Design of underground position detector for winter bamboo shoot based on time domain reflectometry

Winter bamboo shoots are not only of great value but also in great demand, but it is difficult to find in the field. There is no specialized detecting device in the market except expensive ground penetrating radar and the EM-38 earth electromagnetic induction instrument. However, their prices are more than 100 000 RMB. Therefore, in this study, we not merely built a winter-bamboo-shoots-soil permittivity model at high frequencies, but also analyzed traditional time-domain reflectometry (TDR). As such we finally put forward a method of detecting winter bamboo shoots based on time-domain reflection. Then, according to the detecting method, a new winter bamboo boots detector with low cost and convenient operation was designed. Electromagnetic waves have different propagation velocities and attenuation rates in different medium, accordingly the amplitude ratio and the phase difference of microwaves show differences through different medium. Hence, the detector first emitted and received microwaves by two antennas. Then the differences of amplitude ratio and phase difference appeared so that the detector can distinguish whether there were winter bamboo shoots or not. The whole detector was divided into three modules: High frequency module, low frequency module and antenna module. Hardware of the detector included following modules: A high frequency signal source system based on phase locked loop (PLL), wave filters, an amplitude and phase detection system, antennas, microprocessors and related Analog to Digital Converter (ADC), buzzers and buttons, etc. Software system was set up founded on the hardware. The high frequency signal source was mainly based on ADF4350, the amplitude and phase detection module was based on AD8302, and the microprocessor was STC15L2K. The detector used two antennas of the same type to receive and transmit microwaves. The 3×3 Latin square experiment was used to optimize the parameters. In the test, the moisture content of the soil was measured by soil moisture sensor first. Then the processor set thresholds of the amplitude ratio and the phase difference according to the soil moisture content so that the buzzer sounded the alarm automatically as soon as the collected value exceeded the threshold. In addition, winter bamboo shoots were buried in the soil as a control group, and the soil was detected by microwaves of different frequencies. The test showed that stones and bamboo leaves hardly had an impact on the detector. It also showed that the result was most obvious when the microwave frequency was 790 MHz. In this case, the analog voltage generated by the amplitude ratio of the soil with winter bamboo shoots was less than that of the solid without winter bamboo shoots by at least 15%, and the analog voltage generated by the phase difference was at least 15% higher than that of the solid without winter bamboo shoots. Many field detective tests were done in bamboo forest which showed that the winter bamboo shoots detector had an accuracy rate of approximately 70%. In conclusion, the winter bamboo shoots detector had both stable performance and high accuracy, which can save more than 25% of the searching time, and its cost was less than 1/10 of that of ground penetrating radar.