Compensation of Magnetorheological Brake Output Torque under High Temperatures: A Thermally-Driven Shape Memory Alloy Integration Approach

Solving the thermal induced degradation problem of magnetorheological (MR) actuators is crucial for expanding the operational temperature range. Current strategies for mitigating temperature effects primarily focus on developing precise temperature-viscosity models and implementing thermal design optimizations. However, refined numerical models cannot compensate for temperature-induced performance losses, while thermal design solutions face practical limitations due to size constraints, spatial restrictions, and external energy requirements. Notably, the viscous friction heat from MR fluids and Joule heat from coils constitute redundant energy within MR systems. This study aims at establishing a self-regulating torque compensation mechanism utilizing this systemic redundant energy to resolve thermally induced degradation. We present a shape memory alloy-based MR brake (SMA-MRB) that integrates multiple deployable structures on its shaft surface. Thermal activation of the SMA structures transforms the actuator's operational mode from pure shear to combined shear-flow, enabling performance compensation under elevated temperatures. A dedicated rotational speed-torque testing system was developed to evaluate the braking performance of the SMA-MRB. Experimental results demonstrate that SMA deployment increases the maximum braking differential from 191 rpm to 313 rpm. The SMA-MRB achieves up to 64.7% performance enhancement at operating temperatures exceeding 42°C, completely eliminating thermal degradation effects. This demonstrates that the SMA-MRB completely eliminates the effects of thermal degradation, which expands the application boundaries of MR brakes.