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Efficient valve helps people off their seats
Wheelchair cushion control valve has one moving part, uses less than 1 watt
By Bruce Wiebusch, Contributing Editor -- Design News, November 4, 2002
Collinsville, IL — Seat-Rite is a new type of wheelchair cushion control system from G & M Industries that automatically adjusts air cushion by adding or removing air to prevent wheelchair users from developing debilitating pressure sores.
Seat-Rite's sensors regularly monitor seating problems relating to over- and under-inflation of the cushion. "To help prevent sores, the system keeps the user deeply immersed into their cushion, maintaining low pressure on the skin," says William Graebe, an electrical engineer and the founder of G & M Industries. "And we made low-power consumption a priority in everything we put into the system."
Four D-cell batteries power the Seat-Rite's air pump as well as a microprocessor, a mechanical sensor, a pneumatic sensor, and a 0.67W EV pneumatic valve from Clippard Instrument Lab (Cincinnati, OH). The EV valve releases and holds air in the wheelchair cushion.
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| The spider element weighs approximately 0.25g and moves only 0.007inch when opening and closing the valve. |
Important to the EV valve's low power consumption is its only moving part—the spider element. "The element is not a poppet and it's not a plunger," says Paul Gant, a product manager at Clippard. He explains that the spider travels only 0.007 inch when opening and closing the valve. "It's this minimal travel distance and the low mass of the element that account for the valve's efficient use of electrical energy," says Gant. The mass of the spider is approximately 0.25g.
Low power consumption is also attributed to the flexing of the spider element. It has virtually no sliding friction to its movement, according to Gant. The EV valve has a fixed core above the spider. The fixed core and housing become magnetized when the coil is energized and draw the magnetic element away from the input orifice, which opens the valve. The approximate force to lift the spider element is between 3 and 4 oz.
Gant notes that most other pneumatic valves use a plunger or poppet to open and close the valves, but those approaches require moving a poppet or plunger heavier than 0.25g a distance longer than 0.007 inch. He explains that mass and distance both cost power. "Poppet valves do not require a lot of stroke distance to open, but have more weight, which costs power," explains Gant. The spider moves like a poppet in that the element presses against a seat.
The center of the spider is a disk that contains a molded rubber insert, which blocks the passage of air from the inlet when the spider is in the de-energized condition. On the outside of the disk are "spider legs" that provide the element's flexing action. "If the legs were not etched out, it would take an enormous effort to flex the solid spider," explains Gant.
The flexing force of the spider element, putting force on the input orifice, accomplishes the valve's sealing. The molded-rubber insert in the element, made from Buna, Viton, or other optional materials, helps form the seal. Using 0.67W power, the EV valve controls air up to 100 psi.
Automatically adjusting the cushion helps prevent pressure sores, which can be endemic to paraplegics and other people that sit for extended periods of time, but do not have sensation below their waists.
For more information on EV Valves from Clipped Instrument Lab, enter 540 at www.designnews.com/info.
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