1. Overview With the rapid development of modern power electronics and microelectronics technologies, and the continuous improvement of automation and intelligence, the research and application of medium- and high-voltage high-power variable frequency speed control devices have gradually matured. Since high-voltage frequency converters are almost always key equipment in industrial and mining enterprises, the high requirements for their reliability are self-evident. Therefore, ensuring reliability is crucial for the acceptance and widespread application of high-voltage frequency converters. To meet this requirement, some frequency converter manufacturers have designed systems that utilize communication to transmit real-time data, operating status, and fault status from the field to remote terminals, enabling remote monitoring and maintenance of field equipment. Previously, this remote monitoring system was typically implemented using the public telephone network, dialing via a modem, and corresponding software. Its obvious drawback is that communication is only possible after dialing, and simultaneous "point-to-multipoint" communication is not possible. Other methods for achieving remote communication include satellite, microwave, fiber optic, or radio communication. However, these methods not only involve huge equipment investment but also incur high procurement and operation and maintenance costs. With the advancement and widespread adoption of wireless communication technology, GPRS networks are now ubiquitous, and application costs have been significantly reduced. Utilizing GPRS embedded in high-voltage frequency converters for remote monitoring communication not only offers ease of maintenance, scalability, and security, but also retains all the advantages of GPRS communication. 2. Introduction to GPRS Wireless Communication Technology: GPRS (General Packet Radio Service) is a packet-switched data transmission method developed based on GSM, aiming to provide mobile users with high-speed wireless IP or X.25 services. GPRS theoretically boasts a bandwidth of up to 171.2 Kbit/s, with practical applications ranging from approximately 40 to 100 Kbit/s. Providing TCP/IP connections on this channel allows for applications such as internet connections and data transmission. GPRS employs packet switching technology, allowing each user to simultaneously occupy multiple wireless channels, while the same channel can be shared by multiple users, ensuring efficient resource utilization. GPRS allows users to send and receive data in end-to-end packet transfer mode without utilizing circuit-switched network resources. GPRS is always online and billed by traffic, thus providing an efficient and low-cost wireless packet data service. It is particularly suitable for intermittent, bursty and frequent, multi-point and small-to-medium traffic data transmission, and also suitable for occasional large-volume data transmission. GPRS-based remote monitoring has the following characteristics: (1) Fast login, always online, and wide coverage (2) High security, the system adds an encryption mechanism during data transmission, and the data can be transmitted securely on the public network; (3) High reliability, the system has error correction and retransmission mechanisms, thereby ensuring the integrity and correctness of the data. Secondly, the system has an automatic recovery function, which ensures the stable operation of the system without manual intervention when the GPRS network status is unstable; (4) Multi-user service reuse, different network users share the same set of GPRS channels, but the channel resources will only be occupied when a user needs to send or receive data. The system relies on the corresponding software to flexibly realize point-to-point and point-to-multipoint data transmission simultaneously; (5) Power saving function, the device can be in standby mode when not sending data, and log in to the network at regular intervals or when data is triggered. [align=center]Table 1 Comparison of GPRS Access and Telephone Dial-up Access[img=478,347]http://www.xmgkw.com/article/uploadfile/200808/2/9711052587.jpg[/img][/align] Currently, the GPRS service launched by China Mobile is becoming increasingly mature, and the application of GPRS is gradually becoming a research hotspot in the field of monitoring. Moreover, users can further reduce the cost of GPRS communication by using various forms such as monthly rentals and subscriptions according to their needs. Therefore, GPRS, as a networking method that is currently and will be developed on a large scale in the future, has the characteristics of being distributed, centralized, networked, and open, and will inevitably be more and more widely used in industries such as industrial control, environmental protection, water conservancy and hydrological monitoring, and alarms. 3. Implementation of GPRS Remote Monitoring The following describes the implementation method of GPRS remote monitoring, explains how to use AT commands to communicate with the GPRS module, proposes the idea of ​​using a (browser/server) architecture for monitoring, and provides experimental steps and application examples. 3.1 GPRS Remote Monitoring System The GPRS remote monitoring system consists of an intelligent controller, a GPRS communication module, China Mobile's communication network (GPRS network), the Internet public network, a data server, an enterprise LAN, and its computer equipment. [align=center][img=500,183]http://www.xmgkw.com/article/uploadfile/200808/2/0311052313.JPG[/img] Figure 1 Schematic diagram of the GPRS remote monitoring system for high-voltage frequency converters[/align] First, the high-voltage frequency converter system collects field parameters, encrypts and compresses them, and then connects them to the GPRS communication module serially in the form of a data stream to communicate with the China Mobile base station. The base station SGSN then communicates with the gateway support node GGSN, and the GGSN processes the packet data accordingly. The GPRS module transmits data via the GPRS network to China Mobile's intranet (CMNET) in the form of GPRS data packets. China Mobile then forwards the data to the Internet through the GPRS service node (GSN) and searches for a server with a specified IP address on the Internet. Since the GPRS network operates based on IP address addressing, the public Internet data server only needs simple Internet access and a publicly assigned IP address. The data server uses Winsock programming to send and receive network data and forward it to the internal network database. The database facilitates data maintenance and web server access. The system can also perform reverse transmission of data and commands to achieve remote control. Workstations can access the web server via the Internet to view detailed information about each monitoring point. The GPRS-based remote monitoring system relies on the mobile communication network. After on-site parameter collection, GPRS terminal communication programs and related software are written according to actual needs. 3.2 Implementation of GPRS Remote Terminal Hardware block diagram of GPRS remote terminal: [align=center][img=396,220]http://www.xmgkw.com/article/uploadfile/200808/2/6311054591.JPG[/img] Figure 2 Schematic diagram of GPRS remote data acquisition[/align] The GPRS remote terminal mainly consists of a data acquisition unit and a GPRS module. First, the control unit of the high-voltage frequency converter collects the industrial field parameters, displays and controls them on-site, and sends them to the GPRS module to control the operation of the GPRS module itself. In data mode, it sends the encrypted and fault-tolerant data to the data server. The communication protocol between the high-voltage frequency converter and the GPRS module is a serial port protocol. The high-voltage frequency converter sets the GPRS module through the serial port. The setting instructions before establishing the data channel include: AT+CGDCONT=1, "IP", "CMNET" Explanation: Initialize the network session, establish a data account on the module, and access the China Mobile GPRS network. Return: OK AT*E2IPA=1, 1 Explanation: The IP connection on the current session Return: OK AT*E2IPO=1, "218.79.163.201", 5000 Explanation: Initialize a TCP connection, 218.79.163.201 is the server's IP address. Return: CONNECT Indicates that the connection was established successfully. After the connection is established, the GPRS module enters the data state, and the high-voltage frequency converter can send and receive data. 3.3 Composition structure of the remote monitoring system (1) Server side In the GPRS-based remote monitoring system, the data server side uses a SOCKET socket based on the TCP/IP protocol to send and receive data with the GPRS terminal. The software developed and running on the server processes the data and then updates it to the SQL database, allowing legitimate users to view the data at any time and in any place through a browser. The browser views the information that the remote device has just updated to the database through the Internet, achieving functions such as on-site data analysis and display. Workflow: [align=center][img=316,382]http://www.xmgkw.com/article/uploadfile/200808/2/1F11054944.JPG[/img] Figure 3 GPRS Data Server Workflow Diagram[/align] (2) Browser end The browser end does not require special software; the system's built-in internet software can access the WEB interface on the server. 3.4 Communication test of GPRS monitoring system (1) First, insert the SIM card (disconnect the power supply when inserting the card), connect the antenna, and connect the GPRS module to the computer's serial port using an RS232 serial cable. Turn on the GPRS module power. When the signal indicator light starts flashing, it indicates that the module has been powered on and successfully registered with the GSM network. (2) When the GPRS device is powered on, after the device performs a self-test, it enters the command mode, and the indicator light shows that it is working normally. (3) The GPRS module can be configured using third-party software or tested using a terminal emulator. Send the AT command AT+CGDCONT=1, "IP", "CMNET". If data is displayed, press Enter and it will return OK. This indicates that the hardware connection is correct. You can also test it using custom software. (4) Start the application server SOCKET. Assume the server IP address is 218.79.163.201 and the port is 5000. (5) Use AT*E2IPO=1, "218.79.163.201", 5000 to set the data server IP address and port number. Press Enter and it will return CONNECT, indicating that the connection has been successfully established. [align=center][img=367,208]http://www.xmgkw.com/article/uploadfile/200808/2/9A11054992.JPG[/img] Figure 4 Screen of HyperTerminal connecting to GPRS DTU[/align] (6) The server uses software written in WINSOCK to send and receive data, and the GPRS end uses HyperTerminal to simulate remote device sending and receiving data. (7) The server runs the host software program, stores the data transmitted from GPRS into the Microsoft ACCESS database, and displays it. (8) The client starts the browser to view and modify the contents of the database on the server. 4. Host Software on the Server The host software of this system is configured using the general version of MCGS configuration software from Beijing Kunlun Tongtai. The configuration software is used to set up the connection with the GPRS DTU.