Vibration discomfort due to multi-axis excitation

Abstract: This licentiate thesis focuses on vibration discomfort due to multi-axis excitation. It is based on three studies; the first two studies were conducted in a laboratory environment using motion platform and utilizing sinusoidal signals, and the last study was a field study that utilized a fork lift truck on an artificial test track with built in obstacles that allowed controlled transient vibration excitation. The first study investigated the effects of lateral and horizontal vibration on the difference threshold of vertical vibration. Twelve male voluntary subjects sat on a flat rigid seat and were exposed to four different vibration conditions (pure vertical vibration; combination of horizontal and vertical vibration; combination of lateral and vertical vibration; combination of horizontal, lateral and vertical vibration). The vertical vibration for the four conditions was a 5 Hz sinusoidal vibration with a magnitude of 1 ms-2 r.m.s. Horizontal and lateral vibration for the last three condition were sinusoidal vibrations with magnitude 0.5 ms-2 r.m.s. that contained ten frequencies (1 to 8 Hz in third-octave band step) with the ratio of the ten frequencies adjusted by multiplying by the inverse of the weighting function (Wd) defined in ISO 2631-1. The results showed that in comparison to the difference threshold obtained with the first condition, the one obtained with the second condition tended to be lower while the ones obtained with the third and fourth conditions tended to be higher. However, a statistical analysis found that the differences were not significant. Horizontal and lateral vibrations were found to have different effects on the difference threshold of vertical vibration. The combination of vertical and horizontal vibration gave a significantly lower difference threshold of vertical vibration than the combination of vertical and lateral vibration. The second study examined the consistency of Weber's Law for multi-axis whole-body vibration. Three types of stimuli were tested; each stimulus consisted of the combination of 5 Hz sinusoidal vertical vibration, horizontal and lateral vibration of ten third-octave band center frequencies from 1 to 8 Hz with an acceleration level of 0.5 ms-2 r.m.s. The acceleration level of vertical vibration for the first stimulus is 0.5 ms-2 r.m.s., for the second stimulus 1 ms-2 r.m.s. and 2 ms-2 r.m.s. for the third stimulus. Results showed that the relative difference threshold of vertical vibration did not differ significantly between the three vertical vibration magnitudes. It indicates that for multi-axis whole-body vibration, Weber's Law is approximately consistent. The third study evaluated different seat designs in the aspect of minimizing vibration transmission and reducing the level of discomfort experienced by drivers subjected to transient vibration. Two seat designs (sliding or fixed in the horizontal direction) were compared. Results showed that a sliding seat is superior in attenuating vibration containing transient vibration in the horizontal direction and it was perceived as giving less low back and overall discomfort when compared to a fixed seat. It was also found that for pure multi-axis excitations a good model of discomfort requires at least measurements of 3 degrees of freedom.