Driver Seat Ergonomics

Transcription

1 Driver Seat Ergonomics Lecture delivered by: Kiran V. R. Asst. Prof. Dept. of AAE MSRSAS-Bangalore 1

2 Session Objectives At the end of the module, the delegate would have understood Dimensional factors in seating comfort Seating design pitfalls Seat dimensional data Foot controls Hand controls Forward vision Computerized approaches to ergonomic Measuring seat comfort Seat cushion deflection and ride comfort Seating systems 2

3 Dimensional Factors in Seat Comfort Stress must be given on ergonomic consideration for seat design in car All the requirements of seat design must be incorporated to ensure the comfort of driver and passengers The principal body dimensions for vehicle seat design being shown in Figure It ranges from 5 th percentile woman to 95 th percentile man 3

4 Special consideration should be taken for those, who have, for example, long legs and a short trunk, or vice versa, and do not conform to the averaged percentile norms 4

5 The contrast has also to be resolved between the extra relaxation enjoyed by the seated person and the constriction of certain organs and the restriction of blood flow through the veins, from the lower extremities of the body. There is also greater susceptibility to vibration loads due to the absence of damping characteristics of the legs in an otherwise upright posture. 5

6 The best compromise is obtained when the seated posture minimizes the strain on the human organism imposed by external loads as well as equilibrium between the ligaments and the sections of muscle which act in the opposite sense to them. So a degree of freedom of movement must be provided, holding the body in position 6

7 Sufficient rearward recline of the seat is given to displace the body s CG behind the ischial contact point, so that arm and leg movement is free from unwanted feedback effects at the other extremities This can be provided by obtaining comfort angles through subjective survey 7

8 In the seated position the load imposed by the body on the legs rests on the bones of the pelvis which roles to the rear and initially gives a pleasant relaxation sensation. 8

9 The natural uniform load on the elements of the spine is departed from, with attendant health risks. Back muscles are elongated and the abdominal area is compressed. The forces imposed by the seated occupant must be reacted where the pelvic bone structure projects the farthest, in an area where blood flow through the muscle is least affected. 9

10 Lateral location of the pelvis must also be provided. A wide support area should be provided to avoid high pressure areas. Cushion compression effects must be taken into account when transferring the anthropometric data into seat dimensions 10

11 All-foam cushioning systems may provide the necessary compromise in the spring characteristic distribution over the surface of the seat Resonances are stimulated in relaxed seated persons in the shoulder area, which can affect breathing comfort, also over the entire trunk, leading to pains in the lower body, and in the eyes, leading to vision impairment. 11

12 The seat springing may cause a dynamicmagnifier effect if appropriate damping is not provided. Appropriate foam systems should be used in which static seat compression should be 40% of total spring travel 12

13 Seating design Pitfall The figure shows how the wrong backrest inclination has the potential to cause severe discomfort over a long journey. The muscular interaction with vehicle motions, sensed through the seat, is governed by anticipation of (experience) reaction and by a sense of balance. 13

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15 The interaction becomes a reflex condition, considerably more energy is required with conventional seat systems, to maintain a relationship with the controls, than is necessary to actual control the vehicle The choice of cushion springing and damping characteristics as well as the contouring of the seat and squab profiles and the choice of upholstery trimcloth are equally important for ergonomic comfort. 15

16 Seated occupant with tables of seat dimensional data 16

17 Seat Parameters 17

18 Foot Controls Foot controls are divided between the brakes, needing considerable force, for which free leg movement is required, and the precise control of throttle, say where use of ankle articulation is better suited. 18

19 Critical factors for pedal force and movement are: knee angle in leg operation optimum degree and ankle angle in leg and ankle operation, within degree, obtained from spatial relationship of seat and control Optimum lateral offset of pedals from body midline is 635 mm; at 165mm offset, leg force potential is reduced by 75% 19

20 It is best to site pedal mm above floor level with further 7.6 mm clearance for toes; pedal travel should be restricted to 7.6 mm max before pressure is required. 20

21 Ankle controls, such as throttle, are best with pedals hinged at their base. When pedal is hinged at heel-to-floor contact point, angle E should be increased, to allow for heels, by 5 degrees for men and 10 degrees for women compared with the noshoes condition. For example, E = 45 degrees (55 degrees with shoes) for women and E = 55 degrees (58 degrees with shoes) for men, so an acceptable pedal can be arranged. 21

22 Designing controls for ankle operation starts with the determination, from Table 2, of seat height for population extremes and finding corresponding D in this figure. A pedal angle is selected to lie within allowable range. 22

23 Throttle and brake pedal surfaces are then aligned for quick foot interchange, so the throttle pedal is mounted closer to the SRP than the depressed brake pedal (using smaller knee angles than for the brake pedal when calculating the position). Sufficient throttle spring loading is required to support the foot (22kg) 23

24 One example: When E = 55 degrees to suit the smallest woman on an 280 mm seat, A is 115 degrees for the largest man on a mm seat. If depression is 15 degrees, this gives an unacceptable maximum for A of 130 degrees. The remedy is to increase E to 60 degrees and reduce the depression. 24

25 Hand Controls The relation between the areas of hand reach to the SRP is shown in this figure applying to the seat-belt restrained driver. The large stature person seems unusually badly off in relative terms due to fact that he/she must sit far back to ensure proper use of foot controls. 25

26 Several factors affect steering wheel position; applied hand force depends on column rake, perpendicular distance from SRP to wheel centre and seat back inclination. Maximum force is available when pulling longitudinally with fore-arm and shoulder acting together while faster, but low efficiency operation, motion is obtained with movement across the body. 26

27 A wheel tilted 30 degrees to the vertical could have a rim force 1.7 dynes that with a vertical wheel but the rate of turning is halved and the energy expended reduced considerably. 27

28 A competition car has a near vertical wheel and an extended arm position gives maximum speed for a short-term high stress activity. Heavy industrial vehicles, by contrast, favour the flatter wheel. Fig 9 28

29 The location of wheel centre and inclination for population extremes determine reach envelopes using dimensions in Table 2 and fig 2 & fig 6, 7. Wheel centre must then fall in the common area shown in fig 10 29

30 If the column length is non-adjustable, no position meets population extremes; for a 45 degrees wheel the small area X applies and for a 30 degree wheel the larger area Y is common. If column length is adjustable by 88.9 mm, for this example, then a satisfactory condition exists for both driver extremes anywhere with a 30 degree wheel and for a small range within the 45 degree inclination. 30

31 The figure shows the resulting improved woman s vision when an adjustable column is used 31

32 Forward Vision The SAE eyelipse is a common standard used for optimizing forward vision; this is a contour of eye position boundaries for a range of driver statures. 32

33 Tangents drawn from the contours are constructed to obtain vision areas. The figure shows the method for locating the eyelipse & to locate the plan view eyelipses on a longitudinal line passing through the centre of the driving seat. Since the max sideways viewing angle of the eye is 30 degrees, some geometric modification to the plan view contour is required above this angle. 33

34 34

35 This figure indicates the position of the vision points (V) within the vehicle and it can be seen that a plane in which the V- points are located is required to be positioned 0.4 in forward of the hip reference point (joint articulation centre) for each mm of longitudinal seat travel in excess of mm. 35

36 AMA Measuring System AMA measuring system, has been used widely to determine the relative benefits of vehicle space envelops. Fig 14 36

37 Fig 15 Fig 16 37

38 38

39 Standard manikins are used in the system and critical dimensions defining space and comfort for occupants. In passenger service vehicles, the journey time exerts considerable influence on comfort and thus seating requirement varies for vehicles. AMA. American Medical Association 39

40 Computerized Approach to Ergonomic System Design Ergonomic data is used to optimize occupant-packaging design of vehicles, leading to analysis of the occupant s space envelop within the vehicle. Fig 18 40

41 The static anthropometric measurements, such as functional, task-oriented measurements are taken with the human body at work, in motion, in driver- and passenger-space attitudes. The previous human factors are expressed in three-dimensional co-ordinates around the major reference points. Key points are H (hip), BOF (ball of foot) and AHP (accelerator heel point). 41

42 Fig 19 Fig 20 42

43 Selection of design limits is carried out to determine what percentage of the population is to be accommodated and to arrive at the amount of seat adjustment required. Usually the packaging engineer develops the vehicle from the ground up, the seat height and inclination being variable to vehicle category between, say, sports car (low) and van (high) 43

44 Ground clearance and under frame structure define the floor level which locates accelerator heel point and so on. 44

45 Sight lines are drawn from the eyelipse and roof clearances measured to the headposition contours. SAMMIE-CAD models is strong in its ability to specify logical or functional relationships between components, important for the operation of doors, pedals and adjusters. The designer interacts with the system via a menu-driven interface 45

46 These models are constructed from primitive shapes such as cuboids, spheres, cylinders, meshes and poly-prisms which can be specified interactively, by name and key dimension 46

47 Irregular solids can also be modeled and curves are represented by multi-facets. The man model is constructed from 17 pinjoints and 21 rigid links encased in solid modules of flesh ; link lengths and joint constraints are data-driven and can easily be controlled by the user. 47

48 Measuring Seat Comfort Sensitive manikins are used for measuring seat comfort which are based on a distribution of small load cells over the contacting surface. The figure shows the pressure distribution contour maps. Fig 14 48

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50 This kind of manikins are used in conjunction with special seats whose cushion-springing can be adjusted by means of helical springs distributed across the surface which can be adjusted to different rate settings. An optimum comfort distribution is then reproduced as closely as practically possible, on the manikin. The manikin then can be used to rate different seating systems for comfort level. 50

51 Seat Cushion Deflection And Ride Comfort Seat deflection data is utilized for designing and evaluating automotive seats. Indentation load deflection (ILD), D-point penetration (DPP) and thigh point penetration (TPP) are used for specifying desired cushion deflection characteristics. Such data are used as guidelines during the design and evaluation process, and to meet H-point requirements 51

52 Human Factors engineers utilize deflection data for studying posture, lumber position, occupant feel, eye position and clearance between human body and hardware. The load-deflection data may also be utilize for competitive benchmarking, assessing stiffness, and to estimate seat natural frequencies. Local stiffness is considered to influence occupant feel and driving comfort. 52

53 Seat natural frequencies are crucial for ride comfort and quality since these frequencies should not interfere with vehicle or occupant body natural frequencies. Static deflection of a seat system is one of the objective parameters which can be effectively utilized for improving the quality of seats for comfort, as well as optimizing automotive ride and handling characteristics 53

54 If the seat natural frequency matches with vehicle ride frequencies, the seat may affect vehicle ride handling characteristics, causing squeaks, rattles and high magnitudes of vibration, leading to occupant discomfort. If the seat natural frequency matches with human body frequencies, then the occupant body components may undergo large motions resulting in extreme discomfort 54

55 Automotive seat consists primarily of soft foam, trim and suspension and will deflect when an occupant is seated. Therefore, it is the task of deciding the type of spring characteristics required to accomplish design objectives for maximum comfort. The local stiffness properties of the seat cushion influence the pressure distribution at the cushion-occupant interface and therefore is a significant parameter that influences occupant feel at all times. 55

56 Concept designs such as adjustable lumber and adjustable back essentially provide the occupant with ability to control the desired local stiffness. The spring rate or the stiffness coefficient is an important parameter of subjective feeling of the lower back and trunk muscles that varies with changes in seat surface stiffness. 56

57 Proprietary Seating System The seat include ergonomically shaped upholstery to support the spine; extendable seat cushion to ease stress on the thighs and adjustable height and tilt. 57

58 There are shaped side bolsters on the seat cushion, and backrest, also a variable lumbar support system. The latter comprises air-chambers. Safety features include a cross-member in the seat frame to counteract submarining effect on impact. The seat cushion is mounted in suspension bushes to further isolate vibration and there is a vent system to optimize the control of seat temperature. 58

59 The vent system draws moisture through the seat cover and membrane into airchannels in the upholstery. Fans in the cushion and backrest draw the moisture out of the air channels and out of the seat via slots in the backrest. Optional heating elements are also available. The ventilated seat in a car is a world s first for Saab Automobile and this innovation was implemented in the Saab 9-5 and continues M.S to Ramaiah be School available of Advanced Studies as an - Bangalore option today. 59

60 It offers heated seats as an option, both for the front and rear occupants. But, when the weather is hot and sticky, the Saab 9-5 s optional electrically-ventilated seats are there to deliver maximum comfort. Fig 27 60

61 Two flat electric fans at the back of each front seat and one in the lower cushion, extract warm, humid air that is normally trapped between the person and the seat upholstery. The fans operate in three operator-set speed modes, drawing warm air through the perforated seat leather and venting underneath the seat. 61

62 The heated and ventilated seats contribute to safe driving by keeping the driver more comfortable and alert especially on long trips. This seat can automatically adjusts to the occupants shape and size when sat upon. Active surface control is achieved by electronically actuated pneumatics to control 10 air cells positioned between upholstery and foam springing. 62

63 Sensors on the cells measure pressure between the seat and the occupants back/lower-body, sending signals to microprocessors which store a biomechanical model in their memory, for comparison. Any discrepancy causes individual air cell pressures to be adjusted 63

64 Fig 28 64

65 Suspension Seat For commercial vehicles, there is a strong case for suspension seats in which controlled levels of springing and damping can be introduced. A simplified approach is to regard the driver on his or her seat as a single-degreeof-freedom spring/mass/damper system with the familiar response characteristic 65

66 Cushion design should observe the anatomical necessity of providing firm support only for the bones immediately above the buttocks since the surrounding areas have little pressure capacity. Too deep a cushion deflection would overload the surrounding areas and cause discomfort an acceptable maximum is put at 76 mm. 66

67 Fig 29 67

68 Summary In this session following aspects of driver seat ergonomics were explained Dimensional factors in seating comfort Seating design pitfalls Seat dimensional data Foot controls Hand controls Forward vision Computerized approaches to ergonomics Measuring seat comfort Seat cushion deflection and ride comfort Seating systems & suspension seat 68