Geotechnical Engineering Stress Monitoring Based on Sensor Network and GPRS Network

With the rapid development of China's economy, the number of large geotechnical buildings such as highways, long-span bridges, and dams is increasing. However, geological factors, construction quality, and building aging have made it increasingly urgent to monitor the health of geotechnical buildings. The mainstream stress detection method today is mostly to use stress monitoring equipment to conduct field measurements manually at a fixed time, which inevitably leads to untimely data monitoring and human errors.
This system mainly utilizes the advantages of wireless sensor networks, such as convenience, low cost, and low power consumption. Combined with the use of GPRS (General Packet Radio Service) network, it realizes the real-time collection of geotechnical building stress data and remotely transmits the data through the GPRS network. In terms of cost, it greatly saves the resource consumption of previous human monitoring. At the same time, the combination of GPRS network and wireless sensor network WSN (Wireless Sensor Networks) technology makes the monitoring method simple and easy, and more operational [1].
The system mainly uses wireless transceiver chips, low-power single-chip microcomputers, and GPRS modules. Through data acquisition nodes and data transmission nodes, the stress data collected from geotechnical buildings are transmitted to remote monitoring personnel in real time, thereby achieving the purpose of remote real-time monitoring. The system is mainly composed of data collection nodes and data transmission nodes. By writing the protocol stack, setting up the coordinator, router and data terminal, a sensor network based on the ZigBee protocol is established. The data collection range can be expanded as the number of collection node networks increases [2]. The network first aggregates multi-point data through a "multi-hop method" and then sends it to the remote receiving end in the form of short messages through the GPRS network. In addition, by using the network transmission function of the TCP/IP protocol of the GPRS module, the stress data can be synchronously transmitted to the PC terminal, thereby realizing online monitoring. The system diagram is shown in Figure 1.

1 Design of hardware platform
1.1 Overall platform
The system in this paper mainly controls the wireless transmission module and GPRS module respectively through the single chip microcomputer, collects data from the sensor, transmits it to the terminal sending node through the 2.4 GHz frequency band channel, and finally sends the data in SMS mode through the GPRS module. The hardware structure diagram is shown in Figure 2.

1.1.1 Network nodes
The microcontroller MCU (Micro Control Unit) uses the MSP430F1611 , which is a 16-bit microcontroller with ultra-low power consumption. In active mode, the power consumption of the MSP430 can reach 280 Ω. Secondly, the 12-bit A/D conversion module with sampling and holding can provide analog-to-digital conversion for sensor data acquisition. The two serial communication ports USART (Universal Synchronous/Asynchronous Receiver/Transmitter) can control the radio frequency module through the SPI port while also operating the GPRS module [3]. The wireless transceiver chip uses TI's CC2420 , which is compatible with IEEE 802.15.4 wireless transceiver chip. It has excellent performance, low power consumption, and small size, making it very suitable for the field of wireless sensor networks. CC2420 has a fully integrated voltage-controlled oscillator. It only needs to expand the antenna and a few components such as a 16 MHz crystal oscillator on the periphery to use it in the 2.4 GHz frequency band. The chip is equipped with an SPI port, which is easy to connect to the microcontroller. This solution uses MSP430F1611 as the microcontroller to configure the registers of CC2420, read the status bits, and control the sending and receiving of data [4]. The GPRS module uses SIMCOM 300, which has the advantages of supporting AT command control, RS232, and TTL level dual-mode control, which is very conducive to the application of the system. The JTAG interface is mainly used to download and debug programs, and the USB interface can realize direct communication with the computer. 1.1.2 Data acquisition module Sensor: This system uses traditional adhesive strain gauges, and converts the deformation into electrical signals by designing amplification and holding circuits. The stress data acquisition part sticks the strain gauge on the bridge model, and the output voltage is: where ε is the strain gauge bridge excitation voltage, ε is the strain gauge deformation, and A is the signal conditioning circuit amplification factor. In the bridge model used in this article, the output voltage signal range is 1 V to 2 V. The sensor circuit diagram is shown in Figure 3.

1.2 Hardware connection CC2420 mainly uses SFD, FIFO, FIFOP and CCA pins to indicate the communication status in communication. SFD pin indicates whether the data frame is being received or sent; FIFO indicates whether there is data in the receiving buffer during reception; FIFOP is used to indicate that the upper limit of the received data has been reached or the frame has been completely received; CCA is used to check whether the channel is empty. The connection between CC2420 and MSP430 is very convenient. Only SFD, FIFO, FIFOP and CCA pins are needed to indicate the state of receiving and sending data; and the processor exchanges data and sends commands with CC2420 through the SPI interface. The SPI interface consists of CSn, SI, SO and SCLK pins. The processor accesses the internal registers and storage of CC2420 through the SPI interface. During the access process, the clock signal and chip select signal from the processor are received, and input/output operations are performed under the control of the processor. In this design, MSP430 is in master mode and CC2420 is in slave mode. The hardware connection between MSP430 and SIM300 is connected through three wires: RX, TX and GND. The processor uses the USART0 serial communication port to write AT commands to the GPRS module through these three wires in asynchronous communication mode to control it to send text messages. The specific hardware connection diagram is shown in Figure 4.

The connection between the sensor and the microcontroller is achieved by connecting the output end of the sensor to the corresponding analog-to-digital conversion channel interface on the microcontroller.
2 Software Design
The system is mainly programmed through the compiler IAR Embedded Workbench 430 4.21 provided by TI. A series of data transmission and reception are completed by configuring the SPI port, ADC port, timer and CC2420. This article will take point-to-point communication as an example to introduce the system program into a data transmission module and a data reception module.
2.1 Data transmission module
This module is mainly responsible for controlling the sensor to collect data regularly, and converting the collected electrical signals into data through analog-to-digital conversion, and finally sending the data through the 2.4 GHz channel by setting CC2420.
The program first initializes each required module of the microcontroller, and then configures the registers of CC2420 through the SPI serial port and sets the source address, destination address, etc. After initialization, the microcontroller enters a low-power mode and waits for the timing interrupt to arrive. The data collection cycle can be set through software settings. When the number of collected data reaches the predetermined value, the collected data, destination address, etc. will be written into the sending buffer in the predetermined format and then sent out. According to the custom protocol, if the data is successfully received, the sender will receive a confirmation frame. The specific sending process is shown in Figure 5.

2.2 Data receiving module
The function of the data receiving module is to control the GPRS module through the microcontroller to send the data sent from the sending node in the form of text messages. The specific program flow is shown in Figure 6. The initialization process is the same as that of the data acquisition module. After the initialization is completed, the microcontroller enters the low-power mode and waits for data to be received. When configuring CC2420 , the conditions for triggering the FIFOP interrupt have been pre-set. When the length of the received data exceeds the preset value, the FIFOP level changes, triggering the microcontroller interrupt. CC2420 first confirms the address. If the destination address in the data matches the address of this node, the address confirmation is successful, and the hardware automatically sends a confirmation frame. After sending, according to the protocol, the frame length, control frame, and user data are read out in turn. By analyzing the received data, when the stress data value exceeds the preset threshold value, the function is called to send the stress data to the monitoring personnel's mobile phone via text message.

In addition, the integer data sent by the sensor acquisition node needs to be converted into character data through ASCII code conversion before it can be sent. The encoded data is encapsulated into AT commands, and then written to the GPRS module through the serial port to achieve the purpose of sending text messages.
3 Simulation verification
The bridge condition is simulated by steel bars, and the system designed in this paper is used for simulation verification to realize the real-time acquisition of stress data. When powered by a 9 V dry battery, different vertical forces are applied to the steel plate to generate deformations of different strengths, and the voltage signal can vary from 2 V to 1 V. The corresponding digital signal after analog-to-digital conversion and linear processing can indicate corresponding linear changes from 0 N to 80 N. After setting the sampling interval of about 20 s by software and setting a fixed threshold value, the SMS alarm signal can be received about 15 s after the steel plate is deformed by human force, and the corresponding stress value can be displayed.
The stress monitoring system introduced in this paper combines the high-performance, low-power MSP430 microcontroller with a radio frequency module and a GPRS network. By utilizing the convenient communication of the 2.4 GHz frequency band, remote real-time monitoring becomes easier, while greatly reducing the consumption of manpower and material resources. In low-power mode, the node can consume as little as 36 μA, and can be powered for a long time using a single AA battery. The wireless data transmission mode gets rid of the limitations of wiring affected by geographical factors. The system has simple hardware connections, is easy to implement and maintain, has extremely low power consumption, and is easy to use for a long time.
References
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[4] Wang Shuizhang. Design of self-organizing wireless sensor network based on MSP430 and CC2420 [J]. Science and Technology Information Development and Economy, 2008, 18(33): 134-136.