Control system based on Profibus DP fieldbus

1 Introduction
Fieldbus technology embodies the development of information and network technologies in the control field and is one of the hot topics in automation technology development. It has gained widespread application due to its characteristics of digitalization, openness, decentralization, and adaptability to field environments. Currently, the main fieldbus technologies that have matured and are influencing the industrial automation process include PROFIBUS, HART, LONWORKS, and FF. Among them, PROFIBUS is one of the most popular fieldbus technologies, and its products are widely used in automation fields such as industry, power, energy, and transportation. It is a fieldbus standard according to the German national standard DIN 19245 and the European standard EN 50170, and is an international, open, and manufacturer-independent fieldbus. According to the application characteristics, it can be divided into three compatible versions: (1) PROFIBUS-DP is used for communication between equipment-level control systems and distributed I/O with its fast transmission speed and strong anti-interference capability; (2) PROFIBUS-FMS mainly solves the general communication tasks at the workshop level and can provide flexible and large-scale communication services; (3) PROFIBUS-PA is designed for process automation. It is directly connected to field sensors or actuators and can be connected to the DP bus through the DP/PA interface. It is used in situations with high safety requirements and bus-powered applications [1].
2 System topology and configuration [3]
Depending on the connection method from field devices to the controller, fieldbus topologies typically employ three methods: linear, tree, and ring. PROFIBUS uses a linear structure, with a main trunk line connecting the controller to the controlled object. Bus cables branch off from the trunk line to the field devices. The controller scans all inputs on all I/O stations and can send information to output channels when necessary, enabling multi-master and peer-to-peer communication. This article focuses on the design of the PROFIBUS-DP fieldbus control system, whose topology is shown in Figure 1.
2.1 Host Computer
This system uses an Advantech industrial PC as the host computer, connecting it to PROFIBUS-DP via a CP5611 interface card. This connects the IPC and the fieldbus, forming a complete control network system capable of configuration, operation, and other functions. The host computer monitoring software can be either Intellution's Windows 9x & NT-based configuration software or Siemens' WinCC software. It can provide dynamic display, alarms, trend analysis, control strategies, and control network communication functions, and offers a user-friendly interface, allowing users to create corresponding application software based on their actual production needs.
2.2 The SIMATIC S7 master station acts as the DP master station, with the CPU located at the control center. This system uses the CPU315-2DP modular PLC, which integrates a PROFIBUS-DP fieldbus interface and boasts powerful processing capabilities (processing 1024 statements in 0.3ms). After the PLC program is compiled in the host computer using STEP 7, it is downloaded to the CPU315 and stored. The CPU can automatically run the program, read the status words of all I/O modules on the bus according to the program content, and control the corresponding devices.
2.3 Slave Stations (Slave A, B)
In station A, SIMOVERT Master drives are AC frequency converters, and CB1 is the communication processor that matches them. SIMOREG K 6RA24 is a DC converter, and CB24 is the communication processor that matches it. When S7-300 or S7-400 is used as the master station of PROFIBUS-DP network, it can drive 32 such slave stations respectively. If repeaters are added, it can reach up to 127. This AC/DC drive product can perform open-loop and closed-loop control of the motor. The data received from the DP network is stored in the bidirectional RAM through the CB communication board. Each word in the bidirectional RAM is addressed. The bidirectional RAM at the frequency converter end can be sorted by the addressed parameters to write control words, set values ​​or read actual values, diagnostic information and other parameters to the frequency converter [4].
2.4 Distributed I/O ET200M
The ET200M is a modular distributed I/O station that connects to the PROFIBUS-DP fieldbus via an IM-153 interface. For the SIEMENS STEP7 development platform, the address layout of the distributed I/O nodes on the ET200M is consistent with that of traditional centralized systems. Therefore, programming is similar to programming a centralized control program. Furthermore, the module addresses of the distributed I/O can be changed according to user needs to adapt to the requirements of actual field commissioning.
3. PROFIBUS-DP communication function [1,2]
The PROFIBUS-DP network uses a physical layer, a data link layer, and its own user layer. The physical layer employs ETA RS485 transmission technology and uses either shielded twisted-pair copper wire or fiber optic cable. Termination resistors are connected at both ends of the bus to prevent surges. In actual use, care should be taken to ensure that the termination resistors at intermediate nodes are in the OFF position.
The data link layer, also known as the fieldbus data link layer (FDL), generally has two media access methods: token bus (the token bus protocol conforms to IEEE 802.4) and master-slave mode.
A token is a special message used to transfer control between master stations. The token bus system allows a master station that receives a token to gain bus control for a pre-defined time period. During this time, the master station is allowed to perform its tasks and can communicate with all other master or slave stations according to a master-slave relationship table. If a master station has no frames to send, has sent all required frames within the specified time, or its control time expires, it passes the token to the next master station. This ensures that even under heavy load, each master station can obtain bus access for a defined period, avoiding the congestion issues common in Ethernet under heavy load, while retaining the advantages of a simple bus network structure and the ability to freely add or remove stations.
The master-slave data link protocol differs from LAN standards; it conforms to the Unbalanced Normal Response (NRM) mode in HDLC. This mode is characterized by one master station controlling multiple slave stations on the bus. The master station establishes a logical link with each slave station. The master station issues commands, and the slave stations respond. Slave stations can continuously send multiple frames until no more information is sent, the required number of frames is reached, or the master station stops the transmission. The frame transmission process in the data link consists of three phases: data link establishment, frame transmission, and link release.
In the diagram, U represents an unnumbered HDLC frame. U:A,SNRM,P indicates that slave station A is selected in SNRM mode, with P being the polling bit. U:A,UA,F indicates that the master station establishes a data link using the unnumbered acknowledgment command UA of the U frame as a response, with F being the stop bit. I represents an HDLC information frame. N(S)=0 indicates the number, N(R)=0 indicates that slave station A's frame has not been received, N(S)=1, N(R)=0 and N(S)=2, N(R)=0 indicate the 2nd and 3rd consecutive information frames sent by the master station. The polling bit P is used in the 3rd frame to indicate that the slave station also has an information frame to send. N(R)=3 indicates that slave station A has correctly received I frames with sequence numbers 2 and earlier, and the stop bit F indicates that the slave station only sent one frame. Finally, when neither the master nor the slave station has any information frames to send, or when the master station is about to establish a data link with another slave station, the master station uses the U release connection command DISC, and slave station A acknowledges the connection using the UA of the U frame, ending the link transmission process.
4. User Interface Development
The control systems based on PROFIBUS-DP networks mainly have the following human-machine interface interfaces.
(1) Information display screen: mainly displays the current operating status information, such as the motor speed, the inverter's operating frequency, and some fault information;
(2) Equipment control screen: Although the S7-300 can realize data acquisition and control signal output on the fieldbus and has some control algorithms, complex control functions still need to be manually controlled on the host computer. Clicking the corresponding device button in the interface allows for individual control of devices such as frequency converters and inverters;
(3) Real-time alarm processing: The system judges the data collected in real time, issues alarm signals, processes them according to technical requirements, and automatically controls the corresponding equipment;
(4) Report printing and data curve display, etc.
5. Conclusion
PROFIBUS fieldbus is a digital communication network. Digital signals are used for communication between bus devices in the middle layer or different layers of the system. Since digital signals replace analog signals, multiple signals (including multiple operating parameter values, multiple device status, and fault information) can be transmitted on a single wire while providing power to multiple devices. A/D and D/A conversion components are no longer needed outside the field devices, which greatly reduces the number of wires and connecting accessories and improves the reliability and anti-interference capability of the system. Therefore, this bus control system can be applied to almost all industrial control systems and building automation systems, and it is also a "fully integrated automation" system bus [1]. It uses a brand-new concept of distributing control logic throughout the entire system, which greatly improves the level of industrial automation, maximizes production efficiency, and realizes the integration of factory management and control. With its own characteristics and advantages, it will inevitably become the mainstream direction in the field of automatic control.

References:
[1] Yang Xianhui. Fieldbus Technology and Its Applications [M]. Beijing: Tsinghua University Press, 1999.
[2] Pan Xiaoning, et al. Communication principle of PROFIBUS-DP industrial control network [J]. Electrical Drive, 1999(5): 25-28.
[3] SCHICKHUBER. Mccarthy Distributed Fieldbus and Control Network System[J]. Computing & Control Engineering Journal[J]. 1997,8(1): 53-55.
[4] Zhao De'an, et al. A type of tension adaptive control system based on PLC and Profibus-DP bus [J]. Information and Control, 2002, 31(2): 122-126.