## 3 Design and Implementation of the WSN Based AST System
### Wireless Sensor Network Node Performance Tests  
After the fabrication of the sensor node, node performance tests were conducted. The first test was to evaluate the static error and repeatability error of the designed node. Four stain gauges are attached to the upper and lower exterior of the equal intensity beam. The strain gauges are connected to the wireless sensor node. A P3500 strain indicator is used to calibrate the WSN sensor node. The P3500 strain indicator is a portable instrument for stress analysis testing with strain gauge based transducers. Figure 4 shows the calibration setup. Table 1 shows the experimental results of three loading cycles with positive and negative loading. The plots of the sampled voltage values of the wireless senor node with respect to three cyclic load strains measured by the P3500 are shown in Figure 5. These plots clearly show the tested strain changes for every repetitive loading cycle. Figure 4: The picture of tests setup for static error and repeatability error. (a) Tests for output voltage under repetitive Loadings. (b) Tests for strain changes under repetitive Loadings. Figure 3: Cont. The static error for the wireless sensor node can be represented by \(\gamma\) given by:  
\[\gamma\ \ =\pm\frac{3}{y_{{}_{FS}}}\sqrt{\frac{1}{n-1}\sum_{i=1}^{n}\left(\Delta y _{{}_{i}}\right)^{2}}\ \times 100\% \tag{1}\]  
where, \(\Delta y_{{}_{i}}\) is the residual difference for the test points, n is the number of the test points, and \(y_{{}_{FS}}\) is the full scale output. Using the test data in Table 1 to the formula (1), a static error \(\gamma\ \approx\pm 0.1\%\) can be achieved for the wireless sensor node. Experiments show the developed WSN node is accurate and meet the pre-design requirements. \begin{table}
\begin{tabular}{|c|l|l|l|l|l|l|} \hline \multirow{3}{*}{ \begin{tabular}{c} **Load** \\ **Conditions** \\ \end{tabular}  } & \multicolumn{6}{c|}{**Output Voltage (V) / Strain (\(\mu\varepsilon\))**} \\ \cline{2-7}  & **Cycle 1** & & **Cycle 2** & & **Cycle 3** \\ \cline{2-7}  & **Plus** & **Negative** & **Plus** & **Negative** & **Plus** & **Negative** \\  & **Direction** & **Direction** & **Direction** & **Direction** & **Direction** & **Direction** \\ \hline
**0** & 1.144/0 & 1.144/0 & 1.144/0 & 1.143/0 & 1.143/0 & 1.143/0 \\
**2** & 1.218/387 & 1.218/387 & 1.218/387 & 1.218/387 & 1.21  
The other two tests include the wireless sensor node service lifetime and transmission range. Table 2 shows the different service lifetime tests to corresponding AA battery capacity. The service lifetime is divided by active time and low-power time. The transmission period is set to 10 seconds. Power supply load for the active mode is about 40 mA in this test configuration. It can be calculated by totalizing the operation currents of three functional modules. The node total current consumption in the low-power mode is about 100 \(\upmu\)A with the sensor input unit shut-down mode, so a conservative estimation for the anticipated life of the wireless sensor node using four batteries (each 2,000 mAh) is over two years. Table 3 shows the different transmission ranges corresponding to different transmission powers. \begin{table}
\begin{tabular}{c c} \hline
**Each Battery (mA\(\cdot\)h)** & **Lifetime (h)** \\ \hline
1.000 & 185 \\
2,000 & 364 \\
3.000 & 588 \\ \hline \end{tabular}
\end{table} 
Table 2: Different lifetime to corresponding AA batteries. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Test1** & **Test2** & **Test3** & **Test4** \\ \hline Power & -20 & -10 & 0 & 10 \\ Range (m) & 10.1 & 16.5 & 29 & 75 \\ \hline \end{tabular}
\end{table} 
Table 3: The different transmission ranges corresponds to the transmission power. Figure 6: The received signals of the wireless sensor node in the test environment set by (a) no EMI factors, (b) a Bluetooth phone, (c) a GSM mobile phone and (d) five body obstacle. The final performance test of the wireless sensor node was its anti-EMI capabilities.