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.     

Sensitivity for pressure difference on the ischial tuberosity

Although research on sitting and sitting postures has been done for decades, the field of the subjective feeling of comfort or discomfort during sitting is still an unexplored field. On the basis of those studies some manufacturers claim enhanced subjective comfort because of pressure-relieving qualities of the seat cushions. The question is: does a relatively small pressure reduction enhance comfort? Before that question can be answered, the sensitivity for pressure differences applied to the skin must first be determined. The aim of this study was therefore, to determine the sensitivity of the ischial tuberosity for pressure difference in a healthy population. For this study five males and five females aged between 19 and 30 years with no exercise-induced muscle ache were selected. The (Δa) was determined for which stimulus (a + Δa) was judged as exceeding stimulus (a) in 50% of the trials. This value was called (Δa_0.5) and was determined with an adapted simple up?–?down method with forced choice. Two different values for (a) were used: (a)?=?13.3?kPa and (a)?=?26.5?kPa and the pressure was applied with two different contact surface with diameters of 10?mm and 20?mm. For (a)?=?26.5?kPa and d?=?10?mm a Δa_0.5?=?2.7?kPa was found. For (a)?=?26.5?kPa and d?=?20?mm a Δa_0.5?=?3.5?kPa was found. For (a)?=?13.3?kPa and d?=?20?mm a Δa_0.5?=?1.9?kPa was found.     

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.     

Driver sitting comfort and discomfort (part II): Relationships with and prediction from interface pressure

Pressure at the driver–seat interface has been used as an objective method to assess seat design, yet existing evidence regarding its efficacy is mixed. The current study examined associations between three subjective ratings (overall, comfort, and discomfort) and 36 measures describing driver–seat interface pressure, and identified pressure level, contact area, and ratio (local to global) variables that could be effectively used to improve subjective responses. Each of 27 participants was involved in six separate driving sessions which included combinations of two seats (from vehicles ranked high and low on overall comfort), two vehicle classes (sedan and SUV), and two driving venues (lab-based and field). Several pressure variables were identified as more effective for assessing sitting comfort and discomfort across a range of individual statures. Based on the results, specific approaches are recommended to improve the sitting experience: (1) lower pressure ratios at the buttocks and higher pressure ratios at the upper and lower back; and (2) balanced pressure between the bilateral buttocks, and between the lower and upper body. Finally, separate analyses supported that human–seat interface pressure was more strongly related with overall and comfort ratings than with discomfort ratings.

Relevance to industry

Several interface pressure variables were identified that showed associations with subjective responses during sitting. Use of these measures is suggested to improve the quality of car seats.     

Predicting passenger seat comfort and discomfort on the basis of human,context and seat characteristics: a literature review

Abstract

This literature review focused on passenger seat comfort and discomfort in a human–product–context interaction. The relationships between anthropometric variables (human level), activities (context level), seat characteristics (product level) and the perception of comfort and discomfort were studied through mediating variables, such as body posture, movement and interface pressure. It is concluded that there are correlations between anthropometric variables and interface pressure variables, and that this relationship is affected by body posture. The results of studies on the correlation between pressure variables and passenger comfort and discomfort are not in line with each other. Only associations were found between the other variables (e.g. activities and seat characteristics). A conceptual model illustrates the results of the review, but relationships could not be quantified due to a lack of statistical evidence and large differences in research set-ups between the reviewed papers.

Practitioner Summary: This literature review set out to quantify the relationships between human, context and seat characteristics, and comfort and discomfort experience of passenger seats, in order to build a predictive model that can support seat designers and purchasers to make informed decisions. However, statistical evidence is lacking from existing literature.     

Driver sitting comfort and discomfort (part I): Use of subjective ratings in discriminating car seats and correspondence among ratings

Several subjective rating schemes were investigated to determine which might be the most effective for use in designing and evaluating car seats, and what relationships exist among these schemes. Participants (n=27) completed short-term driving sessions, in six combinations of seats (from vehicles ranked high and low on overall comfort), vehicle class (sedan and SUV), and driving venue (lab-based and field). Overall ratings were obtained, as well as separate measures of comfort and discomfort of the whole body and local body parts. No association was found between subjective ratings and a publicly available overall vehicle comfort score (J.D. Power and Associates’ Comfort Score), implying that other factors besides sitting comfort/discomfort (and car seats) account for overall vehicle comfort. Other major results were that contemporary car seats appear to best accommodate those of middle stature, that packages/seats of sedans were preferred over those of SUVs, that separate processes appeared to be involved in determining whole body comfort and discomfort, and that ratings of comfort were most effective at differentiating among the car seats. Finally, a scheme for the use of subjective ratings was suggested: discomfort ratings for ensuring basic seat requirements (pain prevention-oriented) and comfort ratings for promoting advanced seat requirements (pleasure promotion-oriented).

Relevance to industry

Evidence regarding the advantages and disadvantages of different subjective rating schemes can facilitate future design and evaluation of automotive seats.     

Definition and determination of the bus oscillatory comfort zones

The paper defines “equal oscillatory comfort zones” as a novel concept in the sphere of the bus vertical dynamics. Oscillatory zones are determined using the original and validated oscillatory model of the intercity bus and comfort criteria according to the international ISO 2631/1997 standard requirements. The bus spatial oscillatory model with 65 degrees of freedom (DOF) was built in the ADAMS/View module of the multibody software package MSC.ADAMS. The model was excited by two different real road surfaces: poor asphalt-concrete and good asphalt-concrete pavements, registered at the speed of 64 km/h and 90 km/h respectively. It was found by simulation that oscillatory zones with different comfort assessments exist in the bus. The most comfortable oscillatory zone is in the middle part of the bus (between the front and the rear axle), whereas the least comfortable oscillatory zone is on the rear bus overhang. For the purpose of the ride comfort harmonization, using Design of Experiments (DOE) analysis, new oscillatory parameters are proposed for passenger seats which do not ensure satisfactory oscillatory comfort level. It is concluded that harmonization of oscillatory comfort for all bus passengers could be achieved for good asphalt-concrete excitation. For the poor road excitation it is possible to achieve significant improvement of comfort, especially for the assistant driver and passengers in the bus rear overhang. On a poor asphalt-concrete pavement, by using the proposed seat oscillatory parameters, the allowed exposure time for vertical whole body vibration would be considerably extended.Relevance to industryOscillatory comfort has a particular importance for users of intercity buses traveling longer distances. Comfort assessment of each bus user and mapping of comfort zones can indicate the individual seat and group of the seats on which the oscillatory comfort is reduced. Proper selection of seat oscillatory parameters can improve users comfort. Results of such an analysis can significantly help bus designers and manufacturers in order to improve and harmonize oscillatory comfort on the whole vehicle platform.     

The use of pressure mapping to assess the comfort of agricultural machinery seats

Interface pressure measurement gives an objective value to human comfort. Prolonged sitting is known to contribute to musculoskeletal disorders. Driving a tractor involves several actions such as steering, operating levers, buttons, brake and clutch pedals, and looking behind to observe and maneuver the machine. These operations affect sitting posture and create a pattern of loading on the structures of the operator's body.The aim of this study was to study barometric mapping at the operator's buttocks-seat interface for comfort evaluation of the agricultural and forestry machine seats. Three different tractor seats (A “low cost”; B “medium cost”; C “high cost”) were tested during ploughing, harrowing and haying operations, by 8 different operators. Two standardized conditions were used, one on a track with ridges and one on an asphalted surface, with driving tests conducted on both. From each test, the following values were obtained: maximum pressure peak (P_max); average pressure value (P_avg); and the average percentage of cells activated by pressures ranging between 50 and 130 g/cm² (NC_50-130), 131–400 g/cm² (NC_131-400) and higher than 400 g/cm² (NC_401-1000). Mean values of P_avg, P_max, NC_131-400, recorded after the two lab tests (on the road and the ridged track) carried out with seat-A tractors were greater (p < 0.05) than those obtained in tests with more comfortable seats (seats B and C). P_avg and P_max mean values recorded after three field tests carried out with A-seat tractors were greater than those obtained in tests with more comfortable seats (B and C). Similarly, the cell activation in the pressure interval 131–400 g/cm² (NC_131-400) in A-seat tests was significantly greater than that of both B and C seats. Based on our findings, it is possible to conclude that the analyzed pressure indexes in this study are useful instruments to describe the characteristics of seat mapping and compare agricultural machine seats as a function of operator's buttocks-seat interface, thus highlighting the comfort rate obtainable in dynamic situations by the operator.SummaryThe sitting posture is one of the main factors that contribute to musculoskeletal disorders. The methodology is based on barometric mapping for comfort evaluation. Tractors seats were tested in three agriculture operations and on two standardized tracks. The study showed indicators to evaluate barometric maps in dynamics conditions.     

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.     

Evaluation of an intelligent seat system

This study was conducted to evaluate the effect of an intelligent seat system, a microprocessor-based interactive seat that automatically adjusts itself to fit a seated individual by making pressure-sensitive adjustments on its own. First, a standard American automobile seat ('baseline' seat) was assessed for comfort. Subjective ratings of comfort, pressure distribution and seated anthropometric measurements were recorded for 20 test subjects. These measurements were recorded while the subjects maintained a simulated driving position in a seat buck. The comfort scale was based on a rating of 1 to 10, with 1 corresponding to 'very poor' and 10 corresponding to 'very good.' Based on a nonlinear, multiple regression model that had been previously developed, the comfort rating of the seat was predicted based on the subjective ratings and the recorded values of 450 pressure measurements from 20 subjects. The predicted comfort value was 7.46 for the baseline seat. Following the baseline assessment, the intelligent seat system was installed into the standard American automobile seat. The objective and subjective assessments were then repeated for 17 subjects and the new predicted comfort rating was 8.06. A t-test performed on the subjective and objective measures indicated that this was a significant improvement in seat comfort. Overall, subjects felt the self-adjusting seat was more customized and more comfortable, providing a better fit.     

Buttock and back pressure distribution tests on seats of mobile agricultural machinery.

Multidisciplinary effort is usually required in the evaluation of comfort problems in the working environment of mobile agricultural machinery workers. Comfort can be partly assessed from the study of the pressure distribution of the human-seat interface. Four combine foam seats and a new air-based seat were compared with regards to static buttocks and back support pressures. Within and between subject variability and the issue of measurement repeatability were addressed. Significant differences were found in the maximum pressure profiles of the four seating systems with seats 2 and 3 performing better than seat I (seat 4 showed no significant difference when compared to the other seats). There is an almost linear relationship between mean pressure and body mass index. In all cases the air seating system performed better with regards to the static pressure gradients.     

Coach design for the Korean high-speed train: a systematic approach to passenger seat design and layout

Proper ergonomic design of a passenger seat and coach layout for a high-speed train is an essential component that is directly related to passenger comfort. In this research, a systematic approach to the design of passenger seats was described and the coach layout which reflected the tradeoff between transportation capacity and passenger comfort was investigated for the Korean high-speed train. As a result, design recommendations and specifications of the passenger seat and its layout were suggested. The whole design process is composed of four stages. A survey and analysis of design requirement was first conducted, which formed the base for designing the first and second class passenger seats. Prototypes were made and evaluated iteratively, and seat arrangement and coach layout were finally obtained. The systematic approach and recommendations suggested in this study are expected to be applicable to the seat design for public transportations and to help modify and redesign existing vehicular seats.     

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.     

Study of human–seat interface pressure distribution under vertical vibration

The influence of whole-body vertical vibration on the dynamic human–seat interface pressure is investigated using a flexible grid of pressure sensors. The ischium pressure and the overall pressure distribution at the human–seat interface are evaluated as functions of the magnitude and frequency of vibration excitation, and seated posture and height. The dynamic pressure at the seat surface is measured under sinusoidal vertical vibration of different magnitudes in the 1–10 Hz frequency range. Two methods based on ischium pressure and ischium force are proposed to study the influence of seat height, posture and characteristics of vibration. The results of the study reveal that the amplitude of dynamic pressure component increases with an increase in the excitation amplitude in almost entire frequency range considered in this study. The dynamic components of both the ischium pressure and the ischium force reveal peaks in the 4 to 5 Hz frequency band, the range of primary resonant frequency of the seated human body in the vertical mode. The mean values of the dynamic ischium pressure and the ischium force remain constant, irrespective of the excitation frequency and amplitude. The magnitudes of mean pressure and force at the human–seat interface, however, are dependent upon the seat height and the subject's posture. The inter-subject variability of the static ischium pressure and effective contact area are presented as functions of the subject weight and subject weight-to-height ratio. It was found that heavy subjects tend to induce low ischium pressure as a result of increased effective contact area.

Relevance to industry

Pressure distribution at the human–seat interface has been found to be an important factor affecting the seating comfort and work efficiency of various workers. The study of human–seat interface pressure distribution under vibration is specifically critical to the comfort, work efficiency and health of vehicle drivers, who are regularly exposed to vibration. The results reported in this paper will be useful to study dynamic response of the interface pressure and design vehicle seats.     

AN ANALYSIS OF SOME FACTORS INFLUENCING SEAT COMFORT

Abstract

This study presents a re-analysis by correlation and factor analysis of data on the relative comfort of six Air Force pilot and crew seats. On the basis of the results obtained, the following conclusions may be drawn:

1. Seats are rated in the same relative order of comfort after only 5 min of sitting time has elapsed as after 4 or more hours of sitting on the seats.

2. People tend to rate the overall comfort of a seat mainly on the basis of the comfort of their backs and buttocks. The comfort of the neck and shoulders plays a secondary role while thigh und leg comfort seems to have little relationship to judgements of the overall comfort of a seat.

It was found that the following seven types of measurements wore measures of the same quantity, Overall Comfort of a, sent:

(a) Actual time a subject is willing to sit in a scat (up to a maximum of seven hours).

(b) Ratings of overall comfort after 5 min of sitting.

(c) Predictions of total time he would be willing to sit in the seat made after 5 min of sitting.

(d)Time of onset of first signs of discomfort.

(e)Ratings of overall comfort after 4 to 7 honrs of sitting.

(f)Comfort of the back.

(g)Comfort of the buttocks.     

Future vehicles: the effect of seat configuration on posture and quality of conversation

Abstract

The percentage of passengers that prefer travelling in groups is increasing. In most vehicles, passengers sit side by side and need to turn their body to be engaged in the conversation with their fellow travellers. However, rotating the body could lead to discomfort which influences conversation quality. The aim of this research is to study the effect of seat configuration on the (dis)comfort experience, conversation quality and posture. Experiments in which participants were asked to talk to each other while sitting at the same distance (1 m) were conducted in four seating arrangements (with seat-belts on), where the angle between the forward directions of two seats were positioned at 0° (side by side), 22.5°, 90° and 120° (almost opposite each other), respectively. Optical tracking has been deployed and the collected data were processed with MatLab^® to acquire postural angles over time. Questionnaires were also used to evaluate the perceived (dis)comfort and the quality of the conversation. Experiment results indicate that the 120° configuration scored the best in the overall comfort and the quality of conversation, but only slightly better than the 90° configuration.

Practitioner summary: Seating side by side is not optimal to have a comfortable conversation with your seatmate. To improve comfort and quality of conversation in future vehicles, we tested four seating arrangements analysing the effect of seat layout on (dis)comfort experience. Statistical analysis of objective and subjective data shows the optimal configuration for a comfortable conversation.

Abbreviation: LPD: localized postural discomfort; PDF: probability density function; OCRA: occupational repetitive action     

Some observations regarding the vibrational environment in child safety seats

A growing issue in the area of vehicular ride comfort is that of child safety seats. Postural, thermal and vibrational comfort considerations are finding their way into child seat design. This paper makes some observations regarding the current state of child safety seat design, then goes on to present the results of vibration tests performed over two road surfaces using two child seats and two children. The vibration levels measured at the interfaces between the children and their seats were found to be higher than the vibration levels between the driver and the driver's seat. Calculated power spectral densities and acceleration transmissibility functions showed that the vibration transmission characteristics of the coupled system consisting of the automobile seat, child seat and child were different from those of the driver/seat system. Whereas, automobile seats normally reduce vibrational disturbances at most frequencies, the child seats tested amplified vibration at most frequencies up to 60 Hz.     

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.