Control Systems: An Introduction to Open-Loop and Closed-Loop Control

by Joost Nusselder | Updated on:  June 25, 2022
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Control systems are used to maintain a setpoint or desired output by adjusting an input signal. Control systems can be open loop or closed loop. Open loop control systems don’t have a feedback loop and closed loop control systems do.

In this article, I’ll explain what control systems are, how they work, and how they’re used in everyday life. Plus, I’ll share some fun facts about control systems you might not know!

What is a control system

Control Systems- The Art of Designing and Implementing

Control systems involve the process of setting and maintaining a particular output by adjusting the input signal. The goal is to produce a correct and consistent output, despite any initial changes in the input. The process involves a number of stages, including the following:

  • Input stage: where the input signal is received
  • Processing stage: where the signal is processed and analyzed
  • Output stage: where the output signal is produced

The Role of Control Systems in Production

Control systems play a significant role in production and distribution in many industries. Automation technology is often used to implement these systems, which can be highly complex and expensive to construct. The following elements are required to create an excellent control system:

  • A good understanding of the system being controlled
  • The ability to design and implement the correct type of control system
  • A package of standard designs and techniques that can be applied to particular situations

The Steps Involved in Creating a Control System

The process of creating a control system involves the following steps:

  • Designing the system’s structure: This involves determining the type of control system required and the components that will be included
  • Implementing the system: This involves carefully constructing the system and running tests to ensure that it is working correctly
  • Maintaining the system: This involves monitoring the system’s performance over time and making any necessary changes to ensure that it continues to function correctly

Open-loop and closed-loop control: The difference between self-correction and fixed output

Open-loop control systems are also known as non-feedback controls. These systems have a fixed output that is not adjusted based on any input or feedback. The structure of an open-loop control system is typical and includes an input, a set point, and an output. The input is the signal that is used to produce the desired output. The set point is the target value for the output. The output is the result of the process running.

Examples of open-loop control systems include:

  • A toaster: The lever is placed in the “on” phase, and the coils are heated to a fixed temperature. The toaster stays heated until the appointed time, and the toast pops up.
  • A cruise control in a vehicle: The controls are set to maintain a fixed velocity. The system does not adjust based on changing conditions, such as hills or wind.

Closed-loop control: Self-correction for consistent output

Closed-loop control systems, also known as feedback control systems, have the ability to self-correct to maintain a consistent output. The difference between an open-loop and closed-loop system is that the closed-loop system has the ability to self-correct while the open-loop system doesn’t. The structure of a closed-loop control system is similar to that of an open-loop system, but it includes a feedback loop. The feedback loop leads from the output to the input, allowing the system to continually monitor and adjust based on changing conditions.

Examples of closed-loop control systems include:

  • Temperature control in a room: The system adjusts the heating or cooling based on the temperature in the room to maintain a consistent temperature.
  • Amplification control in a sound system: The system adjusts the amplification based on the output to maintain a consistent sound level.

Feedback Control Systems: Bringing Control to the Next Level

Feedback control systems are a type of control system that uses the output of a process to control the input. In other words, the system receives a signal from the process being controlled and uses that signal to adjust the input to achieve the desired output.

Diagrams and Names Associated with Feedback Control Systems

There are several diagrams and names associated with feedback control systems, including:

  • Block diagrams: These show the components of the feedback control system and how they are connected.
  • Transfer functions: These describe the relationship between the input and output of the system.
  • Closed-loop systems: These are feedback control systems where the output is fed back to the input to maintain the desired output.
  • Open-loop systems: These are feedback control systems where the output is not fed back to the input.

Logic Control: Simplified and Effective Control Systems

Logic control is a type of control system that uses Boolean logic or other logical operations to make decisions and control processes. It is a simplified and effective control system that is widely used in various industries, including production, manufacturing, and electrical engineering.

How does Logic Control Work?

Logic control systems are designed to handle a variety of inputs and produce a desired output. The basic method of operation is as follows:

  • The system receives an input signal, which is usually in the form of an electrical current.
  • The input signal is then compared to a set value or point, which is stored in the system.
  • If the input signal is correct, the system will perform a specific action or switch to a specific setting.
  • If the input signal is incorrect, the system will continue to receive input until the correct value is reached.

Examples of Logic Control Systems

Logic control systems are used in a wide range of applications, including:

  • Traffic lights: Traffic lights use logic control to switch between red, yellow, and green lights based on the flow of traffic.
  • Industrial robots: Industrial robots use logic control to perform complex tasks, such as welding, painting, and assembly.
  • Automatic washing machines: Automatic washing machines use logic control to switch between different wash cycles and temperatures based on the user’s input.

On-Off Control: The Simplest Method for Controlling Temperature

On-Off control is historically implemented using interconnected relays, cam timers, and switches that are constructed in a ladder sequence. However, with the advancement of technology, on-off control can now be performed using microcontrollers, specialized programmable logic controllers, and other electronic devices.

Examples of On-Off Control

Some examples of products that use on-off control include:

  • Domestic thermostats that switch the heater on when the room temperature drops below the desired setting and switch it off when it goes above it.
  • Refrigerators that switch the compressor on when the temperature inside the fridge rises above the desired temperature and switch it off when it goes below it.
  • Washing machines that use on-off control to trigger different interrelated sequential operations.
  • Pneumatic actuators that use on-off control to maintain a certain pressure level.

Advantages and Disadvantages of On-Off Control

Advantages of on-off control include:

  • It is simple and cheap to implement.
  • It is easy to understand and perform.
  • It can be used in different types of machinery and operations.

Disadvantages of on-off control include:

  • It produces abrupt changes in the system, which can cause negative effects on the product or process being controlled.
  • It may not be able to maintain the desired setpoint accurately, especially in systems with large thermal masses.
  • It may cause wear and tear on the electrical switches and relays, leading to frequent replacements.

Linear Control: The Art of Maintaining Desired Outputs

Linear control theory is based on several principles that govern how linear control systems behave. These principles include:

  • The principle of ignoring undesirable effects: This principle assumes that any undesirable effects of the system can be ignored.
  • The principle of additivity: This principle adheres to the concept that the output of a linear system is the sum of the outputs produced by each input acting alone.
  • The principle of superposition: This principle assumes that the output of a linear system is the sum of the outputs produced by each input acting alone.

The Nonlinear Case

If a system does not adhere to the principles of additivity and homogeneity, it is considered nonlinear. In this case, the defining equation is typically a square of terms. Nonlinear systems do not behave in the same manner as linear systems and require different methods of control.

The Fuzzy Logic: A Dynamic Control System

Fuzzy logic is a type of control system that utilizes fuzzy sets to convert an input signal into an output signal. It is a mathematical structure that analyzes analog input values in terms of logical variables that take on continuous values between 0 and 1. Fuzzy logic is a dynamic control system that can handle changes in the input signal and adjust the output signal accordingly.

Examples of Fuzzy Logic in Action

Fuzzy logic is used in many fields to perform a wide range of control tasks. Here are some examples:

  • Water treatment: Fuzzy logic is used to control the flow of water through a treatment plant. The system adjusts the flow rate based on the current state of the water and the desired output quality.
  • HVAC systems: Fuzzy logic is used to control the temperature and humidity in a building. The system adjusts the temperature and humidity based on the current state of the building and the desired comfort level.
  • Traffic control: Fuzzy logic is used to control the flow of traffic through an intersection. The system adjusts the timing of the traffic lights based on the current traffic conditions.


So, control systems are used to control processes in many industries, and they involve designing, implementing, and maintaining a system that maintains a consistent output despite changes in the input. 

You can’t go wrong with a control system, so don’t be afraid to use one in your next project! So, go ahead and control your world!

I'm Joost Nusselder, the founder of Tools Doctor, content marketer, and dad. I love trying out new equipment, and together with my team I've been creating in-depth blog articles since 2016 to help loyal readers with tools & crafting tips.