1 Due to the huge equipment and disperse layout, the wiring is too long and the path is unreasonable to cause poor grounding, interference loops, line noise, and mutual influence with related equipment.
2 When the cable was routed, it did not follow the principle of strong and weak electricity shunting, that is, power, control, and communication were combined. The dynamic aspects are easier to distinguish, and the content of control signals is more, and their classification according to different actual requirements is also different.
3 The mutual influence of related equipment and systems, generally electromagnetic interference, is roughly as follows:
a) Equipment with pulse generators such as inverters and servo devices b) Equipment for generating pulses and arcs such as wire cutting machines and electric discharge machines
c) Jitter of the lighting device (fluorescent lamp) during the opening phase d) The counter-peak voltage when the relay and contactor are released e) The frequency of some signals of the peripheral devices and the system are similar
The basic classification of a control signal:
1 Switch (digital) type input a) Dry contact --------- operation button (switch), stroke and limit switch, relay, contactor auxiliary contact and so on. Because the PLC input current is 5-7mA, in order to ensure the stability of the signal, the current at the input must be maintained at 3.5 and 4.5 mA. If the contact is bad or the wire is too long, it is susceptible to interference. In addition, the larger contactor auxiliary contact should consider the contact reliability when the current is small.
b) Proximity switches, photoelectric switches, open-collector type encoders, etc. --------- These components are mostly placed on the site, with a certain distance between the PLC, there is the possibility of interference, should try to use Switching performance products. Generally, it is more susceptible to interference when the level interval/pulse width ratio is large.
c) Differential Encoder --------- This type of sensor is more suitable for long-distance, high-speed applications, with less chance of interference than the former. On the other hand, the open collector type will attenuate the output level due to the long wire length.
2 Switch (digital) type output:
a) Relays ----------- are used to drive contactors, solenoid valves, lights, electronic circuits and other equipment. The load power is below DC30V and below AC240V. If the AC load, in order to facilitate the cable, such control signals can be placed together with the power line under normal circumstances. Under DC load, depending on its voltage level and the nature of the equipment being driven, it depends on the circumstances.
b) Transistor output (DC5-30V) I Drive relays, indicator lights, electronic circuits, and other equipment. Interference factors must be considered when using low voltage loads. II Drives inverters, positioning devices, electronic circuits, etc. in level or pulse form. It is recommended to maintain a proper distance from the power line, at least 30cm. Consider the system as far away as possible from the equipment that may affect each other.
c) Note ----------- PLC output is connected to the external load (contactor, solenoid valve) and other inductive components, should be connected according to the AC and DC properties of resistance absorption circuit or freewheeling diode. Mitsubishi PLC's promotion has not caused interference due to ignoring this work, but it will shorten the life of its internal output components and it will easily affect external electronic devices.
3 Analog input:
Connected to PLC in the form of voltage or current, generally output from various types of testing or control equipment (sensors, instrumentation, etc.), if they are susceptible to interference due to problems such as long lines, improper use, or their quality, etc. Operation. Current-type input is recommended especially for small signals.
4 analog output:
In the form of voltage or current output from the PLC to related equipment, as with analog inputs, attention is paid to the reliability of small signals, such as interference and attenuation phenomena in long distances.
5 Communication lines:
a) Optical fiber cable ----------------------- Because of the way out of level transmission, high-performance, low-loss information transmission is possible. It is immune to electromagnetic interference.
b) The coaxial cable and the twisted pair ----------- are both electrically connected and have the characteristics of level transmission. The coaxial cable has a long transmission distance, a high speed, a large number of link stations, and less interference due to good media and structural refinement, but the cost is high. Twisted pair performance in this regard is lower than the former, but the price is economical, convenient line, you can make their own. In the case of a large system, space is limited (appropriate termination resistance). When arranging the communication cable, it should try its best to separate from the power line and control line and walk independently to ensure the communication of the whole system is unblocked and undisturbed. At present, Mitsubishi's products have not been found to cause communication anomalies due to interference.
Since users choose Mitsubishi products as part of the system when they complete the equipment, they configure a certain number of peripheral equipment, such as low-voltage electrical appliances, motors, solenoid valves, sensors, instrumentation, electronic circuits, computers, various networks, and other industrial control products. . Some of these products or networks do not all have industry standards or have not yet undergone trials. System integration has the potential to affect each other. Therefore, there are problems in system design, component selection, equipment in place, installation, wiring, debugging, and maintenance. There are congenital and artificial factors, or just after normal operation, with many reasons. Causes changes in the environment and causes abnormalities.
In order to reduce the chance of interference, electrical appliances should complete relevant preparations in accordance with relevant industry regulations to eliminate hidden dangers. Since system design, equipment placement and other issues are all weighed by users according to their respective production conditions, application requirements, funding conditions and other factors, inconvenient to change, here are only suggestions from the electrical aspects, including:
1 The equipment is in place ---------- In the case that the overall principle of the system has been determined (including the components in the electrical cabinet), refer to the principle and make every effort to achieve a reasonable layout, so as to reduce the invalid cable path to reduce Harmful factors.
2 Cables ------------ If the conditions are inconvenient, the detailed classification should be roughly divided into strong and weak electricity, and as far as possible to ensure that the cable reduces parallel opportunities to reduce the mutual inductance between the wires. In addition, when the weak signal line is long, the twisted pair can be used as required to reduce the distributed capacitance and mutual inductance.
3 Even if it is impossible to eliminate the interference on the hardware, it may be improved in some cases by adding a corresponding program.
According to the user's current report, interference is likely to occur when products such as high-speed counting, temperature control, inverters, and servo (stepper) devices are used.
1 High-speed counting module AIDD62:
The distance between the encoder and the module is 30m, and there is an error (pulse loss) when counting occurs. Installation wiring requirements should be attached to the encoder and power supply should be individually connected, but not through the power path of the module, after correction to improve the situation.
2 Thermocouple Temperature Module FX2N-4AD-TC:
The temperature value often fluctuates indefinitely and the range is 3-5°C, which affects the unit's operation. The relevant BFM of the average sampling point is repeatedly adjusted in the program and has no effect. After the shield terminal of the compensation wire was changed to 0V of the module power supply, the situation was significantly improved. Its fluctuation range is 0.1-0.3°C. In some cases, if the above measures still have no effect, thermocouple+transmitter+normal A/D module can be used. Because the transmitter itself has formed an isolation link, it can solve the numerical disturbance to a large extent. problem.
3 positioning module FX1S (1N), FX2N-1PG, FX2N-10 (20) GM:
For the interference prevention measures of the positioning module and servo device, please refer to the product manual. Here is a brief explanation of the connection to the stepper motor driver. Many stepper motor drivers have differences in comparison with imported products. The common phenomenon is that the position error is caused by the motor step-out caused by the interference pulse. In addition to the similar method to the servo device, a simpler method may be to install a filter. Improve it. Since the pulse-driven devices have a wide operating frequency range and do not know their input (output) impedance, it is difficult to calculate them. Therefore, it is not easy to add components such as self-made filters. Improper selection will not only absorb normal high-frequency pulses, but also reduce The level of response of the system depends on the specific circumstances.
4 Frequency Converter FR-E500:
The user installed multiple A500 and E500 series inverters in the electric control cabinet. When there are multiple E500 inverters in operation, the frequency on the operation panel shows 0, and the RUN indicator goes out, but the operation is as usual. Press any key to restore the display. During the observation, it was found that all the ground terminals of the inverter were connected to the cabinet of the electrical cabinet, and the cabinet was not grounded. Grounding is a more rigorous job and must be done thoroughly. If it is unconditional, it may not be grounded temporarily. Half of it and half of it will cause interference. After the equipment is shipped to the site, the fault is eliminated.
5 Frequency Converter FR-A500:
When the inverter runs, it interferes with the user's control instrument. After the motor is removed, the inverter runs without load. The system is normal. The initial judgment should be caused by the output line noise. Use the FR-BLF line noise filter to connect the inverter U, V. Between W and the motor. The degree of interference is significantly reduced. If the line noise does not come from the load but originates from the grid input, the filter can be connected to the R, S, T terminals on the incoming side.
6 In some cases, because the interference can not be completely eliminated, a program can be added to the software. The principle is to reduce the sensitivity of the data to be interfered with, and it is relatively slow and difficult to fluctuate by timing average calculation. Of course, it is only suitable for occasions where real-time requirements are not high.
The product functions and technical data mentioned above are based only on Mitsubishi products. The relevant contents are just some of my own experiences in the past. The occurrence of these phenomena far exceeds the scope of a brand product. Due to the complexity and uncertainty of the interference phenomenon and on-site experience, some understandings are not comprehensive. It is hoped that peers with relevant experience will supplement and correct them to better maintain and enhance the reputation of the products they are engaged in.
Mitsubishi Products - Some Considerations in Electrical Packages
Mitsubishi's industrial control products are widely used in various industrial and mining enterprises. Although the quality of the products themselves has been guaranteed, due to the ever-changing conditions in the industry, the laws are not clear, and interference will inevitably occur in the entire system operation after the equipment is complete. Because interference is the process of interaction, the effect of either party on the other side can cause system failure. Therefore, it is very complicated in principle analysis and practical solution. It requires both situation judgment and practical experience. According to the current source of users, the sources of this problem are: