EFFECT OF WHOLE BODY VIBRATION ON ACTIVITY COMFORT

Abstract

Many passengers utilize their time during train journey to carry out sedentary activities like reading, writing etc. There are, however, several important factors inside the train environments that hamper the performance of such activities. The vibration is a major factor influencing the performance of sedentary activities during the travel. Moreover, passengers during travel adopt different postures to attenuate the vibration in order to perform their activities satisfactorily. The available standards (ISO 2631-4, ENV 12299) do not include the effects of vibrations on passenger activities. Therefore, in order to provide a suitable environment for satisfactory performance of sedentary activities, an assessment of activity discomfort due to whole-body vibration needs deliberation. For better understanding of how whole-body vibration influences the comfort, performance and health, better knowledge about the transmission of vibration through the body is required. Furthermore, to minimize the transmission in the body, it is necessary to have knowledge of adverse effects of vibration. In order to maintain economic viability of a transport vehicle, it is essential that the design engineer has knowledge of the effect of vibration stimuli and posture on passenger comfort. This information needs to be available well in advance of the vehicle design since major alterations in design are expensive and, after the vehicle has been built, these are impracticable. The earlier studies have not considered vibration conditions corresponding to those observed in a rail vehicle, in particular the multi-axis vibration. The previous studies have considered vibration in mono axis only; therefore it is difficult to arrive at any conclusion from the results about various axes for a particular task performed under a specific vibration condition. In view of the above, the objective of the present research work is to investigate the influence of mono, dual and multi-axis whole-body random vibrations on the ability to perform various sedentary activities in two subject postures. To achieve the objective, the study has been broadly classified into three major parts namely, field study on various Indian passenger trains, laboratory study on activity comfort under controlled conditions and the biodynamic response and modeling. A field study is conducted to gain knowledge regarding the various sedentary activities of passenger onboard train and various seated postures that passengers normally adopt while traveling. It was also vital to obtain the passengers' views with respect to their experience of the vibrations and noise in relation to interference with activities during normal train journey. The field study also aims at finding a correlation between train vibration and passenger discomfort for sedentary activities and also establishes the relation of vibration level measured among all three translational axes of motion. Carrying out the study on trains would have been impractical, as there would have been very little experimental control. Therefore, an experimental, laboratory based study was performed using the vibration simulator, developed as a mock-up of passenger rail vehicle. Thirty healthy male subjects participated in the laboratory study. Random vibration stimuli have been excited in various axes in frequency range of 1-20 Hz at magnitudes of 0.4, 0.8 and 1.2 m/s2 rms. Various reading and writing related tasks are assigned to subjects in two sitting postures (Forward lean posture with working material on table and backrest posture with working material on lap) under given vibration stimuli. The vibration levels and subject postures observed in field study have been considered for laboratory experiment. Among all sedentary activities found in field study, only reading and writing related tasks has been considered in the study such as reading word chain, reading epaper, reading Hindi newspaper, sketching, handwriting, writing and typing tasks. The subjective measure using Borg CR10 scale and various objective measures have been used to evaluate the effect of vibration stimuli and posture on perceived difficulty and degradation in performance for performing reading and writing related tasks. A biodynamic response study has been conducted to determine the seat-to-head transmissibility (STHT) in three translational axes of head motion with seat vibration in each mono axes. The study also relates STHT data to measures of discomfort or activity disturbance. Similar vibration magnitudes and postures are chosen in biodynamic response study as observed in activity comfort study. The study also proposed a three degree-of-freedom model that can predict the vertical STHT in backrest posture during vertical excitation. The results from the field study indicated that reading, writing as well as working with laptop computer has been found to be the most frequently indulged sedentary activities for short to medium duration train travel. Writing is the activity, which is found most difficult among all other activities. The study shows that greater influence on activity performance was found with vertical and lateral vibration. The study also reported that the vibration levels measured from floor of passenger compartment has found to be in the range of 0.2 — 0.54 m/s2 rms in ii X-axis; 0.28 — 0.68 m/s2 rms in Y-axis and 0.38 — 1.0 m/s2 rms in Z-axis for all the trains. While comparing the mean vibration levels (rss) in three translational directions, it has been found that the ratio of Y-axis to X-axis and Z-axis to X-axis are about 1.3 and 1.8, respectively. The same ratios have been adopted to excite the vibration platform in dual and multi axis vibration. On the whole, the perceived difficulty in task performance increased with the vibration magnitude for all mono, dual and multi axes vibration in both subject postures. The results of subjective evaluation and performance measures revealed that among the entire mono axes vibrations, all the tasks have been affected to a great extent in Y- and Z-axis vibration and the least in X-axis vibration. Moreover, it has also been observed from both subjective and objective measure that the effect of multi axis vibration was similar to the effect of dual axes vibration (p>0.05) and higher than mono axes vibration (p<0.05). The overall result for subject postures from subjective evaluation and objective methods illustrate that the effect of subject postures was direction dependent. Only for X-axis vibration, both the perceived difficulty and degradation in task performance is found to be higher with backrest posture, however, in all other direction of vibration, both the perceived difficulty and task performance affected more in forward lean posture. The study concluded that the font size for reading activity comfort should have a minimum value of 10 point for Arial font, while for Times New Roman; the font size should not be smaller than 12 point. From the biodynamic response study, it is observed that the human body responds differently to motion in each axis and frequency. The results show that the overall STHT is higher in vertical axis, followed by lateral axis and fore-and-aft axis vibration. For seat vibration in fore-and-axis, the backrest posture is able to guide vibration transmitted to head and arm over a wide range of frequencies. In biodynamic modeling study, a close agreement between the experimental and model value has been observed up to the frequency of 15 Hz. The simulation results revealed that the resonance frequency of both the model and target value is 5 Hz. The goodness-of-fits of the model have been obtained as 94.40% for frequency range of 1-15 Hz. iii