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01 May 2004

Single feedback control loop

Any given process will have a number of different controlled variables, and for each controlled variable, there is an associated manipulated variable.

A specific controlled variable ties to a specific manipulated variable through the appropriate feedback control hardware.

The controlled variable value registers on appropriate instrumentation, and this sensed value of the controlled variable compares to the desired value of the controlled variable—the set point.

The difference between these two—the error—is used as input to the feedback controller. This controller then calculates a signal to adjust the manipulated variable. Because the manipulated variable is normally a flow, the output of the feedback controller usually is a signal to a control valve.

While all of this is happening in a continuous fashion, disturbances may enter the process and tend to drive the controlled variable in one direction or another. The single manipulated variable compensates for all such changes produced by the disturbances and, in addition, if there are changes in set point, the manipulated variable also changes accordingly to produce the needed change in the controlled variable.

A sensor actually measures the controlled variable, and the measured value of the controlled variable transmits back to the controller case.

Inside the controller case is the comparator. This functionally important device compares—takes the algebraic difference between—the value of the set point and the value of the variable transmitted back into the controller case to represent the controlled variable.

The comparator, or error detector, is common to all feedback control systems. Note that this is negative feedback control, which is to say that the signal fed back to the comparator subtracts from the signal that indicates the set point.

All applied feedback control is negative feedback control, as positive feedback control is inherently unstable.

The error signal, which is the output of the comparator, becomes the input to the feedback controller. Based on the error signal, the controller calculates a signal to the final control element—typically a valve—and this in turn controls the manipulated variable input to the process.

Nicholas Sheble (nsheble@isa.org ) writes and edits the Control Fundamentals department. This piece came from Paul Murrill's Fundamentals of Process Control Theory, ISA Press.