<title>A thin film magnetorheological (MR) damper/lock</title>

Abstract

This paper presents the design, testing, and application of a thin-film Magnetorheological (or simply MR) fluids damper/lock. This MR damper/lock is designed for the use in a model adaptive fan nozzle system actuated by shape memory alloy wires. The MR damper/lock (a total of 8 in the fan nozzle system) will lock the opening size of the fan nozzle and provides damping when the system vibrates. For this purpose, the MR damper/lock has to have the following characteristics: 1) The device is in lock position when power is off. 2) The device has a small static friction force (less than 1 lbf) when power is on. 3) The device generates a small kinetic friction force when it slides during power-on period. 4) Its damping coefficient can be adjusted. 5) Being compact. To meet these requirements, a new design of a damper/lock using thin MR fluid film is employed. The device consists of five major components: two soft steel bars, two stacks of permanent magnets, two groups of magnetic wires, a soft steel slider, and MR fluid. Utilizing the permanent magnets, the MR fluid is trapped and the device is always in lock position. When the device is powered on, the flux of the electrical magnets partially cancels and re-directs the rest of the flux from the permanent magnets, and then the slider is free to move. In this design, MR fluid reduces the air gap and increases locking force when it is powered off. On the other hand, it also functions as a lubricant to reduce the kinetic friction forces when it is powered on. Extensive tests of the MR damper/lock are conducted to reveal its force-displacement curves and force-velocity curves under different applied voltages. Utilizing these testing results, the MR damper/lock is applied to the model adaptive fan nozzle system to perform both locking and damping tasks with a feedback control. Experimental results show that these tasks are successfully achieved.