Optimum compensation of a position servo with a magnetic clutchactuator

Abstract

In the past decade improvements have been made in the design of dry powder magnetic clutches which now make them competitive with hydraulic devices for missile servo actuator applications. Some of the inherent advantages of magnetic clutch actuators are: (a) increased control system reliability, (b) simplified equipment requirements, (c) properties remain constant with usage, (d) environmental requirements are quite compatible with an orbiting vehicle. The dynamic servo response of the magnetic clutch servo actuator is required in order to incorporate these advantages in missile and space control system design. This paper presents a synthesis procedure for the optimum design of a position servo based upon the closed-loop transient response to a step input. The transient response of the position servo is described by four independent parameters. These parameters are the rise time, damping ratio, undamped natural frequency, and the steady-state gain. The synthesis procedure is based upon a theoretical model of a dry powder magnetic clutch, which is an air-gap, iron-care transformer with a one-turn secondary. The air gap is filled with dry ferrite particles. To provide simple analytic functions, a definition for rise time is developed which corresponds to the first crossing of unity gain to a high order of accuracy. The theoretical model of the magnetic clutch has two, first-order time lags. These two time lags are derived from the properties of the theoretical clutch model. They arise from the inductive properties of the clutch excitation coil and the clutch rotor induced eddy current, which is the current in the equivalent transformer single-turn secondary. The compensation for the excitation coil time lag consists of excitation current feedback, which decreases the effective transfer function time constant. The induced rotor eddy current time lag is optimally compensated by the proper selection of feedback gains as outlined in the synthesis procedure. The position servo drives an undamped inertial load. This is characteristic of a control system for an outer space vehicle or a guided missile which utilizes swivel rocket nozzles for control. It is also a first-order approximation to an airframe control servo utilizing neutral aerodynamic surfaces with low damping. The position servo characteristics are provided by a position feedback branch, and the basic servo stability is provided by a tachometer feedback. The theoretical model results in a third-order system which cannot be unstable. Practical magnetic clutches differ from the theoretical model primarily because they have ferromagnetic hysteresis. At high frequencies, hysteresis appears as a constant time delay which makes servo instability possible. An additional constraint to the synthesis procedure is given which guarantees a stable, closed-loop position servo with the desired performance characteristics. The entire analysis in this paper is based upon the application of Root-Locus analytical methods to extract the theoretical roots of the third-order system describing the magnetic clutch actuator position servo.