Development of a fail-safe magnetorheological fluid device using electro and permanent magnets

An important design strategy in the safety of any mechanical system is the “fail-safe” concept, which is used to prevent inevitable mistakes from resulting in accidents. For a failsafe design, the proper combination or selection of both normally open and closed devices is important. However, most conventional Magnetorheological fluid (MRF) devices are normally open, and their output force/torque disappears in the off state of the current input. Therefore, the development of a normally closed MRF device (NC-MRD) may contribute to a fail-safe mechatronics system. In this study, we analytically and experimentally investigated the effects of separated axial magnetized permanent magnets (PM) on normally closed and rotary-type MRF devices. We used two types of separated axial PM as magnetization resources in the off state of the electric magnet (EM). The distributions of the magnetic flux densities in the off and balanced states were evaluated as representative values of the NC-MRD. As a real testbed, we designed and developed the NC-MRD using three commercially available sectional axial neodymium magnets. The experimental results showed that the device generated a braking torque in the off state of the EM. With a positive input current of approximately 200 mA, the torque was almost balanced, and the minimum torque was less than 0.1 Nm, which was less than 2% of the maximum torque.