Characterization of vehicle seat structural dynamics and its effects on ride comfort

This thesis presents an in-depth analysis of vehicle seat structural dynamics based on detailed experimental modal analysis. The experimental methodology to measure the frequency responses, resonant frequencies, and corresponding mode shapes of the three different vehicle seats in the mounted configuration is described in this thesis. It investigates the effect of the foam cushion, the seated human body and occupant weight on the dynamic characteristics of the bare-frame of the seat. Consequently, the effect of the dynamic properties on subjective ride comfort is then investigated according to the IS02631-1 international standard.

All seat structures were discovered to have three fundamental resonance and mode shapes below 80 Hz. This is within the human sensitivity to vibration range of 0.5 - 80 Hz. The addition of the foam cushions or seated human body does not contribute new resonances or mode shapes to the seat system. However, significant changes in the resonant frequencies were observed. Results indicated the cushion significantly increases the modal mass of the backrest thereby reducing resonant frequencies. The human body, contrary to expected results, increases the modal stiffness of the seat system and generally resonant frequencies increased with the addition of the occupant. Furthermore, the research discovered that increases in occupant weight reduces the seatback lateral frequency while increasing the fore/aft resonant frequencies. These effects of the human body on the seat dynamics was shown to be predominantly caused by the body weight on the seat backrest. The experiments done with volunteers without leaning on the backrest showed no significant changes in resonant frequencies from an unoccupied seat.

In the comfort survey, the twisting resonant frequency was seen to cause the highest discomfort to the seated occupant even though the frequency weighted vibration levels were similar to that of other test environments. Further investigation in the research showed that the accelerometer pad measurement point for the IS02631-1 is located close to the node point of the twisting mode shape. This caused inaccuracy in the measurement of the vibration transmitted to the occupant's back.

The outcome of this research firstly provides a method to characterise and predict the key vibration attributes such as occupied seat structural resonant frequencies and mode shapes from their corresponding unoccupied seat or bare frame characteristics. This alleviates the need for complex modelling or detailed analysis of the human body structure itself. Secondly, this research identified inaccuracies in the IS02631-1 measurement techniques about the twisting resonant frequencies of the seat structure and suggests ways in order to improve ride comfort assessment.