ABSTRACT
A duplex controller has one input and two outputs. It may have two control mechanisms, each with an output, or a single-control mechanism operating two control valves by means of relays or positioners. The control action is pneumatically produced by a conventional controller with proportional action. Auto-selector control is the opposite of duplex in that it allows the automatic selection between two or more measurement inputs and provides a single output to control a single valve. The auto-selector-control system continuously senses all measurements applied to it and provides control action to the valve based on the value of the measurement closest to its particular set point. The objective of the cascade-control system is the same as that of any single-loop controller. Its function is simply to achieve a balance between supply and demand and thereby maintain the controlled variable at its required constant value.
