Understanding Control Systems: Open Loop vs. Closed Loop

Written at April 13, 2024 on English with a size of 6.24 KB.

Let us explore a fascinating field of engineering known as Control Systems Engineering. This captivating subject involves a significant amount of calculation and plays a crucial role in various disciplines, including economics, sociology, biology, and medicine. In this exploration, we will delve into the concepts of open and closed loop control systems, understanding their fundamental differences.
Control System theory has been instrumental in advancing engineering and science. Automatic control has become an indispensable part of modern manufacturing and industrial processes, such as numerical control of machine tools and regulating pressure, temperature, humidity, viscosity, and flow in process industries.

When multiple elements or components are interconnected in a sequence to achieve a specific function, the resulting group is called a system. In a system where the output quantity is controlled by adjusting the input quantity, it is referred to as a control system. The output quantity is known as the controlled variable or response, while the input quantity is called the command signal or excitation.

Types of Control Systems:

Control systems can be categorized into two main types: 1) Open Loop System 2) Closed Loop System

1) Open Loop Control System:

An open loop system is a physical system that does not automatically correct variations in its output. In other words, the output quantity has no influence on the input quantity. This means that there is no feedback mechanism to adjust the input based on the output.

In an open loop control system, the output can be modified by changing the input. However, external disturbances can cause the system output to deviate from the desired value. Since there is no feedback, these changes are not automatically corrected. Manual intervention is required to adjust the input and rectify the output in open loop systems.

2) Closed Loop Control System:

Closed loop systems are control systems where the output has an effect on the input quantity to maintain the desired output value. An open loop system can be transformed into a closed loop system by incorporating a feedback mechanism. This feedback automatically compensates for changes in output caused by disturbances, making closed loop systems also known as automatic control systems. The general block diagram of an automatic control system is illustrated below. It consists of an error detector, a controller, a plant (open loop system), and feedback path elements.

The reference signal (or input signal) represents the desired output. The feedback path elements sample the output and convert it to the same type as the reference signal. The feedback signal, proportional to the output signal, is fed to the error detector. The error detector generates an error signal, which is the difference between the reference signal and the feedback signal. The controller modifies and amplifies the error signal to produce better control action. The modified error signal is then fed to the plant to correct its output.

Advantages of Open Loop Control Systems:

1. Open loop systems are simple and cost-effective. 2. They are easier to construct. 3. Generally, open loop systems exhibit stability.

Disadvantages of Open Loop Systems:

1. Open loop systems lack accuracy and reliability. 2. Changes in output due to external disturbances are not automatically corrected.

Advantages of Closed Loop Systems:

1. Closed loop systems offer high accuracy. 2. They maintain accuracy even in the presence of non-linearities. 3. The sensitivity of the systems can be reduced to enhance stability. 4. Closed loop systems are less susceptible to noise.

Disadvantages of Closed Loop Systems:

1. Closed loop systems are complex and expensive. 2. Feedback can lead to oscillatory responses. 3. Feedback reduces the overall gain of the system. 4. Stability is a critical concern in closed loop systems, requiring careful design to ensure a stable closed loop system.

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