In the field of automatic control, control systems are mainly divided into two categories: open-loop control systems and closed-loop control systems. Open-loop control is the most basic and simple control method, which is widely used in many industrial equipment, household appliances and automation systems. This paper will focus on the theme of “how to realize open-loop control” and introduce its basic principle, implementation mode, advantages and disadvantages.
First, the basic principle of open-loop control
Open-loop control system refers to a system in which the output of the control system has no feedback effect on the control function. That is to say, the controller of the system directly generates control action according to the input signal, and does not depend on the actual output of the system. This control method is simple in structure and low in cost, and is suitable for occasions where the system parameters change little and the control accuracy is not high.
Its basic structure includes four parts: input signal, controller, actuator and controlled object. For example, in a simple electric fan control system, the user sets a gear (input signal), and the controller drives the motor (actuator) to run according to this signal, driving the fan (controlled object), but the system will not detect the actual speed of the fan and make adjustments.
Second, the realization method of open-loop control
The key to realize open-loop control is to accurately generate the corresponding control output according to the input signal, which can usually be realized in the following ways:
1. Hardware circuit implementation
In early control systems, analog circuits or digital circuits are often used to construct open-loop control systems. For example, a potentiometer is used to adjust the voltage signal to drive the DC motor to run, and the speed of the motor is determined by the applied voltage.
2. Realization of program control
With the development of computers and microcontrollers, more and more open-loop control systems are realized by programs. For example, single chip microcomputer or PLC (programmable logic controller) outputs control signals to actuators such as relays and stepping motors according to preset programs.
3. Timing control
In some automation equipment, time is the main control variable. For example, the washing time of washing machine and the heating time of rice cooker all belong to timing control, which is a typical open-loop control form.
4. Look-up table method and experience control
In some complex systems, the controller determines the output value according to the input parameters look-up table or empirical formula. For example, in numerical control machine tools, open-loop control movement is carried out according to the preset machining path and speed.
Third, the application example of open-loop control
1. Household appliances
As mentioned earlier, household fans, washing machines, rice cookers and other commonly used electrical appliances mostly use open-loop control, because these devices do not require high accuracy, but they require high cost and simple structure.
2. Industrial equipment
Open-loop control is also commonly used in some industrial equipment with low requirements on control accuracy, such as speed control of conveyor belt and lighting control.
3. Robot system
In some primary robots, the action path is preset, and no real-time feedback adjustment is made during execution, which belongs to open-loop control.
Fourth, the advantages and disadvantages of open-loop control are analyzed
Advantages:
-Simple structure, easy to design and maintain.
-Low cost, suitable for mass production.
-Fast response and no feedback delay.
Disadvantages:
-The control accuracy is low, and it is greatly influenced by external interference.
-Error cannot be corrected automatically, and the system stability is poor.
-Not suitable for complex or changeable environment.
summary
As the most basic form of control system, open-loop control is not as good as closed-loop control in accuracy and stability, but it still has irreplaceable advantages in many application scenarios because of its simple structure, easy implementation and low cost. With the development of technology, although closed-loop control is becoming more and more popular, open-loop control is still an important basis for understanding automatic control theory. Open-loop control is still an efficient and practical control method for some systems with low precision and little environmental change.