Design and implementation of embedded power monitoring system based on ARM9

The power monitoring system, also known as the power SCADA (Supervisory Control And Data Acquisition) system or telecontrol system, is developed based on the demand for remote status monitoring, data collection and remote control of power supply system equipment. It is used to realize the functions of remote control, remote communication, remote measurement, remote adjustment and remote viewing of the main electrical equipment in the substations along the line. It plays an irreplaceable role in improving the reliability, safety and economic benefits of power grid operation, reducing the burden of dispatchers, realizing the automation and modernization of power dispatching, and improving the efficiency and level of dispatching. The characteristics of the power system are that the stations are relatively scattered, and the normal operation of the stations is very important. Once any abnormal situation occurs at the station, the monitoring system needs to transmit real-time data (temperature, wind speed, vibration, voltage, etc.) to the dispatcher so that timely response measures can be taken. Traditional manual monitoring on site not only wastes a lot of manpower and material resources, but also has very low efficiency [1]. The ARM microprocessor is a high-performance, low-power 32-bit microprocessor that is widely used in embedded systems. ARM9 represents the mainstream processor of ARM and is widely used in digital consumer products, imaging equipment, industrial control, storage devices and network equipment. This paper designs a new type of embedded system. The system uses the Linux operating system and ARM9 hardware platform as the core to realize real-time monitoring of the site, and transmits data to the monitoring center in a timely manner through the GPRS module. The monitoring center stores and processes the data. At the same time, once the monitoring center receives new data, it immediately sends it to the mobile phone (MT) in the form of a short message, and the mobile phone can also send a query request to the monitoring center. The monitoring center sends data to it by authenticating its authority.
1 System composition and principle
1.1 Data acquisition terminal
As shown in Figure 1, the entire system consists of five parts: field monitoring terminal, GSM network, data exchange center, monitoring center, and mobile receiving terminal.

In general, the local monitoring system uses SM service to send the working data of the substation. When a substation fails, the local monitoring system of the substation actively calls the remote monitoring computer and establishes a data connection to send an alarm message; according to the actual production needs, the production manager can decide when to establish a GSM data circuit connection for real-time monitoring.
The sending end of this system is shown in Figure 2. Among them, the chip of the control unit uses the S3C2410 embedded processor, which is a 32-bit microcontroller based on the ARM920T processor core of ARM Company and manufactured by Samsung with a 0.18 μm manufacturing process. The basic features of this processor are: independent 16 KB instruction cache and 16 KB data cache, MMU, LCD controller supporting TFT, NAND flash controller, 3-way UART, 4-way DMA, 4-way Timer with PWM, I/O port, RTC, 8-way 10-bit ADC, Touch Screen interface, I ² C-BUS interface, I ² S-BUS interface, 2 USB hosts, 1 USB device, SD host, MMC interface and 2 SPI. The S3C2410 processor can run at a maximum frequency of 203 MHz.

The GPRS module uses the industrial-grade module SIM300C produced by SIMCOM, which supports GPRS and embedded TCP protocols. It is a complete mobile phone module, belonging to the mobile device side, responsible for signaling exchange with GSM and GPRS networks. It can be controlled and data can be transmitted through the serial port, including short messages and GPRS. This module requires a SIM card with GPRS service enabled to be used with it.
1.2 Server Side
In view of the particularity of the power system, its monitoring center is not allowed to directly access the Internet, so a data exchange center is required to process and forward data. The controller of the information collection point transmits data from the GSM/GPRS module to the monitoring center through the GSM network at regular intervals or according to the instructions of the monitoring center, and the monitoring center stores and processes the data.
1.3 Data publishing end
Once the monitoring center receives new data, it will immediately send it to the on-duty personnel's mobile phone (MT) in the form of a short message. The on-duty personnel can also send query requests to the monitoring center through their mobile phones, and the monitoring center sends data to them by authenticating their authority. As shown in Figure 3.

2 Operating System Customization
Linux is used as the embedded operating system in the design. Linux is a free and freely disseminated Unix-like operating system. It makes full use of the task switching mechanism of the X86 CPU to achieve a truly multi-tasking and multi-user environment, allowing multiple users to execute different programs at the same time and giving urgent tasks a higher priority. It is particularly suitable for embedded systems. The application development tool uses ADS1.2 (ARM Developer Suite) [2].
2.1 Building a cross-compilation environment
Since the required compiler cannot be installed on the embedded hardware, it can only be done with the help of a PC. The PC and embedded hardware are based on two different processor types, so it is necessary to generate software that can run on ARM on the PC, which requires building a cross-compilation environment. This design uses the open source Crosstool to build a cross-compilation tool chain [2].
2.2 Porting Bootloader
Bootloader is similar to the Bios on a PC. It is the first program that runs when the system starts. Its main function is to initialize hardware devices, mask interrupts, set startup parameters, etc. before the operating system kernel runs, so as to create a good environment for the operating system and then load the operating system. This design also uses the open source project U-boot. [page]

2.3 Kernel transplantation
Although the latest Linux kernel has added support for the S3C2410 CPU, it is still necessary to make some modifications to the kernel according to actual needs, and reconfigure and compile to generate a new kernel image.
2.4 Customizing the root file system
The root file system refers to the first file system used when the Linux system starts. When starting the kernel, the root file system needs to be mounted to support external devices, as well as to load and run kernel modules and applications. Generally, customizing the root file system requires the following steps: creating an empty directory tree, transplanting BusyBox, selecting necessary dynamic shared libraries, initialization scripts, etc.
3 Software Design
3.1 Communication between the information collection point and the monitoring center
The control of the GPRS module is completed through AT commands, while the communication between the module and the monitoring center is completed by WinSock. When the information collection point transmits data, the GPRS module will actively access the monitoring center server according to the IP address pre-set in it, and establish a TCP/IP link with the monitoring center through the firewall [3]. At the same time, the monitoring center maintains the IP address and serial number of each information collection point connected, and sends data requests to a certain information collection point at regular intervals as needed. After receiving the request, the information collection point responds to complete the communication process [4]. The specific protocol stack is shown in Table 1.

3.2 Software Structure of Monitoring Center
The software structure of the monitoring center is divided into three layers: operating system and system software layer, system software support layer and basic application layer.
(1) Operating system and system software layer
The operating system shields the specific details of the underlying hardware from the user. Programmers can use the function development package provided by the underlying hardware to serve the upper-layer software.
(2) System support software layer
The system support software layer includes database system, power system model, data acquisition and transmission, etc. Its main function is to realize data storage and forwarding, and to serve the upper-layer software.
(3) Basic application layer
The main function of the basic application layer is to process the collected data, manage the dispatchers and display it through the GUI interface [5].
The monitoring center is shown in Figure 4.

3.3 Communication between the management center and the dispatcher
At the data publishing end, the wireless module communicates with the host computer through RS232, using serial asynchronous communication with a baud rate of 9 600 Kb/s. The serial port program uses the Pcomm serial port communication library of MOXA. Pcomm provides simple and convenient serial port operation functions. Commonly used functions are:
int WINAPI sio_ioctl(int port, int baud, int mode);
//Set serial port parameters, including baud rate, data bits, stop bits, etc.
int WINAPI sio_read(int port, char *buf, int len);
//Read a specified number of characters from the input buffer
int WINAPI sio_write(int port, char *buf, int len);
//Write a specified number of characters to the output buffer
int WINAPI sio_cnt_irq(int port, VOID (CALLBACK *func)(int port), int count);
//Interrupt function, respond to
events This system is based on the S3C2410 platform and Linux operating system. It exchanges information with the remote monitoring center via the GPRS network through SIM300C. It has long transmission distance, high reliability, strong real-time performance, low price and high practical value.
References
[1] Wang Minghu, Ou Wensheng. Introduction to ARM Embedded Linux Application Development (1st Edition) [M]. Beijing: China Electric Power Press, 2008.
[2] Quan Qian, Zheng Xuefeng. Power Line Monitoring System Based on GPRS [J]. Computer Engineering and Design, 2005, 26 (11): 3053-3055.
[3] Wen Zhicheng. GPRS Network Technology [M]. Beijing: Electronic Industry Press, 2005.
[4] Andrew S T. Computer Networks (4th Edition) [M]. Beijing: Tsinghua University Press, 2004.
[5] BLANCHETTE J, SUMMERFIELD M. C++ GUI Qt4 Programming (2nd Edition). Translated by Yan Fengxin. Beijing: Electronic Industry Press. 2008.