Wheelchair design and engineering
Wheelchair use is not the same as the use of a regular chair. Able-bodied people, even in desk jobs where they sit all day, do not experience sitting in the same way that wheelchair users do. Able-bodied people get up periodically, if only to use the restroom, go to the coffee machine, or stretch their legs. Further, people with the use and control of their muscles shift their weight while sitting, something wheelchair users may not be able to do. A wheelchair user may be in the same position all day, something with consequences for blood flow, pressure sore formation, and general discomfort. A "bad fit" in a wheelchair can result in breakdown in tissue, urinary tract infection, unneeded atrophy of muscles, and fatigue. The size of the chair is an important factor in matching the equipment with the user. Height, weight, and muscular mobility play a part in determining the optimal seat length, back length, and angle of repose.
Factors that should be considered in matching a user to a wheelchair include the time the person typically spends in the chair every day, and the terrain they typically navigate, and what type of vehicle is used by the person. There are many types of chairs, and the big manufacturers all make many models. Selection criteria includes diagnosis and prognosis of the user, his or her communication ability and cognitive status, whether he or she has problems perceiving surroundings, and whether edema is present.
Wheelchair propulsion efficiencies are typically low, in the range of 2% to 10%. Mark's Standard Handbook of Mechanical Engineering says the top efficiency for a manual chair is at half the maximum muscle force and a quarter of the maximum muscle speed. From an engineering efficient standpoint, manual wheelchair is inefficient. Bicycles or even skateboards are more efficient. Chair users therefore must put up with high physical strain from short-duration peak loads. A report from the Department of Veterans Affairs stated that "crank- and lever-propelled wheelchairs appear to be less straining forms of locomotion."
Here's the VA report on mechanical advantage in wheelchair propulsion (PDF) (Warning: file size is 1.2 MB)
Wheelchair construction materials
Manufacturers have replaced the wood, cast iron and wicker of early wheelchairs with steel, aluminum, rubber and plastic. Modern chairs are therefore much lighter: in fact, some lightweight aluminium transit wheelchairs weigh no more than 18lbs.
Aluminum, however, is less flexible and more brittle than steel. Wheelchair makers must therefore use comparatively large tubing to ensure a chair’s strength.
On the other hand, steel is strong, widely available and reasonably priced. And it need not be too heavy: standard manual wheelchairs made of steel weigh between 20 and 40lbs.
Sports wheelchairs are built to be light, robust and flexible. The best are made from composite materials such as titanium, carbon fiber and chrome alloy. Titanium in particular is a very tough metal but significantly more expensive than steel.
Manual – transit: $99 - $199
Manual – bariatric: $229 - $399
Manual – push rim: $169 - $299
Electric – standard: $1,199 - $3,895
Electric – wheelbase/scooters: $579 - $2,899
What the future holds
The future of wheelchairs is well illustrated by the iBOT, a powered chair made by a company called Independence Technology. As well as being able to balance on two of its four wheels and raise the occupant, the iBOT can go up and down stairs; climb five inch curbs; tackle rough terrain with a four wheel drive feature; and work remotely (for storage and transport).
While the iBOT is currently available, the Pegasus wheelchair, from the Porsche Design Studio, is still at the concept stage. The Pegasus has just two wheels and uses gyroscopic sensors to maintain balance, and like the iBOT, it can raise the occupant to an upright position.
Other researchers - in Australia, China and England - are also reviewing the principles of robotics and applying them to wheelchairs. One idea in development is a sensor that enables a wheelchair to automatically avoid obstacles in its path. Another proposal is a wireless navigation system that plans a wheelchair’s route.
If technology and production techniques can make such features viable, there is little doubt that the wheelchair user of the future will have greater independence than ever before.
Image from http://www.flickr.com/photos/32099449@N00/2275518435
by Flickr member Squiggle. Used under Creative Common License.