Field-Weakening and Fatigue Management for Explosive Robotics

Explosive motions in robotics require both high strength and high velocity actuators. The electrical dynamics of the Permanent Magnet Synchronous Machine (PMSM) motor can be exploited to increase the peak-velocity. An operating method known as field-weakening uses existing current reserves to produce a counteracting magnetic field. This alleviates the voltage constraints, which in turn allows for an increase in peak velocity. Two field-weakening algorithms were analysed. The Feed-Forward field-weakening algorithm as designed in: ‘Enhanced Explosive Motion for Torque Controlled Actuators Through Field Weakening Control’ by W. Roozing, was selected. Its only disadvantage is that it strongly depends on the motor parameters. It is evaluated how the performance of this algorithm changes if the motor parameters known in the controller do not fully agree with the actual motor parameters. Results show that the performance of the feed-forward field-weakening strategy quickly degrades if the motor parameters in the controller are not in agreement with the actual motor parameters. Furthermore, a lot of power is required to perform an explosive motion, hence there is a risk of overheating the motor. A thermal management strategy is presented that has a variable current limit based on the temperature and thermal time constant of the motor. However, in practice the explosive motion is so short that the nominal current over the whole motion stays well below the maximum continuous current. A jumping leg demo setup is designed in order to apply the field-weakening algorithm to a practical use case. This setup contains a single motor in the hip joint and is constraint such that the leg can only jump up and down along a vertical axis. Simulation results show that the leg can jump about 16 percent higher with the field-weakening control enabled. In practice this improvement was not observed, but the design of the leg can be improved substantially by changing the gear ratio and link length such that field-weakening indeed has an added benefit.