1.Overview
With the development of science and technology, PLC is more and more widely used in industrial control. the reliability of the PLC control system directly affects the safe production and economic operation of industrial enterprises, the system's ability to resist interference is related to the reliable operation of the entire system is the key. Automation system used in various types of PLC, some are centrally installed in the control room, some are installed in the production site and the motor equipment, most of them are in the strong electrical circuits and strong electrical equipment formed in the harsh electromagnetic environment. To improve the reliability of the PLC control system, the designers only pre-understanding of the various interferences in order to effectively ensure the reliable operation of the system.
2. electromagnetic interference sources and interference with the system is what?
Impact on the PLC control system interference from the general impact of industrial control equipment, the same source of interference, mostly generated in the parts of the current or voltage drastic changes in the parts of these charges move violently is the source of noise, i.e., the source of interference.
Types of interference are usually generated by the cause of interference, noise interference pattern and noise waveform nature of the different divisions. Among them: according to the different reasons for noise generation, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the different waveforms and properties of the noise, it is divided into continuous noise, episodic noise, etc.; according to the different modes of acoustic interference, it is divided into common-mode interference and differential-mode interference. Common mode interference and differential mode interference is a more commonly used classification method. Common mode interference is the signal to the ground potential difference, mainly by the power grid string into the ground potential difference and space electromagnetic radiation in the signal line induced by the common-state (same direction) voltage sent to add the formation. Common mode voltage is sometimes larger, especially with poor isolation performance of electrical power supply room, transmitter output signal common mode voltage is generally higher, some can be as high as 130V or more. Common mode voltage through the asymmetrical circuit can be converted to differential mode voltage, directly affecting the measurement and control signals, resulting in component damage (this is the reason why some systems I / O module damage rate is high), this common mode interference can be DC, can also be AC. Differential mode interference refers to the signal between the poles of the interference voltage, mainly by the electromagnetic field in the space between the signal coupling induction and unbalanced circuits to convert the common mode interference formed by the voltage, which allows the direct superposition of the signal, which directly affects the measurement and control accuracy.
3. What are the main sources of electromagnetic interference in the PLC control system?
(1) radiation interference from space
Radiation electromagnetic field (EMI) in space is mainly generated by the power network, electrical equipment transient process, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually referred to as radiation interference, the distribution is extremely complex. If the PLC system is placed in the radio frequency field, it is recycled to the radiation interference, the impact of which is mainly through two paths; one is directly to the PLC internal radiation, by the circuit inductance of the interference; but rather to the PLC communication within the network of radiation, by the inductance of the communication line to introduce interference. Radiation interference with the field equipment layout and the size of the electromagnetic field generated by the equipment, especially frequency-related, generally through the setting of shielded cables and PLC local shielding and high-voltage relief elements for protection.
(2) from the system outside the lead interference
Mainly through the introduction of power and signal lines, usually called conduction interference. This kind of interference is more serious in our industrial site.
(3) from the power supply interference
Practice has shown that the interference caused by the introduction of power supply PLC control system failure in many cases, encountered in a project debugging, and then replace the higher isolation performance of the PLC power supply, the problem is solved.
The normal power supply of the PLC system are powered by the power grid. As the power grid covers a wide range, will be subject to all space electromagnetic interference and induction of voltage and circuitry on the line. Especially the changes within the grid, into the switching operation surge, large power equipment starting and stopping, AC and DC rotating devices caused by harmonics, grid short-circuit transient shock, etc., through the transmission lines to the power side. PLC power supply is usually used to isolate the power supply, but the mechanism and manufacturing process factors that make its isolation is not ideal. In fact, isolation is impossible due to the presence of distributed parameters, especially distributed capacitance.
(4) from the interference introduced by the signal line
Connected with the PLC control system of various types of signal transmission lines, in addition to the transmission of all types of effective signals, there will always be external interference signal intrusion. This interference mainly has two ways: one is through the transmitter or shared signal instrumentation power supply string into the grid interference, which is often ignored; the second is the signal line by the space electromagnetic radiation induction interference, that is, the signal line on the external induction interference, which is very serious. Interference introduced by the signal will cause the I / O signal work abnormally and the measurement accuracy is greatly reduced, and in serious cases will cause component damage. For poor isolation performance of the system, will also lead to mutual interference between the signals, resulting in a common ground system bus backflow, resulting in changes in logic data, error and crash. PLC control system due to the introduction of signal interference caused by the number of damage to the I / O module is quite serious, resulting in system failure is also a lot of cases.
(5) from the grounding system when the confusion of the interference
Grounding is to improve the electromagnetic compatibility of electronic equipment (EMC), one of the effective means. Correct grounding, not only can suppress the impact of electromagnetic interference, but also inhibit the equipment to the outside of the interference; and wrong grounding, but will introduce serious interference signals, so that the PLC system will not be able to work properly. PLC control system ground including the system ground, shielded ground, AC ground and protective ground and so on. Grounding system confusion on the PLC system interference is mainly uneven distribution of the potential of each grounding point, there is a ground potential difference between different grounding points, resulting in ground loop current, affecting the normal operation of the system. For example, the cable shield must be grounded at one point, if the cable shield ends A, B are grounded, there is a ground potential difference, there is a current flowing through the shield, when an abnormal state plus lightning strike, the ground current will be greater.
In addition, the shielding layer, grounding wire and the earth may constitute a closed loop, under the action of the changing magnetic field, there will be induced currents in the shielding layer, through the shielding layer and the coupling between the core wire, interfering with the signal loop. If the system ground and other ground processing confusion, the resulting ground loop may produce unequal potential distribution in the ground, affecting the normal operation of the logic circuit and analog circuitry within the PLC. PLC logic voltage interference tolerance is low, the logic of the ground potential distribution of interference is likely to affect the logic of the PLC operation and data storage, resulting in data confusion, program run or crash. Analog ground potential distribution will lead to a decline in measurement accuracy, causing serious distortion of the signal measurement and control and false action.
(6) interference from within the PLC system
Mainly generated by the system's internal components and circuits between the mutual electromagnetic radiation, such as logic circuits.
Mutual radiation and its impact on analog circuits, analog and logical ground and the mutual influence of the components of the mutual mismatch between the use and so on. This all belongs to the PLC manufacturer of the system within the electromagnetic compatibility design of the content, more complex, as the application of the department is unable to change, may not have to give too much thought to, but to choose to have more application achievements or tested system.
4. How to better, simpler solution to PLC system interference?
(1) the use of isolation performance is better equipment, the use of good power supply, power line and signal line alignment to be more reasonable, etc., can also solve the interference, but more cumbersome, not easy to operate and higher costs.
(2) the use of signal isolator products to solve the problem of interference. As long as there is interference in the place, the input side and the output side of the middle plus this product, you can effectively solve the problem of interference.
5. Why solve the PLC system interference are selected signal isolator?
(1) The use of simple, convenient, reliable, low-cost.
(2) can greatly reduce the designers, system commissioning staff workload, even if the complex system in the hands of ordinary designers, will become very reliable.
6. What is the working principle of signal isolator?
First of all, the signal received by the PLC, through the semiconductor device modulation transformation, and then through the optical or magnetic sensing device isolation conversion, and then demodulation transformation back to the original signal before isolation or different signals, while the isolation of the signal after the isolation of the power supply for isolation processing. Ensure the independence between the transformed signal, power supply and ground.
7. What is the function of signal isolator?
(1) Protect the lower level of the control loop.
(2) Weaken the impact of environmental noise on the test circuit.
(3) Suppress the public ground, inverter, solenoid valve and unknown pulse interference with the equipment; at the same time, the lower level of equipment with a voltage limit, rated current function is a transmitter, instrumentation, inverters, solenoid valve PLC/DCS input and output and communication interfaces of the faithful protection. Standard series rail structure, easy to install, can effectively isolate: input, output and power supply and earth potential between. Can overcome inverter noise and a variety of high and low frequency pulsation interference.
8. Now the market has so many brands of isolators, prices vary, how to choose?
Isolator is located between the two system channels, so the first choice of isolator to determine the input and output functions, at the same time to make the isolator input and output modes (voltage, current, loop power supply type, etc.) to adapt to the front and rear end channel interface mode. In addition, there are still precision, power consumption, noise, insulation strength, bus communication functions and many other important parameters related to product performance, for example: noise and precision, power consumption, heat and reliability, these need to be carefully selected by the user. In short, applicable, reliable, cost-effective products is the main principle of selecting the isolator.