Thermocouple remote monitoring system strategy based on ARM7 wireless transmission

Introduction
At present, many powerful embedded processors are built based on ARM core. Such a configuration system is cost-effective and has strong scalability.
In the application of industrial detection system, as a main temperature measurement element, thermocouple has the characteristics of simple structure, easy manufacturing, convenient use, wide temperature measurement range, high temperature measurement accuracy, etc., and can be widely used in industrial temperature control process. However, the output potential of thermocouple is extremely weak, and there are cold end temperature errors and nonlinear problems between output potential and measured temperature, which can easily cause large measurement errors and require extremely complex signal processing circuits. MAX6675 is a K-type thermocouple serial analog-to-digital converter launched by Maxim. This type of chip can independently complete signal amplification, cold end compensation, linearization, A/D conversion and SPI serial port digital output functions, which can simplify the design of software and hardware modules of thermocouple measurement intelligent devices.
Mobile detection terminals can be quickly put into the field, and the system debugging cycle is short. With the popularization of wireless 3G services, various transmission costs based on wireless communication are rapidly declining, and wireless, mobile, and remote temperature measurement control systems will become more popular. Related research based on wireless communication at home and abroad has also become very popular, and related products have been put on the market. According to the development trend of temperature monitoring, this paper develops an embedded wireless thermocouple temperature measurement control system based on ARM7 processor and GPRS wireless transmission module.

1 System composition and working principle
The temperature remote wireless monitoring system is application-centric. It connects MAX6675 with SPI interface to collect temperature data and sends it to Huawei GTM900A wireless transmission module through serial port. Since GPRS network (2.5G mobile communication technology) supports TCP/IP protocol, it makes wireless data transmission easier and much cheaper than short messages. In order to set parameters remotely, the system supports setting relevant core parameters through SMS. The block diagram of the terminal device system is shown in Figure 1.

The terminal equipment of this system adopts a modular structural design concept, and the terminal equipment is divided into a main control module and various functional modules. There is a unified or specific interface form between the main control module and each functional module, and users can choose different functional modules according to actual conditions. Various types of data can be transmitted simultaneously in the system without interfering with each other, and other functional modules can also be expanded according to market changes.
The functions of several main modules of the temperature remote wireless monitoring system are as follows:
① Sensor data processing module. Collect thermocouple temperature sensor data according to actual needs. When the collected data value is higher or lower than the alarm value, start the automatic alarm module to remind the patrol personnel to pay attention.
② LCD display module. Display the detected temperature data and related system core parameters so that relevant staff can observe the data.
③ Automatic alarm module. After the system detects valid alarm information, the module immediately makes the buzzer on the embedded device sound an alarm and sends an alarm message at the same time. The information is transmitted back to the remote monitoring management center in time through the GPRS network so that the staff can find it in time and take corresponding treatment measures.
④ Parameter configuration module. The core parameter configuration of the system is saved in the Atmel AT24C01 chip through the I2C bus, and the relevant parameters are read when the system starts. Parameters can be set in a variety of ways (serial port, SMS and network).
⑤ARM7 core control module. It uses low-power, high-performance embedded SoC chip Samsung S3C4480X, 8MB RAM and 2 MBFlash ROM.
⑥Thermocouple temperature acquisition and processing module. The general I/O port uses software to simulate SPI operation to realize the communication between ARM7 and MAX6675.
⑦GPRS wireless data transmission module. Considering the actual needs of the system, Huawei GTM900A module is used. This module supports AT commands. The control of the module and the transmission of data can be sent or read through the serial port of the device through AT commands.
⑧Remote monitoring software module. Remote monitoring software acquisition end and control end. The acquisition end needs to be connected to the Internet, has a fixed IP address, and is responsible for receiving and sending data from remote terminals. The control end is used by operators to display the operation status of each terminal and the current real-time temperature data on the interface. The operator's instructions are sent to the acquisition end through the control end, and finally sent to each corresponding terminal through the acquisition end.

2 System software design process
When the system starts, in order to support the setting of relevant parameters through the serial port, the user is prompted to enter the relevant configuration parameters in sequence. If the user does not operate for a certain period of time, the system enters the initialization module, starts the GPRS module, and processes the SMS command (initialization has its own point-to-point protocol module). The process of the main application is shown in Figure 2.

When the system is running, the interrupt handler of TimerO processes its own point-to-point protocol, including processing "login", "heartbeat" and "exit" commands. Read data from the system's send data buffer queue, package the data into a "data" command, and send it to the GPRS module through the serial port. If the GPRS module has data, read out various commands of its own protocol sent by the remote monitoring center in the serial port, and only parse the "data" command and store it in the receive data buffer queue. Other protocol maintenance commands are processed by the corresponding subroutines. The
main application interacts with the data communication module through a few system status global variables, the send data buffer queue and the receive data buffer queue, that is, it interacts with the interrupt handler of TimerO. The main application does not need to understand the encoding and decoding process of its own point-to-point protocol and the maintenance of the data link, which reduces the direct coupling degree of the module. The main application reads the thermocouple temperature data collected by MAX6675 in real time through the SPI bus, checks the GPRS module status in real time (if the GPRS module error exceeds the specified number of times, restart the GPRS module), stores the alarm data in the sending data buffer queue in real time and starts the alarm module at the same time, processes the data in the receiving data buffer queue in real time, processes the GPRS module SMS command data in real time, displays the temperature data and status information collected by the system in real time, and stores the collected data in the sending data buffer queue regularly.

3 MAX6675 and S3C4480X interface implementation
The MAX6675 chip integrates a cold end compensation circuit. The chip has a simple 3-bit serial SPI interface, which can convert the temperature signal into a 12-bit digital quantity with a temperature resolution of 0.25℃. The chip contains a thermocouple disconnection detection circuit. Its cold end compensation temperature range is -20～80℃, and the application ambient temperature range is relatively wide. It can measure the measured temperature of 0～1023.75℃, which basically meets the actual needs of industrial temperature measurement. When the CS pin of MAX6675 changes from high level to low level, the chip will stop any signal conversion and output the converted data through S0 pin under the action of clock SCK. This data is the sum of the digital quantity after the amplified A/D conversion and the cold end compensation; on the contrary, when the CS pin of the chip changes from low level to high level, the MAX-6675 chip will perform new temperature digitization. When the CS pin changes from high level to low level, the first byte D15 appears on pin SO. A complete data reading process of this series of chips requires 16 clock cycles, and the data reading is usually completed at the falling edge of SCK. It should be pointed out that the A/D conversion time of this chip is between 0.17 and 0.22 s, which is much longer than the μs-level conversion time of general A/D conversion chips. The corresponding temperature conversion module needs to be optimized and should not occupy too much CPU resources.
MAX6675 uses the standard SPI serial peripheral bus to connect with ARM7, so the chip can only be used as a slave device (i.e. serial interface chip). SPI (Serial Peripheral Interface) bus system is a synchronous serial peripheral interface, which is a bus standard introduced by Motorola. It enables ARM7 microcontrollers to communicate with various peripheral devices in serial mode. Since the S3C44BOX microcontroller does not have an SPI serial bus interface, a general I/O port can be used when the chip is docked, and software can be used to simulate SPI operations, including serial clock and data input/output.

4 GPRS wireless data transmission module
The data collected by the temperature acquisition and processing module is transmitted to the processor through the SPI bus, and then transmitted to the GPRS module through the serial port AT command. Then, the GPRS module transmits the data to the remote control center host connected to the Internet through the GPRS network and the Internet network. A small distributed control system can be established on the network of the control center through the configuration software. As for the choice of configuration software, commercial configuration software can be selected according to needs, or a small distributed control system control software like the author's master's thesis.
The use of AT commands to operate the GPRS module has been described in relevant literature. Here, Huawei GTM900A module is used as an example to illustrate the relevant operations. Table 1 describes the basic function instruction set of Huawei GPRS module AT commands. By using these instructions, you can perform some common operations such as initializing the GPRS module, opening the link, sending data, receiving data, and closing the link. The
32-bit microcontroller based on the ARM series has rich RAM resources, and application software can be written using high-level languages ​​such as C and C++. Here, C language is used to develop the GPRS module function program. Using the 16-byte FIFO serial port provided on the ARM7S3C4480X chip, AT commands are sent to the serial port to operate the GPRS module. The following is a module program written in C language to open a TCP connection:

The GPRS module is a chip with the function of logging into the GPRS network plus the corresponding digital processing circuit. A mobile phone card with the GPRS function is installed inside the module, and it communicates with the ARM7 processor through a serial protocol. The data is first connected to the operator's Internet access server through PPP in the form of a data packet, and then the data packet is sent to the Internet and finally reaches the control center host. The sending and receiving of data follows its own point-to-point transmission protocol to ensure the safe and reliable sending and receiving of data.
The software structure design of the GPRS wireless data transmission module can be divided into four parts:
① Parameter configuration part. Before installation, connect the system to the PC through the serial port, and set the remote monitoring center host IP address, port, device ID and heartbeat time parameters in the super terminal on the PC. During system operation, the system-related parameters can also be set through the serial port, GPRS or SMS commands.
② Connect to the remote monitoring center host. In this part, the system will initiate a TCP or UDP connection to the remote monitoring center. This system uses a TCP-based point-to-point protocol to handle communication.
③ Own protocol processing. This protocol includes "login", "data", "heartbeat", "exit" and response commands. "Login", "heartbeat", "exit" commands and response commands are the state maintenance part of the protocol. The two-way communication between the system and the monitoring center host is transmitted through the "data" command. The memory configuration based on the ARM7 processor can reach 8 MB, so the point-to-point proprietary protocol based on TCP can be maintained in the software design to make data transmission safer and ensure that key data is not lost.
④ Data transmission. The processor first determines whether the received data is temperature data or an alarm signal: if it is alarm data, it immediately stops the transmission of other data and sends the alarm data quickly; if it is temperature data, it is transmitted back to the monitoring center at a certain interval. The system also reads various commands from the control center through the GPRS module and performs corresponding processing.

5 Remote monitoring software module
The monitoring end is located in the monitoring management center and is divided into two parts: the data acquisition end and the control end. The data acquisition end uses a high-performance PC or commercial server, running Windows XP/2000/2003Server system or Linux system. Here, the Delphi7 language is used to write a data acquisition system based on CLX components. The software based on CLX components can be easily ported to the Linux system. The data acquisition system stores the collected data in the Oracle 9i database for future query and report generation. At the same time, the data is sent to each control terminal in real time through the TCP protocol.
The control terminal module uses the configuration software written by the author in Delphi7 language. Through the relevant interface, the configuration software exchanges data with the data acquisition terminal. The staff can view the temperature collection data and system status of multiple terminals in the system through the control terminal, and can also send relevant commands to the data acquisition terminal according to the actual situation, and finally arrive at the temperature detection terminal for execution. The operation diagram of the control terminal monitoring main interface is shown in Figure 3.

Conclusion
The embedded temperature remote wireless monitoring system based on ARM7 processor is connected to MAX6675 through SPI interface, continuously collects the monitoring temperature on site, and sends the data to the remote monitoring management center through its own point-to-point protocol based on GPRS network. When the temperature data exceeds or falls below the critical value, an alarm is issued in time, and the alarm signal is transmitted back to the monitoring center, realizing wireless remote monitoring of temperature, promoting the wireless and intelligent industrial temperature monitoring, greatly reducing the hardware wiring time on site, and can also be used as a mobile online detection terminal.