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In a tractor-trailer, ride comfort affected by horizontal human body motions, so called “wavy” and “shaky” feelings, is at issue. Insight about “wavy” and “shaky” feelings which is important for efficient vehicle development is not enough. Experiments using 6-axis motion generator and motion capture and inverse-analysis using multi-body human model indicated the characteristics of each feeling. Motion observation and transfer function indicated that while a bad subjective score of “wavy” feeling corresponds to same-phase roll motion of chest and pelvis up to 0.7Hz, “shaky” correlates to an antiphase of them around 2Hz. By multiple regression, dominant vibration components of the human body and the vehicle to subjective evaluation of the feelings above were identified. Explanatory variables for the “wavy” feeling are roll rate and lateral jerk and those for the “shaky” are lateral acceleration and longitudinal acceleration.

This paper presents an estimation method of muscle force which is considered variation of muscle force direction. We define a moment arm vector of muscle in musculoskeletal model. Its direction represents a rotational axis of the joint torque which is generated by contraction of the muscle. The direction of moment arm vector is redefined repeatedly according to joint torque calculated by inverse dynamical analysis. Muscle forces are estimated by optimization with the modified moment arm vector being employed in constraint condition which represents equilibrium between muscle forces and joint torques. We applied this method to analyzing braking motion. Resultant patterns of muscle forces match measured electromyogram. Consequently, it is verified that the variation of muscle force direction is necessary to estimate muscle force accurately.

Four ellipsoid-type dexterity analyses, which have been popular in the field of robotics, were applied to evaluate vehicle driver's steering arrangement. The Human was modeled as three-dimensional rigid bodies. Their length and inertia properties were based on anthropometric data. The newly defined measures were introduced as an effective radius of each ellipsoid along the steering direction, divided by a degree of agreement between the principal axes of the ellipsoid and those of the steering wheel. Sensitivity analyses were done with regard to the steering wheel location in a vertical plane and inclination, and seat back angle. Measures from dynamic dexterity ellipsoids, in particular the measure based on kinetic energy, nearly correspond to the traditional region of steering wheel arrangement from subjective judgement. Optimum inclination angle depending on seat back angle was calculated for three different sizes of drivers with the measures from dynamic dexterity ellipsoids.