PLC applications need to be aware of the problem

PLC is a kind of used for industrial production automation control equipment, generally do not need to do, can be directly used in the industrial environment. However, although as mentioned in the high reliability, strong anti-jamming capability, but when the production environment is too bad, particularly strong electromagnetic interference, improper use or installation, can cause bugs or operational errors, resulting in a input error and the error output, this will lead to equipment is out of control and false action, which can guarantee the normal operation of the PLC, to improve the reliability of PLC control system, on the one hand require PLC manufacturers to improve the anti-interference ability of the equipment; Requirements, on the other hand, cause attaches great importance to the design, installation and use and maintenance, parties cooperate to perfect solve the problem, effectively enhance the anti-jamming performance of the system. So in use should be paid attention to the following questions: 1. The work environment ( 1) PLC for environment temperature in 0 ~ 55 oc, cannot in calorific value big components below when installation, ventilation cooling around the space should be large enough. （ 2) Humidity to guarantee the insulation performance of PLC, air relative humidity should be less than 85% ( No condensation) 。 （ 3) Vibration should make PLC from strong vibration source, to prevent the vibration frequency of 10 ~ 55 hz frequency or continuous vibration. When using the environment inevitably vibration damping measures must be taken, such as using damping rubber, etc. （ 4) Air to avoid corrosion and combustible gases, such as hydrogen chloride and hydrogen sulfide. For more dust in the air or the environment of corrosive gas, PLC can be installed in a closed better control room or control cabinet. （ 5) For the power cord to the disturbance of power PLC has certain ability to resist. In the high requirement of reliability or power interference, especially serious environment can install a shield of isolation transformer, to reduce the interference between the device and ground. Generally PLC has provided to the input dc 24 v output, when the input using an external dc power, dc regulated power supply should be chosen. Because the average power rectifier filter, because of the influence of the ripple, makes the PLC receives the error information. 2. Control system at the scene of the interference source and its electromagnetic interference is the most common in the PLC control system is also the most easily one of the factors affecting the reliability of the system, treats the so-called cure first, find out the problem, to put forward the solution to the problem. So you must know the interference source. （ 1) Interference sources and the general classification of PLC control system interference sources, mostly produced in the part of the current or voltage change sharply, the reason is that current magnetic field changes, the equipment to produce electromagnetic radiation; Creates an electric current magnetic field changes, electromagnetic produce electromagnetic waves at a high speed. Electromagnetic interference usually according to the interference pattern is different, divided into common mode and differential mode. Is common mode interference signal to ground potential difference, mainly by the grid series, epd and the space electromagnetic radiation on the signal induction of common mode ( Same direction) Formed by the superposition of voltage. Common-mode voltage by asymmetric circuit can be converted into a differential mode voltage, a direct impact on measurement and control signals, damaged components ( This is some system I/O module of the main causes of damage rate is higher) The common-mode interference can be used equally for dc, for communication. Differential mode interference refers to the interference voltage applied to signal between the poles, is mainly composed of space electromagnetic field in the signal coupling between induction and unbalanced by circuit switching common-mode interference voltage, which is formed by the superposition of the interference in the signal, directly affect the precision of measurement and control. （ 2) The main source of interference in PLC system and the way to high voltage interfere with the normal power supply of the PLC system consists of power supply. Due to the grid coverage range, it will be all the space electromagnetic interference and online induced voltage on the road. Internal changes, especially in power grid knife switch operation surge, large power equipment start-stop, ac/dc transmission caused by harmonic, power grid short-circuit transient impact, etc. , through the transmission line to supply the original edge. Incase interference control incase of high-voltage electrical appliances, large inductive load, chaos wiring are easy to cause a certain degree of interference of PLC. Interference from signal is introduced into all kinds of signal transmission lines connected to the PLC control system, in addition to all kinds of information transmission effectively, there will always be the external interference signal. There are two main ways: the interference is a power supply via the transmitter or common signal interference instrument series with the power supply of power grid, this often neglected; Secondly, signal interference of electromagnetic radiation induced by space, namely the external induction interference on line, this is very serious. Introduced by signal interference can cause I/O signal abnormal work and measurement accuracy is greatly reduced, serious when will cause components damage. Interference from grounding system chaos when the grounding is to improve the electronic equipment electromagnetic compatibility ( EMC) One of the effective means. Proper grounding, can restrain the influence of electromagnetic interference, and inhibit the equipment sent outside interference; And the wrong ground, on the contrary will introduce serious disturbance signal, make the PLC system will not be able to work properly. Interference from the PLC system is mainly composed of the internal components and circuit between electromagnetic radiation against each other, mutual radiation such as logic circuit and its influence on analog circuits, analog ground and logic to influence each other and don't match each other between the components of use, etc.