Mr-brake with permanent magnet as passive actuator for haptics

In haptics applications, fast, stable and crisp force responses are desired. Magnetorheological (MR) brakes are viable actuators for haptics since they provide high torque-to-volume ratios with fast response. However, they tend to be rather large and use high current inputs. The actuator size is mainly affected by two factors: (1) design of the magnetic flux path, and (2) the number of coil turns inside the device. To address these issues, we developed a new MR-brake combining serpentine flux path design and a permanent magnet. The new design also provides a fail-safe feature with the help of the permanent magnet while reducing the actuator size. To the best of our knowledge, this is the first such rotary MR-brake. The design was optimized using the Taguchi optimization method. A prototype was fabricated with 45.5 mm diameter, 63.5 mm length and 2.0 Nm maximum torque output. The prototype was tested to assess its effectiveness as a 1 degree-of-freedom haptic device in virtual wall collision. The smaller new MR-brake provided a crisp torque response and simulated the virtual wall collision successfully.