Design of Miniaturized Sensing Unit for Underground Displacement 3-D Measurement Sensor

Existing underground displacement monitoring sensors require prior drilling, but the size of borehole will affect the workload of drilling. Meanwhile, the landslide monitoring site is often in the field where the traffic is inconvenient and vehicles cannot be directly reached, miniaturization can reduce the weight and volume of the sensor, making transportation and handling even more convenient. Therefore, miniaturization is carried out on the underground displacement 3-D measurement sensor based on double mutual inductance voltage contour method. This study has combined with COMSOL Multiphysics and actual testing, discussing the influence of the air-core coil parameters on sensitivity and measurement range of the sensor, optimizing the parameters of air-core coil inside sensing unit, improving the internal structure of sensing unit to increase the sensitivity of the sensor. Additionally, this study has designed the sensor for low power consumption. Compared to the original system, the power consumption of the sensing array in this study decreases from 8250 to 43.65 mW $\cdot $ h (about 0.529% of the original system) with a measurement interval of 30 min, which is expected to achieve long-term monitoring in the field. The displacement measurement experiment shows that the sensor can measure small displacement changes while diameter of the sensing unit reduces from 9 to 5 cm. At the same time, when two adjacent sensing units within 60° angular range and 50 mm horizontal and vertical displacement range, the average displacement error is less than 1 mm, the maximum error does not exceed 2 mm.