ZigBee Wireless Strain Collection device

1 Strain Collection method for health monitoring of hydraulic structures

With the construction of large-scale water conservancy and hydroelectric projects, the health monitoring of hydraulic structures and related civil engineering structure becomes more and more important. Especially in recent years, the warning of some major accidents and the development of materials technology, computer technology, communication technology and intelligent control technology have promoted the development and application of structural health monitoring technology.

Structural health monitoring is divided into whole monitoring and local monitoring. and whether the local monitoring or the overall monitoring, are based on the sensor accurate data acquisition and transmission of the premise and foundation, a variety of structural health monitoring data collection is mainly used in the traditional "wired" sensor to achieve, although this collection method with accurate acquisition of signal, anti-jamming, product serialization and other characteristics, but the use of "wired" Sensors consist of a large number of network cabling, high installation and maintenance costs, poor reliability, and even in some structures can not achieve cabling. With the development of sensor technology,ZigBee Digital Transmission Module communication technology and MEMS Technology, wireless sensor technology has been developed and can overcome the cabling capacity and high cost of wired sensor network, and in practical application ZigBee The wireless module has been greatly developed.  

Strain is an important parameter reflecting the local state of the structure, which is very important in the local monitoring of structures. The existing strain-collecting device is either large in size, cannot be placed near the sensing element, or is not configured with a wireless transceiver interface and needs to communicate with the control center via a wired connection. Therefore, a small size, low power consumption of wireless ZigBee Data acquisition device for strain parameter acquisition is very necessary. This paper introduces a resistor-chip strain-collection device based on ZigBee Wireless module .

2 principle of strain measurement

The resistance strain gauges produce mechanical deformation under the external force, which causes the resistance change, which is the resistance strain effect, and the strain measurement is realized by using this mechanism. The Resistance strain gauge is the most commonly used and better sensing element in the local strain measurement of the structure, and it is stable in the structure surface or in the buried human structure, and can form various shapes and area array, prevent electromagnetic interference, and have better durability. And the Resistance strain gauge area is very small, does not affect the embedded material performance, has the production easy, the price , high temperature resistance, impact resistance and high flexural strength, suitable for hydraulic structure monitoring applications. When the resistance strain gauges are connected to the bridge circuit, the change of stress can be transformed into a voltage change. In the actual installation, should be tested on the pieces of the 2 work of the same specifications of the strain gauges, a pull, a pressure, to ensure that their strain is equal in size, the opposite direction, will be the bridge adjacent to the bridge arm, it constitutes a differential half-bridge input circuit. The output voltage of the differential half-bridge circuit is u=e xR/2R, where E is the supply voltage of the half-bridge circuit, and the resistance change of the strain resistor is caused by external force,R is the bridge balance arm resistance, generally I'm in Europe.

3 Hardware Design of strain collection device

This paper intends to adopt Texas Instruments (TI) new chip system CC2530 asZigBee Digital Transmission ModuleThe host controller. The CC2530 is a true 802 for IEEE. A. 4,ZigBee,ZigBee rf4ce and Smart energy should beOn-Chip system solutions. Due to the highKBof high-capacity flash,CC2530Great forZigBee PROapplications, and larger capacity memory will allow on-chip download to support reprogramming within the system. In additionCC2530Also combines the1A fully integrated, high-performance RF transceiver with8051 MCU，8 KB RAM、 +／ -／ -／KBFlash memory and other powerful support features and peripherals.CC2530Provides a101 DBThe quality of the link, excellent receiver sensitivity and excellent anti-jamming performance,4Multiple flash sizes, and a broad set of peripherals, including2AUART、 A-bitADCAnd +A UniversalGPIO。 In addition to passing excellentRFPerformance, selectivity, and industry-standard enhancements8051MCUThe kernel, which supports low-power wireless communication in general,CC2530Can also be equipped withTIA standard compatible or proprietary network protocol stack(Remoti，Z-stackOrSimpliciti)To simplify development.

The hardware portion of the strain-collection device is shown in the function 1 . The half-bridge acquisition unit completes the modulation of the bridge differential voltage signal, which is digitized by a differential amplification and voltage offset link into the CC2530 built-in analog-to-digital converter. Where the power supply section includes 3. 3 V regulated power supply circuit,3. 3 v boost to 5 v Circuit and 3. 3 v Buck to 2 v reference circuit. From the original intention of reducing the static power consumption, should try to use a low quiescent current chip, the design of the use of low ripple charge pump DC/DC transform chip TPS60211, charge pump DC /DC chip TPS60151 and ultra-low noise, high precision voltage reference chip ADR4520.

Figure 1 hardware system functional block Diagram

To reduce the current consumption caused when the 5V voltage of the TPS60151 output of the boost chip is supplied to the bridge circuit, the ENA pin of the power chip is set to high by the microcontroller only when the strain needs to be measured, and the output 5 V In most non-measuring time, the ENA is low, and the power supply chip does not output 5 V voltage. Through this kind of control measure, it can satisfy the design requirement of system low power consumption well.

The bridge circuit supply voltage 5 V, in the 2 resistor configuration under the static operating power Supply, theUbd common-mode voltage is 2. 5 V, the differential mode voltage is ±ten MV. The meter op amp AD623 The differential magnification to 1 vand the AD623 voltage reference pin voltage is shifted by 2 V , The Adin voltage value is within the range of l-3 V . The CC2530 voltage Conversion reference uses its supply voltage of 3. 3 V, the resolution of the digital- to-analog conversion is 0because the AD conversion of the CC2530 is a bit. 8 MV, corresponds to 8uVof bridge voltage variation.

Figure 2 analog signal acquisition circuit

4 protocol stack analysis of Wireless sensor network

ZigBeethe driver software of the digital transmission module acquisition device is based on TI Company protocol stack z-stack ,z-stack protocol stack code is semi-open source, part of the underlying function code to the user screen, but does not affect the user in the application layer of the program development. The entire Z-stack protocol stack is layered, and the hardware Abstraction Layer (HAL) provides drivers for a variety of hardware modules, including timer timer, Universal I/O Port GPIO, Universal Asynchronous transceiver transmitter UART, ADC application Interface for analog-to-digital converters. Operating system Abstraction Layer Osal realizes a non-snatch multi-task operating system platform, realizes task scheduling by time slice rotation function, and provides multi-task processing mechanism. Users can invoke the relevant APIs provided by Osal for multitasking programming and implement their own applications as a separate task. Each layer in the stack has a specific service method and a previous layer of connectivity, called a protocol. The data entity provides the transport service for the data, while the management entity provides all the service types. Service entities for each layer are connected through the service access point, SAP , and the previous tier, and each SAP provides a number of service methods to complete the operation.

5 Embedded Software design

The software development environment uses the IAR Embedded workbenchide, the program uses the C language, the programming adopts the modular structure, ensures the function transplant is good, and facilitates the debugging. The main function is shown in major workflow 3 .

Z-stack the essence of the protocol stack is the multi-task scheduling of operating system osal . Therefore, the initialization of the operating system is particularly important in preparation for the main loop processing function. osaltaskinit (),osaladdtasks (),osalinittasks () 3 function calls form the 7 Large Task List of the protocol stack, which 7 the big tasks were Zdapp_event_loop,nwk_event_loop,hal_processevent,mt_processevent ,aps_event_loop,maceventloop,sapi_processevent.

Figure 3 software Workflow

After entering the main loop handler function, the task list is always scanned, and these 7 tasks are determined by TaskID and taskpriority to scan and process the order. In the process of cyclic scanning, the transmission of data is operated using Direct access (DMA) controller, which can reduce the burden of transmitting data in the 8051CPU Core, and realize the high performance of CC2530 under the condition of efficient utilization of power. The main function, main functions, is registered by the function pointer Tasksarr , and after the system is started by the osal_start_system function, the call relationship 4 of each function is shown.

Figure 4 function call Relationship

The Data acquisition sub-function can be compiled in the sendreport function. Software debugging can be aided by the TI company ZigBee Sensor monito application software to observe the network topology. Due to the compact design, the hardware board cancels the peripheral components such as the keys on the TI Company's Development Board, so it is necessary to change the program of the coordinator and the terminal node to the power-on auto-binding instead of the joystick button. Can be modified by demosensor. C and democollector. The zb_bindconfirm function in C implements automatic binding.

6 Device Implementation and system testing

Based on the above hardware and software design scheme completed the strain ZigBee data acquisition device is shown in Figure 5 . The low power consumption of the device is not only reflected in the selection of hardware chip and circuit design, but also in the embedded software program to make the CC2530 dormant, so that the strain ZigBee Data Acquisition Wireless sensor node power consumption as low as possible. Through the actual network testing, the strain collection results are accurate, the network is reliable, the working power consumption is low, and it is suitable for water Conservancy automation engineering and other structures.

Figure 5 Wireless Data acquisition device physical

7 Conclusion

Based on Ti 's new generation of on-chip system CC2530 ZigBee Digital transmission module and ti 's z-stack protocol stack developed a compact, functional flexibility of the strain ZigBee Data acquisition device. The device uses ZigBee Wireless module can be used in structural health monitoring and other water conservancy monitoring projects, effectively solve the wiring and power supply lightning protection problems, easy to build a large wireless sensor network.

ZigBee Wireless Strain Collection device