Open Loop Control System vs Closed Loop Control System

Open Loop Control System

An open loop control system is a type of control system in which there is no feedback from the output of the system to the input. In an open loop system, the input to the system is set based on the desired output, and the output of the system is not used to adjust the input.

In an open-loop system, the output is not compared to the input in order to make adjustments. Instead, the output is determined solely by the input and the system’s fixed response to it. This means that the accuracy of the system depends on the accuracy of the initial calibration. If the system is properly calibrated, it will produce accurate results. However, if the calibration is off or if the system changes over time, the output may be less accurate. To maintain accuracy in an open-loop system, it may be necessary to periodically recalibrate or adjust the system.

Example of Open Loop Control System

• Manually immersing water heater rod

We can’t set up a certain temperature and heat the water as required. Instead, we have to manually check the water temperature to stop the heater

• A washing machine is also a good example of an open-loop system. The user inputs the desired washing cycle (soaking, washing, rinsing, etc.), and the machine follows a fixed series of steps to complete the cycle. The output (clean laundry) is not compared to the input (the selected washing cycle) in order to make adjustments. Instead, the machine follows the same series of steps every time, regardless of the input.

In this case, the accuracy of the system depends on the initial calibration of the machine. If the water level, temperature, and timing of the steps are all properly set, the machine will produce clean laundry. However, if any of these factors are off, the machine may not be able to effectively clean the laundry. To maintain accuracy, it may be necessary to periodically check and adjust the machine’s settings.

Advantages of Open Loop Control System

• Simplicity: Open-loop systems are typically simpler in design and operation compared to closed-loop systems, which can make them easier to understand and use.
• Lower cost: Open-loop systems may be less expensive to build and maintain compared to closed-loop systems.
• Suitability for certain applications: Open-loop systems may be the best choice for certain applications, such as when the inputs are known ahead of time and there are no disturbances that would require the system to adjust its output.
• Ease of implementation: Open-loop systems do not require feedback, making them easier to implement in certain situations.
• Speed: Open-loop systems may be able to respond more quickly to changes in the input compared to closed-loop systems.
• Convenience: Open-loop systems may be more convenient to use when the output is difficult to measure directly.

It is also worth noting that the accuracy of an open-loop system may depend on the experience of the user, as the system does not adjust the output based on the measured output. Additionally, open-loop systems are generally less stable than closed-loop systems, as they are not able to compensate for disturbances or changes in the system.

Disadvantages of Open Loop Control System

• Lack of accuracy: Because open-loop systems do not continuously adjust the output based on the measured output, they may not be as accurate as closed-loop systems.
• Lack of stability: Open-loop systems are not able to compensate for disturbances or changes in the system, which can lead to instability.
• Inability to adapt to changing conditions: Because open-loop systems do not adjust the output based on the measured output, they are not able to adapt to changes in the system and may produce incorrect results.
• Sensitivity to variations in the input: Open-loop systems are more sensitive to variations in the input, which can affect the accuracy of the output.
• Limited range of applications: Open-loop systems are only suitable for certain applications and may not be suitable for use in more complex or dynamic systems.

Closed Loop Control System

A closed-loop control system, on the other hand, is a type of control system in which the output of the system is used to adjust the input. In a closed-loop system, a sensor is used to measure the output of the system, and this measurement is compared to a desired setpoint value. If the output differs from the set point, a correction is made to the input of the system in order to bring the output back to the set point. This process is repeated continuously, with the output being measured and compared to the setpoint, and the input is adjusted as needed, in order to maintain the output at the desired level.

It is the control system in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated.

It is generally true that closed-loop control systems are more stable than open-loop systems, as they are able to compensate for disturbances or changes in the system. However, it is also possible for a closed-loop control system to become unstable if it is not designed or tuned properly. One common issue that can lead to instability in a closed-loop control system is “overcorrection,” where the system tends to overreact to errors and oscillate at constant or changing amplitudes. This can be caused by a number of factors, such as poor gain selection, high loop gain, or inappropriate system dynamics. To prevent these issues, it is important to carefully design and tune the closed-loop control system to ensure that it operates smoothly and stably.

Example of Closed Loop Control System: 

• Automated water level controller

An automated water level controller is an example of a closed-loop control system because it uses feedback to continuously compare the output (water level) to the setpoint (desired water level) and change the input (water flow) as needed.

The system contains a sensor that measures the water level and communicates the data to the controller. The controller is a device that processes sensor information and determines the proper action to execute. The controller delivers a signal to the actuator based on the difference between the output and the setpoint (the difference between the measured and desired water level).

The actuator is a device that performs the intended action in response to the controller’s signal. The actuator in the case of a water level controller can be a valve that controls the flow of water into the tank. If the water level falls below a certain level, the controller sends a signal to release the valve, allowing more water to enter the tank. If the water level in the tank becomes too high, the controller sends a signal to close the valve, restricting water flow into the tank.

The control system can keep the water level at the specified setpoint by continuously adjusting the input based on the output.

• Air Conditioner

An air conditioner is a type of control system that is designed to regulate the temperature within a specific space, such as a room or building. It does this by cycling a coolant through a system of refrigerant lines, which absorbs heat from the air inside the space and releases it outside. The temperature within the space is monitored by a thermostat, which sends a signal to the air conditioner to turn on or off based on the temperature of the air. The thermostat acts as the feedback element of the control system, providing information about the current temperature and adjusting the air conditioner’s output accordingly to maintain the desired temperature. Essentially, the air conditioner receives input from the thermostat, processes that input, and produces an output (cooling the air) to maintain the desired temperature within the controlled space.

Advantages of Closed Loop Control System

• Increased accuracy and precision: Because closed-loop control systems continuously measure the output and adjust the input as needed, they can achieve higher levels of accuracy and precision compared to open-loop systems. It can handle and process according to the disturbance produced too.
• Greater stability: Closed-loop control systems are able to compensate for disturbances or changes in the system, which can lead to greater stability overall.
• Improved performance: Closed-loop control systems can operate more efficiently and effectively, leading to improved performance.
• Adaptability: Closed-loop control systems can adapt to changes in the system, allowing them to maintain the desired output despite variations in the input.
• Robustness: Closed-loop control systems are generally more robust than open-loop systems, meaning they can tolerate a wider range of operating conditions and are less prone to failure.

Disadvantages of Closed Loop Control System:

• Complexity: Closed-loop control systems are generally more complex in design and operation compared to open-loop systems, which can make them more difficult to understand and use.
• Higher cost: Because closed-loop control systems are more complex, they may be more expensive to build and maintain compared to open-loop systems.
• Sensitivity to measurement errors: Closed-loop control systems rely on accurate measurement of the output, and any errors in measurement can lead to inaccuracies in the control action.
• Risk of instability: If a closed-loop control system is not designed or tuned properly, it can become unstable and may oscillate or exhibit other undesirable behaviors.
• Slower response time: Because closed-loop control systems need to measure the output and adjust the input accordingly, they may have a slower response time compared to open-loop systems.

Open-loop control systems are simpler and less expensive than closed-loop systems but are not as effective at maintaining the performance of a system because they do not take into account any changes in the system that may affect the output. Closed-loop control systems, on the other hand, are more effective at maintaining the performance of a system but are more complex and costly to implement.