On Using a Brushless Motor as a Passive Torque-Controllable Brake

When compared to electrically modulated brakes (EMBs), such as a magnetic particle brake, modern brushless direct current (BLDC) motors have substantially higher power densities, and therefore have become an attractive alternative to EMBs for applications requiring controllable modulated resistance (i.e., torque-controllable braking). Unlike brakes, however, which fundamentally guarantee strict passivity, emulating mechanical resistance with a motor does not guarantee passive behavior. In order to enable the use of a motor as a torque-controllable brake with fundamentally guaranteed passivity, this paper presents a control scheme that provides high-fidelity torque tracking and physically guarantees strictly passive behavior. Specifically, this paper: (1) describes model-based control methods for applications requiring high-fidelity torque control and strictly passive impedance; (2) presents an analysis that characterizes the limits of control and tradeoffs of controllability and energy generation associated with using a motor in this manner; and (3) presents experimental results that validate both the system characterization and control performance for the strictly passive controllable impedance approach.