Control System

What is process control?

A process stands for an operation and this operation could be on solids or liquids, this operation will be an industrial operation. A process is a method of converting raw materials into a product, so in order to do this properly, process control should be done. Process controls are done to ensure that the parameters in control plants are maintained in a proper manner. So basically we can describe process control in an industry as the regulation of all the process aspects. The process control involves the control of water level, temperature control, pressure control, and flow control. The major advantage of doing process control in an industry is that the quality and quantity of the product can be increased, because of this, there will be a good profit. 

What is the purpose of a process control?

  • Safety
  • To maintain a process at the required operating conditions
  • Protection of the equipment
  • Smooth operation
  • Quality can be increased
  • Monitoring and diagnosis
  • Comply according to the environmental requirements
  • Profit

Many big industrial plants would have many process variables such as temperature, pressure, flow, level, and lot more so all these process variables should be controlled in a good manner to improve the product quality.

What are the common process variables that needs to be controlled in a plant?

  • Pressure
  • Flow
  • Level
  • Temperature
  • Density
  • PH
  • Oxygen content
  • Mass
  • Conductivity
  • Liquid interface, if different liquids are added in a vessel

What are the types of variables in a process control system?

Controlled variable

This is the final product of a process or it can also be considered as the process output

Manipulated variable

These are the input variables that can be adjusted dynamically so that the controlled variables can be maintained in the setpoint.

Disturbance variable

This is the disturbance that affects the process variable and the disturbance could be a change in temperature or pressure. Because of the disturbance, the controlled variable will be different from the setpoint.

What are the major components in a process control system?


This device can measure the process variable an example of this would be a thermocouple. A sensor would measure the controlled variable or the disturbance.


This device would maintain the process variable at the setpoint, this device would process the sensor signals, and it would do the control calculations and decision making.

Final control element

Mostly this will be a control valve and it would act as per the control signal which is received from the controller. This device would vary the manipulated variable to get the required controlled variable.

How process control is done?

The process control is mostly done in an industrial plant to increase the efficiency and the safety of the industrial plant. Process control is achieved by a combination of measuring elements, controller, and regulating elements. In order to control certain processes in a plant, equipment like the controller, and the reactor is controlled by setting a control loop. The control loop will control the variable that is needed to be controlled in a process.

The control loop is composed of

  • Measuring devices ( Flowmeter)
  • Feedback or feedforward controller
  • Regulating element
  • Set variable

The controlled quantity in a process is also known as a controlled variable and these are the conditions that the operator likes to control at a level. The controlled variable will be parameters such as temperature, PH, moisture, level, position, flow, weight, etc. In a process, the controlled variable has a required or desired value and it is called a set point. In a process for each controlled quantity, there will be a manipulated variable an example of this will be flow rate, and the manipulated variable is controlled by using a final control element. The controller would use feedback control to determine the process output and this value will be compared to the setpoint and the difference will be known. Thus according to the difference between the values, changes are made in the input section to get the desired output.

What is a disturbance in a process and where does disturbance come from?

The disturbances would cause the process variable to deviate from the setpoint. The common type of disturbances in a process is temperature, pressure, etc. The disturbance can also be caused due to component damage too. So because of this, the automatic control system should vary the manipulated variable in order to reach the setpoint.

The disturbance can be caused due to the variation in the composition of the incoming raw materials. It can also happen due to environmental conditions such as hot, cold, and also pollution. The disturbances can also happen in a process because of the unexpected surges, and also because of the wear and tear of the components. 

What are the different types of control in a process control system?

Feedback control

The feedback control would only operate if the controlled variables are affected, so it would measure the controlled variable and compare it to the desired variable or the setpoint. This value is used to make the changes in the input variable and because of that, the required output will be received. In this type of control the corrective action is taken without considering the disturbance source and also the response is not quick for the disturbances.

Feedforward control

This type of control would act before the controlled variables are affected, so this type of controller would measure the disturbance variable and it would take the required action so that it won’t affect the output. So the feedforward control would respond faster to the disturbances.

What is a process automation and process dynamics?

In process automation, computer technology and software engineering is used to operate the factory effectively and safely. The process dynamics is basically the study of the transient and steady-state behavior of the process.

What are the different control loops which are used for the process control?

A control loop is the basic part of the process control, the control loop is composed of many components and also the control functions which are used to control the final control element of a process. The control loop consists of the process sensor, controller function, and also the final control element which is needed for the automatic control. The control loops could be manual or automatic, during the manual control there will be operating personnel to control.

Types of control loop

There are two types of control loops and they are open-loop systems and closed-loop systems. The open-loop can be described as a manual mode and in this process, an operator will be there to control the process, and if the system is in auto mode which means that there won’t be any operator to control the process and it is called a closed-loop process.

Open loop control

The open-loop control is done manually which means that there will be an operator to control the process. So the operator will check the process and if the controlled variable is not at a set point then the operator would modify the manipulated variable. The open-loop control doesn’t have any info about the real-time data related to the process so it won’t act quickly in case of any disturbances. The open-loop control will be used in a process where the output will be constant and predictable.

Closed loop control

In this type of control, there will be a controller in place of a human being, and this controller would keep the controlled variable at the required setpoint. This type of control has a feedback device and it would check the disturbances which would affect the process. So if there are any disturbances in the process then it will be cleared at the first instant and because of this the closed-loop system is better than the open-loop control system and it can be used in processes where there could be frequent changes in the output.

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