Projectile weapons, whether they are launched by cannons, as long-range rockets, or from aircraft, reach a state of “steady-flying” after their deceleration stage. At this point, the projectiles are able to automatically detect and recognize targets. Extensive testing must be performed during initial development to measure a projectile's spin rate and scanning angle -- collectively known as flight attitude -- that assure accuracy.
Mikrotron EoSens 3CP camera. Image Credit: courtesy of Northwestern Polytechnical University
Measurement of these critical parameters is performed by sensors, either projectile borne or non-projectile borne. Projectile borne sensors, such as accelerometers, gyroscopes, geomagnetic sensors, and solar sensors, are generally installed inside the body of the projectile during testing, while non-projectile borne sensors, including optical measurement instruments, radar, and global navigation satellite systems are usually placed outside the body of the projectile. Regardless of the type of sensor, projectile testing is carried out in vertical wind tunnels to simulate actual flight circumstances.
Optical measurement instruments are increasingly being adopted in the testing of projectiles. Due to high-speed video, it is now possible to smoothly view the steady-state rotation of a projectile spinning at hundreds of revolutions per second with a camera when combined with computer vision software. The one drawback to optical measurement is that while this technique usually captures higher accuracy in spin rate measurement than other sensor types, its lacks precision for scanning angle measurements.
Simple, Versatile Optical Method
Researchers at the School of Aeronautics, Northwestern Polytechnical University and the Xi’an Institute of Modern Control Technology have devised a new optical measurement system that is exceptionally simple yet versatile enough to accurately measure both scanning angle and spin rate during steady-flying. While previous optical methods used multiple cameras, their novel system employs only a single camera, an EoSens 3CXP three-megapixel camera from Mikrotron.
The research team tested their proposed solution in a university laboratory equipped with a vertical wind tunnel. Wind speed in the tunnel ranged from 5 to 50 m/s (meter per second) to reproduce projectile flight. Each projectile was suspended in the air using a flexible rope that passed over a pulley and rotated around a plumb axis. Depending on the projectile type, spin rates were maintained between 4 to 30 revolutions per second. Leveraging the CoaXPress high-speed interface, the researchers set the Mikrotron camera in a fixed position at a resolution of 1280 x 1024 pixels, a focal length of 35 mm, and a frame rate of 1000 frames-per-second. To ensure adequate illumination, a LED light source was also employed.
When a rotating projectile reached a steady state, the Mikrotron camera recorded a total of 5,000 images over a duration of 5 seconds. Rotation rates were determined by tracking the shapes and coordinates of projectile features using tracking algorithms or deep learning algorithms. In addition, a mathematical model for calculating scanning angle of a steady-state projectile was derived which made it possible to measure scanning angle from images recorded by only the camera from a fixed position. The algorithm effectively captured and analyzed the rotational motion of the projectile, providing precise measurements of both the scanning angles and spin rates. The low measurement errors indicate the reliability and robustness of the algorithm in accurately quantifying these parameters.