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.