N EWS

FLUIDPOWER

BREATHEDEEP: Not just for industrial respirators, Clippard’s EVPvalve can control flowing gaschromatographs and anesthesia machines as well.

PNEUMATICS

Proportional Valve CouldImprove Respirators

Cutting parts count boosts accuracy, life

Designers of ventilators, respirators, and blood pressure monitorscould benefit from a new proportional pneumatic valve manufacturer that reportedly offers greater air flow control accuracy.The new valve, introduced by Clippard Instrument Laboratory, Inc.(Cincinnati, OH; www.clippard.com) and known as the EVP, uses adesign that incorporates only one moving part to improve accuracyand boost its life."We have a maximum flowaccuracy of ±10%," notes PaulGant, International Sales Manager. "And at a minimum,we expect its life to be between200 million and 300 millioncycles," he adds.Clippard executives say theyforesee the new pneumatic valvebeing employed in mass flow controllers, gas chromatographs,and anesthesia machines, in addition to respirators and ventilators. The valve's long lifecould make it particularlydesirable in such applicationswhere human lives could dependon its ability to keep operating.The EVPjoins other small (20-25 l/min flow rate) proportionalpneumatic valves in providing avariable output flow that is proportional to an inputcurrent. The company saysthat its valve differs, however,in its ability to consistently andaccurately meter flow."Valves currently on themarket can have a hysteresis ashigh as 15% that can wreak havoc in a closedloop control system,"Gant says.The company claimsthat such problems canundermine flow accuracy,thus placing a burden on equipment designers tomake their products adaptto those weaknesses.Clippard engineers saythey solved thoseproblems by employing avariation on a time-tested design that the company hasemployed in its other pneumaticvalves. By using a diaphragm-likedevice that the company calls a "spider", Clippard engineers say they were able to create a valvethat offers a variable output flowthat is truly proportional to inputcurrent.The flat, circular "spider"accomplishes that by acting as a spring in the one direction and amagnet in the other. Made from a magnetic material, it is attractedto the force of a magnetic field.As a result, when current isapplied to the electrical coil in the valve, the spider is drawn awayfrom the closed orifice of thevalve When current is removed, the spring force causes the spiderto flex back to its originalposition, thus closing the valve's orifice. "The more current that isapplied to the coil, the more thespider moves away from the closed orifice," Gant explains.The orifice's opening is alwaysproportional to the amount ofcurrent applied. Clippard engineers claim that their newproportional valve's hysteresis isless that 10% of full current over its entire flow regime. Moreover,they say that the use of only onemoving part eliminates sliding friction inside the valve, causing itto be consistently accuratethroughout its useful life."Mechanical friction causesaccuracy problems, which weeliminate because the spider isour only moving part," Gant says. "That's the real benefit of thisvalve: It's accurate throughout its life."

SPIDERINSIDE: Using Clippard’s magnet/spring “spider”accurately proportions valve opening to input current. 48 DESIGNNEWS 09.08.03 [designnews.com]