Wireless image-based two dimensional displacement sensors for civil engineering structure

Structural displacement data are useful to perform in both static and dynamic analysis to ensure the structural safety and service ability. Currently, there are a few sensors that can beused to directly measure displacements. These sensors however have some problems which might limit their implementation on large-scale civil structures. In this study, a wireless image-based displacement sensor is developed for monitoring displacements propagating in two dimensions.;This sensor is developed by incorporating a navigation sensor board as a sensing subsystem into a smart wireless platform (processing subsystem and communication subsystem) with apower supply subsystem to provide energy. To measure displacement, the processing subsystem sends a signal to the sensing subsystem to collect a sequence of images reflected from the concrete surface or designed grid pattern. These acquired images can be processed in the sensing subsystem directly (the navigation mode) or sent to processing subsystem for processing (the frame capture mode). The computed displacement can then be transmitted wirelessly to a base station. The design and the construction of this sensor are reported herein followed by some calibration tests on three prototype sensors. Test results show that the sensor can provide sub-millimeter accuracy under sinusoidal and step movement. However, it suffers from drifting problems. Hence, three different computational approaches were proposed and studied: the moving reference, the absolute reference and the hybrid reference. The results show that the hybrid reference could effectively suppress the drifting problem. As the navigation mode could only support moving reference approach, several motion estimation algorithms are studied to implement in the frame capture mode. Fast normalized cross correlation (NCC) method cooperated with Lucas-Kanade method is proposed to have accurate dynamic displacement measurement. Laboratory test results showed that the maximum sampling frequency of the sensor after applying the fast NCC method is 13 Hz. Shake table test results also show that the sensor could have the 2D displacement tracking capability. The displacement sensor was then used to measure mid span displacement on a concrete beam and to measure the response from an expansion joint on a pedestrian bridge to confirm the applicability of the proposed two dimensional (2D) displacement sensor in practical application.