VARIABLE OUTPUT DEVICES
Linear Variable Differential Transformers (LVDT)
A device which provides accurate position indication throughout the range of valve or control
rod travel is a linear variable differential transformer (LVDT), illustrated in Figure 6. Unlike the
potentiometer position indicator, no physical connection to the extension is required.
Figure 6 Linear Variable Differential Transformer
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
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 6 illustrates a valve whose
position is indicated by an LVDT.
If the open and shut position is all
secondary coils could be utilized at
each end of the extensions 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
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.