Analysis of Sources of Electromagnetic Interference in PLC Control System

(1) Radiated interference from space
Space radiation electromagnetic field (EMI) is mainly caused by the power network, the transient process of electrical equipment, lightning, radio, television, radar, high-frequency induction heating equipment, commonly known as radiation interference, the distribution is extremely complex. If the PLC system is placed in the radio frequency field, it will recover the radiation interference, its influence mainly through the two paths; one is directly on the PLC internal radiation, caused by the interference of the circuit induction; but the PLC communication network radiation, The induction of communication lines introduces interference. Radiation interference and field equipment layout and equipment generated by the size of the electromagnetic field, especially the frequency, generally by setting the shielded cable and PLC local shielding and high pressure relief components to protect.
(2) interference from outside the system lead
Mainly through the power and signal lines introduced, commonly known as conduction interference. This interference is more serious in China’s industrial field.
(3) interference from the power supply
Practice has proved that the interference caused by the introduction of power supply PLC control system failure situation, the author encountered in a project debugging, after the replacement of higher performance isolation PLC power supply, the problem was resolved.
The normal power supply of the PLC system is powered by the grid. As the grid coverage is wide,
Will be subject to all space electromagnetic interference on the line induced voltage and circuit. Especially within the grid changes, switching operation surge, large power equipment starting and stopping, AC and DC rotating device caused by harmonics, power short-circuit transient shock, etc., through the transmission line to the power side. PLC power supply is usually isolated power supply, but its mechanism and manufacturing process factors to isolate it is not ideal. In fact, absolute isolation is not possible due to the presence of distributed parameters, especially the distribution of capacitance.
(4) interference from the signal line
And PLC control system connected to the various types of signal transmission lines, in addition to the transmission of various types of signals, there will always be external interference signal intrusion. This interference has two main ways: First, through the transmitter or common signal instrument power supply string into the power grid interference, which is often overlooked; Second, the signal line by the electromagnetic interference of space interference, that is, the signal line on the outside Sensing interference, which is very serious. The interference caused by the signal will cause the I / O signal to work abnormally and the measurement accuracy is greatly reduced, which will cause the component damage. For the isolation of poor performance of the system, will also lead to interference between the signals, causing the common system bus backflow, resulting in logical data changes, malfunction and crash. PLC control system due to signal interference caused by the I / O module damage is quite serious, resulting in a lot of system failure situation.
(5) interference from grounded system chaos
Grounding is one of the effective ways to improve the electromagnetic compatibility (EMC) of electronic equipment. The correct grounding, both to suppress the impact of electromagnetic interference, but also inhibit the equipment out of the interference; and the wrong ground, but will introduce a serious interference signal, the PLC system will not work properly. The ground system of the PLC control system includes systematically, shielded, AC and protected areas. The interference of the grounding system chaos to the PLC system is mainly that the ground potential distribution is uneven, there are ground potential difference between different ground points, causing the ground loop current, affecting the normal operation of the system. For example, the cable shield must be grounded, if the cable shield at both ends of A, B are grounded, there is ground potential difference, there is a current flow through the shield, when the abnormal state of lightning, the ground current will be greater.
In addition, the shielding layer, the ground wire and the earth may constitute a closed loop, under the action of the changing magnetic field, there will be induced current within the shield, through the shield and the coupling between the core, interference signal circuit. If the system and other ground handling chaos, the resulting loop may produce unequal potential distribution on the ground, affecting the PLC logic and analog circuits in the normal work. PLC logic voltage interference margin is low, the logical ground potential of the distribution of interference easily affect the logic of PLC operations and data storage, resulting in data chaos, the program running or crash. The distribution of the simulated ground potential will result in a decrease in the measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.
(6) interference from the PLC system
Mainly by the internal components of the system and the circuit between the electromagnetic radiation generated, such as logic circuits
Mutual radiation and its impact on the analog circuit, analog and logical ground interaction and mutual use between components do not match. This is part of the PLC factory for electromagnetic compatibility within the system design content, more complex, as the application sector can not be changed, do not have to consider too much, but to choose a more application performance or test the system.