Noncontact deflection measurements in structures using diffraction gratings

An optical technique has been developed whereby two angles and linear displacement can be simultaneously measured in a non-contact manner. The method depends upon the usage of a diffraction grating with linear variation of period along its length. The grating is attached to a structure at a point of interest while all other system components are placed at a remote location. Evaluation of this measurement technique has been demonstrated on a laboratory-based structure which simulated conditions found at deep trench (or tunnel) walls or bracing systems. In a construction site configuration, this sensor allows the user to determine if the walls are undergoing structural deformation. In addition, the magnitude of deformation may be measured and alarm conditions may be monitored. Experimental results obtained using this technique are presented and compared with theory.