InstrumentationProcess Control

How does a process control work?


Process control is the automatic control of an output variable by sensing the amplitude of the output parameter from the process and comparing it to the desired or set level and feeding an error signal back to control an input variable.

In order to produce a product with consistently high quality, tight process control is necessary.In any process there are a number of inputs, i.e., from chemicals to solid goods. These are manipulated in the process and a new chemical or component emerges at the output. The controlled inputs to the process and the measured output parameters from the process are called variables

Basics of process control.

Controlled or measured variable is the monitored output variable from a process. The value of the monitored output parameter is normally held within tight given limits.

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.

Set point is the desired value of the output parameter or variable being monitored by a sensor. Any deviation from this value will generate an error signal.

 1. Sensor

Sensors are devices that can detect physical variables, such as temperature, light intensity, or motion, and have the ability to give a measurable output that varies in relation to the amplitude of the physical variable.sensors acquires information about the status of the process variables

Typical examples: thermocouples (for temperature measurements), differential Pressure cells (for liquid level measurements), gas/liquid chromatographs (for Composition measurement), etc.


Actuators are devices that are used to control an input variable in response to a signal from a controller. A typical actuator will be a flow-control valve that can control the rate of flow of a fluid in proportion to the amplitude of an electrical signal from the controller. Other types of actuators are magnetic relays that turn electrical power on and off. Examples are actuators that control power to the fans and compressor in an air-conditioning system in response
to signals from the room temperature sensors.

ie Actuator implements the control command issued by the controller on the process.


The brain or heart of the control system (the decision maker). It is the hardware element with built-in capacity for performing the only task requiring some form of .intelligence. Typical examples: Pneumatic controller, Electronic controllers, digital computers used as controller

4.Feedback Control

The traditional way to control a process is to measure the variable that is to be controlled, compare its value with the desired value (the setpoint) at the controller and feed the difference (error) into a controller, which will change a manipulated variable to drive the controlled variable back to the desired value.

an example of a process control is given below

The importance of these components is that they perform the three basic operations that must be present in every control system. These operations are:

  1. Measurement (M): Measuring the variable to be controlled is usually done by the combination of sensor and transmitter.
  1. Decision (D): Based on the measurements and the set point, the controller must then decide what to do to maintain the variable at its desired value.
  1. Action (A): As the result of the controller.s decision, the system must then take an action. This is usually accomplished by the final control element.

 Different types of process controls 

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