The main sources and channels of electromagnetic interference of Mitsubishi PLC

Most of the interference sources that affect the PLC control system are generated at the parts where the current or voltage changes dramatically. These parts where the charge moves violently are the interference sources. The types of interference are usually divided according to the cause of the interference, the noise interference mode and the nature of the noise waveform. According to the different causes of the noise, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the waveform and nature of the noise, it can be divided into continuous noise, occasional noise, etc.; according to the different noise interference modes, it can be 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 superposition of common-mode voltage induced on the signal line by the series connection of the power grid, the ground potential difference and the electromagnetic radiation in space. The common-mode voltage can be converted into a differential-mode voltage through an asymmetric circuit, affecting the measurement and control signal and causing damage to components. Differential-mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly formed by the coupling induction of the electromagnetic field in space between the signals and the conversion of common-mode interference by the unbalanced circuit. This interference is superimposed on the signal and directly affects the control accuracy.

1. The interference of radiation from space to Mitsubishi PLC.

The electromagnetic field radiated in space mainly comes from the on-site power lines, transient processes of electrical equipment, lightning, wireless signals, high-frequency induction heating equipment, etc., and is related to the on-site equipment layout and the electromagnetic field generated by the equipment. The radiation interference to the Mitsubishi PLC system mainly occurs through two paths: one is the radiation to the Mitsubishi PLC communication network, which is introduced by the communication line induction; the other is the direct radiation to the inside of the Mitsubishi PLC, which is caused by the circuit induction.

2. Interference from power supply to Mitsubishi PLC

Mitsubishi PLC power supply usually adopts isolated power supply, but its isolation is not ideal due to its structure and manufacturing process. In practice, there are many cases where Mitsubishi PLC control system failures are caused by interference introduced by the power supply.

The normal power supply of Mitsubishi PLC system is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by all spatial electromagnetic interference and induce voltage and current on the line, especially the changes within the power grid, the start and stop of large equipment, the switch operation surge, the short-circuit current impact, the harmonics caused by the AC and DC transmission devices, etc., are all transmitted to the primary side of the power supply through the transmission line, causing interference. There are mainly two paths: one is the interference introduced by the Mitsubishi PLC power supply; the other is the interference introduced by the transmitter power supply or the power supply of the shared signal instrument.

3. Interference from the conduction introduced by the signal line to Mitsubishi PLC

4. Interference to Mitsubishi PLC caused by grounding system conduction.

Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can not only suppress the impact of electromagnetic interference, but also suppress the interference emitted by the equipment. Incorrect grounding will introduce serious interference signals, making the Mitsubishi PLC system unable to work properly. The grounding wires of the Mitsubishi PLC control system include system ground, shielding ground, AC ground and protection ground. The interference of the chaotic grounding system to the PLC system is mainly due to the uneven distribution of potentials at each grounding point. There is a ground potential difference between different grounding points, which causes ground loop current. The ground loop current acts on the Mitsubishi PLC system and 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 will be a ground potential difference and current will flow through the shielding layer.

In addition, the shielding layer, grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, which will interfere with the signal loop through the coupling between the shielding layer and the core wire. If the system ground and other grounding processes are messed up, the generated ground loop current may produce unequal potential distribution on the ground wire, affecting the normal operation of the logic circuit and analog circuit in the Mitsubishi PLC. The logic voltage interference tolerance of Mitsubishi PLC is low, and the distribution interference of the logic ground potential can easily affect the logic operation and data storage of Mitsubishi PLC, causing data confusion and crashes. The distribution of analog ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunction of signal measurement and control.

5. Interference from within the Mitsubishi PLC system.

It is mainly caused by the mutual electromagnetic radiation between components and circuits within the system, such as the mismatched use of components, mutual radiation of logic circuits, mutual influence between analog ground and logic ground, etc. These are all design issues of Mitsubishi PLC manufacturers. Although the application department cannot change them, it is necessary to conduct more research and comparison when choosing.

3. Anti-interference measures for Mitsubishi PLC control system

When conducting anti-interference design for a specific project, it is necessary to select products with higher anti-interference capabilities, and adopt measures such as suppressing interference sources, cutting off or attenuating the propagation paths of electromagnetic interference, and using software to improve the anti-interference capabilities of devices and systems.

1. Use a power supply with excellent performance to suppress the interference introduced by the power grid.

For the power supply of Mitsubishi PLC system, non-power line power supply should be adopted, and dedicated line power supply should be adopted directly from the main bus of low-voltage distribution room. Isolation transformer should be selected, and the capacity of transformer should be about 1.2 to 1.5 times larger than the actual need. Filter can also be added in front of isolation transformer. For the power supply of transmitter and common signal instrument, distributor with small distributed capacitance and multiple isolation, shielding and leakage inductance technology should be selected. The controller and I/O system are powered by their own isolation transformers respectively and separated from the main circuit power supply. Mitsubishi PLC's 24V DC power supply should not be used to power various peripheral sensors as far as possible to reduce the interference of short circuit fault inside peripheral sensors or power supply line to Mitsubishi PLC system. In addition, in order to ensure the uninterrupted power supply of power grid, online uninterruptible power supply (UPS) can be used for power supply. UPS has overvoltage and undervoltage protection functions, software monitoring, isolation from power grid and other functions, which can improve the safety and reliability of power supply. For some important equipment, AC power supply circuit can adopt dual power supply system.

2. Correctly select cables and implement laying to eliminate the spatial radiation interference of Mitsubishi PLC.

Different types of signals are transmitted by different cables, using distance technology. Signal cables are laid in layers according to the type of transmitted signals, and the same type of signal cables are twisted in pairs. It is strictly forbidden to use different conductors of the same cable to transmit power and signals at the same time, avoid laying signal cables close to and in parallel with power cables, and increase the angle between cables to reduce electromagnetic interference. In order to reduce the radiated electromagnetic interference of power cables, especially the feeder cables of frequency converters, and block the intrusion of interference from the interference path, shielded power cables should be used.

3. Anti-interference measures for Mitsubishi PLC input and output channels

The filtering of the input module can reduce the differential mode interference between the input signal lines. In order to reduce the common mode interference between the input signal and the earth, the Mitsubishi PLC should be well grounded. When there is an inductive load at the input end, for AC input signals, capacitors and resistors can be connected in parallel at both ends of the load, and for DC input signals, freewheeling diodes can be connected in parallel. In order to suppress the parasitic capacitance between the input signal lines, the parasitic capacitance between other lines, or the induced electromotive force generated by coupling, an RC surge absorber can be used.

If the output is an AC inductive load, an RC surge absorber can be connected in parallel at both ends of the load; if it is a DC load, a freewheeling diode can be connected in parallel, and it should be as close to the load as possible. For occasions with switching output, a surge absorber or a thyristor output module can be used. In addition, the use of an intermediate relay or photoelectric coupling in series at the output point can prevent the Mitsubishi PLC output point from being directly connected to the electrical control circuit, and it is completely electrically isolated.