The difference between open-loop and closed-loop control systems is mainly reflected in the working principle, structural composition, stability and control effect. Below I will explain these differences in detail through specific examples.
1. Working principle
Open-loop control system: Its working principle is that the input directly controls the output without feedback. That is, the output of the system has no effect on the control action, and there is no feedback mechanism between the output and input. Taking the washing process of a semi-automatic or fully automatic washing machine as an example, the washer first observes the degree of dirtiness of the clothes, sets the washing and rinsing time according to his own experience, and the washing machine completes the washing and rinsing tasks according to the set program. The information of the system output (that is, the cleanliness of the clothes) is not fed back to the input through any device, and has no effect on the control of the system, so it is an open-loop control.
Closed-loop control system: Its working principle is to compare the feedback signal of the output with the given signal, calculate the deviation signal, and adjust the control signal accordingly to achieve precise control of the output. Taking the water storage control system of the toilet water tank as an example, the controlled quantity (output) of the system is the water level of the water tank (reflecting the water storage). The water level is measured by the float and acts on the water supply valve through the lever (i.e., feedback to the input end) to control the water supply and form a closed-loop control. When the water level reaches the upper limit of the water storage capacity, the valve is fully closed and the system is in a balanced state. Once water is used, the water level drops, the float sinks, and the water supply valve is opened through the lever. The deeper it sinks, the larger the valve opening and the greater the water supply, until the water level rises to the upper limit of the water storage capacity, the valve is fully closed, and the system is in a balanced state again.
2. Structural composition
Open-loop control system: It consists of a controller and a controlled object. There is no detection equipment to detect changes in the output, so the structure is relatively simple.
Closed-loop control system: In addition to the controller and the controlled object, it also includes a feedback link and a comparison link. The feedback link is used to detect changes in the output and convert it into a feedback signal; the comparison link is used to compare the feedback signal with the input signal to generate a deviation signal as a control basis. Therefore, the structure of the closed-loop control system is relatively complex.
3. Stability
Open-loop control system: Stability is relatively easy to solve, because the system has no feedback mechanism, so the complexity of the system will not be increased due to the introduction of the feedback loop. However, the open-loop system is very sensitive to changes in system parameters, and the control accuracy and interference suppression performance are relatively poor.
Closed-loop control system: Although the introduction of the feedback loop increases the complexity of the system, it has the ability to suppress interference, is insensitive to changes in component characteristics, and can improve the response characteristics of the system. Therefore, the closed-loop control system performs better in terms of stability.
4. Control effect
Open-loop control system: The control accuracy mainly depends on the components used in the system and the accuracy of calibration. Since it is impossible to detect and correct errors, the control accuracy of the open-loop control system is relatively low and the anti-interference ability is also poor.
Closed-loop control system: By introducing a feedback mechanism, it is possible to detect and correct errors in real time, making the correction action more accurate and powerful. Therefore, the closed-loop control system performs well in terms of control accuracy and anti-interference ability.
In summary, there are significant differences between open-loop and closed-loop control systems in terms of working principles, structural composition, stability, and control effects. In practical applications, we need to select the appropriate control system type according to specific needs and conditions to achieve the optimal control effect.