Tracking BOS: shock-wave image measurement of high-speed flying objects

Summary

In our laboratory, we have developed a high-speed gaze/optical axis control device (Saccade Mirror) using rotational mirrors (galvanometer mirrors), and established 1ms Auto Pan-Tilt technology which can continuously track and capture dynamic objects (e.g., ping-pong balls) at the center of the image. The 1ms Auto Pan-Tilt technology can be applied to dynamic sports broadcasting, but in this research we treat much faster objects than the sports balls, which are usually measured by a high-speed camera with tens of thousands of fps.

In this research, we aim to understand the in-flight behavior of the high-speed flying objects in detail for forensic investigation. We focus on optical measurement for actual flying objects rather than methods for fixed objects using wind tunnels and numerical simulation. We have established a new technology introducing the 1ms Auto Pan-Tilt technology based on a high-speed vision.

Compared to conventional methods using fixed cameras, our technology has the advantages of 1) long duration, 2) high resolution, 3) long exposure time, and 4) little motion blur in video recording and image measurement. Thanks to the optical axis control using rotational mirrors, other optical measurements can be performed on the same optical axis at the same time in addition to this image measurement technology. Then, a great contribution of this technology to the further development of the forensic investigation is expected.

This work is the result of collaborative research between our laboratory and National Research Institute of Police Science.

Step 1: Video recording of high-speed flying objects

As step 1, we first developed a video recording system for high-speed flying object (about 250 m/s).

In the conventional 1ms Auto Pan-Tilt technology for sports balls, the object can be kept within the angle of view by high-speed visual feedback and mirror control thanks to high mirror response. Even if the movement of the object and the optical axis of the camera are greatly different at the start of the target tracking, capturing the object within the angle of view can be sufficiently realized. On the other hand, the high-speed flying object will be out of angle of view immediately after the tracking from the stationary state of the mirror. Therefore, it is necessary to introduce trajectory prediction instruction as new optical axis control strategy.

In this system, the trajectory prediction instruction using a function generator is additionally modulated with high-speed visual feedback by the 1ms Auto Pan-Tilt technology. We have realized the continuous video recording of the high-speed flying object of about 250 m/s.

Fig.1 Setup of the proposed system.	Fig.2 Video recording of high-speed flying objects.

Step 2: Shock-wave image measurement of high-speed flying objects

As step 2, we developed a continuous shock-wave image measurement system for high-speed flying objects of even faster transonic speed (about 340 m/s).

Shock waves are generated around objects that fly at sound speed. Especially, it is known that unsteady phenomena such as shock-wave oscillations occur at almost the same speed as sound speed. Our Tracking Background-oriented Schlieren (BOS) system can capture the shock waves around the flying object as refraction of light similarly to the heat haze by introducing a striped retroreflective background.

In step 2, we have achieved shock-wave image measurement of actual high-speed flying object utilizing the Tracking BOS method (trajectory prediction instruction with improved response, high-speed tracking using high-speed visual feedback, and shock-wave visualization image processing to images including periodic stripes of the background). Then, based on the long-duration high-resolution image measurement, we have succeeded in observing the shock-wave oscillations around the actual high-speed flying object as the unsteady phenomena.

Fig.3 Concept of the Tracking BOS method.	Fig.4 Shock-wave visualization: Target A (388.3 m/s), B (419.9 m/s), C (238.2 m/s).

Movie

Reference

1. Tomohiro Sueishi, Masato Ishii, and Masatoshi Ishikawa: Tracking Background-oriented Schlieren for Observing Shock Oscillations of Transonic Flying Objects, Applied Optics, Vol. 56, Issue 13, pp. 3789-3798 (2017)
2. Kohei Okumura, Masato Ishii, Eri Tatsumi, Hiromasa Oku and Masatoshi Ishikawa: Gaze Matching Capturing for a High-speed Flying Object, SICE Annual Conference 2013 (Nagoya, 2013.9.15)/Proceedings, pp.649-654
3. Kohei Okumura, Hiromasa Oku and Masatoshi Ishikawa: High-Speed Gaze Controller for Millisecond-order Pan/tilt Camera, 2011 IEEE International Conference on Robotics and Automation (ICRA 2011) (Shanghai, 2011.5.12) / Conference Proceedings, pp.6186-6191