Design of automatic control system based on fieldbus and Ethernet technology

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Design of automatic control system based on fieldbus and Ethernet technology [Copy link]

Abstract: This paper effectively solves the problem of seamless integration of the three electrics (electrical, instrumentation, and computer) within the basic automation system (L1) and between it and the process and management control system (L2) by adopting ProfiBus and ControlNet fieldbus Ethernet technology.
Introduction
With the development of control, computer communication, and network technology, the structure of automation systems is undergoing tremendous changes. The underlying control network is no longer an isolated control system. The current development direction of the automatic control field is to achieve network integration through fieldbus and Ethernet technology, and connect the basic automation control network (L1), process and management control system (L2) with the enterprise resource planning layer ERP (Enterprise Resource Planning). This paper will introduce the design scheme of the slab warehouse/heating furnace automation control system of a steel plant. By adopting fieldbus and Ethernet technology, the problem of seamless integration of the three electrics (electrical, instrumentation, and computer) within L1 and between it and L2 is effectively solved.

The automation control system of the slab warehouse/heating furnace
area of the steel plant is composed of the basic automation control system and the process and management control system. By combining the two fieldbus technologies ProfiBus and ControlNet, the communication of the instrument and electrical control system in the L1 network is realized; at the same time, the communication between the basic automation system and the process and management control system is realized by using Ethernet technology.

Basic automation control system
The basic automation control system includes the heating furnace electrical PLC control system and the heating furnace instrument DCS control system. The Processlogix hybrid controller developed by RockWell Automation of the United States is used for the heating furnace instrument control system (one set for each heating furnace); the Controllogix controller developed by the company is used for the control of the heating furnace electrical drive (one set for each heating furnace). The instrument and electrical control systems are connected to the server of the operation station through the dual ControlNet, thus forming an instrument-electric integrated control system.
Two mutually redundant ProcessLogix servers are used in the system configuration: SERVER A and SERVER B.
Configuration of instrument DCS control system
The instrument DCS hardware system adopts the ProcessLogix series. According to the process requirements and the characteristics of the on-site equipment layout, each heating furnace is equipped with a DCS control station, which is connected to the local I/O control rack through ControlNet, and is used for the heating furnace 6-stage combustion control, furnace pressure control, two sets of combustion-supporting fans, one set of dilution fans, flue, heat exchanger and cooling water process detection and control.
Electrical PLC control system configuration
The electrical PLC hardware system adopts the ControlLogix series. According to the process requirements and the characteristics of the on-site equipment layout, 5 PLC control stations are set up to complete the electrical control functions of the slab warehouse, No. 1 heating furnace, No. 2 heating furnace, No. 1 heating furnace hydraulic station, and No. 2 heating furnace hydraulic station.

Figure 1 Configuration of the electrical PLC control system for No. 2 heating furnace

Figure 1 shows the configuration of the PLC of the No. 2 heating furnace. The PLC controller controls the steel loading machine, walking beam, steel tapping machine, hydraulic station and other public and auxiliary facilities of the heating furnace respectively, and connects to the remote I/O station through the ControlNet bus to realize the acquisition and signal output of the field signal. At the same time, it communicates with the instrument DCS system through two redundant ControlNet buses and communicates with ABB's ASC600 frequency converter through ProfiBus DP to realize the control of the steel loading machine/steel tapping machine and the front and rear rollers of the heating furnace.
The process and management control system
level L2 must complete the process control functions with strong real-time performance, such as billet tracking, steel loading and tapping settings, etc. On the other hand, it must complete the rolling plan compilation and slab warehouse data management functions with low real-time requirements. Therefore, according to the process characteristics and the requirements of decentralized control functions to reduce risks, the process and management control system consists of a heating furnace process control machine and a slab warehouse control machine, and each computer communicates through Ethernet.
The heating furnace computer exchanges data with the basic automation system by accessing two redundant servers SERVERA and SERVERB. The slab warehouse computer does not communicate through the server, but directly communicates with the slab warehouse PLC through Ethernet. Because the slab warehouse computer is responsible for issuing operation instructions to the cranes in the warehouse, there is a very close signal interlock between these cranes and the rollers controlled by the L1-level PLC. If it passes through the intermediate L1-level network, the interlocking often fails due to communication loss or delay, resulting in unimaginable consequences.
Some characteristics of communication
This automation system consists of two levels, L1 and L2, and contains three communication networks. The operation station adopts a server/client structure, and the Ethernet connection between the server and the client is TCP/IP protocol. The furnace operation room (3CS) is equipped with two sets of HMI (operation stations 1 and 2) to centrally monitor and operate the instrument-electrical of the heating furnace (each operation station can operate the instrument and electrical); the four HMI operation stations and two redundantly configured ProcessLogix servers of the heating furnace basic automation control system, as well as the secondary computer are all built on the same industrial Ethernet, together forming a three-electric automation control system. The basic automation control system communicates with the secondary computer via industrial Ethernet.
Ethernet communication
At present, one method to solve the real-time problem of Ethernet is to use a technology that combines Ethernet and UDP/IP protocol. This system uses general Ethernet technology combined with TCP/IP protocol to realize communication within the L2 level and between L2 and L1. The L2 level is connected via Category 5 twisted pair cables; the L2 switch and the L1 device that is far away and must pass through the cable trench are connected via optical cables.
ControlNet communication
ControlNet is a high-speed deterministic network used for information transmission in applications with strict time requirements. At the same time, it allows the transmission of message data that is not time-critical, but will not impact the transmission of data with time-critical requirements. It supports mixed system structures (any mixture of master/slave, multi-master and peer devices).
This system uses the ControlNet bus mainly for the following three reasons:
(1) As a channel for information exchange between L1 controllers, it facilitates communication between instrumentation DCS and electronic control PLC.
It is extremely difficult for traditional DCS systems, such as HONEYWELL's DCS, to achieve communication with PLC. However, in this system, due to the use of ControlNet, the communication between the ProcessLogix controller and the ControlLogix controller can be completed with simple configuration.
(2) Realize real-time data transmission. For the information that needs to be exchanged in real time between the remote I/O station and the controller that undertakes the signal acquisition task, the communication configuration software is set to a fixed-cycle read-write mode so that the transmission of these signals will not be delayed.
(3) Convenient debugging and maintenance. Programs can be uploaded and downloaded on any ControlNet communication module. This means that debugging or maintenance can be performed by connecting a laptop to the communication module of the master station or the on-site remote I/O station. For occasions such as the bottom of a heating furnace and a hydraulic station where the communication effect is poor, it is very necessary to provide such a means.
Although ControlNet has the above advantages, and the transmission rate can reach up to 5Mbps, and supports periodic and non-periodic scanning methods. However, the ControlNet bus is usually used for communication between the controller and the remote I/O or between controllers, and not for communication between the controller and the transmission device. Therefore, the communication between the controller and the transmission device must be considered to be implemented using another field bus.
ProfiBus DP communication
ProfiBus DP is used for high-speed data transmission at the device level. The central controller communicates with the dispersed field devices (such as I/O, drive, valve, etc.) through high-speed serial lines. The communication rate is 9.6kbps ~ 12Mbps. The transmission medium uses RS-485 twisted pair, cable or optical cable. It supports single-master or multi-master systems. The maximum number of stations on the bus is 126.
In this system, the ProfiBus DP bus completes the communication between the PLC and the frequency converter, and adopts the master-slave communication mode: the ProfiBus communication module (produced by SST) with the model PFB-SST-CLX is configured in the PLC main frame as the ProfiBus DP master station, which performs periodic scanning and reading and writing to the subordinate slave stations (frequency converters) in a polling manner. The module also realizes communication with ControlLogix through the ControlNet backplane bus. In this way, ControlLogix only needs to read and write the PFB-SST-CLX module once in one scanning cycle to transmit the instructions issued to the inverter to the corresponding data area of the PFB-SST-CLX module, and extract the status signal returned by each inverter from the corresponding data area, thereby realizing the monitoring of each subordinate inverter.
In fact, for this system, the transmission system access solution recommended by PLC manufacturer RockWell is to use DeviceNet bus. In the end, we decided to use ProfiBus bus to realize the communication between PLC and inverter based on the following three factors:
(1) Speed considerations. Although the communication speed is related to the length, type, number of stations and data transmission volume of the medium used. However, we noticed that the data transmission rate of DeviceNet is as fast as 1.5Mbps, while the data transmission rate of ProfiBus can reach as fast as 12Mbps.
(2) Considerations for debugging and maintenance. SIEMENS automation equipment-level communication solution-ProfiBus bus has been widely adopted. The management and technical personnel of the enterprise have a certain understanding of the debugging and maintenance of this bus, so they tend to adopt the ProfiBus fieldbus solution.
(3) Consideration of system scalability. In the field of electrical transmission, most transmission devices, whether DC or AC, support ProfiBus communication, while DeviceNet has a relatively low support rate in this field. For the convenience of future system upgrades or expansions, we use ProfiBus.
Of course, if DeviceNet is used, there is no need to insert third-party modules into the ControlLogix rack. The difficulty and workload of the initial debugging stage will be greatly reduced, especially by avoiding writing communication programs that have few sentences but are extremely difficult to understand. However, these minor losses are insignificant compared to the other benefits of ProfiBus.

Conclusion
According to the above system solution, after hardware design, software programming, construction, debugging and trial operation, the No. 2 heating furnace of the first phase of the plant has been put into production with good results. The No. 1 heating furnace of the second phase is under construction.