1. Overall structure of ADSL-based heat network monitoring system 1.1 Overview Urban heat network monitoring and control is an important part of urban municipal engineering. The control nodes of the heat network monitoring and control system are generally geographically distributed over a wide range, making it difficult to achieve access for all nodes using a single access method. Currently, the more common access methods include PSTN access, GPRS access, data radio access, and dedicated line access. [1] These access methods have their applicable occasions, but they all have the disadvantages of low bandwidth and high operating costs. This paper proposes an ADSL-based heat network monitoring system to achieve node communication access using ADSL. Its advantages are: (1) convenient access, and this access method can generally be provided in places with telephone network coverage in cities; (2) high bandwidth, up to 2Mbps, which can greatly improve the real-time performance of monitoring; (3) low investment; (4) low operating cost. 1.2 Overall structure of the system The overall structure of the ADSL-based heat network monitoring system is shown in Figure 1. A typical heat network monitoring system consists of a monitoring center and multiple control nodes. The monitoring server is responsible for data communication with each control node, receiving operating data from the control nodes, and sending instructions to the control nodes based on the operating status of the heating network to adjust the overall heating balance of the network. The database server stores daily and historical operating data, providing support for data analysis and decision-making; the web server displays the heating network operating condition monitoring interface. Figure 1: Structure diagram of a heating network monitoring system based on ADSL. The heating company has several heating stations, and each station typically has one control node, which is composed of an embedded system. On the one hand, the control node collects on-site operating data such as supply/return water temperature, flow rate, and pressure through sensors, and controls solenoid valves, regulating valves, and frequency converters to adjust on-site operating parameters. On the other hand, it connects to the Internet via an ADSL modem to transmit operating data to the monitoring center. The control node can also receive instructions from the monitoring center to adjust on-site operating parameters. [2] 2. Key technologies of ADSL-based heating network monitoring system The ADSL-based heating network monitoring system effectively utilizes the latest achievements in control technology, computer technology and communication technology. The key technologies adopted are: 2.1 Design and implementation of WEBGIS-based monitoring interface of monitoring center The development of GIS (Geographic Information System) technology has put forward higher requirements for the interface of heating network monitoring. It is not only required to display working condition data in tables, curves, etc., but also to realize the data query and display of heating stations in the browser in the form of electronic map navigation. At present, there are two methods for WEBGIS implementation. One is to complete the secondary development on the basis of commercial GIS software. The second method is to complete the secondary development on the basis of open source WEBGIS server. In practical applications, due to the high cost of commercial GIS, open source Mapserver is used as GIS server. 2.2 Research on control algorithm of network control system The heating network remote monitoring system is a network control system (NCS). Its control object is a large lag object. At present, there is no good mathematical model and control algorithm that can solve this control problem. Currently, heating companies generally rely on human experience to adjust the balance of the heating network. Researching a control algorithm suitable for heating network monitoring, while fully considering the impact of ambient temperature, is crucial for energy conservation and improving heating efficiency. In network control systems, bandwidth limitations cause latency, and studying the impact of latency on the control algorithm is also a problem that needs to be solved. 2.3 Control Node Hardware and Software System Design If industrial control computers, PLCs, or other technologies are used to implement the control nodes of a heating station, the cost of the control nodes is high. Using embedded system design to implement the control nodes will reduce the overall system cost and facilitate large-scale deployment. 2.4 VPN Protocol Design and Implementation Figure 2. Control Node Hardware System Structure Diagram After adopting the ADSL node access method, the use of the Internet to transmit control data raises data security concerns. To ensure data security, VPN (Virtual Private Network) technology can be used to guarantee the security of transmitted data. [3] 3. Control Node Software and Hardware System Design 3.1 Control Node Hardware System Design The hardware system of the control node is based on the Samsung ARM7 processor S3C44B0X, as shown in Figure 2: The S3C44B0X is a 32-bit microprocessor with an ARM7 (Advanced RISC Machine) core manufactured by Samsung. It has 8-channel 10-bit A/D converters and other hardware resources. The chip is low in cost and very suitable for use in the heating network monitoring system. A 5V/24V switching power supply is used to power the embedded system and sensors. A 10MHz crystal oscillator module is used. The S3C44B0X has a phase-locked loop inside, which can generate a stable output frequency of 66MHz based on the crystal oscillator. The display part uses a VFD high-brightness display screen, which has the advantages of dot matrix output, high brightness and wide viewing angle. This screen is used to display the on-site temperature, flow rate, pressure and other operating conditions. In addition, in order to meet the requirements of human-machine interaction, indicator lights and keyboard are also extended. This part is implemented through the general I/O of the S3C44B0X. The system expands two RS-232 serial ports using the MAX232 chip, one for debugging and the other for communication with the frequency converter. Since the S3C44B0X lacks an internal network interface, an RTL8019A network control chip is used to implement it. This chip has a communication rate of 10Mbps, which fully meets the system requirements. This chip communicates with the ADSL modem via a network isolation transformer and an RJ45 interface, enabling dial-up and network communication. The data acquisition section works as follows: six 4-20mA analog signals (supply/return water temperature, flow rate, and pressure) are converted into 0-2.5V signals (required by the S3C44B0X's internal A/D converter) through an I/V conversion circuit, completing data acquisition. The actuator section works as follows: An analog signal is output through an expanded D/A converter to regulate the opening of the control valve. The solenoid valve's opening and closing action is controlled via general-purpose I/O and optical isolation. Communication with the frequency converter is accomplished via an RS-232 serial port. Commands are sent to the frequency converter through the serial port to adjust the working status of the booster pump. 3.2 Control Node Software System Design Figure 3: The control node software system structure diagram is shown in Figure 3. The entire system architecture adopts a hierarchical architecture design pattern, with each layer providing calling services to the layer above it. This design pattern has good scalability and maintainability. The bottom layer is the operating system layer, using the VxWorks real-time operating system. This layer also provides TCP/IP protocol encapsulation for the middleware layer to call. Above the operating system layer is the middleware layer, which provides services to the application layer. It includes two parts: a hardware driver module and a communication protocol module. Above the middleware layer is the application layer, which is the system's application software, including three modules: a data acquisition module, an automatic control module, and a remote communication module. This layer is implemented by calling services provided by the middleware layer and services provided by the operating system kernel. The three modules of the application layer have high real-time requirements, which are achieved by designing several independent tasks. The data acquisition module is a periodic task, acquiring data every 100ms, using the operating system kernel for precise timing. When an alarm occurs, it is handled using an interrupt. The data acquisition module communicates with the other two modules using message queues and shared memory. The automatic control module controls the actuators based on real-time data to adjust the operating conditions of the heating network. It can also receive instructions from the remote communication module to adjust the operating conditions. The remote communication module transmits real-time data to the monitoring center via the network and receives control instructions from the monitoring center. Inter-task communication between the remote communication module and the automatic control module is achieved through message queues. 4. Conclusion The ADSL-based heating network monitoring system proposed in this paper is a low-cost, reliable, and high-bandwidth solution for heating network monitoring. This solution is also applicable to the monitoring of other urban pipe networks (such as water and gas), and has a wide range of applications. Currently, there are still some technical issues to be resolved, such as the control algorithm and latency of the network control system, which will be further studied in future work. The innovation of this paper is the use of ADSL technology as the communication method for remote monitoring of the heating network, and the use of embedded system design to implement the control nodes, which has the advantages of low cost and good real-time performance. After two heating seasons of operation in a certain city, the system has proven to be stable and reliable, generating an annual economic benefit of 5.27 million yuan by reducing energy consumption, reducing manpower, and increasing income. References: [1] Sui Qiang. Comparison of data transmission methods in heating network monitoring system [J]. Energy Saving, 2007, 2: 4, 54-55. [2] Liu Hao, Dai Jufeng. Microcomputer monitoring system for centralized heating network [J]. Microcomputer Information, 2005, 10-1: 86-87. [3] Li Qi, Zhu Lin. Design and implementation of heating network monitoring system based on VPN [J]. Journal of Baotou Iron and Steel Institute, 2005, 4: 361-363, 372.