In-vehicle vibration study of child safety seats

This paper reports experimental measurements of the in-vehicle vibrational behaviour of stage 0&1 child safety seats. Road tests were performed for eight combinations of child, child seat and automobile. Four accelerometers were installed in the vehicles and orientated to measure as closely as possible in the vertical direction; two were attached to the floor and two located at the human interfaces. An SAE pad was placed under the ischial tuberosities of the driver at the seat cushion and a child pad, designed for the purpose of this study, was placed under the child. Four test runs were made over a pave’ (cobblestone) surface for the driver's seat and four for the child seat at both 20?km?h^?1 and 40?km?h^?1. Power spectral densities were determined for all measurement points and acceleration transmissibility functions (ATFs) were estimated from the floor of the vehicle to the human interfaces. The system composed of automobile seat, child seat and child was found to transmit greater vibration than the system composed of automobile seat and driver. The ensemble mean transmissibility in the frequency range from 1 to 60 Hz was found to be 77% for the child seats systems as opposed to 61% for the driver's seats. The acceleration transmissibility for the child seat system was found to be higher than that of the driver's seat at most frequencies above 10 Hz for all eight systems tested. The measured ATFs suggest that the principal whole-body vibration resonance of the children occurred at a mean frequency of 8.5, rather than the 3.5 to 5.0 Hz typically found in the case of seated adults. It can be concluded that current belt-fastened child seats are less effective than the vehicle primary seating systems in attenuating vibrational disturbances. The results also suggest the potential inability of evaluating child comfort by means of existing whole-body vibration standards.     

In-vehicle vibration study of child safety seats

This paper reports experimental measurements of the in-vehicle vibrational behaviour of stage 0&1 child safety seats. Road tests were performed for eight combinations of child, child seat and automobile. Four accelerometers were installed in the vehicles and orientated to measure as closely as possible in the vertical direction; two were attached to the floor and two located at the human interfaces. An SAE pad was placed under the ischial tuberosities of the driver at the seat cushion and a child pad, designed for the purpose of this study, was placed under the child. Four test runs were made over a pave' (cobblestone) surface for the driver's seat and four for the child seat at both 20 km h(-1) and 40 km h(-1). Power spectral densities were determined for all measurement points and acceleration transmissibility functions (ATFs) were estimated from the floor of the vehicle to the human interfaces. The system composed of automobile seat, child seat and child was found to transmit greater vibration than the system composed of automobile seat and driver. The ensemble mean transmissibility in the frequency range from 1 to 60 Hz was found to be 77% for the child seats systems as opposed to 61% for the driver's seats. The acceleration transmissibility for the child seat system was found to be higher than that of the driver's seat at most frequencies above 10 Hz for all eight systems tested. The measured ATFs suggest that the principal whole-body vibration resonance of the children occurred at a mean frequency of 8.5, rather than the 3.5 to 5.0 Hz typically found in the case of seated adults. It can be concluded that current belt-fastened child seats are less effective than the vehicle primary seating systems in attenuating vibrational disturbances. The results also suggest the potential inability of evaluating child comfort by means of existing whole-body vibration standards.     

The effect of posture and seat suspension design on discomfort and back muscle fatigue during simulated truck driving

Several studies have shown a relationship between low-back problems and exposure to seated whole-body vibration. The amount of vibration transmitted to the operator is influenced by the posture of the subject in the vehicle. The aim of this study was to determine whether a truck seat with a gas spring in its suspension is superior to the standard spring seat in slowing the onset of muscle fatigue and reducing the level of discomfort experienced during road vibrations while maintaining typical driving postures. The experiment used a 2 x 3 (2 seats x 3 postures) repeated measures design. It was conducted on six males free from low-back pain. Subject comfort was rated before and directly after exposure to typical vibrations. Muscle fatigue using centre frequency was determined during vibration exposure, and the magnitude and phase of acceleration transfer were calculated from the base plate to the seat pan and from the seat pan to the bite bar. None of comfort, fatigue rate or fatigue average were affected by seat type or seat suspension design in the short term, 10 min vibration exposure. Fatigue and comfort measures could continue to be used to detect postural defects, but the more sensitive measures of seat/driver interactions remain mechanical ones using motion-measuring techniques such as accelerometry and correcting for the heavily damped nature of the system. Until more sophisticated manikins are available the characteristics of vibration-attenuating seats should be confirmed using live humans.     

Product design evaluation model of child car seat using gray relational analysis

This paper attempts to explore a design evaluation system dealing with the ergonomic comfort problems for the child car seat designers. The new product design evaluation (abbreviated as PDE) model is developed through two stages. The first stage is to perform an ergonomic experiment to find out the relations between the comfort and the interface pressures on the child subjects. The second stage is to construct a PDE model by applying the method of gray relational analysis with programming languages. Final results show that the new PDE model is very effective for the car safety seat designers to solve the complex problems of ergonomic comfort. Therefore, the new model can provide designers a scientific method supporting decisions on the new product design of child car seats. Some suggestions by using this model for the reference of future researchers are also provided.     

Haptic Seat Interfaces for Driver Information and Warning Systems

The implementation of haptic interfaces in vehicles has important safety and flexibility implications for lessening visual and auditory overload during driving. The present study aims to design and evaluate haptic interfaces with vehicle seats. Three experiments were conducted by testing a haptic seat in a simulator with a total of 20 participants. The first experiment measured reaction time, subjective satisfaction, and subject workloads of the haptic, visual, and auditory displays for the four signals primarily used by vehicle navigation systems. The second experiment measured reaction time, subjective satisfaction, and subjective workloads of the haptic, auditory, and multimodal (haptic + auditory) displays for the ringing signal used by in-vehicle Bluetooth hands-free systems. The third experiment measured drivers' subjective awareness, urgency, usefulness, and disturbance levels at various vibration intensities and positions for a haptic warning signal used by a driver drowsiness warning system. The results indicated that haptic seat interfaces performed better than visual and auditory interfaces, but the unfamiliarity of the haptic interface caused a lower subjective satisfaction for some criteria. Generally, participants showed high subjective satisfaction levels and low subjective workloads toward haptic seat interfaces. This study provided guidance for implementing haptic seat interfaces and identified the possible benefits of their use. It is expected that haptic seats implemented in vehicles will improve safety and the interaction between driver and vehicle.     

Age-related difference in perceptual responses and interface pressure requirements for driver seat design

Due to typical physiological changes with age, older individuals are likely to have different perceptual responses to and different needs for driver–seat interface design. To assess this, a study was conducted in which a total of 22 younger and older participants completed six short-term driving sessions. Three subjective ratings (comfort, discomfort and overall) were obtained, along with 36 driver–seat interface pressure measures, and were used to assess differences and similarities between the two age groups. For both age groups, localised comfort ratings were more effective at distinguishing between driver seats and workspaces. Older individuals appeared to be less sensitive to discomfort than younger individuals. Across age groups, two distinct processes were used in determining whole-body comfort and discomfort perceptions based on localised comfort/discomfort perceptions. Whole-body discomfort levels were largely affected by lower back discomfort in the younger group versus upper back discomfort in the older group. Four specific pressure measures at several body regions differed between the age groups, suggesting distinct contract pressure requirements and loading patterns among these groups.     

Ergonomics modelling and evaluation of automobile seat comfort

Automobile seats are developed in an iterative manner because subjective feedback, which is usually of questionable quality, drives the design. The time and cost associated with iteration could be justified if the process was guaranteed to produce a comfortable seat. Unfortunately, this is not the case. Current practices are based on the premise that seat system design teams need objective, measurable laboratory standards, which can be linked to subjective perceptions of comfort. Only in this way can predictions be made regarding whether or not a particular design will be viewed by the consumer as comfortable. This type of forecasting ability would effectively improve the efficiency with which automobile seats are designed. In this context, the research reported, developed, and validated a stepwise, multiple linear regression model relating seat interface pressure characteristics, occupant anthropometry, occupant demographics, and perceptions of seat appearance to an overall, subjective comfort index derived from a survey with proven levels of reliability and validity. The model performance statistics were: adjusted r(2)=0.668, standard error of estimate=2.308, F (6, 38)=15.728, p=0.000, and cross-validated r (15)=0.952, p=0.000. From the model, human criteria for seat interface pressure measures were established. These findings could not have been attained without first demonstrating that (1) the data collection protocol for seat interface pressure measurement was repeatable and (2) seat interface pressure measurements can be used to distinguish between seats.     

The application of SEAT values for predicting how compliant seats with backrests influence vibration discomfort

The extent to which a seat can provide useful attenuation of vehicle vibration depends on three factors: the characteristics of the vehicle motion, the vibration transmissibility of the seat, and the sensitivity of the body to vibration. The ‘seat effective amplitude transmissibility’ (i.e., SEAT value) reflects how these three factors vary with the frequency and the direction of vibration so as to predict the vibration isolation efficiency of a seat. The SEAT value is mostly used to select seat cushions or seat suspensions based on the transmission of vertical vibration to the principal supporting surface of a seat. This study investigated the accuracy of SEAT values in predicting how seats with backrests influence the discomfort caused by multiple-input vibration. Twelve male subjects participated in a four-part experiment to determine equivalent comfort contours, the relative discomfort, the location of discomfort, and seat transmissibility with three foam seats and a rigid reference seat at 14 frequencies of vibration in the range 1–20 Hz at magnitudes of vibration from 0.2 to 1.6 ms⁻² r.m.s. The ‘measured seat dynamic discomfort’ (MSDD) was calculated for each foam seat from the ratio of the vibration acceleration required to cause similar discomfort with the foam seat and with the rigid reference seat. Using the frequency weightings in current standards, the SEAT values of each seat were calculated from the ratio of overall ride values with the foam seat to the overall ride values with the rigid reference seat, and compared to the corresponding MSDD at each frequency. The SEAT values provided good predictions of how the foam seats increased vibration discomfort at frequencies around the 4-Hz resonance but reduced vibration discomfort at frequencies greater than about 6.3 Hz, with discrepancies explained by a known limitation of the frequency weightings.     

Effect of lumbar support on seating comfort predicted by a whole human body-seat model

Automobile seat greatly affects the ride comfort of drivers in a prolonged driving. Not only the layout parameters of automobile seats, such as seat height, cushion inclination angle, backrest inclination angle, etc, but also the backrest surface related with lumbar support all affect the seating comfort. The human body-seat system includes the three-dimensional data of body based on anatomy and anthropometry, three-dimensional data of seat and adjustable assembly interaction between body and seat based on human body kinematics. Body height and driving posture are adjusted in POSER software, then the solid model of human skin, skeleton and muscle are created in ANSA software, and the integrated model of body-seat system is created in ABAQUS software. The adjustment of the lumbar support parameters is achieved by setting boundary condition of lumbar support region of seats. The finite element model of human body-seat system is validated by comparison to available literature results. At last the finite element model is applied to analyze the effect of lumbar support parameters of seats on the interaction between body and seat under the action of gravity. The pressure value and distribution, contact area, total force of backrest and intervertebral disc stress are obtained. The result shows that the optimal thickness of seat's lumbar support size for the seating comfort is 10 mm after comprehensive comparison and evaluation.Relevance to industry: This study investigated the effects of lumbar support on seating comfort, and can be used to protect the lumbar health. The modeling and simulation method can be applied for the optimization design of vehicle seats.     

Whole-body vibration: Measurement of horizontal and vertical transmissibility of an agricultural tractor seat

The seats may significantly reduce the exposures levels transmitted to the driver, but the European Directive 2002/44/EC (2002) requires only tests on the damping seat capacity along the vertical direction, whereas nothing is required for the longitudinal and transversal directions.Field tests were carried out using a 93 kW tractor to verify the vibrational comfort values given by seat with pneumatic suspension. The tests were executed with the tractor running on different surfaces, at two different forward speed and tire pressures and with different tractor masses. Three repetition were carried out for each configuration. Accelerations were always measured on both the seat and the cabin platform and the calculations were done using the ISO 2631 standard suggestions. The vibration total values and the acceleration transmissibility along the 3 perpendicular axes were calculated and analysed.Despite different boundary conditions (surface, tire pressure, forward speed and tractor mass distribution), along the Z axis the transmissibility was constantly around 0.7, to confirm that the seat worked well to damp the vertical exposures. Different were the situations for the X and the Y axes. Excluding the asphalt, on the other crossed surfaces high transmissibility values were observed (never less than 1), especially along the X axis.Relevance to industry. This paper describes the vibration transmissibility of an agricultural tractor seat. Tests were carried out with the tractor running on different surfaces and with different configurations. The seat transmissibility along the three orthogonal directions was acquired.Results suggest that the tractor manufacturer should consider, during the machine design, also the rolling and pitching movements, because the seat accelerations along the X and Y axes are influenced by them. The seat manufacturer could reduce the rolling and pitching effects using specific suspension systems along the horizontal and lateral directions.     

Analysis and study of an automobile rear seat by FEM

The aim of this work is to carry out the design of an automobile rear seat and the simulation by finite element method (FEM) on its performance using different standard tests. The design is intended for the rear seats of a four-seat vehicle: two front seats and two rear seats. The rear seat set comprises the right and the left seats. Each of these seats is individual and independent so that it can house only a person and is structurally independent of the other one. Two main parts of this work are: (1) design; and (2) analysis by FEM. We carry out the design of the rear seat of a generic automobile, using the CAD Catia V4 software [P. Carman and P. Tigwell, CATIA Reference Guide, OnWord Press, New York, 1998; F. Karam and C.D. Kleismit, Using CATIA, OnWord Press, New York, 2003]. The seat is composed of two very different parts: (a) framework which is the internal structure of the seat, supporting the weight of the occupants and the seat itself, in order to allow the different positions and to assure the durability and correct operation throughout the vehicle's useful life, and (b) foam rubber whose function is to distribute the load, to provide comfort and to give the most aesthetic possible finish to the seat. During the design we have kept in mind criteria such as resistance, safety, durability, aesthetic and lightness; this last property is necessary to achieve fuel economy and a lower inertia in the event of a collision. The standard rules demand that the automobile seats are able to overcome a series of tests, implying the security in the event of a road accident. In this study the tests were carried out by finite element simulations. The software used was Pam-Crash, a well-known non-linear explicit solver of finite elements. From the computer simulation of these tests, stresses and displacement data are obtained for the seat's framework in order to check the correct execution of the demands of the standard rules, carrying out, if necessary, changes in the design within the allowed limits. In this way the use of this tool makes possible to carry out an optimisation of the geometry, materials, etc. of the product before the construction of the prototype, minimising costs and manufacturers’ development time.     

Automobile seat comfort: occupant preferences vs. anthropometric accommodation

Automobile seat design specifications cannot be established without considering the comfort expectations of the target population. This contention is supported by published literature, which suggests that ergonomics criteria, particularly those related to physiology, do not satisfy consumer comfort. The objective of this paper is to challenge ergonomics criteria related to anthropometry in the same way. In this context, 12 subjects, representing a broad range of body sizes, evaluated five different compact car seats during a short-term seating session. Portions of a reliable and valid survey were used for this purpose. The contour and geometry characteristics of the five seats were quantified and compared to the survey information. Discrepancies were discovered between published anthropometric accommodation criteria and subject-preferred lumbar height, seatback width, cushion length, and cushion width. Based on this finding, it was concluded that automobile seat comfort is a unique science. Ergonomics criteria, while serving as the basis for this science, cannot be applied blindly for they do not ensure comfortable automobile seats.     

A subjective framework for seat comfort based on a heuristic multi criteria decision making technique and anthropometry

Consumer expectations for automobile seat comfort continue to rise. With this said, it is evident that the current automobile seat comfort development process, which is only sporadically successful, needs to change. In this context, there has been growing recognition of the need for establishing theoretical and methodological automobile seat comfort. On the other hand, seat producer need to know the costumer’s required comfort to produce based on their interests. The current research methodologies apply qualitative approaches due to anthropometric specifications. The most significant weakness of these approaches is the inexact extracted inferences. Despite the qualitative nature of the consumer’s preferences there are some methods to transform the qualitative parameters into numerical value which could help seat producer to improve or enhance their products. Nonetheless this approach would help the automobile manufacturer to provide their seats from the best producer regarding to the consumers idea. In this paper, a heuristic multi criteria decision making technique is applied to make consumers preferences in the numeric value. This Technique is combination of Analytical Hierarchy Procedure (AHP), Entropy method, and Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS). A case study is conducted to illustrate the applicability and the effectiveness of the proposed heuristic approach.     

A subjective framework for seat comfort based on a heuristic multi criteria decision making technique and anthropometry

Consumer expectations for automobile seat comfort continue to rise. With this said, it is evident that the current automobile seat comfort development process, which is only sporadically successful, needs to change. In this context, there has been growing recognition of the need for establishing theoretical and methodological automobile seat comfort. On the other hand, seat producer need to know the costumer’s required comfort to produce based on their interests. The current research methodologies apply qualitative approaches due to anthropometric specifications. The most significant weakness of these approaches is the inexact extracted inferences. Despite the qualitative nature of the consumer’s preferences there are some methods to transform the qualitative parameters into numerical value which could help seat producer to improve or enhance their products. Nonetheless this approach would help the automobile manufacturer to provide their seats from the best producer regarding to the consumers idea. In this paper, a heuristic multi criteria decision making technique is applied to make consumers preferences in the numeric value. This Technique is combination of Analytical Hierarchy Procedure (AHP), Entropy method, and Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS). A case study is conducted to illustrate the applicability and the effectiveness of the proposed heuristic approach.     

An ergonomics investigation into human thermal comfort using an automobile seat heated with encapsulated carbonized fabric (ECF).

This report presents the results of an ergonomics investigation into human thermal comfort using an automobile seat heated with an encapsulated carbonized fabric (ECF). Subjective and objective thermal comfort data were recorded while participants sat for 90 min in a heated and a non-heated automobile seat in an environmental chamber. Eight male participants each completed eight experimental sessions in a balanced order repeated measures experimental design. The conditions in the chamber were representative of a range of cool vehicle thermal environments (5, 10, 15 and 20 degrees C; in the 20 degrees C trial participants sat beside a 5 degrees C 'cold wall'). Participants in the heated seat condition used the heating controller with separate temperature control over the back of the seat (squab) and bottom of the seat (cushion) in an effort to maintain their thermal comfort while wearing the provided clothing, which had an estimated insulation value of 0.9 Clo. The trials showed that participants' overall sensations remained higher than 'slightly cool' in the heated seat at all temperatures. Participants' overall discomfort remained lower (i.e. more comfortable) than 'slightly uncomfortable' at temperatures ranging down to nearly 5 degrees C in the heated seat. Hand and foot comfort, sensation and temperature were similar in both seats. Asymmetric torso and thigh skin temperatures were higher in the heated seat although no significant discomfort was found in the front and back of the torso and thigh in either seat. Participants reported no significant difference in alertness between the control and heated seat.     

Ergonomics modelling and evaluation of automobile seat comfort

Automobile seats are developed in an iterative manner because subjective feedback, which is usually of questionable quality, drives the design. The time and cost associated with iteration could be justified if the process was guaranteed to produce a comfortable seat. Unfortunately, this is not the case. Current practices are based on the premise that seat system design teams need objective, measurable laboratory standards, which can be linked to subjective perceptions of comfort. Only in this way can predictions be made regarding whether or not a particular design will be viewed by the consumer as comfortable. This type of forecasting ability would effectively improve the efficiency with which automobile seats are designed. In this context, the research reported, developed, and validated a stepwise, multiple linear regression model relating seat interface pressure characteristics, occupant anthropometry, occupant demographics, and perceptions of seat appearance to an overall, subjective comfort index derived from a survey with proven levels of reliability and validity. The model performance statistics were: adjusted r ²?=?0.668, standard error of estimate?=?2.308, F (6, 38)?=?15.728, p?=?0.000, and cross-validated r (15)?=?0.952, p?=?0.000. From the model, human criteria for seat interface pressure measures were established. These findings could not have been attained without first demonstrating that (1) the data collection protocol for seat interface pressure measurement was repeatable and (2) seat interface pressure measurements can be used to distinguish between seats.     

Vibration attenuation performance of suspension seats for off-road forestry vehicles

Four different types of vertical suspension seats were evaluated in the laboratory and in the field in order to measure their adaptability for attenuating whole-body vibration in log skidders used in the forest industry. Laboratory testing first consisted of determining the static and dynamic characteristics of the seats such as the static stiffness of the cushions and suspension systems and the hysteresis parameters and damping properties of the cushions. The vibration attenuation characteristics of the seats were then measured using a laboratory test rig simulating a driver work station. The influence of amplitude of excitation and the variations in seat height on the vibration attenuation performance of the suspension seats was evaluated for sinusoidal excitations in the frequency range of 0.2–8.0 Hz. The seats were then field tested during normal skidding operations to determine their vertical transmissibility characteristics and to compare the vibration exposure that results from operating a skidder while being equipped with a suspended seat, as opposed to having an unsuspended one. There was generally good agreement between the transmissibility characteristics measured in the laboratory and in the field. The results of vibration transmissibility and exposure are helpful in identifying one of the suspension seats as being the most appropriate for attenuating vertical whole-body vibration on skidders, while conforming at the same time to the basic dimensional characteristics and stability required for safe operation of such vehicles.     

Whole body vibration exposures in forklift operators: comparison of a mechanical and air suspension seat

Using a repeated measures design, this study compared differences in whole body vibration (WBV) exposures when 12 forklift operators drove the same forklift with a mechanical suspension and an air suspension seat. A portable PDA-based WBV data acquisition system collected and analysed time-weighted and raw WBV data per ISO 2631-1 and 2631-5 WBV measurement standards. Tri-axial measurements of weighted vibration (A(w)), crest factor, vibration dose values, time-weighted average-peak, raw (+) peak, raw (-) peak and static compression dose (S(ed)) were compared between seats. There were significant differences in z-axis WBV exposures with the air suspension seat, yielding lower WBV exposures. In addition, there were differences between seats in how they attenuated WBV exposures based on the driver's weight. In the mechanical suspension seat, WBV exposures were weight-dependent, with lighter drivers having higher WBV exposures, whereas with the air suspension seat, the same trends were not as prevalent. STATEMENT OF RELEVANCE: This study contributes to the understanding of how different seat suspensions can influence WBV transmission and how some components of vibration transmission are dependent on the weight of the driver. Additional systematic studies are needed to quantify how various factors can influence WBV exposures.     

Effects of elastic seats on seated body apparent mass responses to vertical whole body vibration

Apparent mass (AM) responses of the body seated with and without a back support on three different elastic seats (flat and contoured polyurethane foam (PUF) and air cushion) and a rigid seat were measured under three levels of vertical vibration (overall rms acceleration: 0.25, 0.50 and 0.75 m/s²) in the 0.5 to 20 Hz range. A pressure-sensing system was used to capture biodynamic force at the occupant-seat interface. The results revealed strong effects of visco-elastic and vibration transmissibility characteristics of seats on AM. The response magnitudes with the relatively stiff air seat were generally higher than those with the PUF seats except at low frequencies. The peak magnitude decreased when sitting condition was changed from no back support to a vertical support; the reduction however was more pronounced with the air seat. Further, a relatively higher frequency shift was evident with soft seat compared with stiff elastic seat with increasing excitation.     

The effects of the duration on the subjective discomfort of a rigid seat and a cushioned automobile seat

The subjective discomfort caused by the seat would affect the judgements of discomfort for the seated subjects. However, there have been few studies concerned with the discomfort on the rigid seat in static states, especially for a relatively long duration. This paper investigated the subjective discomfort caused by a rigid seat and a cushioned automobile seat for an hour. Twelve students (eight males and four females) rated the overall discomfort on a category-ratio scale and the local body discomfort on a 6-point rating scale every 10 min caused by two seats in two separate days. The static discomfort increased with increasing time, and the rigid seat caused greater discomfort than the cushioned seat. The local discomfort on the back dominated on the automobile seat, whereas the local discomfort on the buttock area dominated on the rigid seat. We established the empirical equations to predict the relations between the discomfort and duration of the two types of seats, for benchmark in the future studies on vibration and noise discomfort.