fluctuations such as those caused by thermal expansion. A powered solenoid valve
with a low differential pressure producing
relatively low flow will have a greater internal
temperature rise than the same valve with a
higher pressure differential and higher flow.
The larger resultant flow rates actually cool
the internal components of the valve. At a
given power setting, seals in a valve without
internal thermal compensation will expand
over time and as temperature increases.
As the seal expands, flow is reduced as the
cylindrical area above the orifice decreases
(Figures 2 and 3).
that the orifice is subject to thermal expansion that will affect performance.
Flow is proportional to the cylindrical area
above the orifice (= Π x d x h). This area is
referred to as the effective metering orifice,
and flow stays proportional until the cylindrical area becomes greater than the machined
orifice or other limiting restriction. Valves
that incorporate thermal compensation
features can reduce seal expansion and control the area above the orifice, maintaining
repeatability and minimizing hysteresis.
Closed Loop Feedback Does Not
Solve All Ills
Even in a closed loop system, the thermal
expansion of the valve can be greater than the
ability of the control circuit to correct for it.
In a miniature valve with a relatively large
orifice, such as 0.065 inches, thermal expansion comprises a small percentage of the
total flow. Compensation is usually unnecessary in this instance. In small orifice valves,
however, such as in the 0.003 to 0.030 inch
Figure 2. Valve in open state
Figure 3. Heat swollen elastomer reducing
Internal Thermal Compensation
Internal thermal compensation should be
a foremost requirement when selecting proportional solenoid valves. In typical “spider
spring” poppet-style proportional valves
born of digital valve heritage, published
repeatability and hysteresis numbers are
often incorrectly related to the accuracy of
the machined orifice. Engineers must realize
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