F
LUIDPOWER
Proportional valveseliminate designheadaches
New design produces consistent flow and gain. Paul GantRich Humason
Clippard Instrument LaboratoryCincinnati, Ohio www.clippard.com
P
Proportional valves are a great alternative whenan application requires more than on-off control,
but not the high precision and expense of
servo valves. As such, proportional valves are widelyused in a multitude of applications, including the control of
gas and air flow from medical
industrial respirators, dialysis machines,
and paint-delivery and semiconductor-manufacturing systems.The beauty of the valves is the ease with which they can varythe rate of flow, with output proportional to input current.
However, most such valves have several major characteristics
that can make precise control difficult, including hysteresis
and variable maximum flow and gain.Hysteresis can be as high as 15%, which can wreak havoc ina closed-loop control system. The second problem ismaximum flow can vary significantly from one unit to another
of the same size and from the same manufacturer. Finally,
gain tends to vary from one unit to another with the current
generation of valves. Gain for a proportional valve is the rate
of flow change in response to a change in valve input current.
Variations in gain result in a large performance envelope for a
given family of valves. This puts the burden on the OEM toadapt its design to compensate for these variations whenperformance of the end product is to be identical from one
machine to another. Design, installation, and testing can be
laborious and expensive when trying to achieve matched
performance levels.Gain varies from valve to valve mainly because ofmechanical and geometrical tolerances in normal The EVPproportional valve hasstandard and manifold mountingoptions, a variety of orifice sizes and
control voltage ranges, and threedifferent electrical
connectors manufacturing processes. Most manufacturers usea chemically-etched or stamped flat spring to
counter-act plunger or armature movement.
Electric current in the valve coil creates a magnetic
force that pulls the armature. Manufacturers tend to
compensate for variations in gain and flow by
making the valve's specification window quite
large. MACHINEDESIGN 72 • JULY10, 2003 www.machinedesign.com