The Kettering University Crash Safety Center (Flint, MI) conducts vehicle subsystem tests using a pneumatic deceleration sled. The research and testing includes frontal, rear, or side impact occupant protection systems containing airbags, pretensioners, and/or child safety seats. The sled system can carry up to a 2,000-pound payload to a speed up to 42 miles per hour, and reach peak decelerations over 70 Gs. Access to high-quality imagery and dependable on-board and off-board high-speed digital video is as vital to the success of the data collection as is the anthropomorphic test devices (ATDs, also known as “crash dummies”) and their instrumentation (Figure 1).
High-speed video analysis is necessary to visualize kinematics during a crash event. Displacement can be measured directly from the video and compared to “Pass/Fail” criterion, such as head excursion in the Federal Motor Vehicle Safety Standard (FMVSS) 213. Displacement can also be calculated from acceleration measures. But, the mathematical process is prone to error buildup from measurement noise, and is less reliable than the direct measure of displacement from the video.
Having a high-speed video camera that is “stable” on the sled during the crash event is a necessity. Stability eliminates error from the camera motion with respect to the object being measured. The lighter the camera head, the less fabrication needed for the attachment of the instrument to the fixture. Therefore, the risk that the inertia of the camera and attachment will affect the kinematics of the event is lessened.
The high-speed video cameras used at the Crash Safety Center at Kettering are provided by Photron (San Diego, CA). The Fastcam MH4 (Figure 2) is a tiny, multi-head camera system utilized to visualize kinematics during a crash event. It’s important that the cameras are small and unobtrusive, yet rugged enough to withstand the high G event. The MH4’s 10-micron-pixel CMOS sensor supplies excellent sensitivity throughout the camera’s 60 to 10,000 frames per second (fps) operating range with full 512 x 512 pixel resolution available from 60 to 2000 fps. From one to four camera heads can be connected via the flexible cables up to 33 feet in length to the compact DC processor to enable high-speed imaging even in the most difficult-to-access and hostile environments, such as crash testing.
Photron’s camera also features a built-in battery that provides data retention for up to a full hour in the event that external power is lost during or after a test. The camera has an optional expansion battery pack that enables umbilical-free, complete camera operation and data retention up to 60 minutes. The rear- or top-mounted camera head brackets and the highly flexible camera cable simplify the positioning of the heads in confined spaces, making it especially suitable for deceleration sled testing and analysis.
The smaller the camera head, the greater the flexibility. In the assessment of seatbelt webbing payout, seat attachment points, belt attachment brackets, or imaging other “hidden” objects, the resultant slow-motion imagery captured may be key to understanding the occupant protection (Figure 3).
Having the ability to change the video speed, measured in frames per second, is vital to assess the kinematics. While 1,000 fps is usually enough to understand the gross kinematics of the ATDs, higher frame rates are necessary to assess airbag punch-out or seatbelt pretensioner timing.
This article was written by Dr. Janet Brelin-Fornari, Director of the Crash Safety Center at Kettering University, and Andrew Bridges, Director of Sales and Marketing for Photron, Inc. For more information, visit http://info.hotims.com/22918-167.