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LVDT Construction and Principle, Study Guides, Projects, Research of Electronics

This is LVDT construction. At this file you will learn how the LVDT works and the structure of it.

Typology: Study Guides, Projects, Research

2018/2019

Uploaded on 12/15/2019

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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
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The LVDT: construction and principle of operation

Measurement Specialties, Inc. www.meas-spec.com 1-757-766-

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 main 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

The LVDT: construction and principle of operation

Measurement Specialties, Inc. www.meas-spec.com 1-757-766-

When the primary coil is excited with a sine wave voltage (Vin), it generate a variable magnetic field which, concentrated by the core, induces the secondary voltages (also sine waves). While the secondary windings are designed so that the differential output voltage (Va-Vb) is proportional to the core position from null, the phase angle (close to 0 degree or close to 180 degrees depending of direction) determines the direction away from the mechanical zero. The zero is defined as the core position where the phase angle of the (Va-Vb) differential output is 90 degrees.

The differential output between the two secondary outputs (Va-Vb) when the core is at the mechanical zero (or “Null Position”) is called the Null Voltage; as the phase angle at null position is 90 degrees, the Null Voltage is a “quadrature” voltage. This residual voltage is due to the complex nature of the LVDT electrical model, which includes the parasitic capacitances of the windings. This complex nature also explains why the phase angle of (Va-Vb) is not exactly 0 degree or 180 degrees when the core is away from the Null Position.

Primary Excitation

Differential output Va-Vb Direction 2: Out-of-phase with excitation (180 degree)

Differential output Va-Vb Direction 1: In-phase with excitation (0 degree)

LVDT Schematic

LVDT waveforms