuations and the effectiveness of HCI. Finally, HCI is applied to reduce the torque and force ripple in load. Unlike the previous studies, the method proposed here can be used without pre-measuring the motor's back electromotive force. By directly utilizing the measured torque or force information and combining it with the previous judgment on the distortion of the magnetic flux, the torque or force ripple can be reduced more precisely.