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PLC control system in actual use, precautions

In Electronic Infomation Category: P | on March 02,2011

Years programmable logic controller (hereinafter referred to as PLC) from its creation to now, to achieve a connection to the storage logical leap in logic; its function from weak to strong, to achieve a logic control to digital control of the progress; its applications small to large, simple control device to achieve a single to qualified motion control, process control and K9F1G08UOA-PCBO datasheet and distributed control across the various tasks. PLC in dealing with todays analog and K9F1G08UOA-PCBO price and digital computing, human-computer interfaces, and K9F1G08UOA-PCBO suppliers and networks have been a substantial increase in the capacity to become the mainstream of the field of control of industrial control equipment, in all walks of life playing an increasingly important role.

1 PLCs applications

Current PLC has been widely used in domestic and foreign iron and steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural entertainment and other industries. There are chemicals in Yuncheng of water plants, sewage treatment, industrial wastewater treatment, condensate polishing processing. About the use of PLC is divided into the following categories.

1.1 switch logic control

Replace traditional relay circuit, logic control, sequential control, both for the control of a single piece of equipment can also be used for multi-group control. Such as start and stop pumps, valves, switches, sequential control of water systems, dry ash handling system.

1.2 Industrial Process Control

In industrial production process, there are some, such as temperature, pressure, flow, level and speed, the amount of continuous change (ie, analog), PLC using the appropriate A / D and D / A converter module and a variety of different kind of control algorithm to handle analog, complete closed-loop control. PID closed loop control system adjustment is used generally have more of a conditioning method. Process control in metallurgical, chemical and other places have a very wide range of applications, Yuncheng Power Plant is mainly used in central air conditioning, heating and heating systems.

1.3 Motion Control

PLC can be used in a circular motion or linear motion control. Generally use a dedicated motion control module, for example a stepper motor or servo motor driven single-axis or multi-axis position control module, is widely used in a variety of machinery, machine tools, robots, elevators and other occasions.

1.4 Data processing

PLC with mathematics (including matrix operations, functions, operations, logic operations), data transfer, data conversion, sorting, look-up table functions, you can complete the data collection, analysis and processing.

1.5 Communication and networking

PLC communication with the communication between PLC and the PLC and other intelligent communication between devices. With the development of factory automation network, now has a communication interface PLC, communication is very convenient.

2 PLC characteristics of the application

2.1 high reliability, strong anti

High reliability is the key to the performance of electrical control equipment. PLC as the use of modern large scale integrated circuit technology, strict production process, the internal circuit to take the advanced anti-jamming technology, with high reliability. Constitute a control system using PLC, and the same scale relay contactor systems, electrical wiring and switch contacts has been reduced to hundreds or even thousands of times, the failure will be greatly reduced. In addition, PLC hardware failure with self-detection, failure to give timely warning information. In the application software, application are also incorporated into the peripheral device fault diagnosis procedure, the system in addition to PLC circuits and devices other than the failure to obtain protection from the diagnosis. Thus, the overall system reliability is extremely high.

2.2 is complete, functional applicability

PLC to today, has formed a series of products of all sizes can be used for all sizes of industrial control applications. In addition to processing logic other than, PLC most of the computing power has improved the data can be used for a variety of digital control areas. A wide variety of functional units in large numbers, so that penetrated into the position of PLC control, temperature control, CNC and other industrial control. Coupled with enhanced communication capabilities PLC and human-machine interface technology, the use of PLC control system composed of a variety of very easily.

2.3 easy to use, by the engineering staff welcomes the

PLC is facing the industrial and mining enterprises in the industrial equipment. It interfaces easily, programming languages easy for engineers to accept. Ladder language, graphic symbols and expressions, and the relay circuit is very close, are not familiar with electronic circuits, computer principles and assembly language do not know who opened the door to industrial controls.

2.4 small amount of work system design, easy maintenance, easy to transform

(1) design and maintenance

PLC wiring logic instead of using stored logic, significantly reducing the control equipment external wiring, the control system design and construction of the much shorter period, while routine maintenance is even easier, more importantly, so that the same equipment has been changed procedures to change the production process possible. Particularly suitable for many varieties, small batch production applications.

(2) Installation and wiring

Power lines, control lines and power lines and PLC I / O lines should be separately wiring, isolation transformer and PLC and I / O should be used between the twisted pair connection. The PLCs I / O lines and power lines go separate lines, such as to be in the same wiring, separate AC line bundle, DC line, if conditions allow, the best sub-groove alignment, not only can it have as much as possible large distance, and can minimize interference.

PLC should stay away from strong sources of interference, such as welding, high power rectifier devices and large power equipment, not with high-voltage electrical installations in the same switch cabinet. PLC in the cabinet away from power lines (the distance between them should be greater than 200 mm). Fitted with the PLC within the same cabinet inductive load, such as large power relay, contactor coil, arc should be parallel RC circuit.

PLCs better to use separate input and output wiring, and analog switch must be laid separately. Transmission of analog signals should be shielded wire, one end or both ends of the shield should be grounding the shield resistance should be less than 1 / 10.

AC output line and DC output lines do not use the same cable, the output should be kept away from high voltage lines and power lines, to avoid parallel.

(3) I / O side of the connection

Input wiring: Input terminal generally not too long. But if the environment is less interference, less voltage drop, the longer the input terminal may be appropriate; input / output lines can not use the same cable, input / output lines to be separated; as far as possible the form of normally open contacts connected to the input the preparation of the relay ladder diagram with the same easy to read.

Output connections: the output terminal is divided into separate output and common output. In different groups, using different types and voltage levels of output voltage, but the output in the same group can only use the same type, the same voltage level power supply. The output of the PLC components are encapsulated in the printed circuit board and connected to the terminal board that if the load circuit connected output components, printed circuit boards will be burned. With relay output, inductive load borne by the size, will affect the life of the relay, therefore, the use of inductive load should be a reasonable choice, or additions isolation relays. PLCs output load can cause interference, so to take measures to control, such as the continued flow of control DC output protection, exchange of the output RC snubber circuit, the output transistors and two-way thyristor shunt resistor protection.

3 PLC application problems that need attention

PLC is used for industrial automation and control equipment, generally do not need to take what measures can be directly used in industrial environments. However, despite the high reliability as mentioned above, anti-interference ability, but when the production environment is too harsh, especially strong electromagnetic interference, or improper installation, may result in program errors or operator errors, resulting in incorrect input and cause false output, which will result in loss of control and malfunction of equipment, which can not guarantee the normal operation of PLC. PLC control system to improve reliability, ask the PLC equipment manufacturers to improve anti-jamming capability; the other hand, to design, installation and maintenance of close attention, multi-party cooperation to improve problem solving, anti-effectively enhance the system interference performance. Therefore, in use should be aware of the following.

3.1 environment

(1) Temperature: PLC requires the ambient temperature at 0 ~ 55 , heating installation, large components can not be placed below the space around the ventilation should be large enough.

(2) Humidity: PLC in order to ensure the insulation performance, air relative humidity should be less than 85% (no condensation).

(3) Vibration: PLC should be made from a strong vibration source, to prevent vibration frequency is 10 ~ 55 Hz for frequent or continuous vibration. When the environment is inevitable vibration, damping measures must be taken, such as the use of damping rubber.

(4) Air: to avoid a corrosive and flammable gases, such as acid-base chemistry and so on. For more dust in the air, or corrosive gas, can be installed in a closed PLC good control room or control cabinet. For example, the dry plant slag, dry ash, etc., added late in the closed cabin infrastructure.

(5) Power: PLC for the interference caused by power line has a certain ability to resist. Or power in the high reliability requirements of a particularly serious environmental interference, you can install a shielded isolation transformer, to reduce interference between equipment and ground. General DC 24 V PLC output by the input provided, when using an external DC input power should be used in DC power supply. Because the average power rectifier filter, the ripple effects, easy to PLC receive an error message.

3.2 control system interference and its source

Site EMI PLC control system is the most common and most likely one of the factors affecting system reliability.

(1) interference sources and the general classification

PLC control system of interference sources, most of the current or voltage produced dramatic changes in the location, the reason is the current change in magnetic field, the electromagnetic radiation on equipment; magnetic field change generated current, high-speed generation of electromagnetic waves. Usually different modes of electromagnetic interference by interference, into common mode interference and differential mode interference. Common mode interference is the signal-to-ground potential difference, mainly by the string into the power grid, ground potential difference and the space electromagnetic radiation in the induction of the common mode signal line (in the same direction) voltage superimposed formed. Common-mode voltage circuit can be converted by asymmetric differential mode voltage, directly affect the measurement and control signals, resulting in component damage (which is some of the system I / O module main reason for the higher failure rate), this can dc common-mode interference also for the exchange. Differential mode interference is the effect on the signal interference voltage between the poles of the main signal from the electromagnetic field coupling between the sensor and converted by the imbalance of the circuit formed by the common mode interference voltage, such interference superimposed on the signal, directly affect the measurement and control accuracy.

(2) PLC system, the main source of interference and ways

Strong electrical interference: PLC system, the normal power supply by the grid. Because a wide range of network coverage, it will be all the space on the line of electromagnetic interference induced voltage. Internal changes, especially power, knife switching surge, start and stop large-scale power equipment, AC and DC transmission caused by harmonics, transient short-circuit the impact of such power, the power transmitted through the primary transmission lines.

Cabinet interference: high-voltage electrical control cabinet, a large inductive load, confusing layout are easy on the degree of interference caused by PLC.

Interference from the signal wires are: connected with the PLC control system for various types of signal transmission lines, in addition to all types of information transmission beyond effective, there are always external interference signal intrusion. This interference There are two main ways: first, shared by the transmitter power supply or power supply string instrument signal into the power grid disturbances, which are often neglected; second signal line induced by the interference of electromagnetic radiation of space, the signal external line induced interference, this is very serious. Introduced by the signal interference caused by I / O signals and accuracy of the work exceptions greatly reduced, will cause serious damage components.

Chaos from the time of grounding system interference: electronic equipment grounding is to improve the electromagnetic compatibility (EMC) is one effective means. Properly grounded, can inhibit the impact of electromagnetic interference, but also outgoing interference suppression devices; the wrong ground, it will introduce significant interference signal to PLC system will not work properly.

Interference from the PLC within the system: the main internal components and circuit by the system interaction between electromagnetic radiation, such as logic circuits between radiation and its effects on analog circuits, analog and logic components to the interaction and mismatch between the use of each other.

Frequency interference: first, start and run the inverter harmonics generated in the process conducted interference generated on the grid, causing power grid voltage distortion, affecting the quality of power supply; Second, the inverter output will produce a strong electromagnetic radiation interference, affect the normal operation of peripheral equipment. Major anti-jamming measures


(1) the power of rational processing, interference suppression for the power grid into the grid disturbance can be installed into a shield with a 1:1 turns ratio isolation transformer to reduce interference between equipment and ground, You can also power input LC filter circuit in series.

(2) correct choice of access sites, improve the grounding system is to ensure good grounding of the important conditions for reliable PLC, to avoid accidental voltage shock hazard. The purpose of grounding usually

Two, one for security, the other is to suppress interference. Grounding system is the perfect anti-PLC control system is one important measure of electromagnetic interference.

PLC control system including the system ground, the shield, the exchange of land and protect the land. Confusion on the PLC system, grounding system is mainly the interference of the uneven distribution of the various potential access sites in different locations connected to the potential difference between the rise to ground loop currents, affecting the system work properly. For example, to point grounding cable shield, if cable shield at both ends A, B are grounded, there is ground potential difference, a current flows through the shield, when the abnormal state, such as lightning, the ground current will be greater.

In addition, shield, ground wire and the earth might constitute a closed loop, in a changing magnetic field, the shielding layer may also appear in the induced current, through the shielding layer and the coupling between wires, interference signal circuit. If the system is chaotic with other ground handling, resulting in circulation may be produced in places ranging potential of online distribution of PLC logic and analog circuits within the normal work. PLC logic voltage interfere with the work of a lower tolerance, logic ground potential interference likely to influence the distribution of PLC logic operations and data storage, and the data confusion, the program ran to fly or crash. The distribution of simulated ground potential measurement accuracy will lead to decline, causing a serious distortion of the signal monitoring and malfunction.

Safety ground or power ground: the power cord connecting earth ground terminal and cabinet for the safety ground. If the power leakage or cabinet charged, can be imported from the safety ground, G, will not cause harm to people.

System grounding: PLC controller to control various devices with the same potential as the ground, called the system ground. Grounding resistance not more than 4 , which normally takes the PLC device and control system to switch power supply cabinet with negative termination, as the control system ground.

Signal and shield ground: General requirements signal line must be the only reference to the "single point ground", could have shielded cable conducted interference encountered situations, but also the only ground in the place or the control room, prevent the formation of "ground loop." Signal source is grounded, the signal side of the shield should be grounded; is not grounded, should be grounded in the PLC side; signal line when the middle connector, shielding

Layer and the insulation should be firmly connected to processing, multi-point ground must be avoided; multiple measuring points signals and multi-core shielded twisted-pair shielded twisted pair cables total, each shield should be connected with each other, and after insulated, select the appropriate ground at a single point of contact.

(3) of the frequency interference suppression

Frequency interference processing generally have the following ways: adding an isolation transformer, is mainly directed against conducted interference from the power supply, you can block most of the Conduction disturbances before the isolation transformer; use of filters, the filter has strong anti-interference ability in, the device itself also has to prevent transmission to the power of interference, and some absorption of both peak voltage; using output reactors, the motor in the drive to increase the AC reactor between the main drive is to reduce Output in the energy transfer process lines produce electromagnetic radiation, affect other devices work properly.

4 Conclusion

PLC control system interference is a very complex issue, so the interference should be considered in the design of various factors, reasonably and effectively suppress interference, the PLC control system can work properly. With the continuous expansion of PLC applications, and how efficient and reliable use of PLC has also become an important factor in its development. In the future, PLC will have a greater development, product variety will be more rich, more complete specifications, by the perfect man-machine interface, a complete communication equipment will better meet the needs of various industrial control applications, PLC as the automation and control network and the network of internationally important part to play in the field of industrial control an increasingly important role.

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