Design of support structure safety monitoring system based on ZigBee and GPRS

As a common method in road and bridge construction, the support method has been widely used due to its mature and convenient construction technology, low cost and long service life. As the carrier of road and bridge structure construction, the support structure not only bears the load of reinforced concrete and various building materials and construction equipment, but also serves as a vertical traffic channel and working platform for construction personnel.

At present, my country has very limited means of construction supervision and in-service monitoring of support structures. Usually, the support structure components are subjected to offline destructive mechanical property tests before assembly, and on-site inspections by supervisors are carried out after construction is completed. The inspection tools are mostly traditional measuring tools such as theodolites, tape measures, and angle rulers. These mechanical destructive detection methods are destructive tests, and the on-site inspection method increases the labor intensity of engineering supervisors. Moreover, it can only perform sampling inspections, which cannot meet the actual needs of fast, real-time, and comprehensive inspections of the project. Therefore, there is an urgent need to develop a wireless safety monitoring technology to achieve comprehensive health monitoring of the support structure.

Common wireless communication technologies can be divided into long-distance wireless communication (such as GPRS, GSM, LTE, CDMA, etc.) and short-distance wireless communication (such as UWB, WiFi, ZigBee, IrDA, Bluetooth, etc.) based on the communication distance. Among them, GPRS technology is widely used in mobile commerce, mobile Internet, industrial control and other fields due to its characteristics of permanent online, long-distance transmission, high data transmission rate, and flow-based billing. ZigBee technology is widely used in smart homes, wireless meter reading, industrial control, mobile terminals, building automation and other fields due to its low cost, low power consumption, low complexity, low transmission rate and long transmission distance. It has obvious advantages and potential application value in the field of wireless communication. On the basis of making full use of GPRS long-distance communication technology and ZigBee short-distance wireless networking technology, the advantages of the two are combined to develop a bracket structure safety monitoring system based on ZigBee and GPRS.

1 System Overall Architecture

The whole system is mainly composed of wireless sensor network, GPRS network and Internet. Among them, the wireless sensor network adopts ZigBee technology to carry out wireless networking at the support construction site, and different types of equipment are scattered at the measured position of the support structure. Among them, the intelligent wireless sensor device is responsible for collecting data from the monitoring area of ​​the support itself, and sending the collected data through the wireless radio frequency module. The routing device receives the collected data and forwards it to the ZigBee coordinator device. The coordinator device then transmits the data forwarded by the routing device to the data transmission unit (Data Transmit Unit, GPRS DTU) through the RS 232 serial port. The CPRS network is connected to the Internet network to send the data to the PC network port of the remote monitoring center (i.e., the command center), and display it on the LabVIEW human-computer interaction software interface, finally realizing the wireless safety monitoring of the support structure. Figure 1 is the overall framework diagram of this system.

2 System Hardware Circuit Design

2.1 Smart Wireless Sensors

Intelligent wireless sensors are dispersed in key parts of the support structure. Based on the analysis of the reasons for the collapse of the support structure, the required physical quantities to be measured are summarized. Common reasons for collapse are:

(1) The bearing capacity of the support does not meet the requirements, and the local vertical poles are bent and unstable, resulting in overall collapse;

(2) The vertical height error of the vertical pole is too large, some fasteners are not tightened, and the horizontal pole connection is not overlapped;

(3) In actual construction, uneven local foundation sinking occurs (uniform sinking is another matter), and the load that the sinking poles should bear is transferred to the unsinking poles, causing the unsinking poles to be overloaded and unstable;

(4) Uneven loading;

(5) Abnormal vibration occurred during the concrete pouring process, but no attention was paid to it.

Therefore, by measuring the physical quantities such as stress, displacement, strain, vibration, and inclination of the support structure, the overall measurement of the support structure can be achieved. The above physical quantities can be converted into each other, and finally can be summarized as measuring the strain and inclination of the support structure. By measuring these two physical quantities, the effect of comprehensive monitoring of the support structure can be achieved. The intelligent wireless sensor consists of four parts: data acquisition module, data processing module, wireless radio frequency module, and power management module. Figure 2 is a hardware framework diagram of the intelligent wireless sensor. The data acquisition module collects strain and inclination information of key parts of the support monitoring area. The strain gauge is mainly responsible for collecting the strain value of the support pole structure and converting it into a slightly changing voltage through a Wheatstone bridge. The inclination sensor is responsible for monitoring the changes in the inclination angle of the support structure, and judging the safety and stability of the overall structure of the support by the relative value of the angle change. [page]

The main function of the data processing module is to amplify and filter weak electrical signals, and then convert the processed analog electrical signals into digital signals through the ADC inside the microcontroller. Here, the AD626 of the American AD company is selected for implementation. AD626 is a differential amplifier composed of a precision balanced attenuator, a low drift preamplifier and an output buffer amplifier. It can work under a single power supply of 2.4 to 10 V, and can also work under a dual power supply of 1.2 to +6 V. It is used to accurately amplify small differential signals and does not use other active components to filter large common-mode voltages. At the same time, the chip has the characteristics of low cost, low power consumption, and low power supply. AD626 has 8 pins. Among them, pins 1 and 8 are used for differential voltage input, pin 5 is used for output of amplified voltage, pin 2 is grounded, pins 3 and 6 are the positive and negative terminals of the power supply respectively, and pin 4 is connected to a capacitor to achieve low-pass filtering. The filter cutoff frequency fw calculation formula is shown in formula (1):

Where Cf is the capacitance of the external capacitor at pin 4.

The circuit amplification factor can be adjusted by correcting the external resistor value at pin 7. The pin connection diagram of the AD626 chip is shown in Figure 3.

The wireless RF module uses the CC2530 chip launched by TI in the United States. It is a wireless RF microcontroller that is fully compatible with the 8051 core and supports the IEEE802.15.4 protocol. It is a true system on a chip (SoC) CMOS solution. The CPU inside the chip analyzes and processes the ADC conversion data and sends the data results in the form of data packets through wireless RF.

The power management module is the guarantee for the normal operation of the entire device. Battery power supply can meet the requirements of small size, low power consumption and low cost of wireless sensors. In order to ensure the accuracy of collected data and the working performance of the entire intelligent wireless sensor module, the REG1117-3.3 voltage regulator chip is used to build a power supply voltage regulator circuit to reduce the impact of power supply fluctuations on the entire hardware circuit.

2.2 Routing Devices

The routing device is mainly responsible for assisting the communication between the connected smart wireless sensors and the coordinator devices, and relaying the transmission through multi-hop routing to expand the communication distance. The routing devices are also installed on the bracket structure in a dispersed manner, and the installation is based on the principle of covering the largest number of smart wireless sensor devices with wireless connections. The routing device is only responsible for transmitting data, so compared with the smart wireless sensors, it does not have a data acquisition module and a data processing module.

2.3 Coordinator Device

The coordinator device is responsible for the establishment and maintenance of the entire network, and manages the addition and deletion of routing devices or intelligent wireless sensors. The coordinator device is preferably installed at the center of the monitoring site, so as to ensure that the number of topological layers of the entire ZigBee network is as small as possible and reduce the waste of device resources. When the startup and configuration functions of the entire network are completed, the coordinator device degenerates into an ordinary routing device. At this time, it can receive data packets sent by routing devices or intelligent wireless sensors, and forward these data packets to the GPRS DTU through the RS 232 serial port. The hardware framework diagram of the coordinator device is shown in Figure 4.

The coordinator device is the core of the entire wireless network and plays a key role in its operation and maintenance. In order to ensure the normal operation of the entire network, the coordinator device adopts an external power supply.

2.4 GPRS Communication Module

The GPRS module connects the wireless sensor network to the Internet network, realizing the wireless transmission of data from the monitoring site to the remote monitoring center. The module uses the CM316 0P of a communication company in Xiamen. The device has TCP transparent data transmission and UDP transparent data transmission, online detection, online maintenance, automatic redialing when offline, etc. By using the corresponding configuration software, the local serial port configuration of the GPRS DTU can be realized to prepare for the sending and receiving of data.

3 System Software Programming

3.1 Lower computer program design

The lower computer program design uses the IAR Embedded Workbench integrated development environment, and the system application is developed based on the ZStack-CC2530-2.2.2.1.3.0 protocol stack provided by TI.

For the coordinator device connected to the GPRS DTU. After the system is powered on, the hardware and protocol stack are initialized first, then energy detection is performed, the appropriate working parameters are selected, and finally the device is allowed to connect and the network is started. After that, the coordinator device is in a state of constantly monitoring the wireless signal in the air. When a data request is detected, it will receive and forward the data to the serial port. When the coordinator finishes sending data, it will be in an idle state. At this time, if a new device joins the network, the coordinator will establish a connection with it and assign it a network address.

After the routing device successfully joins the network, it is always in the state of monitoring the wireless signal in the air. When a data request command from other devices is detected, the data packet is routed and forwarded. After the intelligent wireless sensor device successfully joins the network, it periodically collects and sends the strain value and tilt angle value according to the time interval set by the internal timer of the program. Figure 5 is a program flow chart of three device nodes, namely the coordinator device, routing device and intelligent wireless sensor device. [page]

3.2 Host computer software design

This system is based on the LabVIEW software development platform of the American NI company, and the host computer software of the bracket structure wireless security monitoring system is designed. Figure 6 shows the system host computer software design process.

First, the coordinator node in the ZigBee network transmits the collected data wirelessly to the remote monitoring center PC through the RS 232 serial port and GPRS network. The host computer software listens to the network port number information, and displays the collected strain and inclination information of each position of the support structure in real time to the corresponding position of the human-computer interaction interface according to the received data packet identifier, and saves it to the Microsoft Office Access database at the same time. When the user needs to analyze the strain value and inclination value, the historical data can be queried according to the acquisition time and device number, and the query results can be displayed in the form of a curve. In addition, the software uses a graded threshold alarm mechanism to classify the data. When the data exceeds the specified limit, an alarm prompt dialog box pops up, and the status indicator lights for strain and inclination are used to display different levels of alarms, so that users can take corresponding measures in time to avoid accidents.

4 System Testing

The built support structure safety monitoring experimental system consists of a coordinator device, a routing device, two intelligent wireless sensor devices, a GPRS DTU, a SIM card and a PC monitoring software. The intelligent wireless sensors are installed on different poles of the support structure, and the strain gauges are connected in a half-bridge form to the intelligent wireless sensors, as shown in Figure 7.

Place the SIM card in the GPRS DTU and enable the GPRS Internet access function. The coordinator device is connected to the GPRS DTU through the RS 232 serial port. When starting the test, the host computer software sets the network port number and establishes a wireless connection with the GPRS DTU. Then, start the coordinator device, routing device, and intelligent wireless sensor in sequence. Apply pressure to the bracket pole and observe the strain and inclination values ​​obtained by the host computer monitoring software. The software test output results are shown in Figure 8.

According to the software test results, the system has initial strain and inclination values ​​at the initial position. This is because when the bracket is built, the vertical poles, horizontal poles, bakelite boards, etc. of the bracket itself will form initial stress caused by gravity at the measured position. In Figure 7, the smart wireless sensor is installed approximately vertically, so the test results also show that the angle at the bracket vertical pole is 89°.

As time goes by, by gradually adding weights to the top of the pole, it can be seen that the strain and inclination curves have changed significantly at the same time. The strain curve changes more obviously because the strain bridge has high sensitivity and the weak signal is amplified, so it can be easily detected. The inclination angle is caused by the weight on the top of the pole, which causes the pole to tilt in a small range until the pressure is no longer applied, and the strain value and inclination angle value tend to be stable.

Through the above analysis, the system can realize online, fast and accurate measurement of strain and inclination in the monitoring area of ​​the support structure through wireless communication, thereby meeting the requirements of long-term real-time monitoring of the support structure.

5 Conclusion

This paper establishes a support structure safety monitoring system based on ZigBee and GPRS wireless communication technology. The system collects data such as strain and inclination of the support structure through a distributed wireless sensor network, and realizes the functions of real-time data collection and threshold alarm of the PC monitoring software. The system is flexible and simple to network, with strong reliability and real-time performance. The functional modules of the upper computer monitoring software run independently and orderly, and the operation interface is friendly. Compared with traditional wired monitoring, this system is more intelligent and convenient, allowing users to grasp the on-site conditions at any time without leaving home.