Page 9 - Instrumentation and Measurement

Manipulated variable is the input variable or parameter to a process that is varied by a control
signal from the processor to an actuator. By changing the input variable, the value of the measured
variable can be controlled.

1.5 Control Loops

An open loop system has no feedback and controller action is not related to final result. Consider a
domestic central heating system as an example, in which fuel supply is varied manually or
automatically by external ambient temperature. Room temperature will be maintained at a
reasonable value related to outside conditions. However, room temperature does not control fuel
supply so that this is open loop.
Now to the open loop shown, add human operator, so closing the loop (dotted lines on Figure 1.1).
This is a manually controlled closed-loop system.

Figure 1.1 Control Loops.
The automatically controlled closed-loop system is actuated by a signal dependent upon deviation
(error) between input (set) and output values. Deviation only exists when monitored output
(negative feedback) differs from input and this signal controls power supplied to output. For a
closed-loop system, as sketched, output power is only controlled by, and not supplied by, the
actuating signal. Closed loops have a self- regulating property.

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