How to solve PLC anti-interference problem

With the development of science and technology, PLC is more and more widely used in industrial control. The reliability of PLC control system directly affects the safe production and economic operation of industrial enterprises. The anti-disturbance ability of the system is the key to the solid operation of the entire system. Various types of PLCs used in the automation system are gathered equipment in the control room, equipment in the production site and various electrical equipment. They are mostly in the harsh electromagnetic environment formed by strong electric circuits and strong electrical equipment. To improve the reliability of PLC control system, planners need only know in advance a variety of disturbances can be useful to ensure that the system runs solidly.

2. What is the source of electromagnetic disturbance and disturbance to the system?

The disturbances affecting the PLC control system originate from the same disturbance sources that generally affect the industrial control equipment. Most of them occur at locations where the current or voltage changes drastically. The locations where these charges move violently are the noise sources, that is, the sources of disturbance.

Disturbance type generally according to the causes of interference occurs, the form of noise interference and noise of the waveform of the different nature of the distinction. In the meantime: According to different causes of noise, it is divided into discharge noise, surge noise, high frequency vibration noise, etc.; according to the waveform and nature of noise, it is divided into continuous noise, occasional noise, etc.; Mode interference and differential mode interference. Common-mode interference and differential-mode interference are more commonly used classification methods. Common mode interference is the signal of the ground potential difference, mainly by the grid string into the ground potential difference and the space electromagnetic radiation in the signal line induced by the common mode (in the same direction) voltage sent to form. Common mode voltage is sometimes larger, especially the use of poor selection barrier electrical power supply room, the transmitter output signal common mode voltage is higher, and some can be as high as 130V. The common mode voltage can be changed to the differential mode voltage through the asymmetrical circuit, which will directly affect the monitoring and control signal, resulting in the damage of the component material (which is the reason for the higher damage rate of some system I / O modules). The common mode disturbance can be DC Or for communication. Differential mode interference refers to the disturbing voltage between the north and south poles of the signal. It is mainly caused by the coupling of the space electromagnetic field in the signal and the voltage formed by the unbalanced circuit and the common mode interference. The direct superimposed on the signal directly affects the measurement. Control accuracy.

3. What are the chief sources of electromagnetic interference in the PLC control system?

(1) Radiation from space disturbance

The radiated electromagnetic field (EMI) in space is mainly caused by the transient process of electric power networks and electrical equipment, lightning, radio broadcasting, television, radar, and high-frequency induction heating equipment. It is generally called radiation interference, and its dispersion is extremely messy. If the PLC system is placed in the RF field, it will be back to the radiation interference, the impact of the first two ways; one is directly to the PLC internal radiation interference by the circuit sensor; second is the radiation of the network within the PLC communication from The introduction of communication lines induces interference. Radiation disturbance and field equipment placement and equipment occurred in the electromagnetic field size, especially the frequency, generally set by shielded cable and PLC local shielding and high pressure relief components for maintenance.

(2) Disturbance from leads outside the system

First introduced through the power and signal lines, commonly referred to as conductive interference. This disturbance in China's industrial scene more severe.

(3) Disturbances from the power source

Practice has proved that due to the introduction of the disturbance of the power supply PLC control system failure of many conditions, I encountered in the commissioning of an engineering, after replacing the higher barrier PLC power supply, the problem was solved.

The normal power supply of the PLC system is powered by the grid. Because the grid covers a wide area, it will be susceptible to electromagnetic interference on all lines and induce voltage and circuitry on the line. In particular, changes within the grid, switching surge, start and stop of large power equipment, harmonics caused by AC-DC rolling equipment, transient short-circuit impact of the power grid, etc., are all transmitted through the transmission line to the power supply side. PLC power supply is generally used to block the power supply, but its organization and production process elements so that the barrier is not ideal. In fact, because of the spreading parameters, especially the presence of scattering capacitance, it is impossible to surely block.

(4) interference from the introduction of signal lines

The various signal transmission lines connected to the PLC control system, in addition to the transmission of various types of useful signals, always have external disturbance signals intruded. There are two main ways for this turbulence: one is the interference of the power grid through the power supply of the transmitter or the shared signal, which is often neglected; the other is that the signal line is disturbed by the space electromagnetic radiation induction, that is, the outside of the signal line. Induction disturbance, this is very serious. The interference introduced by the signal will cause abnormal I/O signal operation and greatly reduce the measurement accuracy. When it is severe, it will cause damage to the equipment. On the poor barrier function of the system, will also lead to signal interference between each other, causing the bus back to the common ground system, constitute a logical data changes, malfunction and crash. The number of I/O module failures due to the introduction of signal disturbances in the PLC control system is rather severe, resulting in a number of system failures.

(5) Disturbances when the grounding system is disturbed

Grounding is one of the useful ways to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding, both inhibit the impact of electromagnetic interference, but also inhibit the equipment announced disturbance; and fault the ground, but will introduce harsh interference signal, the PLC system will not be able to work properly. PLC control system grounding system, shielding, communication and maintenance and so on. Disturbance of the grounding system Disturbance of the PLC system is mainly due to the uneven distribution of the address potentials, and the difference in potential between the connected addresses, causing the loop current to affect the normal operation of the system. For example, if the cable shielding layer needs to be grounded a little, if the cable shielding layers A and B are grounded, there is a potential difference and a current flows through the shielding layer. When an abnormal condition occurs and a lightning stroke occurs, the grounding current will be greater.

In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the action of changing the magnetic field, there will be induced current in the shielding layer, which will disturb the signal circuit through the coupling between the shielding layer and the core wire. If the system and other grounding disorder, the occurrence of the earth circulation may occur unequal potential spread on the ground, affecting the PLC logic and analog circuits normal operation. Logic operation of the PLC voltage disturbance tolerance lower, the distribution of logic ground disturbance briefly affect the logical operation of PLC and data storage, constitute a data disorder, the program ran or crashed. The distribution of imitation ground potential will lead to the drop of measurement accuracy, causing serious distortion and misoperation of signal measurement and control.

(6) from the internal system of PLC disturbance

It is mainly caused by the electromagnetic radiation between the internal equipment and circuits of the system, such as the mutual interference of logic circuits and their influence on the imitation circuits, the mutual influence of imitation grounds and logic, and the unmatched application of metadevices. This is all part of the PLC manufacturing plant's plan for electromagnetic compatibility within the system. It is relatively cluttered and cannot be changed as part of its use. There is no need for too much consideration, but it is necessary to select systems that have more operational performance or have been tested.

4. How can we do better and more simple handling of PLC system disturbances?

1) Choose a better barrier device, use the excellent power supply, power lines and signal lines to be more reasonable and so on, but also to deal with interference, but more cumbersome, difficult to operate and costly.

2) The use of signal blockers such products deal with interference problems. Simply disturb the local, input and output center with this product, you can effectively deal with the problem of disturbance.

5. Why deal with PLC system interference signal blockers are elected?

1) The use of simple and convenient, reliable, low cost.

2) Much more can be done to reduce the workload of planners and system commissioning staff. Even messy systems can become very solid in the hands of average planners.

6. What is the principle of signal blocker operation?

Firstly, the signal received by the PLC is modulated by the semiconductor device, and then it undergoes a barrier change through a light sensing or magnetic sensing device, and then demodulates and changes back to the barrier original signal or different signals to block the power supply of the blocked signal. . To ensure that the signal after the change, power, certainly independent between.

7. What is the function of signal blocker?

A: Maintain the lower control loop.

Second: Weaken the impact of environmental noise on the test circuit.

Three: press 捺 public ground, frequency converter, solenoid valve and unknown pulse to disturb the equipment; together with the function of limiting pressure and flow of lower-level equipment is transmitter, appearance, frequency converter, solenoid valve PLC/DCS input and output, and communication Interface loyalty protection. Standard series of rail structure, easy equipment, useful blocking: input, output and power and ground potential. Can overcome inverter noise and various high and low frequency pulsation disturbance.

8. Now there are so many brand mall barrier, the price varies greatly, how to pick it?

The blocker is located between the two system channels, so the selection of the blocker must first determine the input and output functions, together with the input and output of the blocker (voltage type, current type, loop-powered type, etc.) to adapt to the front and rear channel interface types. In addition, many important parameters such as accuracy, power consumption, noise, dielectric strength, bus communication function, etc., affect product functions. For example, noise and accuracy, power consumption and reliability are closely related to these requirements. Always, applicable, solid, cost-effective product selection is the primary criterion barrier.