EFFECT OF LOW FREQUENCY VIBRATIONS ON HUMAN COMFORT

Abstract

Nowadays, the urban populace often spends a significant amount of their time for travelling and there is an increasing demand for comfort, both in private and public transportation. The quality of life in on-road vehicles is influenced by the level of ride comfort which is basically related to vibration levels. The ride comfort has developed facets that are as significant as safety and speed in assessing the physical characteristics of road transportation. The road roughness and vehicle vibration play a predominant role in the subjective evaluation of the ride comfort and activity comfort. A field study was therefore conducted by incorporating a questionnaire based study on public transport buses in India, in three different bus routes between Roorkee to Hardwar, Hardwar to Roorkee, and Saharanpur to Roorkee together with vibration measurements on the seat and floor. The subjective study involved reading of a national Hindi newspaper, to obtain a subjective opinion and to quantify the difficulty in reading and also from the vibration measurements the seat accelerations were measured for suggesting the proper design of seats in public transportation buses. The Preference technique method was adopted and the level of discomfort analyzed in 7- point semantic scale. The conclusions from the seat location, postures adapted for reading and the vibration measurements served as useful guidelines for conducting experimental work in the Laboratory. There are many environments in which people stand upon vibration platforms. The commuters of metro trains and public transport buses often adopt standing postures, wherein they hold the handrail or the handle affixed to the handrail. In the present work, of floor to head transmissibility (FTHT) and floor to knee transmissibility (FTKT) of the standing subjects holding handrail and handle have been studied for sinusoidal vibration magnitude of 1m/s2 The transmissibility results from the dynamic response of human body help in understanding the undesirable stimuli and the feelings of occupants on their activities caused by vibration. Both the vertical transmissibility values (FTHT and FTKT) were found to be higher with the handle as compared with the handrail. However the lateral vibration transmissibility is comparatively higher with the handrail posture. It was also observed that for normal standing posture of the subjects, the transmissibility is higher in lateral and vertical sinusoidal vibration in both handle and handrail postures. rms in vertical as well as in lateral directions in the low frequency range 3-15 Hz. ii The past decades have produced numerous studies on response of human body in vibrating environment which were supported by a number of experimental and analytical reports. Many of these focused on mathematical modeling for describing the biodynamic response of humans. Several biomechanical models have been proposed for responses to whole body vibrations (WBV), with different idealizations of the body structure and distribution of vibration, which are dependent on extrinsic and intrinsic variables, interfaces between the body and the vibration environment. Even though a few previous studies reported the use of finite element method in whole body vibration on specific human body segments to analyze mode shapes, deflections and their dynamic responses to the force excitation on standing posture models using finite element method was deficient. Hence, in the present work a continuum – 3 dimensional biomechanical model of the human body in standing posture with truncated ellipsoidal shaped body segments was developed and analyzed its mode shape characteristics using finite element method (FEM) software ABAQUS 6.10. The main focus of the work was to find natural frequencies for whole body vibrations (WBV), mode shapes and its deflections from the starting mode to the final mode. The principal resonance frequency obtained computationally is found at 6.45 Hz. Mode shapes and deformations due to whole body vibration in all resonance frequency ranges are numerically obtained and found to be within the range of available literature. The model is also analyzed analytically using governing equations and found natural frequencies. The dynamic responses of the standing human body subjected to vertical force were analyzed using finite element method. The objective of this study is to analyze the vibration signatures in time domain and frequency domain under low frequencies from 0 – 25 Hz under excitation conditions of 1m/s2 acceleration and body mass of 75 kg. A vertical force was applied under the foot and the accelerations at the knee and head were computed. The acceleration levels were found to be lower at the head than the knee acceleration which could be attributed to damping in upper body parts. The frequency responses show that the resonances at head and knee occur around frequencies of 5 Hz and 12.5 Hz with the acceleration levels of 0.47 and 3.35 m/s2, respectively. The low frequency vibration analysis of standing posture of human has been analyzed computationally and its results are verified with the experimental study performed on the vibration simulator at IIT Roorkee, India. iii In seated human the experimental study has been conducted in the laboratory to provide supporting information concerning the effect of inter-subject variability, magnitudes, frequencies and postures. The experiments were performed to measure vertical vibration transmitted to the occupants head in two representative postures erect and inclined backrest under three magnitudes of vibration under the frequency range 3 – 12 Hz in vertical direction. The STH transmissibility and BTH transmissibilty were recorded for each frequency and magnitude undertaken in the experiment. The measured data of each subject were collected using sound and vibration analyzer (SVAN 958). The observed result revealed the shift in the frequency was more evident in the back supported postures than in erect posture. This suggests that the upper body supported against a back support exhibits more softening tendency under higher magnitudes of vertical vibration. The STHT also revealed that primary resonance frequency decrease by approximately1Hz (from 5Hz to 4 Hz) from the erect posture to inclined posture for 1.2m/s2 r.m.s vibration magnitude. Human beings are most sensitive to whole body vibration under low frequency in a seated posture, therefore, biodynamic responses of a seated human body have been the topic of interest over the years and a number of mathematical models have been established. Based on this a 15 degree of freedom (DOF) lumped parameter model for standing human body and 5 degree of freedom (DOF) lumped parameter model for sitting human body is developed and solved for natural frequency, eigen values and mode shapes using derivation of governing equations and free body diagrams. The modeling procedure presented in this paper involves several simplifying assumptions. The foremost being the approximation of the body segments to ellipsoidal bodies, truncation of the ellipsoids for evaluating segment stiffness, and the approximation of segment elastic modulus as a geometric mean of bone and tissue elastic moduli. Nevertheless, with the adopted approximations the proposed modeling procedure seems to give a fairly good estimate of the low order mode natural frequencies. The fundamental resonant frequency of the 15 DOF model was found at 13.85 Hz. A 5 DOF lumped parameter model has been developed for a human in seated posture under sinusoidal excitation to determine the dynamic response characteristics such as driving point mechanical impedance (DPMI) and the seat to head transmissibility. As a part of this study analytical transmissibility data is validated with the experimental data. The resonant frequencies of the human subjects computed on the basis of transmissibility function are found to be close to that expected for the human body. It is concluded that the change in the iv human body mass, pelvic stiffness and pelvic damping coefficient gives remarkable change in the biodynamic response behaviors of the seated human body.