Combination Open-Cycle Closed-Cycle Systems

The ancient idea of a combination coarse and fine adjustment is shown to be applicable to the design of precision automatic control systems. In the particular class of systems discussed, the coarse adjustment is taken to be a separate element operated by the input, but outside the feedback loop. Its position outside the feedback loop qualifies the coarse controller as an open-cycle system, and makes it possible to introduce such elements without affecting the system's transient response adversely. In this way, interference equalization of dynamical distortion errors is possible without such critical dependence being placed on a knowledge of series elements of the system as is required for interference equalization by a controller in the feedback loop. Three broad types of open-cycle systems are discussed: series, parallel, and partially parallel. Each of these may be "algebraic," "differential," or a combination. The algebraic controllers are useful when the average value of the input signal is predictable-particularly where a repetitive duty cycle is encountered. The advantages of adding completely parallel open-cycle elements for improving speed range and reducing over-all cost are shown. Idea is also applicable to nonlinear and multiplicoupled systems.