Application Note
The LVDT: construction and principle of operation
Measurement Specialties, Inc. www.meas-spec.com 1-757-766-1500
1000 Lucas Way, Hampton, VA 23666 U SA 1 of 3 1-800-745-8008
An LVDT, or Linear Variable Differential Transformer, is a transducer that converts a linear displacement
or position from a mechanical reference (or zero) into a proportional electrical signal containing phase (for
direction) and amplitude information (for distance). The LVDT operation does not require electrical
contact between the moving part (probe or core rod assembly) and the transformer, but rather relies on
electromagnetic coupling; this and the fact that they operate without any built-in electronic circuitry are the
primary reasons why LVDTs have been widely used in applications where long life and high reliability
under severe environments are a required, such Military/Aerospace applications.
The LVDT consists of a primary coil (of magnet wire) wound over the whole length of a non-ferromagnetic
bore liner (or spool tube) or a cylindrical coil form. Two secondary coils are wound on top of the primary
coil for “long stroke” LVDTs (i.e. for actuator m ain RAM) or each side of the primary coil for “Short stroke”
LVDTs (i.e. for electro-hydraulic servo-valve or EHSV). The two secondary windings are typically
connected in “opposite series” (or wound in opposite rotational directions). A ferromagnetic core, which
length is a fraction of the bore liner length, magnetically couples the primary to the secondary winding
turns that are located above the length of the core.
Even though the secondary windings of the long stroke LVDT are shown on top of each other with
insulation between them, on the above cross section, Measurement Specialties actually winds them both
at the same time using dual carriage, computerized winding machines. This method saves manufacturing
time and also creates secondary windings with symmetrical capacitance distribution and therefore allows
meeting customer specifications more easily.
LVDT cross-section, short stroke LVDT cross-section, long stroke
