All control systems are subject to constraints on both the magnitude of the corrections and the rate at which they can be applied to a process, and violation of these constraints, in response to severe disturbances, then leads to a deterioration in control performance just when it is most unwelcome.
An attempt is therefore made to explain these effects in simple terms, and it is shown that a change in controller configuration could provide a complete solution. Because this cannot be generally applied, however, the use of a design criterion taking account of the rate constraint, and of high speed valve actuators as a means of economising in capital cost, are discussed with examples.
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