It depends upon what you want to control. Pressure, temperature, level, flow, density etc. Then you get open loop where your controller has a fixed response and sort of guesses (technical term) at the result. Closed loop controllers can have feed forward control, measures what is going in and makes changes to reduce any error, and feedback control which measures what is coming out and makes the changes to reduce error.
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What's the significance, if any, of 'pilot plant'?
A controller is a controller is a controller. A Portsmouth Valve (a.k.a. 'ballcock') is a combination of a level sensor and a proportional controller.
The earliest ones in industry were largely pneumatic. These have been superseded by electronic stand-alone units, electronic custom units and, these days, by algorithms embedded within a control unit with much greater functionality.
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controllers
Re: controllers
