Calibration of velocity light screens
conference paper
Correct projectile velocity measurement is essential for shooting ranges, to determine the ballistic performance of sample materials. It determines the accuracy of the provided measurement result and therefore requires flawless and precise functioning of the velocity measurement system, including light screens, counters and calculations.
A regular, traceable calibration of the velocity measuring instrumentation is mandatory for ISO 17025 accredited laboratories. This could be achieved by returning the complete system to the manufacturer for calibration. At TNO, infrared (IR) light screens are mounted on the wall inside the shooting ranges at a fixed position. Removing and re-installing the light screens could induce small deviations in the distance between the light screens, causing an error in the velocity measurement. For this reason it was chosen to calibrate the light screens in their original position. With this approach potential effects of outdoors calibration, like transportation and re-positioning are prevented.
Because a constant projectile velocity cannot be generated over the measurement range, considering the effect of drag resulting from variance of air pressure, temperature and humidity, a high speed video system is used for calibration. The in-house developed and precise velocity measurement device visualizes the projectile position during its flight. With the known projectile position over time and the resulting displacement between two recordings, the projectile velocity can be accurately measured. It results in an accurate average velocity over the distance between infrared light screens, for comparison with the reading obtained with the to be calibrated device.
The results of different sets of calibrations with two different projectiles show that projectile shape and velocity can influence the velocity measurements of the IR light screen. Also effects potentially resulting from vibrations due to moving of mobile velocity measurement equipment to different locations have been found.
A regular, traceable calibration of the velocity measuring instrumentation is mandatory for ISO 17025 accredited laboratories. This could be achieved by returning the complete system to the manufacturer for calibration. At TNO, infrared (IR) light screens are mounted on the wall inside the shooting ranges at a fixed position. Removing and re-installing the light screens could induce small deviations in the distance between the light screens, causing an error in the velocity measurement. For this reason it was chosen to calibrate the light screens in their original position. With this approach potential effects of outdoors calibration, like transportation and re-positioning are prevented.
Because a constant projectile velocity cannot be generated over the measurement range, considering the effect of drag resulting from variance of air pressure, temperature and humidity, a high speed video system is used for calibration. The in-house developed and precise velocity measurement device visualizes the projectile position during its flight. With the known projectile position over time and the resulting displacement between two recordings, the projectile velocity can be accurately measured. It results in an accurate average velocity over the distance between infrared light screens, for comparison with the reading obtained with the to be calibrated device.
The results of different sets of calibrations with two different projectiles show that projectile shape and velocity can influence the velocity measurements of the IR light screen. Also effects potentially resulting from vibrations due to moving of mobile velocity measurement equipment to different locations have been found.
TNO Identifier
959497
Source title
PASS 2020 - Personal Armour Protection Symposium, Copenhagen, Danmark, 11-15 October 2021
Pages
242-250
Files
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