Field-weakening and fatigue management for explosive robotics

Highly dynamic and explosive motions of robots (e.g. jumping) require very high peak power output and velocities of their joints. However, the necessity for gearboxes to achieve sufficient joint torque capability limits their achievable peak velocity due to supply voltage constraints.

One method to improve the peak velocity of electrical motors is field weakening, that exploits the electrical dynamics of Permanent Magnet Synchronous Machine (PMSM) motors. By leveraging existing current reserves, a counteracting magnetic field may be generated that alleviates voltage constraints, thereby increasing achievable peak velocity.

Although initial work has shown the effectiveness of field-weakening on torque-controlled actuators for robotics [1]-[2], several important questions remain. For instance, [1] achieved field weakening in a feedforward fashion using given system parameters such as resistance, inductance and flux linkage. Suitable identification and/or adaptation of these parameters is important for optimal performance.

Furthermore, explosive motions in robotics are usually of short duration, much smaller than thermal time constants of motors. As the allowable winding current is mostly given by temperature, this necessitates a suitable strategy to measure and predict future winding temperatures and determine the achievable performance. The current thermal and power states of the joint can be referred to as its fatigue, which needs to be managed to maintain optimal performance.

This assignment focuses on achieving field weakening in an intelligent manner that includes thermal management of the motor. Several avenues may be pursued depending on the expertise and interest of the student. Ideally, the developed strategy will be experimentally demonstrated in the lab.

[1] W. Roozing, N. Kashiri, and N. G. Tsagarakis, ‘Enhanced Explosive Motion for Torque Controlled Actuators Through Field Weakening Control', in IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), doi: 10.1109/IROS.2018.8593608, 2018.
[2] M. Mohammadnia, N. Kashiri, F. Braghin, and N. G. Tsagarakis, ‘Flux Regulation for Torque-controlled Robotics Actuators', in 19th International Conference on Advanced Robotics (ICAR), doi: 10.1109/ICAR46387.2019.8981613, 2019.