Measurement of wide frequency range structural microvibrations with a pocket digital camera and sub-pixel techniques

Abstract

Analysis of vibrations and displacements is a hot topic in structural engineering. Although there is a wide variety of methods for vibration analysis, direct measurement of displacements in the mid and high frequency range is not well solved and accurate devices tend to be very expensive. Low-cost systems can be achieved by applying adequate image processing algorithms. In this paper, we propose the use of a commercial pocket digital camera, which is able to register more than 420 frames per second (fps) at low resolution, for accurate measuring of small vibrations and displacements. The method is based on tracking elliptical targets with sub-pixel accuracy. Our proposal is demonstrated at a 10 m distance with a spatial resolution of 0.15 mm. A practical application over a simple structure is given, and the main parameters of an attenuated movement of a steel column after an impulsive impact are determined with a spatial accuracy of 4 μm.

Parametric Study of the Errors Obtained from the Measurement of the Oscillating Movement of a Bridge Using Image Processing

“…11). This result was already observed in numerical simulations done by Mas et al [8] and in experimental results obtained Busca et al [7]. The variation of relative error with distance to target is shown in Fig.…”

“…11), as it was already observed in numerical analysis done by Mas et al [8] and in the experiments done Busca et al [7]. The rise in distance to target give an increase in relative error for both cameras, but for Casio camera that trend is stronger (Fig.…”

“…Target tracking is accomplished by an automatic algorithm that searches and fits one ellipse shape in every image. This algorithm was already used in previous works, in which its accuracy was checked [8]. Also, in that work it is demonstrated that this procedure is able to detect movements with subpixel accuracy thus increasing the final resolution of the method.…”

“…By convenient choosing the target shape, one can locate the centroid of the target by simple least squares fitting, obtaining subpixel accuracy even with a pocket low-resolution camera, as it was demonstrated by Mas et al [8]. Measuring the movement from a distance of 10 m and in laboratory conditions, the error obtained was below 0.1 mm.…”

“…Once the ellipse is located, the centre of it is selected as reference point. The complete description of the procedure has been already done in [8].…”