Characterization and control of a new high-torque motor for autonomous wearable robotics

Abstract

A new 'axial-transverse flux' motor (ATFM) topology is of interest to autonomous lower-extremity robotics designers for its high torque density and low winding resistance. Unfortunately, deliberate asymmetries in the design make finite-element modeling of this topology largely intractable. An ATFM prototype was characterized experimentally using a custom dynamometer and controller. The prototype was found to have a torque constant Kt of 7.26 Nm/A and a per-phase winding resistance of 0.59 Ohms. It is characterized by high AC and DC zero-current torque, as well as significant torque ripple (M: 12.9%, SD: 0.6%) when driven with balanced three-phase sinusoidal commutation. A set of optimized commutation waveforms are developed based on an independent phase control strategy, and it is shown that this strategy can eliminate ripple in simulation and reduce it in practice (M: 7.8%, SD: 0.5%), without reduction of mean torque or increased conduction losses relative to sinusoidal commutation.