Selection and implementation of optimal magnetorheological brake design for a variable impedance exoskeleton robot joint

Next-generation exoskeleton and humanoid robots are expected to behave similar to the human neuro-muscular system to perform stable, flexible, and biomimetic movements. To achieve this goal, the variable stiffness actuators have been widely used in various robots. Using variable damping actuators along with variable stiffness actuators will be extremely beneficial for wide range of stable movements. Magnetorheological (MR) brakes are one of the most promising electromagnetic structures that can provide such variable damping in a relatively small actuator volume. In this paper, we focused on the design, characterization, selection and implementation of T-shaped, inner coil and outer coil multi-pole MR brakes to the ankle of an exoskeleton robot. Analytical models are developed using the magnetic circuit analysis to determine the braking torque. Then, magnetic finite element models are developed and coupled with an optimization algorithm to determine the optimal set of parameters of each MR brake design. Prototypes are manufactured in same size and tested experimentally to characterize the actuators' torque-to-volume ratio, transient response, hysteresis, torque tracking, energy consumption, and damping performances. The performance comparison of the brakes showed T-shaped multi-pole MR brake design has superior characteristics compared to two other designs. Therefore, T-shaped multi-pole MR brake design is coupled with a variable stiffness actuator and implemented in an ankle joint of an exoskeleton robot and experimentally tested. The results show that the developed new hybrid robot joint is capable of stable movement with a simple control algorithm by changing its stiffness and damping independently.