Shaft Instantaneous Rotational Speed Vision Sensing Method Using Projection Fringe

Abstract

The measurement of instantaneous rotational speed (IRS) for rotating machinery is essential for ensuring operational stability, enabling predictive maintenance, and guaranteeing the safe operation of rotating equipment. Encoders, as a common measurement method, face challenges such as inconvenient installation and load effects on the shaft. To tackle these issues, a novel vision-based measurement method for the IRS of the shaft using projection fringe was proposed. The proposed method only requires processing a single row of data, thus offering advantages in measurement efficiency and data volume. A pattern with a sine white edge was first preset to the circumferential surface of the shaft. Then, a constant density fringe from the projection lamp was projected onto the white pattern, in which the fringe center will be sine-modulated by the white pattern during the rotation of the shaft. Cross correlation was utilized to track the coordinate variation of the fringe center, thereby obtaining its variation curve. An improved short-time Fourier transform with adaptive window length was proposed to accurately estimate rotational frequency from the fringe center variation curve (CVC), enabling the determination of shaft IRS. In this study, a system simulation model was established to analyze the various factors influencing its performance. Then, the effectiveness of the proposed system and algorithm was verified through experiments, proving that its measurement accuracy was comparable to that of encoders. Constant speed measurement of 300 r/min showed that the average speed error was 0.99 r/min with a relative error of 0.33%. Linear-varied speed measurement revealed that the average speed error was 3.01 r/min with a relative error of 0.48%. Sine-varied speed measurements indicated that the average speed error was 5.80 r/min with a relative error of 0.89%. Consequently, the proposed system offers a promising and reliable avenue for IRS measurement of rotating equipment.