For many years, programmable controllers (hereinafter referred to as PLC) have realized the leap from wiring logic to storage logic from its generation to the present; its functions from weak to strong, achieving the progress of logic control to digital control; its application field from small to small Large, it realizes the simple control of single equipment to the various tasks such as competent motion control, process control and distributed control. Today's PLC has greatly improved its ability to handle analog, digital computing, human-machine interface and network, and has become the mainstream control device in the field of industrial control, playing an increasingly important role in all walks of life.
Second, the application field of PLCAt present, PLC has been widely used in various industries such as steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and culture and entertainment. The usage is mainly divided into the following categories:
1. Switching logic control
It replaces the traditional relay circuit and realizes logic control and sequence control. It can be used for control of single equipment, multi-machine group control and automatic assembly line. Such as injection molding machines, printing machines, bookbinding machines, combination machine tools, grinding machines, packaging production lines, electroplating lines, and so on.
2. Industrial process control
In the industrial production process, there are some continuous changes such as temperature, pressure, flow, liquid level and speed (ie analog quantity), PLC adopts corresponding A/D and D/A conversion modules and various controls. The algorithm program processes the analog quantity and completes the closed loop control. PID regulation is an adjustment method used in general closed-loop control systems. Process control has a wide range of applications in metallurgy, chemical, heat treatment, boiler control and other occasions.
3. sport control
The PLC can be used for the control of circular motion or linear motion. Special motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors, and are widely used in various machines, machine tools, robots, elevators, etc.
4. data processing
PLC has mathematical operations (including matrix operations, function operations, logic operations), data transfer, data conversion, sorting, table lookup, bit manipulation and other functions, can complete the data collection, analysis and processing. Data processing is commonly used in some large control systems such as paper, metallurgy, and food industries.
5. Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of the factory automation network, the current PLC has a communication interface, and communication is very convenient.
Third, the application characteristics of PLC1. High reliability and strong anti-interference ability
High reliability is a key feature of electrical control equipment. Due to the adoption of modern large-scale integrated circuit technology, PLC adopts strict production process, and the internal circuit adopts advanced anti-interference technology and has high reliability. The use of PLC to form a control system, compared with the same size of the relay contactor system, electrical wiring and switch contacts have been reduced to hundreds or even thousands of, the fault is greatly reduced. In addition, the PLC has a hardware fault self-detection function that can issue an alarm message in time when a fault occurs. In the application software, the application can also be programmed into the fault self-diagnosis program of the peripheral device, so that the circuit and the device other than the PLC in the system also obtain the fault self-diagnosis protection. In this way, the entire system will be extremely reliable.
2. Completely furnished, fully functional, and highly adaptable
Since the development of PLC to date, serialized products of various scales have been formed, which can be used in industrial control occasions of various scales. In addition to the logic processing functions, PLCs have perfect data computing capabilities and can be used in various digital control fields. A large variety of functional units have emerged, allowing PLCs to penetrate into various industrial controls such as position control, temperature control, and CNC. Coupled with the enhancement of PLC communication capabilities and the development of human-machine interface technology, it is very easy to use PLC to form various control systems.
3. Easy to learn and use, popular with engineers and technicians
PLC is an industrial control equipment for industrial and mining enterprises. Its interface is easy and the programming language is easy for engineers to accept. The graphical symbols of the ladder language are quite similar to the expressions and relay circuit diagrams. It opens the door for industrial control who are not familiar with electronic circuits, who do not understand computer principles and assembly language.
4. System design, small workload, easy maintenance, easy to transform
PLC replaces the wiring logic with storage logic, which greatly reduces the wiring outside the control device, shortens the design and construction cycle of the control system, and makes the daily maintenance easier. More importantly, the same device changes after changing the program. The production process is possible. This is especially suitable for production occasions of many varieties and small batches.
Fourth, the need to pay attention to the PLC applicationPLC is a kind of equipment used for industrial production automation control. Generally, it can be used directly in industrial environment without any measures. However, although the reliability is high as described above and the anti-interference ability is strong, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation is improperly used, it may cause a program error or an operation error, thereby causing an erroneous input. Causes false output, which will cause the equipment to run out of control and malfunction, thus can not guarantee the normal operation of the PLC, to improve the reliability of the PLC control system, on the one hand requires PLC manufacturers to improve the anti-interference ability of the equipment; on the other hand, requires design In the installation, use and maintenance, it is highly valued, and multi-party cooperation can solve the problem and effectively enhance the anti-jamming performance of the system. Therefore, you should pay attention to the following issues during use:
1. working environment
(1) Temperature
The PLC requires an ambient temperature of 0~55oC. It should not be placed under the component with large heat output during installation. The space for ventilation and heat dissipation should be large enough.
(2) Humidity
In order to ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration
The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibrations with a vibration frequency of 10 to 55 Hz. When the use environment is unavoidable, shock absorption measures must be taken, such as damping rubber.
(4) Air
Avoid corrosive and flammable gases such as hydrogen chloride, hydrogen sulfide, etc. For environments with more dust or corrosive gases in the air, the PLC can be installed in a well-closed control room or control cabinet.
(5) Power supply
PLC has a certain resistance to the interference caused by the power line. In environments with high reliability requirements or particularly severe power disturbances, an isolation transformer with a shield can be installed to reduce interference between the equipment and the ground. General PLC has DC 24V output provided to the input terminal. When the input terminal uses external DC power supply, DC stabilized power supply should be selected. Because of the ordinary rectified and filtered power supply, it is easy for the PLC to receive error information due to the influence of ripple.
2. Interference and its source in the control system
On-site electromagnetic interference is one of the most common and most likely factors affecting system reliability in PLC control systems. The so-called palliative treatment of the first problem, to find out the problem, can propose a solution to the problem. It is therefore necessary to know the source of the on-site interference.
(1) Interference sources and general classification
The interference sources that affect the PLC control system are mostly generated in places where the current or voltage changes drastically. The reason is that the current changes to generate a magnetic field, which generates electromagnetic radiation to the device; the magnetic field changes to generate current, and the electromagnetic high-speed generates electromagnetic waves. Generally, electromagnetic interference is classified into common mode interference and differential mode interference according to different interference modes. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signals, causing component damage (this is the main reason for the high damage rate of some system I/O modules). This common mode interference can be DC. It can also be an exchange. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Precision.
(2) Main sources and ways of interference in PLC system
Strong electrical interference
The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltage on the line. In particular, internal changes in the power grid, surges in knife switch operation, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines.
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