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The idea of fail-safe is to prevent the inevitable mistake from turning into an accident. In the case that the conventional MRF devices were used for the transmission part of the rotational joint of some care-robots, the torque is lost when its electric current is unexpectedly shut down. Therefore, additional safety mechanism is required for fail-safe assistive robot. In this study, we developed a torque-control MRF device using electro and permanent magnets as a highly safety mechanism. This device can generates its torque without power supply. The device applies a magnetic flux to the MRF by means of a magnet. The magnetic flux is reduced by the coil current. To confirm the reduction of the magnetic flux, magnetic field analyses were carried out for several patterns. As a result, a segmented model magnet with axial magnetization was adopted. If the magnets are too close to each other, the direction of the magnetic flux is affected. For this reason, a 45° magnet was employed rather than a 75° magnet. Torque measurements were performed on the devices using a characteristic test machine. The device generated about 0.5 Nm in off state. When current was applied, the torque was reduced to a very small value of 0.05 Nm at 200 mA. When the reverse current was applied, a torque of 5 Nm was generated at 1000 mA.
 

MR fluid,fail-safe,design