Design, Analysis, and Characterization of a Small-Scale High-Torque Magnetorheological Brake for Haptic Applications.

Achieving both high torque for convincing kinesthetic feedback and a compact form factor for user comfort in magnetorheological (MR) actuators remains challenging due to the rapid degradation of torque with reduced dimensions. To address this limitation, a small-scale, high-torque MR brake with a novel dual multi-drum configuration is proposed for haptic applications. In this configuration, two identical multi-gap MR shear regions are located on both sides of an electromagnetic coil. Once an excitation is applied, the MR shear regions are simultaneously activated, effectively maximizing active shear areas within the limited volume. This configuration facilitates a significant torque output without compromising the brake's compactness. The optimized brake prototype has compact dimensions of Ø29.2 × 44 mm and a mass of 171.5 g, with a peak torque of 1165.4 mN$\cdot$m. The torque-to-volume and torque-to-mass ratios are 39.6 kN/m$^{2}$ and 6.8 mN$\cdot$m/g, respectively, which are higher than those of other MR brakes of comparable size. In a practical scenario, a handheld haptic device based on the brake prototype was constructed. The experimental results demonstrated controllable torque rendering, with a just noticeable difference of 54.18 $\pm$ 17.73 mN$\cdot$ m and a Weber fraction of 11.91 $\pm$ 3.90%, thereby highlighting its potential for small-scale, high-torque haptic actuation.