Temperature measurements on fast-rotating objects using a thermographic camera with an optomechanical image derotator

Increasing requirements concerning the quality and lifetime of machine components in industrial and automotive applications require comprehensive investigations of the components in conditions close to the application. Irregularities in heating of mechanical parts reveal regions with increased loading of pressure, draft or friction. In the long run this leads to damage and total failure of the machine. Thermographic measurements of rotating objects, e.g., rolling bearings, brakes, and clutches provide an approach to investigate those defects. However, it is challenging to measure fast-rotating objects accurately. Currently one contact-free approach is performing stroboscopic measurements using an infrared sensor. The data acquisition is triggered so that the image is taken once per revolution. This leads to a huge loss of information on the majority of the movement and to motion blur. The objective of this research is showing the potential of using an optomechanical image derotator together with a thermographic camera. The derotator follows the rotation of the measurement object so that quasi-stationary thermal images during motion can be acquired by the infrared sensor. Unlike conventional derotators which use a glass prism to achieve this effect, the derotator within this work is equipped with a sophisticated reflector assembly. These reflectors are made of aluminum to transfer infrared radiation emitted by the rotating object. Because of the resulting stationary thermal image, the operation can be monitored continuously even for fast-rotating objects. The field of view can also be set to a small off-axis region of interest which then can be investigated with higher resolution or frame rate. To depict the potential of this approach, thermographic measurements on a rolling bearings in different operating states are presented.