This section of the PLC of the structure and characteristics of the hot issues of the solution, so that beginners PLC has an overall knowledge.
Q: what is a PLC (programmable logic controller)?
Answer: PLC can series program device (Programmable Logical Controller) is a digital operation operation of electronic system, designed for industrial environment Xia application and design of industrial computer; it used a can programming of memory, for its internal storage program, implementation logic operation, and order control, and timing, and count and arithmetic operation, oriented user of instruction; and through digital type or simulation type entered/output control various type of mechanical or production process. Is easy with industrial control systems as a whole, easy to expand the principle of functional design, universal automatic control device based on the microprocessor, PLC is now towards the rapid expansion of other industries.
Second, ask: what PLC made?
A: the PLC consists of hardware (the equivalent of the human body) and software (the equivalent of human thought). Its core is a single board computer (CPU boards), single board computer peripheral configuration of interface circuits (hardware) is configured in the single board computer monitoring program (software). SBC‘s role is to control the work of the PLC, its work is divided into two parts: the first is to manage the system, such as diagnosis, error checking, such information, count, flush; the second is the detection field input, program the user program execution, output operations, and so on.
Third, ask: what is the unique advantages of PLC?
A: the PLC‘s unique advantages:
1, high reliability, high reliability of PLC, average trouble–free run time than the IEC (International Electrotechnical Commission) provides 100,000 hours. Such as Japan‘s Mitsubishi FX series PLC can be run continuously for more than 30 years of trouble.
2, easy to program, easy to use
Software: for general electric control lines, use the ladder diagram programming, intuitive and easy to understand;
Specific issues for more complex designs such as step ladder instruction, flow chart (SFC) instruction set.
Hardware: using PLC, both wiring and configuration are very convenient, only with a screwdriver to complete all wiring work.
Typically, is control by PLC system programming, simulation and debugging in laboratory after the site will soon be able to install successfully.
3, the environments requiring low PLC to temperature and humidity changes within the larger scope of work; its good anti-vibration, anti-impact properties; on the stability of power supply voltage low; anti-electromagnetic interference ability, suitable for harsh working environments.
4, interface with other devices, configure the connected PLC principle: for external wiring, level shift as little as possible.
PLC inputs are analog, digital output driving external circuits with relays, SCR (SSR), several different forms such as transistors, can be connected directly to a variety of different types of load. In the area of data communication, only used coaxial cables and ordinary PS232 or RS422 interface, users don’t need to worry about technical issues in data communications. (J) (Not to be continued)
Problems in PLC described in the application and use of
First, brief
For many years, PLC (hereinafter referred to as PLC) from its origin to the present, realized the connection to storage logical leap of logic; its function from weak to strong, realizing logic control to control progress; its applications from small to large, realize a single simple device control to qualified motion control, process control and distributed control of a variety of tasks, such as across. PLC today in dealing with analog and digital operation, all aspects of the human–machine interface and network capacity has increased significantly, becoming the mainstream of industrial control field control equipment, in all walks of life play an increasingly large role.
Second, PLC application
At present, the PLC has been widely used in iron and steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural and entertainment industries, usage can be divided into the following categories:
1. Switch logic control
Instead of the traditional relay circuits, logic control, order, control, and can be used for device control, can also be used for fleet control and automation lines. Injection molding machine, printing machine, book machine, combination machines, grinding machines, packaging line, plating lines and so on.
2. Industrial process control
In the industrial production process, there are things such as temperature, pressure, flow, liquid level and speed continuous variation (analog), the PLC using the corresponding a/d and d/a conversion module and various control algorithms to handle analog signals, complete closed loop control. PID regulation is generally used more of a closed–loop control system regulation. Process control in metallurgical, chemical, heat treating, boiler control and so on have a very wide range of applications.
3. Motion control
PLC used in circular or linear motion control. Generally use a dedicated motion control modules, such as drive stepper motors or servo motors for single or multi-axis position control module, is widely used for all kinds of machinery, machine tools, robotics, elevator and so on.
4. Data processing
PLC with mathematical operations (including matrix operations, functions, operations, logical operations), data transfer, data transformation, sort, table, operations and other functions, you can complete the acquisition, analysis and processing of the data. Data processing is commonly used, such as paper-making, metallurgy, some major control systems in the food industry.
5. Communications and networking
PLC communications PLC‘s communication and communication between PLC and other smart devices. With the development of factory automation network, PLC now has communication interfaces, communication is very convenient.
Three features, PLC application
1. High reliability and strong anti-interference ability
High reliability is the key to electrical control equipment performance. PLC as a result of modern VLSI, strict manufacturing standards, internal circuit adopt advanced anti-jamming technology, with high reliability. Using PLC control system, and the same size of relay contactor system, electrical wiring and switch contact has been reduced to one of the hundreds of even thousands of failure is greatly reduced. In addition, the PLC with a hardware fault self testing features, fails to send alert information in a timely manner. In application software, applications can also be integrated into the peripheral device fault diagnosis procedures in the system but the PLC circuits and access to fault diagnosis of equipment protection. Thus, the whole system will be extremely high reliability.
2. Complete, fully functional, strong applicability
PLC today, big and small, have formed a series of products that can be used for industrial applications of all sizes. In addition to logic processing function, PLC has a sophisticated data processing capability, and can be used in various fields of digital control. A wide variety of functional units have emerged, PLC into the position control, temperature control, CNC and other industrial control. Add PLC communications capacity enhancement and the development of man-machine interface technology, using PLC control system composed of a variety of very easily.
3. Easy to learn and use, by the engineers and technicians welcome
PLC is facing the industrial and mining enterprises of industrial equipment. It interfaces easily, programming language easy for engineering technicians. Ladder diagram language graphical symbol and expression and relay circuit diagram is very similar, is not familiar with electronic circuits, do not understand computer principle and assembly language to engage in industrial control opens the door.
4. System design of work is small, easy to maintain, easy to transform
PLC store logic logic instead of wiring, greatly reduces the wiring of external control device, control system design and construction of cycle is much shorter, and daily maintenance becomes easier, more important is to make the same device after changing the procedures to change the production process becomes possible. This is particularly suitable for more variety and small batch production.
Four, problems in PLC application
PLC is an apparatus for automation and control of industrial production are generally not what measures need to be taken, you can directly use in industrial environments. However, despite has as mentioned above of reliability high, anti-interference capacity strong, but Dang production environment too bad, electromagnetic interference special strongly, or installation using improper, on May caused program errors or operation errors, to produced errors entered and caused errors output, this will will caused equipment of runaway and errors action, to cannot guarantee PLC of normal run, to improve PLC control system reliability, while requirements PLC production manufacturers improve equipment of anti-interference capacity; on the, requirements design, and Installation and maintenance of attention, work together in order to improve problem solving, System anti-interference performance effectively. In use should be aware of the following issues:
1. Work environment
(1) temperature
PLC required ambient temperature 0~55oC, installation cannot be placed in the heat element below, should be big enough ventilation space around.
(2) climate
In order to ensure the insulation performance of PLC, the relative humidity of air should be less than 85% (no condensation).
(3) motion
PLC should be away from strong vibration source to prevent vibration frequency for 10~55Hz frequent or continuous vibration. When using the environment when the inevitable shock, shock absorption measures must be taken, such as the use of damping rubber.
(4) air
Avoid corrosive and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc. For more dust in the air or corrosive gas environment, you can install the PLC in the control room or control cabinet in better.
(5) power supply
PLC power line interference has brought some resistance. Where reliability is very high or a power disturbance particularly severe environments, you can install a isolating transformer with a shield to reduce interference between equipment and ground. PLC have in General provide 24V DC output to input, when the input is an external DC power supply, DC power supply should be used. Ordinary rectifier power source, because of the ripple effect which makes the PLC receives an error message.
2. Sources of interference in the control system and its
Electromagnetic interference in PLC control system is the most common and most likely one of the factors that influence the reliability of the system, the so-called root first symptom, identify the problems and to propose ways to solve the problem. So you must know the sources of interference. (1) the interference sources and general classification
Effect of disturbance sources of PLC control system, mostly produced in current or voltage parts of dramatic change, the reason is the current creates a magnetic field, the devices produce electromagnetic radiation; change of magnetic field current, electromagnetic speed produces electromagnetic waves. Usually electro-magnetic interference according to the interference pattern is divided into common–mode interference and differential mode interference. Common–mode noise signal to ground potential difference, mainly by the power series, potential difference and space electromagnetic induction in the signal lines common mode (same direction) voltage is superimposed with form. Common–mode voltage circuit through asymmetry can be converted to differential mode voltage, directly affects the control signal, and cause damage to the components (which is why some system I/O modules damaged the main reason for higher rates), the common–mode noise for DC, and also to Exchange. Differential mode interference refers to the interference between the signal poles voltage, primarily by electromagnetic coupling between signal induction and by unbalanced circuit formed by the converted common mode interference voltage, noise superimposed on the signal, directly affect the accuracy of measurement and control.
(2) the main source of interference in PLC system and approach
Strong electrical interference
PLC system, the normal power supply by the power supply. Due to network coverage, it will be all space electromagnetic interference induced voltage on the wire. Especially changes in the power system, knife switching surges, large-scale power equipment starts and stops, AC and DC transmission caused by harmonics, power short-circuit transient shocks, through transmission lines to power original side.
Cabinet interference
High voltage apparatus within the control cabinet, large inductive loads, messy cabling is easy to cause some degree of disturbance to PLC.
Interference from signal line introduced
Connection with PLC control system signal transmission cable, in addition to transfers effective all types of information, there will always be external interference signal intrusion. This interference there are two main ways: through the transmitter power supply or total signal power supply for instrument strings into the power grid disturbances, it tends to be ignored; the second is interference of signal lines by space radiation sensors, namely external inductive interference on the signal line, which is very serious. Introduced by signal interference can lead to I/O signals working and accuracy is greatly reduced, serious damage caused when components.
Interference from grounding system disorder
Grounding is to improve the electronic equipment electromagnetic compatibility (EMC) is one of the most effective methods. Proper grounding, which can inhibit the effects of electromagnetic interference and suppression equipment issued to outside interference and error of grounding, but will introduce serious interference, the PLC system will not work properly.
Interference from PLC system
Primarily by system internal components and circuit interaction between electromagnetic radiation, such as logic circuits between radiation and its effects on analog circuits, analog and logic of interaction and components do not match each other and so on.
Transducer interference
Is inverter startup and running processes of conduction interference generated harmonic to power grid, causing voltage distortion, affect the quality of the power supply; the second is the output of the inverter will generate strong electromagnetic interference affecting peripherals to work properly.
Major anti-jamming measures
(1) reasonable processing power, suppressed the introduction of interference
For power into the grid disturbance can install a shielded isolation transformer with ratio of 1:1 to reduce interference between equipment and ground, can also power input series LC circuit. As shown in Figure 1
(2) installation and wiring
Power line, the line of control and PLC‘s power cord and I/O cable should be equipped with lines, isolating transformer and rubber should be used between the PLC and I/O cable. PLC IO and power cables separate alignment must be on the same line, separate banding AC cable, DC cable, if conditions allow, tank line is best, not only to give them the greatest possible distance, and interference to a minimum.
PLC should stay away from interference sources such as welding machines, high-power Silicon rectifier device and large power plants, cannot be in the high voltage electrical installation in a switch cabinet. PLC in the cabinets should be kept away from power lines (should be greater than the distance between the two 200mm). With the PLC installed in the same closet of inductive load, such as large power relays, contactor coils should be parallel RC circuit arc.
PLC best separate the input and output wiring, switches and analog to be laid separately. Analog signal routing shielded cable should be used, cover should be at one or both ends ground shielding resistance of grounding resistance should be less than 1/10.
AC output and DC output cables do not use the same cable, the output line should be kept away from power lines and power lines to avoid parallel.
(3) connection of I/O port
Input wiring
Input wiring is not too long. But if the disturbance is small, the voltage drop when input wiring can be appropriately longer.
Input/output line cannot use the same cable, the input/output lines
PLC ONE 