Rotating machinery speed extraction through smartphone video acquisition from a radial viewpoint

Abstract

This article investigates the viability of using a smartphone as a cost-effective tool for rotational speed extraction. The proposed method exploits the geometrical image deformations induced by a smartphone’s rolling shutter camera, which reads pixel lines sequentially with a delay equal to the rolling shutter period. This characteristic period is the basis for the developed methodology and allows measuring constant and varying speeds above the Nyquist limit related to the camera’s frame rate. With respect to the rotating shaft, the smartphone is pointed to the side surface of the shaft (radial viewpoint). A method is proposed to measure the speed from the deformation of a zebra pattern as a consequence of the sequential readout of a rolling shutter camera. A mathematical model, which also takes into account a possible misalignment between the shaft and the camera’s rolling shutter reading direction, is developed and validated on an in-house test rig. It is shown that the proposed method is robust against changing lighting conditions and a misalignment between the shaft and the smartphone camera. Normalized Root-Mean-Square Errors (NRMSE) of about 3% (or less) are reached for the smartphone aligned with the shaft with peaks in the percentage error of about 5% to 8%. Nevertheless, even for a misalignment of 60°, the NRMSE is only 4% with variations in the percentage error to about 10%.The proposed method shows promising results with room for improvements, making it possible to develop a smartphone application to measure a shaft’s rotational speed in the future.