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Authors: Kumar, Vikas
Keywords: Railway Vehicles
Issue Date: Apr-2015
Publisher: Dept. of Mechanical and Industrial Engineering iit Roorkee
Abstract: In railway vehicles, vibrations are transmitted to the passengers through the various interfaces such as floor, seat, backrest etc. These vibrations affect the passengers’ comfort as well as their performance to do any work such as reading, writing, typing etc. The effects of vibration magnitude, direction of vibration, postures, backrest inclination and reading formats have been studied on the performance of reading activity. Thirty healthy male subjects were exposed to uni-axial whole body vibration (WBV) in 1-20 Hz frequency range at 0.5, 1 and 1.5 m/s2 rms vibration magnitude. The experimental task involved reading a paragraph under the WBV exposure. The task performance has been evaluated in terms of time taken by the subjects to read a given paragraph and also the subjective evaluation of perceived difficulty on Borg’s CR 10 scale. Perceived difficulty and percentage reduction in reading have been found to increase with the increase in vibration magnitude in each direction of vibration. The highest perceived difficulty and percentage reduction in reading have been experienced by the test subjects in the fore-&-aft direction in with-backrest posture. In vertical and lateral vibration, perceived difficulty and percentage reduction have been higher in without-backrest posture compare to with-backrest posture. The perceived difficulty and percentage reduction have been found lower in the lateral vibration for the triple-column format. With the inclination of backrest, the perceived difficulty in reading and percentage reduction in reading performance are found to increase in vertical direction but decreases in fore-&-aft direction of vibration. Transmission of vibration from the vibrating interfaces to various organs of the human body may influence their functioning under WBV exposure. Therefore, an experimental study has been performed to relate reading performance and seat-to-head transmissibility (STHT) under WBV exposure. Twelve seated male subjects were exposed to sinusoidal vibration with three magnitudes (0.5, 1.0 & 1.5 m/s2 Prior understanding of the dynamic behaviour of the human body in the vibration environment is very important for the vehicle system design. The STHT responses of the test subjects in four rms) at seven different frequencies (4, 5, 6.3, 10, 16, 20 & 25 Hz) in three independent directions. The results show that three output measures; STHT, % reduction in reading performance and perceived difficulty in reading are significantly affected by the frequency of vibration in each direction. The results of all the three measured responses are in agreement that the greatest disturbance to WBV is perceived at low frequencies of 4-5 Hz. For vibration in vertical direction, reading performance is affected to some extent around 25 Hz frequency. The STHT responses have shown non-linear behaviour with respect to vibration magnitude of vertical WBV. II seated postures have been studied under WBV exposure in three independent directions. The experiments were performed on ten healthy male subjects by exposing them to 0.5, 1.0, and 1.5 m/s2 rms vibration magnitudes in the 1-20 Hz frequency range. The vertical backrest support increases the STHT magnitude under the vertical and fore-&-aft vibrations exposure at all levels of vibration magnitude. The nonlinear effects of vibration magnitude on STHT responses are found under the vertical and fore-&-aft vibration exposure. A force plate model has been designed and analysed by performing the modal, stress and strain analysis using finite element analysis. The results of modal, stress and strain analysis were under the limits prescribed for the biodynamic studies of under WBV exposure. The physical model of the force plate was manufactured and used in the experimentation for the evaluation of apparent mass (APMS) responses of the ten recruited subjects standing in an erect posture. The platform was excited with sinusoidal vibration at vibration magnitude: 1.0 and 1.5 m/s2 rms at different frequency of 2, 3, 4, 5, 6, 8, 10, 12.5, 16 and 20 Hz. The primary resonance peaks in the normalised APMS responses have been observed between 4 and 5 Hz frequencies. A secondary peak of lower magnitude was also observed for some subjects. The nonlinearity with respect to vibration magnitude has been also observed in the normalised APMS responses. The vertical STHT and APMS characteristics were measured for the human subjects seated in an upright posture with feet supported on the floor and hands positioned in the lap, exposed to vertical vibration at 1.0 m/s2 vibration magnitude. The mean and associated envelop of maximum and minimum values of magnitude and phase related to STHT and APMS were identified. A biodynamic model for the seated human body under the prescribed conditions has been proposed for which bio-mechanical parameters were estimated to satisfy both the experimentally identified STHT and APMS characteristics. The parameters identification technique employed genetic algorithm for the solution of the function comprising sum of squared magnitude and phase errors related with target values of STHT and APMS. The developed model presents a closer agreement with the target values of magnitude associated with APMS and STHT. The model also provide the resonant frequencies calculated on the basis of both biodynamic response functions very close to that found for seated human body experimentally.
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