Mitsubishi plc's main source and path of electromagnetic interference - News - Global IC Trade Starts Here.

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Most of the interference sources that affect the plc control system are generated in places where the current or voltage changes drastically. The part where these charges move violently is the source of interference. The type of interference is usually divided by the cause of the interference, the noise interference mode, and the nature of the noise waveform. According to different causes of noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the waveform and nature of noise, it can be divided into continuous noise, sporadic noise, etc.; according to different noise interference modes, it is divided into common mode. Interference and differential mode interference.
Common mode interference is the potential difference between the signal and the ground, which is mainly formed by the grid-in, ground potential difference and the common-state voltage induced by the space electromagnetic radiation on the signal line. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which affects the measurement and control signals and causes component damage. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly formed by the coupling of the space electromagnetic field between the signals and the common mode interference by the unbalanced circuit. This interference is superimposed on the signal and directly affects the control accuracy.
1. Interference from space radiation to Mitsubishi plc.
Space radiated electromagnetic fields mainly come from on-site power lines, transient processes of electrical equipment, lightning, wireless signals, high-frequency induction heating equipment, etc., which are related to the electromagnetic field generated by the field equipment arrangement and equipment. The radiation interference of the Mitsubishi PLC system is mainly through two paths: one is the radiation to the Mitsubishi PLC communication network, and the interference is induced by the communication line; the other is the radiation directly inside the Mitsubishi PLC, which is caused by the circuit induction.

2. Interference from power supply to Mitsubishi Plc Mitsubishi PLC power supply usually uses isolated power supply, but its isolation is not ideal due to its mechanism and manufacturing process. In practice, there are many situations in which the Mitsubishi PLC control system fails due to interference introduced by the power supply.

The normal power supply of Mitsubishi PLC system is powered by the power grid. Due to the wide coverage of the power grid, it will be subject to all space electromagnetic interference and induce voltage and current on the line, especially the internal changes of the power grid, large equipment start and stop, switching operation surge, Short-circuit current surges, harmonics caused by AC/DC transmissions, etc., are transmitted to the primary side of the power supply through the transmission line, causing interference. Mainly through two paths: one is the interference introduced by the Mitsubishi PLC power supply; the other is the interference introduced by the power supply of the transmitter power supply or the shared signal meter.

3. The interference from the signal line introduction to the Mitsubishi plc and the various signal transmission lines connected with the Mitsubishi PLC control system, in addition to transmitting various types of effective signals, will also be invaded by external interference signals. The interference introduced by the signal will cause the I/O signal to work abnormally and the measurement accuracy is greatly reduced. In severe cases, the component will be damaged. There are two main ways of interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter; the other is that the signal line is interfered by the space electromagnetic radiation, that is, the external inductive interference on the signal line. For systems with poor isolation and shielding performance, signals will also interfere with each other, causing the common system bus to reflow, causing system misoperation and crashes in severe cases.

4. The interference from the grounding system conduction to Mitsubishi plc.
Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can both suppress the influence of electromagnetic interference and suppress the external interference of the equipment. The wrong grounding will introduce serious interference signals to make Mitsubishi PLC system. can not work normally. The grounding line of Mitsubishi PLC control system includes system ground, shield ground, AC ground and protection ground. The disturbance of grounding system to PLC system is mainly the uneven distribution of potential at each grounding point. There is ground potential difference between different grounding points. The loop current and ground loop current act on the Mitsubishi PLC system, which affects the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends of the cable shielding layer are grounded, there is a ground potential difference, and a current flows through the shielding layer.
In addition, the shielding layer, the grounding wire and the ground may constitute a closed loop. Under the action of the changing magnetic field, an induced current may appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system ground and other grounding treatments are confusing, the generated ground circulation may generate unequal potential distribution on the ground line, which affects the normal operation of logic circuits and analog circuits in Mitsubishi PLC. The logic voltage interference tolerance of Mitsubishi PLC is low, and the distributed interference of logic ground potential easily affects the logic operation and data storage of Mitsubishi PLC, resulting in data confusion and crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.

5. Interference from the internal Mitsubishi PLC system.
It is mainly caused by mutual electromagnetic radiation between internal components and circuits of the system, such as mutual mismatch between components, mutual radiation of logic circuits, and interaction between analog ground and logic ground. This is a design problem for Mitsubishi PLC manufacturers. Although the application department cannot be changed, it is necessary to add more research and comparison.

Third, Mitsubishi PLC control system anti-interference measures When carrying out anti-interference design of specific projects, it is necessary to select products with high anti-interference ability, take measures to suppress interference sources, cut off or attenuate electromagnetic interference, and use software measures to improve The anti-jamming capability of the device and system.

1. Use a power supply with excellent performance to suppress interference introduced by the power grid.
For the power supply of the Mitsubishi PLC system, it should be powered by non-power lines, and directly from the main bus of the low-voltage power distribution room. The isolation transformer is selected, and the transformer capacity should be 1.2 to 1.5 times larger than the actual requirement. It is also possible to add a filter before the isolation transformer. For transmitters and common signal instrumentation, choose a distributor with small distributed capacitance, multiple isolation and shielding and leakage inductance technology. The controller and I/O system are powered by their respective isolation transformers and are separated from the main circuit power supply. Mitsubishi PLC's 24V DC power supply should try not to supply power to all kinds of peripheral sensors to reduce the interference of the external sensor or the short-circuit of the power supply line to the Mitsubishi PLC system. In addition, in order to ensure that the grid feed is not interrupted, the online uninterruptible power supply (UPS) can be used for power supply. The UPS has the functions of overvoltage, undervoltage protection, software monitoring, and isolation from the grid, which can improve the safety and reliability of the power supply. For some important equipment, the AC power supply circuit can use a dual power supply system.

2. Correctly select the cable and implement the laying to eliminate the space radiation interference of Mitsubishi Plc.
Different types of signals are transmitted by different cables, and the signal cables are layered according to the type of transmission signals. The same type of signal lines are twisted. It is strictly forbidden to use the different wires of the same cable to transmit power and signals at the same time, to avoid the signal wires and power cables being laid close to each other, and to increase the angle between the cables to reduce electromagnetic interference. In order to reduce the radiated electromagnetic interference of the power cable, especially the feeder cable of the inverter, and to block the intrusion of interference from the interference path, a shielded power cable is used.

3. Anti-jamming measures of the Mitsubishi plc input and output channels The filtering of the input module can reduce the differential mode interference between the lines of the input signal. In order to reduce the common mode interference between the input signal and the earth, Mitsubishi PLC should be well grounded. When there is an inductive load at the input, for the AC input signal, the capacitor and the resistor can be connected at both ends of the load. For the DC input signal, the freewheeling diode can be connected in parallel. In order to suppress parasitic capacitance between input signal lines, and induced electromotive force generated by parasitic capacitance or coupling between other lines, an RC surge absorber can be used.
The output is an AC inductive load, and the RC surge absorber can be connected in parallel across the load; if it is a DC load, the freewheeling diode can be connected in parallel, as close as possible to the load. For the case of a digital output, a surge absorber or a thyristor output module can be used. In addition, the use of output point series intermediate relay or photoelectric coupling measures can prevent the Mitsubishi PLC output point from directly accessing the electrical control loop, and is completely electrically isolated.

4, Mitsubishi plc anti-interference software measures Due to the complexity of electromagnetic interference, it is not enough to take hardware anti-interference measures, to use Mitsubishi PLC software anti-jamming technology to cooperate to further improve the system reliability. The use of digital filtering and power frequency shaping sampling, timing correction reference point potential and other measures to effectively eliminate periodic interference and prevent potential drift. Use information redundancy technology to design the corresponding software flag bits; use indirect jumps, set software protection, etc. For example, for the digital input signal, the timer delay is used to read in multiple times, and the result is consistent and valid, which improves the reliability of the software.

5. Correctly select the grounding point and improve the grounding system.
Good grounding is an important condition to ensure the reliable operation of Mitsubishi PLC, which can avoid the accidental voltage shock hazard and also suppress interference. The perfect grounding system is one of the important measures for the anti-electromagnetic interference of Mitsubishi PLC control system.

The Mitsubishi PLC control system is a high-speed low-level control device and should be grounded directly. In order to suppress the interference applied to the power supply and the input and output terminals, a dedicated ground wire should be connected to the programmable controller, and the grounding point should be separated from the grounding point of the power equipment. If this requirement is not met, it must be grounded in common with other equipment, and it is forbidden to be grounded in series with other equipment. The grounding point should be as close as possible to Mitsubishi PLC. The centrally arranged Mitsubishi PLC system is suitable for parallel grounding, and the grounding point of the cabinet center of each device is led to the grounding pole by a separate grounding wire. The Mitsubishi PLC system with distributed arrangement should adopt a one-point grounding method in series. The grounding resistance of the grounding pole is less than 2Ω, the grounding pole is preferably buried 10~15m away from the building, and the grounding point of the Mitsubishi PLC system must be more than 10m away from the grounding point of the strong electrical equipment. If an expansion unit is to be used, its grounding point should be connected to the grounding point of the base unit.
When the signal source is grounded, the shielding layer should be grounded on the signal side; when the signal source is not grounded, it should be grounded on the Mitsubishi PLC side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and the shielding layers should be connected to each other. Select a proper grounding point for single point grounding to avoid multi-point grounding.

6, equipment selection.
When selecting equipment, first understand the anti-interference indicators given by the manufacturer, such as common mode rejection ratio, differential mode suppression ratio, withstand voltage capability, and allow for working in the field of large electric field strength and high frequency magnetic field strength. Choose products with high anti-interference ability, such as Mitsubishi PLC system with floating technology and good isolation performance.
The anti-interference problem of the Mitsubishi PLC control system in the field application is complex and meticulous. Anti-interference design is a very complicated system engineering, involving specific input and output equipment and the specific environment of the industrial site. It requires us to comprehensively consider various factors, and must reduce the interference source and cut off according to the actual situation on site. Comprehensive considerations were made in terms of interference pathways, and various anti-jamming measures were fully utilized to design the Mitsubishi PLC control system. In order to truly improve the anti-interference ability of Mitsubishi PLC system in the field application, ensure the safe and stable operation of the system.