Factors affecting the dynamic response of the body and the vibration transmitted through seats

The vibration transmitted through a seat is influenced by the dynamics of the seat and the<br/>dynamics of the occupant. The principal objective of this thesis is to understand how the dynamics<br/>of the body and factors affecting the dynamics of the body influence the vibration transmitted<br/>through seats. Previous studies have shown that the apparent mass of the body and seat<br/>transmissibility are affected by the seating environment (e.g. vibration input spectra, backrest,<br/>hands position, foot position) and variability between people (i.e. physical characteristics), but<br/>these effects have not previously been systematically explored for realistic seating conditions.<br/>The apparent masses of 12 subjects were measured during exposure to random vertical vibration<br/>(from 0.125 to 40 Hz) to investigate the effects of the seat backrest, the footrest and steering<br/>wheel, and input spectra. In a rigid seat with no backrest, there were resonances in the apparent<br/>mass of the body around 5 and 10 Hz (with 1.0 ms-2 r.m.s broadband vibration). In the same seat<br/>with a rigid backrest, the median resonance frequency in the apparent mass increased from 5.47 to<br/>6.35 Hz as the backrest was reclined to 30 degrees in 5 degrees increments; with a 100-mm foam<br/>backrest, the median resonance frequency decreased from 5.18 to 4.49 Hz as the backrest was<br/>reclined to 30 degrees. When subjects held a steering wheel, the mass supported on the seat<br/>surface decreased and there was an additional resonance at 4 Hz in the apparent mass. Moving<br/>the steering wheel away from the body reduced the apparent mass at resonance and increased the<br/>apparent mass around the 4 Hz resonance. As the feet moved forward, the mass supported on the<br/>seat surface increased, indicating that the backrest and footrest supported a lesser proportion of<br/>the subject weight. Applying force (0, 50, 100, 150, 200 N) to either the steering wheel or the<br/>footrest reduced the apparent mass at resonance and decreased the mass supported on the seat<br/>surface. Narrowband inputs at ½-octave intervals (from 1 to 16 Hz) presented at five magnitudes<br/>(0.25, 0.4, 0.63, 1.0 and 1.6 ms-2 r.m.s.) showed that the extent of nonlinearity previously observed<br/>with broadband vibration was frequency-dependent: the magnitude of vibration at frequencies less<br/>than 4 Hz had the greatest effect on the apparent mass at resonance, while vibration at<br/>frequencies less than 8 Hz had the greatest effect on the resonance frequency.<br/>A simple lumped parameter model was used to demonstrate that changes in the apparent mass<br/>with backrest contact, backrest inclination, hand position, foot position and vibration magnitude<br/>could be closely represented by changing the parameters in the model. Trends in model<br/>parameters, the damping ratios, and the damped natural frequencies were identified as a function<br/>of the model variables.<br/>A study was designed to determine how the physical characteristics of 80 seated adults (41 males<br/>and 39 females aged 18 to 65) affected their apparent mass and the transmission of vibration<br/>through a seat. Multiple regression models showed that while the strongest predictor of the vertical<br/>apparent mass at 0.6 Hz, at resonance, and at 12 Hz was bodyweight, weight was not strongly<br/>associated with seat transmissibility. A lumped parameter seat-person model was used to show<br/>that the dynamic stiffness of the seat increased with increased loading so as to compensate for<br/>increases in apparent mass associated with increased sitting weight. As age increased from 18 to<br/>65 years, the apparent mass resonance frequency increased by up to 1.7 Hz. This change was<br/>greater than the 0.9-Hz increase in resonance frequency between sitting without a backrest and<br/>sitting with a backrest reclined to 15° and greater than the 1.0-Hz reduction in resonance frequency<br/>when the magnitude of vibration increased from 0.5 to 1.5 ms-2 r.m.s. Subject age was much the<br/>strongest predictor of the seat transmissibility resonance frequency and the transmissibility at<br/>resonance. The model was used to show that changes in the seat transmissibility with age could be<br/>predicted from changes in the apparent mass with age.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Toward, Martin G.R.

Advisor dc:contributor.advisor

• Griffin, M.J.