## 3 Design and Implementation of the WSN Based AST System  
### High-Precision Wireless Strain Node Design  
The fundamental objective of the wireless sensor network based AST system is the design of a dedicated high-precision wireless strain sensor node. High-precision means the testing random error is small and replicated measurements can provide closely similar results. In this paper, this design aims to achieve a node testing accuracy of \(\pm\)0.1%. The wireless sensor node in this paper is designed with integrated bridge voltage circuit which enables precise strain measurement, so the resistance strain gauges might be directly connected to the designed wireless sensor nodes and need no other additional instruments. In order to have good testing precision, the bridge voltage provided to the Wheatstone-bridge must have enough accuracy. Because the strain measurement has low-voltage and varying-load features, the series reference scheme is selected for better initial tolerance, temperature coefficient and power dissipation than using shunt reference [7]. Specially, a series reference IC, REF5030, is used for providing the constant voltage for the Wheatstone-bridge circuit. The REF5030 is able to provide a 3V high precision power with excellent temperature drift (3 ppm/\({}^{\circ}\)C) and high accuracy (0.05%). The bridge circuit output corresponds to structural strain monitored. Since the sensitivity of strain gauge is low, the instrumentation amplifier AD623 is adopted to amplifier the bridge circuit output. As a low-power instrumentation amplifier, the AD623 can offer excellent accuracy. The maximal input offset drift of AD623 is no more than 2 \(\mathrm{\SIUnitSymbolMicro V}\)/\({}^{\circ}\)C. The maximal supply current of AD623 is no more than 575 \(\mathrm{\SIUnitSymbolMicro A}\). Since strain gauges are usually adopted to monitor static signals, a low-pass filter is designed to eliminate the high frequency noise. The voltage follower is adopted to output the filtered voltage signal. Figure 2 shows the schematic circuit diagram for the designed high-precision strain measurement. A 10-bit analog-to-digital (A/D) converter with a sampling rate of 15 KSPS integrated in an Atmel Mega128 MCU is adopted directly in the node design. It has eight multiplexed single ended input channels. Detailed design information on the processing core is outlined in our previous paper [8]. Specifically, in the wireless communication design a TI CC2420 RF transceiver is chosen instead of a CC1000 for the communication capability improvement provided. The CC2420 is a true single-chip 2.4 GHz IEEE 802.15.4 compliant RF transceiver designed for low-power and low-voltage wireless applications. With 2.4 GHz RF frequency and -25 dBm output power, the wireless transceiver only draws 8.5 mA of current while actively transmitting, guaranteeing the low-power characteristics of the designed wireless node. The circuits of the WSN node are divided to be manufactured on three four-layer printed circuit boards. A dedicated installation box is designed to attach these boards. The reason for adopting the four-layer circuits and divided design is to sufficiently separate the analog and digital circuit components. The other benefit obtained from the design is that each part can easily be upgraded according to different application requirements. In order to make the WSN hardware systems light-weight, easy-assemble and anti-EMI, this paper designed an all-aluminum shock-proof encapsulation for the WSN measurement in AST applications. Figure 3(a) shows the aluminum encapsulation size chart with dedicated shock-proof slot design. Figure 3(b) shows the picture of the designed wireless sensor node and the encapsulation for lightweight installation. All the interfaces of the encapsulation use reliable aviation plugs. The power to supply the wireless sensor node is designed to use 5 V direct current (DC) power since all the components are low-power and the 5 V DC power is easy to use in AST systems.