Development of Low-Cost IoT-Based Displacement Sensor using Laser-Camera Interaction

Accurate, continuous displacement monitoring is critical for structural‑health‑monitoring (SHM) applications, yet conventional contact sensors (e.g., LVDTs) and high‑end optical devices (e.g., laser Doppler vibrometers) remain prohibitively expensive or difficult to deploy on large civil structures. This study presents a low‑cost, IoT‑enabled displacement sensor that couples a class‑2 650 nm laser with a consumer USB camera housed in a matte‑black view box. Sub‑pixel centroid detection of the laser spot is performed on an open‑hardware Orange Pi Zero, enabling real‑time displacement calculations that are independent of sensor‑to‑target distance. A one‑time calibration yields millimetre‑level precision. Laboratory validation on a uniaxial shaking table compared the proposed node with a reference LVDT across stepped displacements from 0 mm to ±10 mm. The IoT sensor achieved a mean absolute error of 0.18 mm, a root‑mean‑square error of 0.21 mm, and a coefficient of determination ??2 =0.998, matching the performance of the LVDT while eliminating cabling and rigid mounting requirements. Processed displacements are transmitted via wireless/ethernet at up to 1 Hz, supporting seamless integration with cloud‑based dashboards. With a bill of materials under USD 100 and power consumption below 2.5 W, the proposed device offers an affordable, deploy‑and‑forget alternative for dense, long‑term SHM deployments, paving the way for cost‑effective digital‑twin and predictive‑maintenance frameworks in civil infrastructure.