An experimental investigation of the vibrational comfort of child safety seats.

The research of this thesis was performed to understand the vibrational dynamics of
stage 0&1 child safety seats and of the children who occupy them. Since no previous
vibration data for small children or child seats was found, the investigation took the
form of experiments designed to shed light on the behaviour of the system consisting
of child, child seat, vehicle safety belt and vehicle seat. To provide a background for
interpreting the results a literature review was performed of child seat characteristics,
of human whole-body response and of primate whole-body response. An industrial
test procedure for measuring the vibration isolation properties of vehicular seats is
also presented as an illustration of the concepts involved.
A whole-body vibration bench for testing children in the vertical direction was built
and apparent mass and absorbed power functions were measured for 8 children of
age less than 24 months and mass less than 13 kg. An algorithm was developed for
identifying the parameter values of a single degree of freedom mass-spring-damper
model of the seated body using Differential Evolution optimisation. The parameter
values were determined for each child and compared to those of adults and
primates. This thesis also presents the results of modal testing of 2 child seat units
and of operational deflection shape testing of 1 unit in an automobile under 3 loading
conditions (empty, sandbag or child). In-vehicle transmissibility measurements were
also performed in the vertical direction for 10 children and child seats using 9
automobiles. The floor-to-human transmissibilities were determined for each child
and driver when passing over a reference road surface at both 20 and 40 km/h.
Except for the damping ratio, all child mechanical response parameters were found
to differ with respect to those of adults or primates, with the differences being greater
with respect to adults. The first resonance frequency of children was found to be
located at 8.5 Hz as opposed to 4.0 Hz for adults, raising questions regarding the
applicability of standards such as ISO 2631 towards the evaluation of child
vibrational comfort. The child seats were found to have higher transmissibilities on
average than the vehicular seats occupied by adults. A characteristic low frequency
rigid body rocking motion was noted at 1.8 Hz as were multiple flexible body
resonances starting from frequencies as low as 15 Hz. Areas of possible
improvement and topics for further research have been identified.