With the development of society and the continuous progress of science and technology, DCS/PLC and other automation equipment is more and more widely used in industrial control. The reliability of such control systems directly affects the safe production and economic operation of industrial enterprises, therefore, the anti-interference ability of the system is the key to the reliable operation of the whole system. At present, with the development of the diversity of equipment, the problem of interference is becoming more and more prominent. How can we effectively eliminate and resist interference in automatic control is also getting more and more attention. This paper only briefly analyzes the common interference problems in the automation control system, and hopes that it can help you.
Automation system used in various types of DCS/PLC and other automation equipment, some are centrally installed in the control room, some are installed in the production site and a variety of motor equipment, most of which are in the harsh electromagnetic environment formed by strong electrical circuits and strong electrical equipment. To improve the reliability of this type of control system, it is necessary to eliminate all kinds of interference in order to effectively ensure the reliable operation of the system.
PLC / DCS control system in the main sources of electromagnetic interference
1. Radiation interference from space
Space radiation electromagnetic field (EMI) is mainly from the power network, electrical equipment, transient processes, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually known as radiation interference, its distribution is extremely complex. If the PLC / DCS system is placed in the radio frequency field, it will receive radiation interference, its impact is mainly through two paths; one is directly to the DCS / PLC internal radiation, by the circuit inductance of the interference; the second is to the DCS / PLC communication network radiation, by the induction of the communication line introduced by the interference. Radiation interference and field equipment layout and equipment generated by the size of the electromagnetic field, especially the frequency, generally through the setting of shielded cables and system local shielding and high-voltage relief components for protection.
2. Conducted interference from the lead outside the system
Mainly through the introduction of power and signal lines, usually called conduction interference. This kind of interference is more serious in our industrial site.
A. Interference from the power supply
Practice has shown that the power supply interference caused by the introduction of the PLC/DCS control system failure of many cases.PLC/DCS system of the normal power supply from the power grid power supply. Due to the wide coverage of the power grid, will be subject to all the space electromagnetic interference. And in the line induced voltage and current, especially within the grid changes, switching operation surge, large power equipment start and stop, AC and DC rotating devices caused by harmonics, grid short circuit transient shock, etc., are transmitted through the transmission line to the power supply side. The system power supply usually uses isolated power supply, but its mechanism and manufacturing process factors make its isolation is not ideal. In fact, due to the presence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.
B. Interference from the introduction of the signal line
With PLC/DCS control system connected to the various types of signal transmission lines, in addition to the transmission of effective signals, there will always be external interference signal intrusion. This interference mainly has two ways: one is through the transmitter or common 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. The interference introduced by the transmission signal will cause abnormal operation of the I / O signal and the measurement accuracy is greatly reduced, and in serious cases will cause component damage. For the poor isolation performance of the system, it 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 / DCS control system due to the introduction of signal interference caused by the number of I / O module damage is quite serious, which causes a lot of system failures.
3. From the grounding system confusion when the interference
Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment (EMC). Correct grounding, both to inhibit the impact of electromagnetic interference, but also to inhibit the equipment to the outside world to send out interference; and the wrong grounding, but will introduce serious interference signals, so that the PLC / DCS system will not be able to work properly. DCS / PLC control system ground including the system ground, shielded ground, AC ground and protective ground and so on. Grounding system confusion on the DCS / PLC system interference is mainly uneven distribution of the potential of each grounding point, the 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 a point of grounding, if the cable shield ends A, B are grounded, there is a ground potential difference, there is a current flowing through the shield, when anomalies occur plus lightning strikes, the ground current will be greater.
In addition, the shielding layer, grounding wire and 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 core wire between the convergence, interference 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 circuitry and analog circuitry within the PLC. PLC logic voltage interference tolerance is low, the logic of the ground potential distribution interference is likely to affect the logic of DCS / PLC calculations and data storage, resulting in data disruption, the program runs 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.
4. From the DCS / PLC system within the interference
Mainly by the system components and circuits within the mutual electromagnetic radiation generated, such as logic circuits radiate each other and its impact on the analog circuit, analog ground and logic ground and the mutual influence of components and the use of mutual mismatch. This all belongs to the DCS / PLC manufacturer of the system within the electromagnetic compatibility design content, more complex, as the application of the department is unable to change, can not be too much to consider, but to choose the system with more application practice or after the test.
Common interference phenomenon and judgment method
Phenomenon:
A. When the system sends commands, the motor rotates irregularly.
B. When the signal is equal to zero, the value of the digital display table jumps chaotically.
C. sensor work, DCS/PLC collected signals and the actual parameters corresponding to the signal value does not match, and the error value is random, irregular; D, and AC servo system sharing the same power supply (the same power supply), the same power supply (the same power supply), the same power supply (the same power supply).
D. and AC servo systems share the same power supply (such as monitors, etc.) is not working properly.
Judgment steps are as follows:
A. with a multimeter AC gear detection of the receiving port, such as interference will produce an AC signal. If this signal is not large, the signal acquisition is very small, almost no impact. If this AC signal is large, it will affect the value, need to find ways to solve.
B. see whether the port end of the ground? If grounded whether there is a hanging or poor grounding. Use a multimeter to measure the voltage difference between the port and ground (can be the system ground, can also be the signal ground). If there is AC voltage, it means that there is interference, if there is no AC voltage, there is a DC voltage difference. This voltage difference is large, affecting the system; the difference is small, the impact is small and negligible.
C. and then see whether the shielding layer is grounded, is a single point of grounding or double-point grounding? Generally single-point grounding.
Better, simpler solution to system interference
(1) strong electrical interference:
Instrumentation signals, DCS/PLC control signals are weak, vulnerable to strong electrical interference. Therefore, the requirements of the cabinet wiring (in the cable trench, cable tray, through the pipe and other laying methods), communication lines, signal lines, control lines and other weak electrical signals away from the strong power, the spacing shall not be less than 20CM. cable trench multi-layer, the requirements of the weak cable laying in the strong cable below.
(2) cabinet interference:
DCS/PLC system can not be installed with high-voltage electrical appliances in the same switch refused, PLC output using intermediate relays to achieve the isolation of external switching signals. If the site conditions, the input signal can not be effectively isolated from the strong cable, available small relays to isolate the input switching signals. Of course, DCS/PLC input signals from the control cabinet and input signals not far from the control cabinet generally do not need to be isolated by relays.
There are many signal lines in the control cabinet. If the wiring is chaotic, it will cause equipment malfunction, but it is quite troublesome to check. So in the design of the control cabinet should take into account this situation, the equipment is arranged in layers, the alignment is clear. Sets, the PLC's old and high-power line separate lines, such as must be in the same slot, separate bundles of AC lines, DC lines, such as conditions allow, the best slot alignment, and make it as large as possible spatial distance, and strive to minimize the interference. Different signal lines are best not to use the same plug adapter, such as must use the same plug, to use the spare terminal or ground terminal to separate them to reduce mutual interference.
PLC can not be installed in the same switchgear cabinet with high-voltage electrical appliances, in the cabinet PLC should be away from the power line (the distance between the two should be more than 200mm). With PLC installed in the same cabinet inductive loads, such as relays, contactor coils, should be connected in parallel with the RC arc-canceling circuit.
(3) Signal line anti-interference
Signal lines bear the task of detecting signals and control signal transmission, transmission quality directly affects the accuracy, stability and reliability of the entire control system. Signal line interference is mainly from the space of electromagnetic radiation, differential mode interference and common mode interference.
Differential mode interference refers to the measurement of the signal line superimposed on the interference signal, this interference is mostly higher frequency alternating signals, the source is generally coupled interference. Methods for suppressing normal mode interference are:
A. in the input circuit connected to the RC filter or double T filter.
B. try to use double integral type A/D converter, due to the characteristics of this integrator work, has a certain role in eliminating high-frequency interference:
C. the voltage signal is converted into a current signal and then transmitted.
Common-mode interference is a common interference signal on the signal line, generally caused by the measured signal ground terminal and the ground terminal of the control system there is a certain potential difference caused by this interference in the two signal lines on the cycle, amplitude is basically equal to the case, using the above method can not be eliminated or suppressed. Methods are as follows:
A. the use of dual differential input differential amplifier, this amplifier has a very high common mode rejection ratio;.
B. the input line using stranded wire, stranded wire can reduce the common mode interference, and its induction cancel each other.
C. the use of optoelectronic isolation method, can eliminate common mode interference;
D. the use of shielded wire, and one-sided grounding.
To avoid signal distortion, pay attention to impedance matching for signals transmitted over longer distances.
Switching signals (such as buttons, limit switches, proximity switches and other signals provided) generally have no special requirements for the cable, you can use the general cable, signal transmission distance, you can use shielded cable.
Analog signals and high-speed signal lines (such as pulse sensors, counting digital disk and other signals provided) should choose shielded cable.
Communication cables require high reliability, some communication cable signal frequency is very high, generally should choose the PLC manufacturer to provide special cables, in the requirements are not high or low signal frequency, you can also choose a twisted pair cable with shielding, but the quality should be good.