THE EFFECTS OF ACCELERATION ON THE MECHANICAL IMPEDANCE RESPONSE OF APRIMATE MODEL EXPOSED TO SINUSOIDAL VIBRATION

Criteria for developing active and passive isolation mechanisms for re ducing the effects of whole-body vibration exposure rely on a thorough understanding of the stiffness, damping, and resonance behaviors of t he human or human surrogate body. Three Rhesus monkeys were exposed to seated whole-body sinusoidal vibration between 3 and 20 Hz at 0.69 an d 3.47 msec(-2) rms (0.1 and 0.5 g peak) accelerations. The mechanical impedance magnitude and phase were calculated as the ratio and phase relation between the transmitted force and input velocity, respectivel y, at the seat. The resultant profiles showed a significant decrease i n the primary resonance frequency with increasing acceleration. At the lower acceleration level, a second lower impedance peak was observed at approximately 5 Hz. A three-mass, two degree-of-freedom model, whic h included upper torso and leg representation, was used to determine t he mechanical parameters that best described the measured responses. T he mean stiffness coefficients and the mean undamped natural frequenci es associated with the upper torso and leg subsystems showed a signifi cant decrease with increases in the acceleration level. The results of this study strongly suggested that nonlinear stiffness properties wer e responsible for the observed differences in the biodynamic response of the Rhesus monkey with acceleration level.