Determining angular velocities of fast rotating objects based on motion blur to control an optomechanical derotator

Abstract

AbstractInvestigations of machine components are important for a faultless functioning of the machine. Ideally, the components are investigated during operation. Thus, a realistic performance of the machine can be analyzed. However, measurements of fast rotating machine components can be quite challenging especially under bad illumination conditions leading to the necessity of high exposure times. In case high‐speed cameras reach their limits due to motion blur resulting of high exposure times or high velocities, an optomechanical image derotator can still lead to sufficient results. This is due to its operating principle consisting of a rotating reflector assembly inspired by a Abbe‐Koenig‐prism which optically eliminates the rotational movement. To generate a stationary image, the optomechanical image derotator containing the reflector assembly has to rotate exactly half as fast as the rotating object. Therefore, the angular velocity of the object has to be determined. A common way for this is the use of rotary encoders although not every machine can be equipped with such sensors. A flexible solution for this is to identify the angular position and thereby the velocity with a high speed camera capturing images of the rotating object. Using image processing algorithms the marker can be detected and thus the velocity can be calculated from the image data. However, due to high rotational velocities and long exposure times motion blur of the marker can occur. Generally, motion blur can be treated as an artifact since it impedes the image analysis. However, motion blur can be used to determine the velocity of the rotating objects in knowing the shutter speed as well as the length of the streaking of a blurred object. This work presents an approach which determines the rotational velocity from image blur for different velocities. The resulting information will be used as the input source for a velocity control of the derotator. Furthermore it will be discussed whether the velocity information is precise enough to control the derotator leading to a resting image of the rotating object.