RSS Feeds
Molded Valves Pressurize Pulsating Mattress
Design handles high-flow, low-pressure air in a system that prevents and treats bed sores
By Robert N. Boggs, Southern Technical Editor -- Design News, April 9, 1995
San Antonio, TX-- The Dynapulse™ mattress is a portable patient support system that fits on either hospital beds or standard beds. The computerized system feeds air from a blower to eight zones that provide varying degrees of support to different parts of a patient's body. Pre-programmed, slow pulsation patterns vary support pressures to combat bed sores. Warm, dry air bleeding from the support cushions passes through the Gore-Tex® cover to help keep the patient dry. This air movement also helps prevent bed sores.
With eight air zones to control, air valves become a critical and potentially expensive portion of the system. Design engineer Felipe Gonzales of KCI needed an inexpensive assembly of eight, three-way, proportional valves to provide low air pressure and high flow.
An eight-valve block, molded of acetal resin, includes the inlet plenum and the eight outlets. Molded-in passages conduct air to and from the eight valve bores. Each valve assembly includes an acetal spool with acme threads molded inside and elastomeric seals at each end. An acme screw drives the spool axially in the valve bore. Powering each valve is an inexpensive, 12V, reversible gearmotor.
Early prototypes encountered problems in aligning the gear-motor, valve bore, screw, and spool. When aligned, they worked fine. With imperfect alignment, however, the valve would sometimes bind. Enlarging the screw permitted Gonzales to use a bore larger than the drive shaft. Now, a coupling that extends through the screw connects the gearmotor and the screw. This coupling arrangement accommodates any misalignment.
-
Low-pressure water
-
Natural gas
-
Any high-flow, low-pressure system
The valves operate under a computer's control, with the computer receiving pressure feedback from each element. To maintain pressure, the computer operates the valves to increase or decrease air flow to each segment. As the spool reaches either extreme of travel, the spool begins to turn with the screw rather than advancing. This feature simplifies the design and prevents damage to the mechanism.
"This valve meets all the specs for cost and performance," says Gonzalez. "Solenoids can be expensive, noisy, and usually don't provide proportional flow. Steppers and their controls are expensive, particularly when we already have the computer running the whole system."
Additional details...Contact Felipe Gonzalez, KCI, 3440 E. Houston St., San Antonio, TX 78219, (210) 225-7386.
