Directional damping control: vibration reduction and optimality for precision slide motion

Abstract

When driving a lightly damped system with a servo motor, the design of a controller that reduces disturbance-excited vibration can be undermined by the need to maintain the system bandwidth to achieve good tracking performance. The use of classical lag/lead compensation and conventional filtering techniques to dampen the oscillatory modes often has adverse effects on the system bandwidth. This is the case with precision hydrostatic fluid-bearing slide systems. A control scheme designated as “directional damping control” is developed using classical root locus and frequency response techniques for systems with a lightly damped mode at a frequency considerably higher than the servo bandwidth. The scheme separates the design of the system bandwidth from that of the damping of the highly underdamped mode. The design guidelines are first presented for the general case, followed by a design example of a slide motion controller for precision machining. The scheme is compared with conventional PID control and the H ∞ optimal disturbance rejection controller. The procedure produces a vibration reduction controller similar in nature to the H ∞ optimal solution, but with some practical advantages. Using the scheme, the slide vibration is significantly less than when using conventional PID control, and a corresponding improvement is seen in the surface roughness of machined parts.