Abstract: Profibus is a widely used fieldbus in process control and manufacturing automation. Addressing the challenges of numerous monitoring points, scattered pumping stations, and diverse environments in urban water supply systems, this paper designs an urban water supply monitoring network system using Profibus fieldbus technology, GPRS technology, sensor detection technology, and PLC technology. This system enables centralized management and remote control of multiple distributed pumping stations, achieving integrated urban water supply monitoring and management. Keywords: Profibus, urban water supply system, GPRS technology 1 Introduction Urban water supply systems commonly feature scattered data acquisition points and highly varied field working environments. Figure 1 shows a flow chart of a certain urban water supply system. The source water is drawn from the Yellow River; two-stage sedimentation tanks (four sedimentation tanks each) are used for flocculation and sedimentation. Flocculant is automatically added based on the effluent turbidity and flow rate. The entire water supply system is designed with five pump houses (10KV high-pressure pumps), undergoing three stages of pressurization and boosting to a head of 385 meters. According to the urban water supply process, the manufacturer requires the establishment of a water supply network monitoring system. This system primarily collects parameters such as flow rate, turbidity, and pressure from the pipelines of various dispersed pumping stations. These data are then transmitted to the monitoring center via data communication, allowing monitoring personnel to monitor the entire network's operation and analyze relevant data in a timely manner to identify the causes of water quality non-compliance. Adjustments can then be made through control measures to ensure water supply quality. Therefore, we designed a "Profibus-based Urban Water Supply Network System" using Profibus bus, GPRS technology, sensor detection technology, and PLC. This system enables remote monitoring of the operation of each pumping station, ensuring both the normal operation of the urban water supply system and the quality of urban water supply under low-investment conditions. 2. System Composition The urban monitoring and management system is shown in Figure 2. The system consists of a three-level network. The top-level network is an industrial Ethernet network, using a communication protocol to transmit production management information. The middle level is a Profibus fieldbus, employing a combination of token and master-slave polling control methods to achieve three levels of communication: field, control, and monitoring. Dispatchers can remotely monitor the operation of each pumping station's equipment at any time using a Siemens industrial computer. The lowest level is the actuator-level bus AS-I, which is responsible for communication with field sensors, instruments and actuators[1]. [align=center] Figure 2 Urban water supply network monitoring system[/align] 2.1 Introduction to S7-300 and Profibus S7-300 is a programmable controller produced by Siemens[2], which has powerful computing and processing functions and automatically integrates the Profibus bus interface device. It has strong real-time characteristics, high reliability and fast operation speed. This system uses three sets of S7-300 to complete the real-time control of the system. Profibus is an open, digital, multi-point communication low-level control network. According to the fieldbus standard EN50170 formulated by the ISO/OSI reference model, it adopts the access control method of combining command tokens and master-slave polling. The Profibus protocol structure adopts the first layer, second layer and seventh layer of the OSI reference model. The first layer, the physical layer, mainly handles mechanical, electrical and process interfaces. It adopts the RS-485 protocol and can use standard twisted pair or optical fiber as the transmission medium. The second layer is the fieldbus data link layer (FDL), whose main task is to enhance the physical layer's ability to transmit raw bits, making it appear as an error-free line to the network layer. The seventh layer is the application layer, used to provide reliable transmission. Devices on the Profibus bus include masters and slaves. The master has control over the bus within a limited time and can send data to or receive data from slaves. Slaves only respond to requests from the master and acknowledge data sent by the master. Based on application characteristics, Profibus can be divided into three compatible versions: Profibus_FMS (Fieldbus Message Specification), Profibus_DP (Decentralized Periphery), and Profibus_PA (Process Automation), enabling three levels of communication: field, control, and monitoring. Since Profibus_DP is used for high data transmission between distributed devices, Profibus_DP is selected according to the system monitoring characteristics. The dispatching master station, S7-300, and detection device form a local area network. The communication protocol is completed by the Profibus protocol. The communication network uses RS-485 standard twisted pair cable to realize multi-master communication between the control unit and the monitoring host [3]. 2.2 The control operating system of the dispatching master station is WINDOWS NT WORKSTATION Ver 4.0, KERNEL SP4. The monitoring platform uses the FIXMMI industrial control software of InteLLution Company of the United States, which is directly embedded in the NT operating system. Dynamic monitoring of the production site is realized. The host computer of the dispatching master station has the functions of monitoring, real-time control, data recording, and report printing. The intuitive configuration graphics can enable dispatching managers to monitor the dynamic process flow on site; the operating status of important equipment; the process parameters, electromechanical parameters, and filter backwash process parameters of each workstation; the start and stop of equipment and the action of valves; real-time control realizes the setting and modification of filter backwash process parameters; including the setting of water level; and the setting of alarm upper and lower limits. The data recording function records 200 days of data from each measurement point, which fully meets the needs of historical query. The report printing function can print the work log of the past valid time, and can also print it in real time, which meets the needs of production data archiving. The scheduling master station operating system has operator permissions, which takes into account the convenience of scheduling operation and effectively prevents unauthorized personnel from misoperating [4]. 2.3 V-type filter automatic control part my country began to introduce the V-type filter developed by the French company DEGREMONT in the 1980s. The automatic control of V-type filter filtration and filter media regeneration is the guarantee for the normal production operation of the filter. We adopted a real-time multi-task distributed control system composed of programmable logic controller and industrial computer (PLC+IPC) to automatically control the filter filtration and backwashing. Its functions are: to realize constant water filtration of six systems; to automate filter backwashing; and to realize real-time monitoring of the process by the scheduling. The entire control system is based on Siemens series programmable logic controllers. The control of the six filters is realized by two sets of S7-300 PLCs. S7-300PLC1 controls filters #1, #2, and #3, S7-300PLC2 controls filters #4, #5, and #6, and S7-200PLC3 controls the backwash pumps and blower systems of the six filters, thus completing the automatic control of backwashing of the six filters. The control of the constant water level process of the filters is mainly achieved by adjusting the filter water regulating valve. Water level sensors and head loss sensors are installed in the corresponding parts of the filters. The filtration of the filters is achieved by measuring the water level and head loss of the filters through these sensors. The water level value and the opening degree of the filter water valve are sent to the PLC. After calculation by the PID module in the PLC, the filter water regulating valve is adjusted to achieve constant water level. The design uses the PID module [2], only adopts proportional and integral control, closes the differential loop, and achieves good constant water level control [5]. 2.4 Backwashing control of the filters The backwashing of a group of filters is controlled by S7-300. When the filtration cycle is reached or the filter tank pressure differential (head) set value is reached, the filter tank sends a backwashing request. The S7-300 forms a backwashing request queue according to the priority order of the filter tanks. Once a request from a filter tank is responded to, the PLC implements the entire backwashing process. Within a set of filter plates, two filter tanks are not allowed to be backwashed simultaneously. When one filter tank is being backwashed, the backwashing requests from other filter tanks are stored in a shared PLC, and then the filter tanks are backwashed sequentially according to the stored order. Monitoring of the filter tanks is achieved through PROFIBUS bus communication. During filter backwashing, the PLC control process is as follows: Open the backwash drain valve; when the filter water level drops to the top of the sand washing drain trough, close the post-filtration water control valve; start the blower; after 5 seconds, open the filter backwash air valve for a 1-minute air pre-wash; open the backwash water valve, start the backwash water pump, and perform a 7-minute simultaneous air and water backwash; close the backwash air valve; after 5 seconds, stop the blower, open the air diaphragm valve to vent air, perform a 5-minute clean water backwash, and then stop the backwash water pump. After 5 seconds, close the water backwash valve, then close the backwash drain valve, open the filter water inlet valve, and the filter resumes filtration. The entire backwashing process takes approximately 25 minutes. Additionally, the PLC can control the number of filters that are open. It determines the number of filters to be opened based on the filter inlet flow rate, and determines the opening and closing of a specific filter cell according to the first-to-stop principle. The backwashing process execution time can be changed by the monitoring microcomputer or operated on-site by the TD200. The monitoring of the filter tank is achieved through PROFIBUS bus communication. The automatic control system of the V-type filter tank is designed with both automatic and manual functions. When the automatic part fails, the manual part can still control production, ensuring normal production. 3 GPRS Technology In the field application, the No. 1 to No. 6 pump stations and the booster pump station are distributed over a long distance, and the data transmission volume is small. Establishing a dedicated wired data exchange system is obviously not cost-effective, while using wireless transmission methods results in a high error rate due to geographical differences. With the advancement of communication technology, GPRS (General Packet Radio Service) has become the main Internet access method promoted by China Mobile. GPRS utilizes the networks of China Mobile or China Unicom, requiring no network maintenance from the user. The network signal is reliable and has authentication and encryption functions. Its core network layer adopts IP technology, and the lower layers can use various transmission technologies, which can be easily and seamlessly connected to the IP network. Since the urban water supply project has already installed Internet access, the data receiving end has a static IP address. Therefore, the system uses China Mobile's GPRS public network to securely transmit data to each monitoring point, which not only reduces equipment prices but also reduces equipment installation costs and subsequent maintenance work. [align=center] Figure 3 Remote data transmission process[/align] The system data transmission process is shown in Figure 3: The data collection point sends the collected data to the GPRS module via RS232. The GPRS module adopts the Falcom TWIST industrial-grade module from the German company Funkanlagen Leipoldt OHC. The GPRS module packages the data into PDU (Packet Data Unit) and sends it to the base station controller. The PDU is processed by the SND layer (subnet dependency layer) to become SNDC data unit, and then processed by the LLC layer (Logical Link Control layer) to become LLC frame. It is transmitted to the SGSN (Serving GPRS Supporting Node) GPRS service support point where the mobile station of the GSM network is located through the air interface. The SGSN sends the data to the GGSN (Gateway GPRS Support Node) GPRS gateway support node through the GSM backbone network. The GGSN decodes the received message and converts it into a format that can be transmitted in the public data network. Finally, it is sent to the user in the public data network. After receiving the data, the server sends the data to the monitoring program, displays the data of the monitoring point and saves it in the database [6]. 4. Conclusion The above system has been applied to the water supply system of a certain city, with a daily water supply of 100,000 tons. It has 36 collection points, 4 sedimentation tanks, and 5 booster pump stations, distributed across an area of ​​80 square kilometers, including the source of the water source, suburbs, and urban areas. Due to geographical differences (the head from the water source to the supply point is 385 meters), wired communication is not possible at the water source, and high-rise buildings in the urban area hinder the establishment of shortwave communication. Therefore, the construction of the network project made GPRS the primary means of communication. This system combines a microcomputer monitoring system, detection and control technology, and high-speed network communication technology to achieve integrated management and control of the water supply system, providing a correct basis for optimized decision-making and online control of the urban water supply system's production process. The innovations of this paper are: the successful application of Profibus bus technology to realize the remote management and monitoring network of the urban water supply system; and the use of GPRS technology to solve the problems of dispersed data collection points, large distances between collection points, and significant environmental differences in the water supply system. Because GPRS is a public network information platform, its network signal is secure and reliable, and its equipment installation and debugging are convenient (GSM networks are built by mobile companies and do not require user attention). Construction cycles are short, network maintenance is simple, and communication quality is reliable. GPRS billing is based on data transmission volume, making it particularly suitable for non-real-time systems with many data collection points, wide geographical distribution, and low transmission volume. 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