The displacement measurement in structural health monitoring (SHM), though important, was not popular due to inaccessibility of the civil infrastructures and high installation cost. The currently popular approaches use accelerometer, strain gauge, PZT, GPS, etc., most of which indirectly measure the displacement and require high cost to install and maintain. Thus the development of SHM system that directly measures the displacement of the structure using low-cost sensors is urgently needed. In this article, a multiple paired structured light (SL) system is proposed as a displacement measurement system for a massive structure. The proposed paired SL module which consists of cheap cameras and lasers is inexpensive to implement and can directly measure the accurate relative displacement between any two locations on the structure. Based on various simulations, a minimal configuration of the paired SL module is found. And Newton—Raphson and extended Kalman filter-based displacement estimation methods are proposed by deriving a kinematic equation and its constraints. By building a prototype of the paired SL module, some real experiments are performed to show the feasibility of the system for long-span structural displacement measurement.

No abstract

No abstract

No abstract

We employ an optical pump-probe technique to study coherent phonon oscillations in Ca2RuO4. We find that oscillation-amplitude of an Ag symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its phase, such that the oscillation even flips over with a 180 • change across the temperature. Density functional theory calculations indicate that the Ag phonon has an eigenmode of octahedral distortion with conventional tilting along the a-axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in lowtemperature structures, which may play a crucial role for the phase transition at 260 K.