Use of subpixel techniques in pocket cameras to measure vibrations and displacements

Abstract

Analysis of vibrations and displacements is a hot topic in structural engineering. Video cameras can provide good accuracy at reasonable cost. Proper system configuration and adequate image processing algorithms provide a reliable method for measuring vibrations and displacements in structures. In this communication we propose using a pocket camera (Casio) for measuring small vibrations and displacements. Low end cameras can acquire high speed video sequences at very low resolutions. Nevertheless, many applications do not need precise replication of the scene, but detecting its relative position. By using targets with known geometrical shapes we are able to mathematically obtain subpixel information about its position and thus increase the system resolution. The proposal is demonstrated by using circular and elliptic targets on moving bodies The used method combines image processing and least squares fitting and the obtained accuracy multiplies by 10 the original resolution. Results form the low-end camera (400 €) working at 224x168 px are compared with those obtained with a high-end camera (10000 €) with a spatial resolution of 800x560 px. Although the low-end camera introduces a lot of noise in the detected trajectory, we obtained that results are comparable. Thus for particular applications, low-end pocket cameras can be a real alternative to more sophisticated and expensive devices.