The world of Human beings is surrounded by variety of complicated systems and these systems create vibrations. Each human perceive them differently and finds some to be very pleasant and some to be very annoying.
The effect of vibrations on a human body is studied to understand and provide safer working environment. ‘Whole body Vibration’ (WBV) is the term used to represent vibrations experienced by a person at exposed body parts. WBV can adversely affect the comfort, health and the performance of a system and depends upon the magnitude, exposure time and most importantly on the frequency range.
Human response to vibration, Vibration Dose Value (VDV), Seating Comfort, Whole Body Vibration (WBV),
Transmissibility and SEAT Factor
Areas of learning required to understand Whole Body Vibration (WBV)
Above figure shows the interdisciplinary nature of Human body vibration as it demands the knowledge of human biology/anatomy, engineering mechanism involved in Vibration transmission in human body and how human body responds to understand the problem associated with WBV.
Classification of Human Body Vibration over a frequency Range
Vibration exposure to the human body effects different body parts in different ways. These effects can be categorized into motion sickness, whole body vibration and hand transmitted vibration based on the frequency range and area of exposure. Frequency range is important as each human part and organ is sensitive to different frequencies of excitation.
The short term and long term WBV exposure has consequences to occupational health and safety. Severe cases may impart the damages to the musco-skeletal, spinal and gastro-intestinal disorders. In general cases exposure to the WBV will cause irritation, low back pain, fatigue, reduced comfort and poor performance.
Based on the frequency of vibration- displacement, velocity and acceleration will influence the percipience by Human being. For example at lower frequency displacement value is most significant while at higher frequency acceleration is significant.
How to reduce Whole Body Vibration
There are two main strategies to reduce the whole body vibration and its effect-
- Reduce the magnitude of vibration.
- Reduce the duration of exposure to vibration
Vibration magnitude experienced by human is reduced by source modification, isolation at the path and receiver modification (component in touch with human).
In case of forklift operator, whole body vibration exposure can be reduced by
- Reduction at Source: Reducing engine vibrations by optimizing block, cylinder pressure, pre and post injection, etc.
- Reduction at Path: Using isolators on the path and by keeping the natural frequency of path components away from the excitation frequency.
- Reduction at Receiver (component in touch with human): This is achieved by keeping the natural frequencies of the component (steering wheel, seat, floor etc.) away from the engine excitation frequency.
Reduction of time of exposure to vibration is sort of management task keeping operator’s shift for a shorter duration. It involves exhaustive task of routine management of shift and operator.
This strategy of avoiding whole body vibration is effective in helping the operator to work efficiently without discomfort.
The way in which the human experiences the vibration depends upon many factors such as direction of propagation and frequency of vibration, which requires weighting filters to be applied to the measured vibration.
Whole Body Vibration Filters
A frequency weighting filter for WBV is the frequency response function that models the response of the body to the wave phenomenon. For example occupant is 10 times more sensitive to same level vibration at 5 Hz than at 100 Hz so in order to maintain the subjective parity between two measures, weighting is used.
In case of whole body vibration three major weighting filters are used Wk, Wb and Wd.
Wk – Weighting for vertical whole body vibration, Z-axis seated, standing or recumbent person.
Wb – Weighting for vertical whole body vibration , Z-axis seated, standing and recumbent person based ISO 2631.
Wd –Weighting for horizontal whole-body vibration x-axis, y-axis seated, standing or recumbent person based on ISO 2631.
Vibration Dose Value (VDV)- Quantification of whole body Vibration
One of the very important factors in the whole body vibration comfort is the waveform of the signal. Shock waveform causes more discomfort than the other stimulus type of the same frequency weighted RMS level. RMS value fails to consider shock and longer time of exposure events, so to have better assessment factor, vibration dose value is used.
Vibration dose value is the fourth root of the time integral of the fourth power of the weighted acceleration.
An advantage of the VDV is that its level never goes down unlike RMS value which tries to maintain a baseline even for longer duration of the vibration exposure.
Seat Design for Reducing whole body vibration
Seat designers consider transmissibility as one of the important criteria to reduce the transfer of vibration from floor to occupant.Transmissibility is the ratio of response to the input excitation,
- Unweighted acceleration
Transmissibility more than unity means seat is amplifying the floor vibration. In such cases occupant experience the very high discomfort because seat makes occupant to jump higher than input excitation.
Seat effective amplitude transmissibility (SEAT Factor)
However the transmissibility often fails to provide the proper picture at lower and extreme higher frequencies. To rectify such drawbacks SEAT factor is used.
SEAT factor signifies the vibration and shock isolation efficiency of any typical seat. It is the ratio of the vibration at the top of the seat to the base of the seat. SEAT factor consists of three major contributing parameters of:
- Dynamic seat performance viz., Vibration Spectrum,
- Human response frequency weighting.
a(w) - Frequency Weighted Acceleration
Automotive Seat Design Strategies to reduce Whole Body Vibration
Seat plays a vital role in the isolation of the whole body vibration as it is the primary and last point contact between the human and seat structure. Proper selection of a seat contributes largely in terms of reduction in the whole body vibration levels. Seat should be designed in such a way that the driver can drive the vehicle safely and effectively. It is important to know that the seat which is best for static comfort may be worst in case of dynamic comfort. For example, generally if the seat in static case is very soft/ less stiff it feels better; however it may lose its elasticity over the period of time and may make the seat very stiff (it may so happen in driving condition) . This makes seat very uncomfortable.
Above figure shows overall comfort characteristics of two different seats, one being good at static and the other being good for driving case. In first case, static comfort is high and dynamic comfort is less. However in second case, initial static comfort is less but its dynamic comfort is high. It is up to the manufacturers to optimize the comfort for both cases. Seat comfort is a brand image for any automotive manufacturer and thus choosing a correct seat product wisely is a very essential task
Advanced Structures India offers expertise, know-how and ‘state of the art’ capabilities to provide the solution for the WBV problems and is your engineering companion for seat comfort evaluation in both static as well dynamic conditions.
Advanced Structures India Pvt Ltd is an independent automotive product development company based out of Bangalore, India with operations in India, China and US. Above is a blog entry from our engineers about Human Response to Vibration. We can be contacted on firstname.lastname@example.org for business enquiries and email@example.com for open positions.