Scanner of dynamic deflections (SCADD): a new approach for field data acquisition of the vibration of civil structures

We describe a novel instrument for the remote measurement of dynamic deflection shapes of structures several tens of meters long, based on geometrical optics techniques with scanned laser illumination, which we have named Scanner of Dynamic Deflections (SCADD). A set of aligned control points is measured in each scan, each point being defined by a retroreflector attached to the structure. By measuring the delay of the optical signal reflected from each point, the system renders a component of the displacement of that point which is transverse to the illumination direction. The intended application of SCADD is the field data acquisition for diagnosing the structural health of civil infrastructures, either as a stand-alone instrument or integrated in a non-destructive structure testing system comprising several data sources, typically an array of accelerometers and a SCADD unit. The foreseen measurement accuracy and the spatial and temporal sampling density of SCADD are adequate to the application of modal analysis techniques. For the purpose of locating our proposal in its technological context, we include firstly a brief description of the most usual methods (optical and non-optical) for the field measurement of vibrations of civil structures. Then, the SCADD principle of measurement and architecture are detailed. In the experimental section we describe a SCADD prototype and a series of measurements of a control point located 18 m away from the SCADD head, from which we extract the repeatability and a calibration curve of the prototype. Finally, the main advantages of SCADD are detailed.