Evaluation of a coplanar 6a3ω configuration in the Hybrid III 50th percentile male head
Objectives: In order to understand the mechanisms of traumatic brain injury (TBI) and develop proper safety measures, it is essential that accurate instrumentation methods are utilized. The brain injury criterion (BrIC) has been developed and validated to predict brain injuries in combination with the head injury criterion (Takhounts et al. 2011, 2013). Because the validated BrIC is heavily dependent on angular motion, the accuracy of any head instrumentation technique should be judged in part by its ability to measure angular motion. The main objective of this study was to evaluate a method of accurately measuring 6-degree-of-freedom (DOF) anthropomorphic test device (ATD) head kinematics using a coplanar 6 accelerometers and 3 angular rate sensors (6a3ω) configuration.
Methods: A coplanar 6a3ω configuration (c6a3ω) was implemented via a newly designed fixture. The c6a3ω fixture was placed at the center of gravity (CG) of a Hybrid III 50th percentile ATD (HIII 50) head. In addition, a tetrahedron fixture with 9 installed accelerometers (tNAAP) was externally mounted on the posterior surface of the HIII 50 skull cap. The c6a3ω setup also allowed for comparison to the 3a3ω configuration (i3a3ω) by subsequently treating the c6a3ω fixture as an i3a3ω fixture by only using accelerations and angular rates from select sensors. A total of 63 tests were conducted by impacting the head–neck apparatus at various high speeds and directions by a pneumatic ram. Normalized root mean square deviation (NRMSD), peak differences, and uncertainty were used for quantitative evaluation of the 3 configurations (e.g., c6a3ω, i3a3ω, and tNAAP).
Results: The average NRMSD and peak differences between the calculated angular accelerations were less than 5% between the tNAAP and the c6a3ω with 5.6% of uncertainty but greater than 18% for NRMSD and 20% for the peak differences between the tNAAP and i3a3ω with 58.2% uncertainty. Average NRMSD and peak differences between transformed resultant linear accelerations and gold standards (accelerations directly measured by accelerometers at the origin of tNAAP or c6a3ω fixtures) were also calculated. The c6a3ω had both NRMSD and peak differences less than 3% (uncertainty of 2.5%), and i3a3ω had NRMSD, peak values, and uncertainty on the order of 20% and higher. The tNAAP was slightly less accurate than the c6a3ω for transformed accelerations (NRMSD and peak differences <6%, uncertainty of 4.6%) and showed NRMSD and peak differences in the 7–8% range for angular velocity and rotation (uncertainty of 4.3 and 6.7%, respectively).
Conclusions: The c6a3ω configuration exhibited much better accuracy for calculating angular acceleration and transformed linear acceleration than the i3a3ω configuration. The tNAAP showed slightly less accurate transformed linear acceleration than the c6a3ω and was demonstrated to have less accuracy than c6a3ω and i3a3ω for calculating angular velocity and rotation. The c6a3ω configuration could be a potential alternative to specialized NAAP ATD heads because all kinematics can be measured near the head CG, and 6a3ω instrumentation provides the most comprehensive 6DOF kinematics (i.e., accelerations, velocities, and displacements) with accuracy.
