Computer Mechanics. Positioning Control of an Actuator with Two Degrees of Freedom.

Precise and accurate positioning is required by high-density disk storage devices, such as magnetic disk drives and optical disk drives. Nonlinear friction in a guide system reduces positioning accuracy. In this paper, we introduced a swing arm rotary actuator with two degrees of freedom which overcomes the problem of nonlinear friction and improves positioning accuracy. The moving part of the rotary actuator has two members. The first member runs on ball bearings around the fixed shaft, and the second is suspended from the first by a leaf spring. Driving torque is applied to the second member. Small displacements are achieved by bending the leaf spring, and large displacements are realized by rotation around the ball bearings. We analyzed our actuator theoretically and found that its positioning is stable with a suitable lead lag compensator. We conducted a numerical analysis of the control system, including the influence of nonlinear friction, using the Runge-Kutta-Gill method. The positioning accuracy of our system is clearly higher than that of a conventional rotary actuator positioning system. We also present the performance of an actual actuator. We obtained an error suppression ratio of better than -60dB for a sinusoidal input of 1 μm at 60Hz.