Automatic Detection Method of Camera Direction from Target Normal Direction in Sampling Moire Method and its Application to Displacements Measurement of Large Structures

Large structures, such as bridges or unloader cranes in a steel plant, require health inspection methods to monitor their operation. Optical inspection methods offer numerous advantages for these large structures compared to traditional contact-based inspection methods. The sampling moire method is one such phase analysis method. Its features include high precision and non-contact field inspection. When measuring large structures, the measuring instruments are often positioned far away, sometimes several hundred meters, from the target. Calibration of such larger structures while in motion is nearly impossible. In this paper, the authors propose a calibration-free sampling moire method for measuring 3-axis displacements. This method initially measures the angles between cameras and the target. Subsequently, these angles are used to calculate a coefficient array, which translates phase differences into displacement values. Thus, the proposed method enables measurement of larger structures situated at a distance. Indoor experiments were conducted to verify the principles of the proposed method, and outdoor experiments were performed in a real steel plant to measure the displacements of an actual unloader crane. The results demonstrate that the proposed method can accurately measure the displacements of a working unloader crane.