Implementation of GSM Remote Monitoring System Based on ARM

　　At present, vehicle information systems have gradually become an indispensable part of intelligent transportation systems (ITS) that integrate advanced electronic, computer and communication technologies.

　　The in-vehicle information system integrates multiple information services such as vehicle instrumentation, anti-theft monitoring, automatic navigation, media playback, and web browsing on a unified hardware platform. Compared with the separate development and integration of multiple components in the vehicle, it saves material and communication costs, is conducive to the digitization of vehicle information, and is also conducive to improving the cost-effectiveness of the entire vehicle. [1]

　　The car monitoring system is an important part of the vehicle information system and provides an important guarantee for the anti-theft of the car. At present, the principles of car monitoring devices are mainly divided into the following three categories: short-range monitoring, wireless remote monitoring and GPS positioning monitoring. Commonly used short-range sound monitoring devices generally sense abnormal vibrations of the protected object (such as a car or a safe) through a vibration sensor, and then drive a high-power sound alarm. The disadvantages of this monitoring method are short monitoring distance, noise pollution to the environment, high false alarm rate, and the monitor is easy to be disassembled or damaged. Ordinary wireless monitoring devices use proprietary radio frequencies to monitor information, which has the disadvantages of short transmission distance and susceptibility to interference from obstacles. GPS positioning alarms [1] use modern electronic information technology, aerospace technology and network technology to achieve real-time information feedback between cars and car owners, but they also have disadvantages such as "blind spots" and high costs.

　　GSM is a relatively mature, complete and widely used system in the current mobile communication system based on time division multiple access technology. The GSM system integrates modern source coding technology, channel coding, interleaving, equalization technology, digital modulation technology, voice coding technology and slow frequency modulation technology, and introduces a large amount of computer control and management into the system, thus ensuring the correctness, security and reliability of data transmission. [2]

　　ARM processor is currently the most commonly used processor in embedded systems. As the hardware platform of vehicle information system, it has the characteristics of small size, low cost, high performance and rich embedded resources.

　　Therefore, a GSM remote monitoring system based on ARM is designed, which uses GSM network as the communication carrier and ARM processor S3C2410 as the basic hardware platform to realize automatic monitoring and timely control of vehicle status.

　　2 System Analysis

　　The ARM-based GSM car monitoring system is a new type of car monitoring system with a dedicated communication module embedded inside. The working principle of the monitor is shown in Figure 1: When the sensor detects a door opening or engine start signal, the signal is converted into a digital signal through the A/D conversion module and transmitted to the ARM processor. The ARM processor analyzes and processes the information and sends the processing result (alarm information) to the wireless communication module in the form of AT commands. The wireless communication module sends the alarm signal to the predetermined car owner's mobile phone in the form of a text message. The car owner replies to the corresponding text message based on the received alarm information to achieve control of the car, such as forcing the engine to shut down or locking the car door. Using the GSM network, the alarm information can be transmitted to any place covered by mobile communications, overcoming the defects of the limited transmission distance and noise pollution of the alarm signal in the existing technology.

　　The monitor should also have instructions for changing the system password, dynamically setting the monitor's control and unlocking instructions, setting the owner's mobile phone number, etc. When making these settings, the user is required to provide a password in a text message for identity verification.

Figure 1 Schematic diagram of the remote monitor hardware structure

　　3 Hardware Design

　　The ARM-based GSM remote monitoring system mainly consists of S3C2410 target board, TC35i communication module, SIM card module, photoelectric coupler, sensor, controller, power supply system and other parts, as shown in Figure 2. The whole system can be divided into three parts:

　　1) The monitoring part of the door switch sensor and the engine vibration sensor are connected to the GPIO port of the S3C2410 target board through a photocoupler, and the port connected to the sensor is configured as an input mode. The corresponding GPIO port can be bound to an external interrupt source. When a corresponding signal is generated, an interrupt is triggered to process the corresponding alarm event.

　　2) Control part The controller is connected to the GPIO port of the target board through an optocoupler, and the port connected to the controller is configured as an output port. The control signal sent by the target board is amplified by the controller to control the locking/opening of the door, or the shutdown and start of the engine.

　　3) The target board of the communication part is connected to the GSM mobile communication module through the serial port to send alarm information or receive control instructions and system configuration instructions sent by the car owner. There is a SIM card in the communication module to determine the identity of the sender or receiver of the SMS.

　　4) Power supply The power supply of the monitoring system includes the power supply of the target board, the controller and the sensor. Due to the difference in voltage and current between the two, a photocoupler is used to isolate them to prevent the large current in the controller from damaging the target board. The entire power supply system can come from the car battery, so that normal monitoring can be performed even after the car is turned off.

　　The monitoring program is stored in the target board memory. The monitoring program is cross-developed using a computer and finally downloaded and solidified into the target board.

Figure 2 Remote monitor hardware connection diagram

　4 Software Design

　　The remote monitoring system is developed in C language under the embedded Linux platform. The remote monitoring system is mainly divided into the following modules:

　　1) The wireless communication module is mainly responsible for sending and receiving communication text messages. The sending and receiving of text messages is achieved by sending the AT command set to the serial port. The AT command set is the communication protocol between the GSM module and the embedded computer. The AT command is a string that starts with AT and ends with characters. Each command has a corresponding return whether it is executed successfully or not. For other unexpected information (such as someone dialing in, no signal on the line, etc.), the module will also have corresponding information prompts, and the receiving end can do corresponding processing. [page]

　　The short message specification used is GSM07.05, and the mode is PDU mode [4]. The PDU string is a string of ASCII codes, consisting of numbers and letters such as '0'-'9' and 'A'-'F', which are 8-bit hexadecimal numbers. The PDU string not only contains the displayable message itself, but also contains a lot of other additional information, and the structure of the sent and received PDU strings is not exactly the same. The format of the PDU string for sending a short message is shown in Table 1. The format of the received short message is slightly different from this, so it will not be repeated here.

　　Table 1 Sending short message PDU string format

　　The generation and decoding of SMS PDU codes are written into corresponding functions according to the GSM07.05 specification for calling by monitoring modules and system settings modules.

　　2) Monitoring and control module The monitoring module mainly completes the detection of sensor information and generates an interrupt when the conditions are met. In the interrupt service program, an alarm SMS is generated and the SMS sending function is called to send the alarm information to the owner's mobile phone. The control module sends the corresponding control signal to the controller according to the content of the received SMS.

　　3) The system setting module mainly completes the modification of the owner's mobile phone number, SMS center number, system password, alarm information content, reply instruction content, etc. to increase the security of the system. The SMS content format is shown in Table 2.

　　Table 2 Remote monitoring system application layer instructions

　　The information about system settings is stored in the system memory in the form of a data file. When the settings are modified, the contents of the file will be modified.

　　The program flow of the entire system is shown in Figure 3. The entire monitoring system starts running as soon as it is powered on. As long as a detected interrupt event occurs, the interrupt is processed. When a new SMS arrives, a specified message will also be sent to the serial port, so the reception of serial port data should also adopt the interrupt method. In this way, the system efficiency is higher and the continuous loop of the program is avoided to waste processor resources.

Figure 3 Remote monitoring system program flow chart

5 Conclusion

　　The ARM-based GSM remote monitor is currently a more advanced car monitoring system, which has the following characteristics: 1) Using text messages as a transmission method, it can achieve "zero-time alarm" of car status information; 2) With the help of the global GSM network, "long-distance alarm" can be made no matter where the car owner is; 3) As part of the vehicle information system, the ARM-based car monitoring system has a "high cost-effectiveness" as a whole; 4) Using a mobile phone as the operating terminal of the monitor and mobile phone text messages as the carrier of control commands, it is easy to operate.

　　The ARM-based GSM remote monitoring system is not only widely used in car anti-theft systems, but it also provides a new model of modern remote monitoring system, and has broad application prospects in railways, transportation, electricity, industry, environmental protection, hydrology and other fields.