Open and Closed-Loop Feedback Systems: Rate Action

In the early pneumatic and solid-state controllers, derivative action was referred to as rate because it controlled the rate of change for the controller by providing more or less time delay to the response. For solid-state, op amp controllers, this was done by adding capacitance to the system. In pneumatic controllers the time delay was changed by adjusting small valves to bleed off or bypass air that the controller was sending to the output device.

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