Anti-interference Problems of Papermaking Fieldbus

According to the definition of IEC1158 of the International Electrotechnical Commission, the digital, serial, multi-point communication data bus between the field devices installed in the manufacturing or process area and the dynamic control devices in the control room is called fieldbus. The most popular existing buses in the world today are FF bus (FieldbusFoundatiON), Profibus, Modbus, etc. In the papermaking industry, ABB's AF100 is also widely used. However, no matter which fieldbus is used, it is a digital signal. When it is transmitted on the medium, due to interference noise, "1" becomes "0" and "0" becomes "1", which affects the performance of the fieldbus and makes it unable to work normally. Therefore, it is very important to study the anti-interference problem of the fieldbus and improve the anti-interference ability of the fieldbus.

1 Interference sources in paper machine workshop

(1) The paper machine drive system is the biggest source of interference in the paper machine workshop. The total load of the paper machine drive system accounts for more than 1/3 of the total load of the papermaking workshop. In the rectification and inversion of the system, high-power power electronic components (IGBT, etc.) are switched on and off at high speed, generating a large amount of high-frequency electromagnetic waves, polluting the entire workshop, and generating a large amount of high-order harmonics, polluting the industrial frequency power grid.

(2) Transformers, MCC cabinets, power cables and power equipment. These devices are all industrial frequency, with low frequencies. Interference generally occurs in the near field. In the near field, the electric field component and the magnetic field component vary greatly depending on the characteristics of the interference source. In particular, when the power equipment is started, the instantaneous current can reach 6 to 1 times the rated current, which will produce transient interference with large current impact.

(3) Interference from industrial power supply. If the waveform distortion and high-order harmonics of the industrial power supply are not isolated or filtered, they will enter the control system through the power supply to the paper machine control system and affect the field bus signal.

(4) Sparks, arcs, etc. caused by poor wire contact.

(5) Ground current caused by unbalanced three-phase power supply and ground loop current caused by the non-common grounding of the shielding layer.

2 Interference propagation pathways

(1) Transmission by wires is called conducted interference. In fieldbus, it mainly manifests as ground impedance interference and interference from industrial frequency power supply.

(2) Transmission through space in the form of radiation is called radiation interference.

3 Anti-interference measures for fieldbus

(1) Stay away from interference sources. The interference of power equipment and power cables to the field bus is inversely proportional to the square of the distance, that is, as the distance increases, the interference decays very quickly. Therefore, the field bus equipment is far away from the power equipment, and the field bus cable and power cable are arranged in layered bridges, which can play a good role in preventing interference. Staying away from interference sources is an important measure to prevent radiation interference.

(2) Shielding of fieldbus equipment and cables The shielding mechanism of fieldbus is that, first, external electromagnetic waves generate eddy currents on the metal surface, thereby offsetting the original magnetic field; second, electromagnetic waves generate reflection losses on the metal surface, and another part of the transmitted waves attenuates and generates absorption losses during the propagation process in the metal shielding layer. The shielding of fieldbus uses a shield made of conductive materials combined with grounding to cut off the interference source.

(3) Using UPS power supply or isolation transformer can prevent interference from industrial frequency power supply.

(4) Use optical cables to transmit signals. When the fieldbus transmission speed is high, the transmission distance is long, and the interference is large, optical cables should be used as much as possible. After using optical cables, many problems of radiation interference and conduction interference are effectively solved. If the fieldbus signal is transmitted between two points that are not in common ground, or when the grounding condition is very poor, using optical cables to transmit fieldbus signals can effectively prevent interference such as ground loops. Especially between the paper machine control system and the paper machine drive MCC, there is a high probability of ground potential difference or ground loops. Pay special attention to the use of optical cables rather than twisted pair cables.

(5) Correct selection and installation of the rectifier part of the paper machine drive system using 12-pulse rectification can effectively eliminate the 5th and 7th harmonics. The power cable between the drive cabinet and the motor must be a well-shielded and three-core symmetrical power cable.

4 Fieldbus grounding

Good grounding is an important measure to prevent interference in fieldbus. The so-called "ground 0" is generally defined as the zero potential reference point of a circuit or system; grounding is to establish a conduction path between two points. Its purpose is to protect the safety of operators and equipment, that is, to protect the ground 0; secondly, to suppress electromagnetic interference and provide a potential reference in electronic measurement, that is, "working ground 0". Figure 1 shows the grounding method of the working place:

Single-point grounding means that all electronic devices or components in the system are connected in parallel to a ground reference point. The advantage of this grounding method is that the ground potential of each electronic device or component is only related to the ground impedance of the electronic device or component, and is not affected by other loops; the disadvantage is that multiple ground wires are required, which increases the length of the ground wire, increases the interference coupling between the ground wires, and greatly increases the ground wire impedance under high frequency conditions. Multi-point grounding means that each point in the system that needs to be grounded is directly connected to the ground point closest to it to minimize the length of the ground wire. Because the ground wire is very short, it is suitable for high-frequency conditions; the disadvantage is that it is easy to form various ground loops, causing low-frequency ground loop interference. When the operating frequency is below 1MHz, a single-point grounding method can be used; when the operating frequency is above 10MHz, a multi-point grounding method can be used; of course, a combination of single-point grounding and multi-point grounding can also be used. When the ground wire is too long, multi-point grounding should also be used to prevent the ground wire impedance from being too large and the interference noise voltage from being too high. Among many fieldbuses, the development of Profibus fieldbus is particularly rapid and is widely used in the papermaking industry. Now take Profibus2DP as an example. The communication rates of Profibus2DP are 9.6, 19.2, 87.5kbit/s and 0.5, 1.5, 3, 6, 12Mbit/s. The most commonly used communication rate is 1.5Mbit/s. At this time, the length of the Profibus twisted pair cable can reach 200m (can be extended using a repeater). When the Profibus2DP communication rate is 1.5Mbit/s, it can be considered that the communication frequency is 7.5MHz. The frequency is relatively high, close to 10MHz, and multi-point grounding is suitable, and the grounding impedance should be minimized as much as possible.
Profibus2DP cable uses RS248 twisted pair shielded cable. Due to the high communication frequency, the noise current generated by interference only flows on the surface of the shielding layer (skin effect). The shielding layer usually adopts multi-point grounding, generally grounded at both ends of the cable shielding layer, that is, the beginning and the end are grounded at the same time. If one end is grounded (the beginning or the end), it is only conducive to eliminating low-frequency interference. However, when there is no equipotential grounding plate in the entire workshop and equipotential grounding cannot be achieved, grounding at one end is better than grounding at both ends, and it can prevent the formation of ground loops.

5 Anti-interference application of fieldbus in papermaking control

Huatai Group PM, 0 paper machine, with a design speed of 1800m/min, a roll width of 635mm, and an annual production capacity of 200,000 tons, was put into production on July 20, 200. The DCS system was provided by an internationally renowned company, the controller used AC450, and the MCC used the INSUM system. The DCS and MCC used Profibus2D communication connection, with a communication rate of 115Mbit/s. Since the internal communication of INSUM uses LONbus, LONbus must be converted to Profibus through the Gateway gateway, and the controller side is connected to the CI541 card. In the early stage of debugging, Profibus twisted pair shielded cable was used, with a total length of about 150m. The connection used a 9-pin D-type plug, and both ends of the cable were grounded through the D-type plug. It was soon discovered that not all Profibus stations could be found, and sometimes even one station could not be found. Many experiments were done to eliminate all other reasons. Finally, it was found that when the cable shield on the DC side was not grounded, all stations could be found, so single-end grounding on the MCC side was adopted. Although it was able to operate normally, a new situation emerged. Every once in a while, a Gateway suddenly crashed during operation and returned to normal after reset. Analysis of the cause: The Gateway is located in the MCC cabinet, where a large number of power equipment exist, resulting in large electromagnetic interference. This part of the MCC is close to the transmission, and the interference generated by the paper machine transmission is affected. There is a grounding potential difference between the DCS and the MCC, so that when both ends of the cable shield are grounded, a loop is formed, which interferes with the bus signal. A large number of CI541 card error messages also appeared on the controller side engineer station, and it is suspected that a large number of bit errors exist. In view of the above reasons, it was decided to replace the Profibus shielded cable with an optical cable. After the replacement, the situation improved and the Gateway no longer crashed.

6 Conclusion

In the design, installation and debugging of paper machine control system, special attention should be paid to the anti-interference problem of field bus. Only by taking correct anti-interference measures can the bus operate normally to provide favorable guarantee for production.