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6 September, 2003 Design Product News

Hydraulics &PneumaticsHydraulics &Pneumatics

Proportional valve design meets control system challenges

EVP Valve Overview - 7116 C

lippard InstrumentLaboratory, long a pioneer inthe miniature pneumaticindustry, has achieved a break- though in proportional valve technology with the introduction ofthe EVPseries proportional valve.Since 1974, Clippardclippard.com has produced the EVseries valve, said to combinesimplicity of design, low power, fastoperation and long life (applicationshave exceeded 3 billion cycles witha single valve), resulting in one ofthe most successful and reliableproducts in its 60+ year history.The heart of the valve is thearmature, which is the valve's onlymoving part. Dubbed the 'spider' because of its flexing 'legs,' it hascontributed to make the EVvalveboth very reliable and versatile. This valve has been used in thousands of applications by OEMs with exacting requirements.Now the spider has again providedthe spark for a new product series for Clippard. By re-engineering the now conventional spider element,an engineering team at Clippard hascreated a valve which provides a variable output flow that is proportional to input current. Theunique and patented design of thenew proportional valve delivers anaccurately positioned spider giving a precise, variable output flow. This provides the OEM design engineer what he needs to solve controlsystem challenges for a wide varietyof applications.When considering the existingstate of proportional control in analytical machines, mass flowcontrollers, and medicalapplications (especially non-invasive blood pressure monitoring,dialysis, and respirators), Clippardsaw some challenges. Valvescurrently on the market can have ahysteresis as high as 15% that can wreak havoc in open- or closed-loop control systems. Valve-to-valve variation in the maximum flow cancause problems, as well. Lastly, thevariation in gain that is typical withthe current generation of valves can cause the design engineer the most headaches.The gain of a proportional valve isthe rate of flow change to valveinput current. Differences in thegain of a given valve cause theperformance envelope to be quitelarge, putting the burden on theOEM to make its equipment adaptto those weaknesses.In order to improve all three ofthese characteristics, while stillproviding a good value, Clippard'sredesign of the proven spiderelement and the sloped ramp that it acts upon needed to achieve a betterresult. As it tackled thesechallenges, one of the unique characteristics of the original EV valve, its factory-adjustable core, proved to have a decisive advantageover existing proportional valves.It gives Clippard the ability to adjust the magnetic field to compensate for the mechanicaltolerance limitations with which allproportional valve manufacturers have had to contend. As a result, theClippard proportional valve has an improved maximum flow variation- as little as ±10% - and also hasmore consistency and linearity of gain. These two improvements should prove to make the OEM design engineers job much easier,due to better controllability.The main cause of varying gainsfrom valve to valve is themechanical and geometrical tolerances that occur in normalmanufacturing processes. Mostmanufacturers use a chemically-etched or stamped flat spring toresist plunger or armaturemovement. The spring, inconjunction with the surface itmakes contact with, is used to create the proportionality between flow rate and valve coil current. As electrical current is increased thearmature is subjected to a magneticfield. The magnetic field creates a magnetic force on the armature that pulls on it. The flat spring resists this travel until the magnetic forceon the armature equals the force thatthe flat spring subjects on thearmature.The flat spring is the criticalcomponent in maintainingconsistent gains from valve tovalve. But typical tolerances for flatsprings can cause changes in springrate. For example, the spring rate offlat springs is inversely proportionalto the thickness of the material it ismade of to the third power. Whichmeans that if manufacturer'stolerances vary by 5% in thickness,a spring rate fluctuation of 35%could exist. Therefore, large swingsin gains will exist when there isonly slight variations in flat spring thickness.Clippard's unique designincorporates a low mass element, properly selected materials and virtually no friction. These designelements are combined to produce avalve that is highly responsive, has a long life and exhibits a hysteresis of less than 10%. This means amuch improved platform for use inopen- and closed- loop controlsystems.Ventilators, anesthesia machines,non-invasive blood pressure monitoring, gas chromatographequipment, and a host of othercontrol applications are suitable forthe Clippard valve. In addition,applications from paint delivery systems and flow controllers tosemiconductor CMPandenvironmental analysis can use theEVPproportional valve.Technically speaking, this valvecontrols flow – but add a pressuresensor and comparator circuit, andyou have an electronic pressure controller.

Core plateCore plate crimpedinto valve housingControl voltageranges include upto 5, 10 or 20 VValve body Adjusting ring maybe loosened for positioning toorient connectionsValve baseArmaturePatented designuses singleinternal movingpartStandard orifices are 0.009,0.013, 0.025, 0.040and 0.060 in. Sectional view of the EVPproportional valve: By press fitting thecore closer to the armature, Clippard has increased the magnetic field to create more pull on the armature. Theresult is a tolerance of ±10% ofmaximum flow or better
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The EVPproportional valve isoffered with standard and manifold mounting options, avariety of orifice sizes andcontrol voltage ranges, and three difference electrical connector options.

Above: Two typical valve performancecurves of a competitor's valve. To compensate for the extreme variations ofvalve gains, the specification window has been made quite large. Since thegains from valve to valve vary, so doesthe maximum flow of the valve. Below: Atypical Clippard proportional valvehysteresis curve. Arelatively consistentlinear gain over the entire flow range ismuch better than the typical "S" shaped gain offered in the market now.