汽车座椅舒适性：乘客偏好与人体测量调节【中文7310字】
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中文7310字
汽车座椅舒适性:乘客偏好与人体测量调节【中文7310字】
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Applied Ergonomics 34 (2003) 177184 Automobile seat comfort: occupant preferences vs. anthropometric accommodation Mike Kolich* Department of Industrial accepted 28 September 2002 Abstract Automobile seat design specifi cations 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 fi ve 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 fi ve seats were quantifi ed 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 fi nding, 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. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Automobile seat; Comfort; Anthropometry 1. Introduction The ergonomics of seat comfort has been studied from a number of different perspectives (Zhang et al., 1996; Yamazaki, 1992). As a generalization, the current practiceistodesignautomobileseatstosatisfy ergonomicscriteria(synonymouswithergonomics guidelines). This approach is assumed to translate into positive consumer comfort ratings. For the purposes of this paper, there are two categories of ergonomics criteria. They are physiological and anthropometric. The physiological factors, which deal with muscles, vertebral discs, joints, and skin, have traditionally been quantifi ed using electromyography (Bush et al., 1995; Lee and Ferraiuolo, 1993; Sheridan et al., 1991), disc pressure measurement (Andersson et al., 1974), vibra- tion transmissibility (Ebe and Griffi n, 2000), pressure distribution at the occupantseat interface (Kamijo et al., 1982; Hertzberg, 1972), and microclimate at the occupantseatinterface(Diebschlagetal.,1988). Ergonomics criteria related to physiology have, how- ever, come under scrutiny, particularly in the past decade. Reed et al. (1991), for example, described the automobile seat designers dilemma as the need for a balance between prescribing a physiologically appro- priate seated posture and accommodating a driver in a preferred posture. They reasoned that prescribed pos- tures sometimes compromise long-term comfort. Later, Reed et al. (1995), based on their preliminary data, highlighted the incompatibility between the traditional practice of designing automobile seatbacks to induce a large degree of lumbar lordosis (which is, according to Andersson et al., 1974, appropriate from a physiological perspective and the ideal of satisfying occupant-selected spinal confi gurations (which, for some occupants, are more kyphotic). Reed and Schneider (1996) verifi ed this incompatibility in a follow-up study. Kolich et al. (2000), in the context of their investigation, came to a similar conclusion. These investigations all suggest that the human body has a great plasticity to adapt to a large variety of sitting conditions. For this reason, ergonomics *Corresponding author. Automotive Systems Group, Johnson Controls Inc., 49200 Halyard Drive, Plymouth, MI 48170, USA.Tel.: +1-734-254-5911; fax: +1-734-254-6277. E-mail address: michael.kolich (M. Kolich). 0003-6870/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. PII: S 0003 -6870(02 )0 0142-4 criteria based on physiology, because they do not ensure comfort, may unnecessarily limit automobile seat de- sign. Due in large part to Akerbloms (1948) work, ergonomics criteria related to anthropometry have long been considered a key aspect of comfortable seating. From this perspective, designers must ensure that a range of people, from small to large, fi t in the seat. In general, automobile seat designs are specifi ed by noting, for a target population, the constraining values of appropriate anthropometric dimensions (usually 5th percentile female and 95th percentile male). Comfortable accommodation in the lumbar region is best achieved through adjustability. This is, in the context of most applications, often impractical, due to the associated cost. According to Reed et al. (1994), the apex of the lumbar contour should be positioned between 105 and150mm from H-Point. As an aside, in the automotive seating industry, many anthropometric dimensions are referenced from H-Point, which is based on the hip point of a manikin that represents how medium-sized men sit in, and interact with, different vehicle seats and vehicle environments (Society of Automotive Engineers, 1995). This aforementioned range is thought to capture the L3 joint level for both small females and large males in the sitting posture. In the upper seatback (at approximately chest height), the minimum width should support the chest breadth of a large male when reclining. The interscye distance, measured across the back between the posterior axillary folds,isanappropriateanthropometricreference measurement. According to Reed et al. (1994), 471mm should accommodate the 95th percentile male interscye distance. Failure to satisfy this criterion may compro- mise seatback lateral support. Cushion length is an important determinant of thigh support. A cushion that is too long can put pressure on the posterior portion of the occupants legs near the knee. Pressure in this area will lead to local discomfort and restricted blood fl ow to the legs (Reed et al., 1994). Cushionlengthisconstrainedbythebuttock-to- popliteal length of the 5th percentile female segment of the population. This dimension is measured on the seated occupant from the rearmost projection of the buttocks to the popliteal fold at the back of the knee. Gordon et al. (1989) reported a 5th percentile female buttock-to-popliteal length of 440mm. This equates to approximately 305mm from H-Point. This dimension/ criterion is a maximum. In the case of cushion width, the 95th percentile female sitting hip breadth is used as a specifi cation limit, since this measure exceeds the 95th percentile male sitting hip breadth. Using the principle of anthropo- metric accommodation, the minimum cushion width must be greater than the 95th percentile female sitting hip breadth of 432mm (Gordon et al., 1989). However, a larger minimum cushion width is required, mainly because the cited anthropometric measurement does not include a margin for clothing (an automobile seat must generally be suitable for use in cold climates where heavy clothing is worn). Reed et al. (1994) believe that automobile seats should provide a clearance of 500mm at the hips. This characteristic affects cushion lateral support. Subjective perceptions of comfort must be quantifi ed before they can be compared to ergonomics criteria related to anthropometry. In the automotive seating industry, structured surveys are commonly used for this purpose. The lack of emphasis on seat comfort survey design (exceptions include Reed et al., 1991; Shen and Parsons, 1997; Kolich, 1999) is surprising given (1) the extent to which seat comfort development relies on survey data and (2) the fact that many of the problems related to the collection of subjective data have been well known for some time. A good survey is reliable and valid. This involves reducing the survey measures into two components: a true score component and a measurement error compo- nent. A reliable survey item contains little measurement error. It is, however, impossible to directly observe the true score and error components of an actual score on a survey item. Instead, correlation techniques are used to give an estimate of the extent to which the survey item refl ects true score rather than measurement error. Validity refers to whether the number/score obtained from the survey truly refl ects what the researcher intended to measure. Validity is related to, although different than, reliability. A reliable measure provides consistent readings but is not necessarily valid. On the other hand, a measurement is unlikely to be valid unless it is also reliable. In general, reliability is a necessary but not suffi cient condition for validity, with reliability setting the upper bound to the level of validity that one can expect to fi nd in a measure. Important indicators of reliability and validity are testretest reliability, internal consistency, criterion-related validity, construct-related validity, and face validity (Kolich, 1999). Reliability and validity can be assured by considering the following principles: (a) the wording of survey items (Oppenheim, 1966), (b) the number of rating scale categories (Guilford, 1954; Grigg, 1978), (c) the verbal tags associated with the categories (Osgood et al., 1957), and (d) the interest and motivation of the respondent, as a function of survey length. The type of rating scale (i.e. nominal, ordinal, interval, or ratio) must also be considered, since seat comfort surveys are, typically, subjected to some form of quantitative analysis, whether it is a simple frequency count or a more complex statistical treatment (Stevens, 1946; Cozby, 1989). The type of statistical analysis employed is dependent on the manner in which the data were collected. Failure to attend to the quantitative aspects of survey design will M. Kolich / Applied Ergonomics 34 (2003) 177184178 produce results that are, at best, biased and, at worst, totally invalid. In fact, Kolich (1999) believes that the lack of quality subjective data has hindered advances in automobile seat comfort development. While research- ers may be tempted to devise surveys with many items, if reliability and validity are not considered, then there is limited confi dence that can be placed in the results. Kolichs (1999) position is that, in the case of surveys items, more is not always better. 2. Objective This paper, on the basis of subjective data collected using portions of Kolichs (1999) survey and spurred by what the automotive seating industry has realized may be questionable ergonomics criteria related to physiol- ogy, intends to challenge the published ergonomics criteria related to anthropometry. The thought is that design specifi cations developed using anthropometric considerations do not contribute to the production of comfortable seats. That is, consumer expectations of automobile seat comfort are not necessarily satisfi ed through anthropometric accommodation. 3. Method To obtain design data, fi ve 1997 model year vehicles were obtained from rental agencies and the driver seat contours were scanned, while in the actual vehicles, using a portable coordinate measurement machine (CMM), known as a FaroArm (displayed in Fig. 1). The FaroArm had a 3.7m spherical diameter, weighed 7kg, and was, according to the manufacturer, accurate to within 0.18mm. The seats, which were evaluated approximately 1 month apart, were base level i.e. cloth with manual track (2-way) and recliner. The vehicles, each produced by a different manufacturer, were selected from the North American compact car segment. Seats from the same market segment are assumed to have comparable seat heights, which is a primary determinant of occupant package. Owing partly to the difference in seat height and partly to the difference in feature content, seats from different market segments are diffi cult to compare. Allthingsconsidered,theseatswerethought to accurately refl ecttherangefoundinthemarket segment. The reader will note that, in this paper, the seats are distinguished using the letters A through E. The seats were not named because permission was not sought from and therefore granted by the vehicle manufacturers. To fairly compare the contour and geometry char- acteristics, the fi ve seats were similarly set-up. In the automotive seating industry, because seat designs vary, manufacturer-specifi ed design position is the standard way to compare seats. This information could not be obtained for the purposes of this research. As a consequence, a protocol was established to estimate each seats design position. It was as follows: 1. The seatback angle was set to 251 from vertical. 2. The track position was set to full rear. 3. The H-Point manikin (Society of Automotive En- gineers, 1995) was placed in the seat (without weights). 4. The seat was adjusted until the H-Point manikin was adequately positioned in front of the pedals and steering wheel. 5. The H-Point manikin was loaded (i.e. weights were added) according to the standard developed by the Society of Automotive Engineers (1995). 6. In this position, the H-Point to heel point relation- ships and the H-Point manikins critical angles (i.e. torso, hip, knee, and foot) were determined for each seat. Table 1 outlines this information and, by default, defi nes limits that can be considered repre- sentative of the compact car segment. After setting the seat to the estimated design position (shown in Table 1), an alignment was created with the FaroArm. This alignment was used to establish a coordinate system (x; y; and z plane). The coordinate system, in relation to the vehicle, can be visualized in Fig. 2. An XZ plane was used to defi ne the centerline of the seat (i.e. between the inboard and outboard edges of the seat). Two separate YZ planes, one for the seatback and one for the cushion, defi ned the cross car sections.Fig. 1. FaroArm used to scan automobile seats. M. Kolich / Applied Ergonomics 34 (2003) 177184179 For each seat, the seatback plane was rotated to the estimated design position torso angle (refer to Table 1). The cushion plane was not rotated. The minimum distance between points was set to 0.1mm. This, basically, served to fi lter through points and delete redundant data. As part of the actual scanning process, the probe was passed back and forth over the selected plane. Each time the probe passed over the plane a point was digitized. Once enough, data points were collected, AnthroCAMTM(Faro Technologies, Inc., 1998) was used to connect the dots in each of the specifi ed planes. Points were taken to the center of the probe. For this reason, the scan lines, in a post processing operation, were offset by the radius of the probe (i.e. 3mm). Each scan line was offset individually. This was an AutoCAD function (Autodesk, Inc, 1996). In addition to the contour, the H-Point (in estimated design position) was digitized. To perform this task, the H-Point manikin was, once again, placed in the seat. The H-Point was, as part of the analysis, related to some of the seat contour and geometry characteristics. The fi nished scan, an example of which is included in Fig. 3, was then dimensioned to defi ne design para- meters. For this study, cushion width at H-Point (corresponding to hip breadth) and seatback width 300mm superior to H-Point (corresponding to chest height) were measured between the two widest points on the contour scan (Fig. 4which represents a typical cross car section). Cushion length was measured as the Table 1 Compact car limits for H-Point machine angles and H-Point to heel point relationships Seat ASeat BSeat CSeat DSeat EMeanSTD Torso angle (deg)24242423.52423.90.2 Hip angle (deg)96.198969597.396.51.2 Knee angle (deg)129.8131127.5127128128.71.7 Foot angle (deg)87.9858789.587.587.41.6 H-Point to heel pointx (mm)887833868837857856.422.3 H-Point to heel pointz (mm)223246222169243220.630.9 Fig. 2. Coordinate system used for the scanning process (adopted from Society of Automotive Engineers, 1998). Fig. 3. Example of fi nished seat scan (isometric view). M. Kolich / Applied Ergonomics 34 (2003) 177184180 horizontal distance from H-Point to the leading edge of the cushion. The location of the apex of the lumbar contour was measured as the most prominent point on the seatback contour tangent and parallel to the design position torso line. Once identifi ed, a line was drawn through the apex that was perpendicular to the torso line. The height of the apex was measured from this line along the torso line to the H-Point. Cushion length and lumbar height are operationally defi ned in Fig. 5, which represents a typical centerline section. After each seat was scanned, 12 subjects completed the survey shown in Table 2. The survey was designed to assess showroom comfort. While it is acknowledged that short-term evaluations do not capture all aspects of automobile seat comfort the physical properties of foam, for example, change over time, which is probably more important to long-term comfort (i.e. ride quality), the survey was appropriate in the context of this studys purpose. In other words, it was felt that the short-term, subjective data collected as part of the experimental protocol, because they were focused on specifi c aspects of seat contour/geometry, could be used to compare occupantpreferencesandcriteriaassociatedwith anthropometric accommodation. It is also worth stating that reference values for what constitutes just right or uncomfortable (i.e. the verbal qualifi ers associated with the rating scales in this study) were not provided. This was justifi ed by the fact that reference values are not defi ned for consumers presented with an opportu- nity to rate a vehicle/seat in the market place. In this way, the study was thought to refl ect real world comfort ratings. Subjects were allowed to adjust the seat to a comfortable position prior to completing the four survey items. The same 12 subjects evaluated each of the fi ve seats. The number of survey items
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