Linear Variable Differential Transformers (LVDT):

A device that gives accurate position indication by the range of valve or control rod travel is a LVDT (linear variable differential transformer), described in next Figure. Different the potentiometer position indicator, no physical connection to the extension is needed.

The extension valve shaft, or control rod, is created of a metal appropriate for acting as the movable core of a transformer. By Moving the extension among the primary and secondary windings of a transformer causes the inductance among the two windings to vary, thus varying the outcome voltage proportional to the position of the valve or control rod extension. Figure describes a valve whose position is denoted through an LVDT. There are two small secondary coils that could be utilized at every end of the extension's travel if the open and shut position is all which is desired.

LVDTs are extremely reliable. As a regulation, failures are limited to rare electrical faults that cause erroneous or erratic indications. An open primary winding will cause the indication to fail to a few predetermined values equivalent to zero differential voltage. This commonly corresponds to mid-stroke of the valve.  A failure of either secondary winding would cause the outcome to denote either full open or full closed.

Figure: Linear Variable Differential Transformer