Transient Thermal Analysis of a Magnetorheological Knee for Prostheses and Exoskeletons during Over-Ground Walking

Abstract

Proper knee movement is essential for accomplishing the mobility daily tasks such as walking, get up from a chair and going up and down stairs. Although the technological advances in active knee actuators for prostheses and exoskeletons to help impaired people in the last decade, they still present several usage limitations such as overweight or limited mechanical power and torque. To address such limitations, we developed the Active Magnetorheological Knee (AMRK) that comprises a Motor Unit (MU), which is a motor-reducer (EC motor and Harmonic Drive) and a MR clutch, that works in parallel to a magnetorheological (MR) brake. Magnetorheological fluids, employed in the MR clutch and brake, are smart materials that have their rheological properties controlled by an induced magnetic field and have been used for different purposes. With this configuration the actuator can work as a motor, clutch or brake and can perform similar movements than a healthy knee. However, the stability, control, and life of magnetorheological fluids critically depend on the working temperature. By reaching a certain temperature limit, the fluid additives quickly deteriorate, leading to irreversible changes of the MR fluid. In this study, we perform a transient thermal analysis of the AMRK, when it is used for walking over-ground, to access possible fluid degradation and user’s discomfort due overheating. The resulting shear stress in the MR clutch and brake generates heat, increasing the fluid temperature during the operation. However, to avoid overheating, we proposed a mode of operation for over-ground walking aiming to minimize the heat generation on the MR clutch and brake. Other heat sources inside the actuator are the coils, which generate the magnetic fields for the MR fluid, bearings, EC motor and harmonic drive. Results show that the MR fluid of the brake can reach up to 31°C after a 6.0 km walk, so the AMRK can be used for the proposed function without risks of fluid degradation or discomfort for the user.