A METHOD FOR INTRA-EXPERIMENTAL VALIDATION OF HEAD IMPACT ACCELERATION
MEASUREMENTS
RWG Anderson
Centre for Automotive Safety Research, The University of Adelaide;
email: [email protected], web: www.casr.adelaide.edu.au
INTRODUCTION
Methods of measuring head kinematics during short duration
impacts include the so-called 3-2-2-2 method [1]. The
method uses an array of linear accelerometers to estimate
linear and angular acceleration of the head. The use of these
and similar arrays in PMHS and animal models can be
problematic because non-rigid effects can drastically affect
the measurement of angular acceleration [2]. Problems
include non-rigid attachment of the array to the head. One
must also consider whether or not the rigid body is actually
rigid, and to what extent the surface to which the array is
attached is likely to exhibit non-rigid behaviour [3].
However, if the acceleration of a rigid body is known, the
acceleration at any point on the rigid body can be calculated.
Therefore it is possible to quantify the validity of the
acceleration measurement using the output of one or more
reference accelerometers. An accelerometer placed on the
skull can record the acceleration experienced by a single
point on the head during the impact. The output of the array
can be used to predict the acceleration of that point. The
degree of correlation between the prediction of the array and
the acceleration measured by the reference accelerometer
provides a statistical measure of the validity of the
acceleration measurement in any given impact. If the
predicted and measured acceleration correlate well, one can
have increased confidence that the array successfully
measured the rigid body motion of the head.
THEORY
The acceleration ün at any point n on a rigid body is given by
where ρ n is the vector
from the origin of the body to the point n and
is the
acceleration of the origin. The component of acceleration
along a vector a at point n is given by
where
.
The acceleration measured by an accelerometer placed at n
may be cross-correlated with the acceleration predicted by an
accelerometer array placed on the rigid body; the correlation
coefficient provides a measure of the extent to which the
array and the reference accelerometer (and by inference their
common substrate) acted as a rigid body throughout the
impact. A poor correlation coefficient indicates poor
attachment of the array or the reference, or other non-rigid
behavior or any combination of these non-rigid effects.
METHODS
The methodology was confirmed by attaching the
instrumentation to a clean and dry skull that was attached to
a mechanical shaker, and measuring the transfer function and
coherence between the array and the reference.
Subsequently, head impact acceleration was measured in
eight experiments where sheep subjected to head impact
were used as a surrogate for human brain injury. An array
of 9 accelerometers arranged in a 3-2-2-2 configuration
was mounted to the head of each animal, and an
independent accelerometer was also attached. The relative
arrangement of the array and the reference accelerometer
was recorded with a 3-D digitizer (Figure 1).
Figure 1:
Arrangement of array and reference
accelerometer mounted the head of the sheep.
Cross correlations between the array prediction and the
reference acceleration were performed on the first 3.5 ms
of data, which was the duration of the impact.
The University of Adelaide Animal Ethics Committee
granted ethical approval for these experiments.
RESULTS AND DISCUSSION
Reference coefficients greater than 0.9 were registered in
six of the eight experiments, another registered a value of
0.88, and the other 0.48. The highest value was 0.967. The
reference signal in this experiment closely matched that
predicted by the array and the peak values of acceleration
were within 5%. An examination of the reference signal
and its predicted signal reveal that those experiments that
had poorer reference coefficients tended to have
significant discrepancies between the reference signal and
prediction of the array.
CONCLUSIONS
Head acceleration measurements in PMHS impact tests
and in-vivo animal tests can be of variable quality because
of inter-experimental variation in the quality of the
mounting. Angular acceleration measurements are
particularly sensitive to mounting problems. The use of a
reference accelerometer provides an intra-experimental
validation of the measurement and helps to identify
unreliable measurements.
REFERENCES
1. Padgaonkar AJ et al. J App Mech 42(3): 552-556, 1975.
2. Yoganandan N et al. J Biomech 39(7): 1354, 2006.
3. Nusholtz GS, et al. 28th Stapp Car Crash Conf, 41-74,
1984