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Linear Variable Differential Transformer ( LVDT ) - A device which provides accurate position
indication throughout the range of valve or mechanical travel
is a linear variable differential transformer (LVDT),
illustrated in Figure F1. Unlike the potentiometer position
indicator, no physical connection to the extension is
required.
Figure F1
The extension valve shaft, or control rod, is
made of a metal suitable for acting as the movable core of a
transformer. Moving the extension between the primary and
secondary windings of a transformer causes the inductance
between the two windings to vary, thereby varying the output
voltage proportional to the position of the valve or control
rod extension. Figure F1 illustrates a valve whose position
is indicated by an LVDT. If the open and shut position is all
that is desired, two small secondary coils could be utilized
at each end of the extension’s travel.
LVDTs are extremely reliable. As a rule,
failures are limited to rare electrical faults which cause
erratic or erroneous indications. An open primary winding
will cause the indication to fail to some predetermined value
equal to zero differential voltage. This normally corresponds
to mid-stroke of the valve. A failure of either secondary
winding will cause the output to indicate either full open or
full closed.
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