MR-brakes are used in many applications including prosthetics, automotive, vibration stabilization and haptics. To obtain large braking torques, the brakes are required to be rather large and use high input current. This research explored a new approach, called serpentine flux path, to shape the magnetic flux path leading to more compact rotary designs. Our prototype MR-brake is about 2.7 times more powerful than the only commercially available rotary brake and about 33% smaller in diameter. Furthermore, with 10.9 Nm torque output, it is more powerful than the closest brake in the literature with 6.9 Nm torque but only approximately half the diameter and requires 62.5% less current to activate.
The serpentine flux path approach, developed at our laboratory, strategically places magnetically conductive and non-conductive elements to decrease the cross-sectional area of the magnetic flux path by bending the magnetic field and forcing it to pass through the MR fluid gap several times. This leads to a more compact brake design because more surface area of the MR fluid can be activated in a given volume. We could increase the braking torque without increasing the size of the brake.
Another contribution of the research was a ferro-fluidic seal. In general, MR-brakes use O-rings to prevent leakage of the fluid. The O-Ring increases the off-state friction of the brake. In our design we used a ferro-fluidic sealing technique which reduced the off-state torque and sealed the fluid. These ferro-fluidic seals work by using the MR fluid itself as a sealing element. Ring magnets placed at both ends produce magnetic flux paths that cross the gap between the shaft and the chassis. The magnetic field increases the yield stress of the MR fluid inside the gap, which builds a pressure differential that keeps the MR fluid from leaking out. As there is no active contact between solid bodies this approach helps decrease the off-state friction tremendously which is critical in haptic displays.
This MR-brake also uses the serpentine flux path approach in its design. The active section of the brake is about 1.1 inches in diameter, 1.4 inches long and can produce about 1.5 Nm torque with 1A of current.
- ” Compact MR-brake with serpentine flux path for haptics applications,” Third Joint Eurohaptics Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems Salt Lake City, UT, 2009.
- ” Serpentine flux path for high torque MRF brakes in haptics applications,” Mechatronics, vol. 20, pp. 377-383, February 2010.
- ” Haptic interface with hybrid actuator for virtual needle insertion and tissue cutting,” IEEE Haptics Symposium, Vancouver B.C., Canada, March 2012.