Development of a precisely controlled magnetorheological fluid brake for medical ergometers

This contribution presents the development of a brake based on magnetorheological fluids (MRF) for the use in medical ergometers. In contrast to eddy current brakes, MRF allows the required braking torque to be generated more independently of the rotational speed. By varying the intensity of the magnetic field, the braking torque is to be adjusted depending on the required pedalling resistance. Making it interesting for both rehabilitation and competitive sports. The conceptual design of the brake considers requirements for service life, installation space and production costs. An asymmetric T-drum concept, consisting of radial and axial shear gaps, proves to be the most suitable. A hybrid magnetic circuit consisting of an electromagnet and a permanent magnet is used. The resulting magnetic flux and thus the generated braking torque at different coil currents is determined by simulations and compared with experimental results. The actual breaking torque can be predicted with a high accuracy. A defined load case is simulated to estimate the thermal load on the brake and the need for additional cooling.