Control

Control is the engineering process of regulating a machine or industrial process so that a measured variable remains at or reaches its desired value—the setpoint.

See the control loop

Female control engineer monitoring process graphics, alarms and control-loop trends in an industrial control room — A control engineer monitors process variables and adjusts the system to maintain safe, stable operation.

The central idea

Compare, decide and correct.

A control system continuously compares what is actually happening with what should happen, calculates the difference and produces the action needed to reduce it.

Desired condition

Setpoint

The target value established by the operator or control strategy.

Actual condition

Process variable

The current measured value returned by the instrumentation.

Difference

Error

The difference between the setpoint and the measured process variable.

Corrective response

Controller output

The command sent to a valve, drive, heater or other final control element.

How it works

A continuous feedback loop.

The measurement returns to the controller after every process response. This feedback allows the system to keep correcting disturbances and maintain the desired condition.

01Setpoint
The operator sets the desired process value.
02Measurement
A sensor and transmitter report the actual process variable.
03Comparison
The controller compares the measurement with the setpoint.
04Correction
The controller calculates and sends the required output.
05Response
The final control element changes the process and feedback repeats.

Example setpoint80°C

A temperature controller compares the measured temperature with 80°C, then adjusts a control valve or heater until the process remains close to the setpoint.

Control strategies

Different ways to guide a process.

The appropriate control method depends on the equipment, process dynamics, operating risk and degree of precision required.

Manual control

An operator observes the process and makes the adjustment.

On–off control

The output switches completely on or off.

PID control

The controller continuously calculates corrective action.

Sequential control

Operations occur in a defined order.

Logic & interlocks

Equipment operates only when required conditions are satisfied.

Advanced control

Coordinated strategies optimize multiple process variables.

Female control engineer tuning a PID loop and verifying the response of a pneumatic control valve in an industrial process plant — Loop commissioning combines PID trend analysis with field verification of the control valve and process response.

The profession in action

What control engineers and technicians do.

Control professionals design, configure, test, tune and maintain the systems that keep industrial processes stable, safe and efficient.

Configure PLC, DCS and control-room displays
Develop and test control logic, sequences and interlocks
Tune PID control loops for stable process response
Analyze process trends, alarms and disturbances
Test control valves, actuators and variable-speed drives
Troubleshoot control-system and communication problems

In simple terms

Control gives an industrial process the ability to compare what is happening with what should happen—and then correct the difference.

Return to the PICS homepage