Introduction
Logic control is also called inference control. It is a form of controlling the system by applying logic principles and methods on the basis of random control and empirical control. It can be divided into logical reasoning and mathematical reasoning. The former focuses on qualitative reasoning and the latter focuses on quantitative reasoning. People find that although they don't know a certain property of a certain thing, they have experience of controlling this property of other things. When these two types of things are consistent in some respects, the control of the latter can be used for the control of the former. For example: Cao Chong weighed the elephant, led the elephant to the boat, carved the waterline, and then replaced the elephant with a stone. After the boat sank to the same waterline, the weight of the elephant was measured by weighing the stone. This method is actually a logical control method. Since this control method is ultimately the use of experience elsewhere, it is also called experience transfer. It realizes control through a transitional medium in the middle, so it is also called conjugate control.
PLC has logic operation functions. It is equipped with logical operation instructions such as "AND", "OR", and "NOT". It can describe various connections of relay contacts in series, parallel, series/parallel, parallel/series, etc. Therefore. It can replace relays for combinational logic and sequential logic control.
Basic logic relationship
Logic AND relationship
The logic "and" can be compared with the effect of two switches connected in series on the power supply of a lamp. For example, if there are two people who have opinions on something, you can use this simplest voting circuit. There is a switch in front of each person. After the two switches are connected in series, they are connected to a light. Only when both of them put the switch on, the light will come on. When the light is on, it means that both A and B agree.
Usually the bathroom and kitchen share the same water heater, no matter [bath] or [washing dishes], the water heater can supply hot water. From the perspective of these two valves, this is the logical "or" relationship. If any one of the two conditions is established, hot water can be obtained, and it is not necessary to have both.
The relationship of logical OR
The relationship of logical “or” can also be compared with a circuit, which is the effect of two switches in parallel to supply power to the same lamp. If the switches in front of A and B are connected in parallel, as long as any one of them puts the switch in the on state, the light will be on. Bright means that at least one of A and B agrees (perhaps both agree).
The relation of logical negation
In addition, there is also the relation of logical negation, which means negative. As long as the definition of a certain condition is reversed, it means "not". For example, [No extinguishing] means "No" [Extinguishing]. Then, [stop water] is "non" [no water stop]. For the circuit, the "on" and "off" of the switch are reversed, and it becomes a "non" relationship.
Control system
Control system means that it can maintain and change any interesting or variable quantity in a machine, mechanism or other equipment in a desired way. The control system is also implemented to make the controlled object reach a predetermined ideal state. The control system makes the controlled object tend to a certain required stable state.
There are several classification methods for control systems:
1. According to different control principles, automatic control systems are divided into open-loop control systems and closed-loop control systems.
Open-loop control system
In an open-loop control system, the output of the system is only controlled by the input, and the control accuracy and interference suppression characteristics are relatively poor. In the open-loop control system, the logic control based on the sequence is called the sequential control system; it is composed of sequential control devices, detection elements, actuators and controlled industrial objects. Mainly used in the control of machinery, chemical industry, material handling and transportation, as well as manipulators and automatic production lines.
Closed-loop control system
The closed-loop control system is based on the principle of feedback. The deviation between the output and the expected value is used to control the system, and better control performance can be obtained. Closed loop control system is also called feedback control system.
2. According to the classification of the given signal, the automatic control system can be divided into a constant value control system, a follow-up control system and a program control system.
Constant value control system
The given value is unchanged, and the system output is required to be close to the given desired value with a certain precision. For example, automatic control systems such as temperature, pressure, flow, liquid level, and motor speed in the production process are constant value systems.
Follow-up control system
The set value changes according to an unknown time function, and the output is required to follow the change of the set value. Such as a radar antenna system following satellites.
Program control system
The given value changes according to a certain time function. Such as program-controlled machine tools.
Development
The earliest mechanical and electrical control system appeared in the 1920s. At first, buttons and switches were used for manual control. Later, contactors and relays and their control systems appeared. Start, stop, step-by-step speed regulation and automatic working cycle control of the controlled object. This kind of control device is simple in structure, intuitive and easy to understand, easy to maintain, and low in price. Therefore, it has been widely used in the control of mechanical equipment and has been applied to this day. The disadvantage is that it is difficult for the control system to change the control program, and mechanical contacts are used to achieve switch control. The contacts are prone to looseness and electrical wear. If the control system is a little more complicated, the reliability is lower.
In the mid-1960s, with the emergence of group technology, it is required to process parts with similar but different structures on the same automatic machine tool. The production process and process often change. The contactor-relay control system has been Cannot meet this need, so a sequence controller composed of logic gate circuits and relays appeared. This kind of controller uses diode matrix or diode matrix pin board to compile the program, which can easily change the program. At the same time, this kind of control system overcomes the shortcomings of contactor-relay control system such as short life, low working frequency, simple function and poor reliability. One of the commonly used sequence control systems.
With the rapid development of computer technology and automatic control technology, in the late 1960s, programmable logic controllers (PLC) with computing functions and power output capabilities appeared. It is a special micro-electronic computer composed of large-scale integrated circuits, electronic switches, power output devices, etc. It has the functions of logic control, timing, counting, arithmetic operations, programming and storage. It is easy to program and modify, and the input and output wiring is simple and universal. Flexible, strong anti-interference ability, suitable for industrial environment, high working reliability, and small size and a series of advantages. By the mid-1980s, PHC has been widely used in the automatic control of machinery and equipment in all walks of life, and has become the mainstream controller in the field of industrial automation. At present, the general development trend of PLC is high integration, small size, large capacity, high speed, ease of use, and high performance.
Fuzzy logic control
Traditional Boolean set theory uses non-“0” or “1” logic to judge things, such as judging the height of an adult man, who is taller than 1.80 m , 1.799m is not a tall person, which is obviously unreasonable in real life, and it cannot express the concept of degree.
Fuzzy logic uses a more precise method to represent and analyze inaccurate and incomplete information. For example, if a person's height is 1.75 m, we say that he is relatively tall. 46% of this person is tall. This is more in line with people's thinking.
Fuzzy logic control is a method of implementing process control based on fuzzy logic theory. It is more intelligent than traditional PID control. It can modify parameters according to changes in the use environment to make the output value infinitely close to the set value.
The realization of fuzzy logic control is generally to fuzzify the precise input quantity first, so that each input quantity corresponds to a fuzzy set; then formulate fuzzy control rules based on expert experience, carry out fuzzy reasoning, and obtain a fuzzy set. A collection of output quantities; finally, defuzzification is performed, and the output of the fuzzy control is clarified, and an accurate output quantity is output.