Development of a Small Rotary MR Brake Working in Mixed Mode for Haptic Applications

In recent years, magnetorheological (MR) brakes have been used to provide passive force/torque feedback in various haptic devices due to their safety and low power consumption. However, how to make MR brakes more compact in structure to provide a wide range and high-precision feedback force/torque for haptic interaction remains to be studied. To further miniaturize haptic devices and offer a wide range of feedback force/torque for interaction, this study combines the advantages of linear and rotary MR brakes to design a novel rotary MR brake that operates in mixed mode. The brake utilizes a cam mechanism to convert the rotating motion of the shaft into the reciprocating linear motion of the piston, thereby significantly improving the torque-to-volume ratio (TVR) of the MR brake. The test results on the self-built experimental platform show that the brake can achieve a maximum output torque of 1.446 Nm, a TVR of 42.4 kN/m(2), and good dynamic characteristics. Furthermore, in order to improve the accuracy of torque feedback, a hybrid actuator composed of the MR brake and a dc motor is designed, and a closed-loop control scheme is constructed to manage the output torque of the hybrid actuator. Psychophysical experiments were conducted to evaluate the subjective perception of the output torque of the two actuators. The results show that improving the accuracy of torque feedback is beneficial to enhance the realism of haptic interactions and the transparency of haptic systems.