ergonomic seat design full report
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17-02-2010, 12:15 PM
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The advancements made in the field of technology have helped the automotive sector in last decade, which has been its key customer. Safety and comfort of the passengers has emerged as one of the prime concerns for the vehicle manufacturers due to the stringent government regulations, which are updated with time.
The paper provides a brief overview of the conditions under which the body is completely comfortable. Hence they have been taken into account while designing the seats. The proper design of seats increases the aesthetics and ergonomics of the vehicle and also add to its ability as a safety feature. The feasibility of implementation of such sophisticated seats in Indian conditions has also been gauged.
The term ergonomics comes from the Greek syllables ERGON which means WORK, AND NOMOS which means LAWS, first appeared in a Polish article published in 1857. The study of human factors did not gain much attention until World War 11. Accidents with military equipments were often blamed on human errors, but the investigations revealed that some were caused by poorly designed controls. The modern discipline ergonomics was born in United Kingdom on July 12, 1949, at a meeting of those interested in human work problems in the British navy. At another meeting held on February, 16, 1950, the term ergonomics was formally adopted for this growing discipline.
Today in the United States, ergonomics professionals belong to Human Factors and Ergonomics Society (HFES), an organization with over 5000 members interested in topics ranging from aging to aerospace to computers. Ergonomic design make consumer products safer, easier to use, and more reliable.
Ergonomics or human factors are the scientific discipline concerned with the understanding of interactions among humans and other elements of a system. i.e. it deals with the scientific study of the relationship between the humans and its environment. It is fitting a job to a worker.
ERGONOMICALLY DESIGNED PRODUCTS
An ergonomically designed toothbrush has a broad handle for easy grip, a bent neck for easier access to back teeth, and a bristle head shaped for better tooth surface contact. Ergonomic design has dramatically changed the interior appearance of automobiles. The steering wheel once a solid awkward disc â€œ is now larger and padded for an easier, more comfortable grip. Its center is removed to improve the driverâ„¢s view of the instruments on the dashboard. Larger, contoured seats, adjustable to suit a variety of body sizes and posture preferences, have replaced the small, upright seats of the early automobiles. Equipped with seatbelts and airbags that prevent the face and neck from snapping backwards in the event of a collision, modern automobiles are not only comfortable but they are also safer. Virtually, all automotive and component manufacturers already recognize ergonomics as an important part of the vehicle design process.
An ergonomically designed chair
APPLICATIONS OF ERGONOMICS
Size and shape
Some years ago, researchers compared the relative positions of the controls on a lathe with the size of an average male worker. It was found that the lathe operator would have to stoop and move from side to side to operate the lathe controls. An Ëœidealâ„¢ sized person to fit the lathe would be just 4.5 feet tall, 2 feet across the shoulders and have an arm span of eight feet
This example epitomizes the shortcoming in design when no account has been taken of the user. People come in all shapes and sizes, and the ergonomist takes this variability into account when influencing the design process.
The branch of ergonomics that deals with human variability in size, shape and strength is called anthropometry.
Vision is usually the primary channel for information, yet systems are often so poorly designed that the user is unable to see the work area clearly. Many workers using computers cannot see their screens because of glare or reflections. Others, doing precise assembly tasks, have insufficient lighting and suffer eyestrain and reduced output as a result.
Sound can be a useful way to provide information, especially for warning signals. However, care must be taken not to overload this sensory channel. A recent airliner had 16 different audio warnings, far too many for a pilot to deal with in an emergency situation. A more sensible approach was to have just a few audio signals to alert the pilot to get information guidance from a visual display.
One goal of ergonomics is to design jobs to fit people. This means taking account of differences such as size, strength and ability to handle information for a wide range of users. Then the tasks, the workplace and tools are designed around these differences. The benefits are improved efficiency, quality and job satisfaction. The costs of failure include increased error rates and physical fatigue - or worse.
In some industries the impact of human errors can be catastrophic. These include the nuclear and chemical industries, rail and sea transport and aviation, including air traffic control.
When disasters occur, the blame is often laid with the operators, pilots or drivers concerned - and labeled 'human error'. Often though, the errors are caused by poor equipment and system design.
Ergonomists working in these areas pay particular attention to the mental demands on the operators, designing tasks and equipment to minimize the chances of misreading information or operating the wrong controls, for example.
Drivers spend a great deal of time behind the wheel and encounter a wide range of road conditions. Consequently, they are frequently exposed to shocks when their vehicles encounter irregularities. Shocks are transmitted to the driver when the seat suspension runs out of a travel, a phenomenon called as bottoming and topping. Heavy drivers who adjust the seat height away from the centre of seat travel are at increased risk of bottoming and topping. Researchers generally agree that exposure to shock increases the risk of spinal injury and lower back pain for drivers. Extremely high shock levels, such as those encountered in an accident, can cause compressive fracture of the spine, while chronic exposure to lower levels can lead to disc degeneration and lower back pain. In addition to increased health risks, drivers who experience frequent bottoming and topping report increased levels of fatigue. Topping and bottoming also presents a safety risk, as these events can cause the driver to temporarily lose control of the vehicle as his feet and hands are thrown off the pedals and steering wheel.
PASSIVE SEAT DESIGN
Today, most driver seats have an air â€œ ride suspension and a passive damper to isolate the driver from vibration. The seats are typically designed to isolate the driver from moderate levels of vibration between 4 and 8 Hz, because the human body is most sensitive to seat vibrations in this range. However, seat suspensions designed to effectively isolate moderate vibration at 4 â€œ 8 Hz are too soft to prevent the suspension from bottoming and topping when the vehicle encounters severe road conditions. Although some seat designs employ elastomer snubbers to absorb some of the impact energy of bottoming and topping, snubbers generally do not provide adequate protection for the driver. In addition, when a seat bottoms out, energy is stored in the snubbers and air spring and then released, propelling the seat and driver upward and often causing the suspension to top out. Stiffening the spring and / or damper provides additional protection from bottoming and topping, but at the expense of overall vibration isolation. Thus, passive seat design always sacrifice some degree of either vibration or shock isolation.
In order to design a seat it is necessary to consider the structure of the human body. Various aspects such as seat height, width, depth, backrest and armrest depend on the dimensions of the body. The further discussion aims at understanding the anthropometrics of seat design.
The branch of ergonomics that deals with human variability in size, shape and strength is called anthropometry. Tables of anthropometric data are used by ergonomists to ensure that places and items that they are designing fit the users.
ANTHRAPOMETRIC ASPECTS OF SEAT DESIGN
SEAT HEIGHT ( H )
As the size of the seat height increases, beyond the popliteal height of the user, pressure is felt on the underside of the thighs. The resulting reduction of circulation to lower extremities may lead to Ëœpins and needlesâ„¢, swollen feet and considerable discomfort. As the height decreases the user will (a) tend to flex the spine more (due to the need to achieve an acute angle between thigh and trunk); (b) experience greater problems in standing up and sitting down, due to the distance through which his centre of gravity must move; and © require greater leg room. In general, therefore the optimal seat height for many purposes is close to the popliteal height and where this cannot be achieved a seat that is too slow is preferable to one that is high. If it is necessary to make a seat higher than this, shortening the seat and rounding off its front edge in order to minimize the under â€œ thigh pressure may mitigate the ill effects. It is of overriding importance that the height of a seat should be enough for comfortable driving.
SEAT DEPTH (D)
If the depth is increased beyond the buttock â€œ popliteal length, the user will â€œ not be able to engage the backrest effectively without unacceptable pressure on the backs of the knees. Furthermore, the deeper the seat, the greater are the problems of standing up and sitting down. The lower limit of seat depth is less easy to define. As little as 300 mm will still support the ischial tuberosities and may well be satisfactory in some circumstances. Tall people some â€œ times complain that the seats of easy chairs are too short â€œ an inadequate backrest may well be to blame.
For purposes of support a width that is some 25 mm less on either side than the maximum breadth of the hips is all that is required â€œ hence 350 mm will be adequate. However, clearance between armrests must be adequate for the largest user. In practice, allowing for clothing and leeway, a minimum of 500 mm is required .
BACKREST DIMENSIONS ©
The higher the backrest, the more effective it will be in supporting the weight of the trunk. This is always desirable but in some circumstances other requirements such as the mobility of the shoulders may be more important. We may distinguish three varieties of backrest, each of which may be appropriate under certain circumstances : the low â€œ level backrest; the medium â€œ level backrest and the high â€œ level backrest.
The low â€œ level backrest provides support for the lumbar and lower thoracic region only and finishes below the level of the shoulder blades, thus allowing freedom of movement for shoulder and arms. e.g. Old â€œ fashioned typists chairs generally had low â€œ level backrests. To support the lower back and leave the shoulder regions free, an overall backrest height © of about 400 mm is required.
The medium â€œ level backrest also supports the upper back and shoulder regions. Most modern seats fall into this category. For support to mid â€œ thoracic level an overall backrest height of about 500 mm is required and for full shoulder support about 650 mm. A figure of 500 mm is often quoted for office chairs.
The high â€œ level backrest gives full head and neck support â€œ for the 95th percentile man an overall backrest height of 900 mm is required. Whatever its height, it will generally be preferable and sometimes essential for the backrest to be contoured to the shape of the spine, and in particular to give Ëœpositive supportâ„¢ to the lumbar region in the form of a convexity or pad. To achieve this end, the backrest should support you in the same place as you would support yourself with your hands to ease an aching back.
A medium â€œ or high â€œ level backrest should be flat or slightly concave but the contouring of the backrest should in no cases be excessive in fact a curve that is too pronounced is probably worse than no curve at all. It was found that a lumbar pad that protrudes 40 mm from the main plane of the backrest at its maximum point would support the back in a position that approximates to that of normal standing.
BACKREST ANGLE OR RAKE (A)
As the backrest angle increases, a greater proportion of the weight of the trunk is supported â€œ hence the compressive force between the trunk and pelvis is diminished. Furthermore, increasing the angle between trunk and thighs improves lordosis. However, the horizontal component of the compressive force increases. This will tend to drive the buttocks forward out of the seat counteracted by (a) an adequate seat tilt; (b) high â€œ friction upholstery; © muscular effort from the subject. Increased rake also leads to increased difficulty in the stand â€œ up sit â€œ down action. Interaction of these factors, together with a consideration of task demands, will determine the optimal rake, which will commonly be between 100Ã‚Â° and 110Ã‚Â°. A pronounced rake is not compatible with a low â€œ or medium â€œ backrest since the upper parts of the body becomes highly unstable.
SEAT ANGLE OR TILT (B)
A positive seat angle helps the user to maintain good contact with the backrest and helps to counteract any tendency to slide out of the seat. Excessive tilt reduces hip/ trunk angle and ease of standing up and sitting down. For most purposes 5 - 10 is a suitable compromise.
Armrests may give additional postural support and be an aid to standing up and sitting down. Armrests should support the fleshy part of the forearm, but unless very well padded they should not engage the bony parts of the elbow where the highly sensitive unlar nerve is near the surface; a gap of perhaps 100 mm between the armrest and the seatback may, therefore, be desirable. If the chair is to be used with a table the armrest should not limit access, since the armrest should not, in these circumstances, extend more than 350 mm in front of the seat back. An elbow rest hat is somewhat lower than sitting elbow height is probably preferable to one that is higher, if a relaxed posture is to be achieved. An elbow rest 200 â€œ 250 mm above the seat surface is generally considered suitable.
In a variety of sitting workstations the provision of adequate lateral, vertical, and forward legroom are essential if the user is to adopt a satisfactory posture.
Lateral legroom (e.g. the Ëœknee holeâ„¢ of a desk) must give clearance for the thighs and knees.
Requirements will, in some circumstances, be determined by the knee weight of a tall user. Alternatively, thigh clearance above the highest seat position may be more relevant â€œ adding the 95th percentile male popliteal height and thigh thickness gives a figure of 700 mm. Standards quote a minimum of 650mm for a normal seat.
This is most difficult thing to calculate. At knee level clearance is determined by buttock â€œ knee length from the back of a fixed seat. In this case clearance is determined by buttock â€œ knee length minus abdominal depth, which will be around 425 mm for a male who is a 95th percentile in the former and a 5th percentile in the latter. At floor level an additional 150 mm clearance for the feet gives a figure of 795 mm from the seat back or 575 mm from the dashboard. All of these figures are based on the assumption of a 95th percentile male sitting on a seat that is adjusted to approximately his own popliteal height, with his lower legs vertical. If the seat height is in fact lower than this he will certainly wish to stretch his legs forward. A rigorous calculation of the 95th percentile clearance requirements in these circumstances would be complex but an approximate value may be derived as follows.
Consider a person of buttock â€œ popliteal length B, popliteal height P, and foot length F sitting on a seat height H. He stretches out his legs so that his popliteal region is level with the seat surface. The total horizontal distance between buttocks and toes (D) is approximated by
D = B+ (P2 - H2)1/2 + F
(Ignoring the effects of ankle flexion.) Hence, in the extreme case, of a male who is a 95th percentile in the above dimensions, sitting on a seat that is 400mm in height requires a total floor level clearance of around 1190mm from the seat back or 970 mm from the table edge. (if he is also a 5th percentile in abdominal depth). Such a figure is needlessly generous for most purposes; most ergonomic sources quote a minimum clearance value of between 600 and 700 mm from the table edge. (Standards quote minima of 450 mm at the underside of the desktop and 600 mm at floor level and for 150 mm above).
The photograph shows the seat foam sectioned down the centerline. The angle of the seat surface is about 17 from the horizontal with an extra slightly softer area to resist softer slipping of the seat bones. The area behind the seat bones has been carefully designed to provide an upward force on the buttock behind the seat bones to provide extra pelvic support, i.e. to resist backward rolling of the pelvis. To make this part of the foam feel soft, although the foam is hard, there is a gap between the foam and the steel of the seat pan.
If the various synthetic materials available they prefer FLEXIBLE POLYURETHANE FOAM (FPF). The reasons for this selection and the properties of this material have been discussed below.
FLEXIBLE POLYURETHANE FOAM:
Comfort, durability, safety and economy of operation are requirements of every modern mode of transportation. Manufacturers of private and commercial vehicles meet these prerequisites by using flexible polyurethane foam (FPM) in seating systems. It is one of the most versatile manufacturing materials today with proven reliability and flexibility. Polyurethane foam can be formulated to dampen the vibration that causes discomfort for the operator of a vehicle effectively.
Improvements and refinements to the miracle material introduced in the early 1950â„¢s continue to expand FPF use and add valuable benefits within the transportation industry. Protecting the environment by recycling vehicle seats to keep them out of landfills is of paramount importance. Elimination of springs in vehicular seating has helped cut the cost of recovery for recycling by about two â€œ thirds. However, the move to deep, all â€œ foam seating brings challenges as well as progress. New research focuses on FPF varieties that dampen the vibration created by the dynamics of the vehicle and irregularities of the roadbed. Some designers specify that a seat should be highly resilient. Others are much more concerned about vibration â€œ dampening qualities of the seat. The two specifications appear to be in opposition, but methods have been developed control vibration and provide resiliency.
H â€œ POINT AND TRANSMISSIVITY
Transmissivity is the transportation industryâ„¢s term for the amount of vibration transmitted through the seating platform to the driver by the motion of the vehicles.
H â€œ Point is the term used by the industry to identify the height at which the driver has adequate visibility for safety. The H â€œ point is influenced by several situations that may develop in the foam with extended use. The primary influence is creep (settling or compression); which, in turn, is influenced by the amount of work put into the foam as measured by dynamic modulus (a measuring of the dynamic firmness), and dynamic hysteresis (a measuring of the change in dynamic firmness, providing information about the foamâ„¢s ability to maintain original dampening properties).
MECHANICAL EQUIVALENT OF A SEAT
Spring and dashpot models are used to predict foam-cushioning behavior.
ACTIVE SEAT DESIGN:
The active Management System of seat design overcomes the limitations of passive suspensions by sensing changing vibration characteristics and instantly adjusting damping force, providing effective vibration damping and shock protection across a much wider range of road conditions and driver weight than passive systems. This system consists of a controllable damper filled with magnetorheological fluid (MR fluid), a sensor arm that measures the position of the seat suspension, a controller with a programmed algorithm that adjusts the damping force in response to changes in seat position. The Motion system senses suspension position and adjusts damping force 500 times per second.
The system includes a ride mode switch with three settings (firm, medium, and soft) to allow the driver to easily adjust the feel of the ride. Testing performed by vibration experts on a popular on â€œ highway seat model showed that replacing the standard passive shock absorber with Motion Master reduced the Vibration Dose Value and maximum acceleration (or shock) transmitted to a 200 â€œ pound driver by up to 40% and 49%, respectively.
Though the condition of Indian roads is improving, the uncomfortability and danger caused to the passengers is not a hidden fact. In such a scenario the advancements made in the field of automobile design should be included in all the cars introduced on the Indian roads. Though this may increase the cost of production marginally, the aesthetics and ergonomics of the vehicle improve. This also increases the comfort level of the vehicle and provides a sense of satisfaction to the customers. Market research has showed that the purchasing power of an Indian has increased over the past few years. Hence he certainly wouldnâ„¢t mind spending the extra bit on his comfort. Eventually it is the customer who is the king in this liberalized economy and any loss to the customer is loss to the company. Driver comfort and accessibility of the vehicleâ„¢s controls during the carâ„¢s operation maximizes the performance capabilities of the car.
Customers value Ergonomics and they are ready to pay for it
Â¢ motion â€œ master. com
Â¢ pfa. org
Â¢ THE MECHANICAL DESIGN PROCESS a book by DAVID G. ULLMAN
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13-10-2010, 01:10 PM
Definition: The study of fitting the task to the
Goal: Prevent musculoskeletal disorders by
surveying the workplace, take preventive steps,
relying on employee input and addressing problems
Musculoskeletal Disorders (MSDs)
Recognizing signs and symptoms of MSDs and how
Common Causes of MSDs
Safe Lifting Principles
MSD Injuries in general
Caused by overuse or repetition involving:
For more information about this article,please follow the link:
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04-02-2011, 06:38 PM
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