# Wireless Strain Gauge for Monitoring Bituminous Pavements
## 5 Results and Discussion  
Figure 13 shows the signal measured during the passage of the free-rolling wheels. Simultaneously, we measured the longitudinal strain with a classic wired Wheatstone bridge, and with the wireless measurement channel. Both measurements give the same waveform and the same maximum and minimum values, so we consider this test as conclusive. The waveforms of both measurements are also close to the work of [PERSON] et al. [11; 34]. That confirms the previous test, the electronic design, and the gluing technique. Indeed, [PERSON] et al. [11] also used a pavement core instrumented with a fiber optic network on a small proof polymeric body. Epoxy glue is used to fix the proof body on the asphalt core, and the latter to the pavement with a glue joint of 1 mm. The described sensor is designed only for the measurement of dynamic load. As a result, it is impossible to determine the long-term strain that appears between the two measurements. As explained in the third section, we use a basic alignment to set the longitudinal gauge axis. To avoid misalignment issues, we might use strain gauge rosettes. With rosettes, we can compute and compensate for misalignment errors; however, it needs to add one measurement channel and this increases the energy consumption and decreases the maximum sampling frequency. As previously announced, the gauges were chosen as long as possible to measure the average strain directly on asphalt concrete to avoid the use of an intermediate core. The drawback of this technique is the overlapping of longitudinal and transversal gauges. Gauges overlapping decrease the independence between longitudinal and transversal measurement. Indeed, [PERSON] et al. [11] observed a lower strain than expected with an optical fiber sensor embedded in polymeric body proof due to energy losses occurred by load transfer from the pavement to the sensor. Our choice is a compromise. It might be better if we add a temperature sensor to know the temperature of the gauges and test body. Figure 12: Measured and calculated longitudinal strain from in situ pavement testing. Indeed, the temperature impact gauges sensibility (around 10% at 40 \({}^{\circ}\)C when the gauge is calibrated at 25 \({}^{\circ}\)C) and pavement modulus. Concerning the performance and limitations of the electronic section, compromises had also to be made. To save board space and power consumption, we choose to use the built-in voltage reference of MCU to calibrate the ADC. Unfortunately, the output voltage drift due to temperature or supply voltage variation is not specified in the datasheet. As a consequence, the only available information is the output voltage range, which is 1.0 to 1.2 V for a typical value set to 1.1 V. The worst case of voltage reference gives 10% of error, which is the major contribution to inaccuracy. The second contribution is inaccuracy on resistors, which defines the amplifier gain with 2% in the worst case. Concerning gauges, there are the following three parameters: transverse sensitivity (1% of longitudinal sensitivity), gauge factor (\(2.14\pm 0.5\%\)), and grid resistance (120 \(\Omega\pm 0.6\%\)). We neglected the inaccuracy of grid resistance and gauge factor because we calibrated our instrument. It remains sensitive to transverse (that corresponds to \(\pm\)2 \(\upmu\)m/m when the wheel is fully loaded) and gauge factor variation due to temperature (\(\approx\)0.1% at 50 \({}^{\circ}\)C). The absolute accuracy of ADC is \(\pm\)2 LSB (least significant bit), which corresponds to 2 \(\upmu\)m/m. As a result, the sum of the uncertainties of the different stages amounts to the following: 13% and \(\pm\)4 \(\upmu\)m/m. Concerning wireless data transmission, we use a class 1 Bluetooth module that is qualified for IEEE standards 2.1/2.0/1.2/1.1. This part is used to transmit data at 2 Mbps in a 100 m range in free space. In practice, the Bluetooth transceiver is close to the soil and does n0t operate in open fields. As a consequence, the real range is reduced.