What needs attention in PLC application

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) sources of interference effects of disturbance sources of PLC control system and general classification, 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 commonmode interference and differential mode interference. Commonmode 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. Commonmode 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 commonmode 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: the normal power supply by the power supply of the PLC system. 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.
The introduction of interference from the signal cable: 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.

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