Research On Vision Measurement Technology For Rotational Speed Of A Disc

The measurement of the rotational speed is widely used in the field of state monitoring and process control of the rotating machinery.So far,a variety of rotational speed measurement methods have been developed according to a variety of different applications.However,the rotational speed measurement still faces many problems and challenges.Among the current variety of non-contact rotational speed measurement methods,as a new emerging kind of technology the vision measurement methods have attracted attention of some scholars in recent years because of theirs unique advantages and good prospects for development.However,the existing rotational speed vision measurement methods are still highly immature.They cannot or can hardly be applied to measurement images with perspective effects.And in the published research literature on rotational speed vision measurement,there exist an issue that the experimental verification data is seriously insufficient.In industry,a large amount of the rotating parts with rotational speed measurement demands have a flat end face,such as the shaft end face and the gear end face,which can be abstracted into a disc surface.According to the shortcomings of the existing rotational speed vision measurement methods,this paper takes the rotational speed of a disc as the object of study and proposes a vision method for measuring the rotational speed of a disc: The disc surface is pasted a checkerboard pattern and a calibrated camera is used to take at least three still disc images in which the disc is at different angular positions and one blur image of the rotating disc.The rotational blur image of the rotating disc contains information of the instantaneous rotational speed to be measured.The blur image is then analyzed by motion blur identification technique and the instantaneous rotational speed is estimated.In this paper,the method proposed applies the inverse perspective mapping projection transformation technique(IPM)to remove the perspective effects on the disc images,so it can be applied to the general industrial situation where the disc shaft is not perpendicular to the camera imaging plane and so reduces the installation requirement on the camera.At the same time,the checkerboard pattern is easy to make and replace with low cost which also brings stability and accuracy guarantee to measurement results.In theory,as long as appropriate shutter speed is chosen,this method can measure any rotational speed.The proposed method has wide speed measurement range,low requirements on measurement environment.In this paper,using the proposed method,a real experiment on measuring the average rotational speed of the disc and a simulated experiment on measuring the instantaneous rotational speed of the disc was conducted to verify the feasibility and explore the accuracy and its influencing factors of the motion-blur-identification based method.These experiments provide quite adequate experimental data.Experiments show that the proposed method is feasible in practice,with moderate measurement accuracy(average rotational speed measurement with maximum relative error of 2.1872%,average relative error of about 0.5% to 1%;when rotational blur angle is greater than 6°,the instantaneous rotational speed measurement is with average relative error of about 1.2%,the maximum relative error of less than 3.5%)and real-time rotational speed measurement potential(if rotational blur can be controlled within 20°,the executing time of the algorithm of the proposed method can be controlled within 0.5 second).Experiments also show that the rotational speed measurement algorithm of this method is less sensitive to the estimation error of the rotation center with instantaneous rotational speed measurement error less than 2% when the random rotation center deviation is within 5 pixels from the real rotation center.The research of this paper promotes the development of the video rotational speed measuring instruments and the application of the vision measurement technology from laboratory environments to industrial environments.