Linearly Variable Differential Transformer - LVDT
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01-11-2010, 04:04 PM

.ppt   LVDT.ppt (Size: 887 KB / Downloads: 271)
Linearly Variable Differential Transformer - LVDT


1831: Michael Faraday discovered current could be induced in a conductor by a magnetic field
U.S. Patent of G.B. Hoadley (1940)
First noted that the LVDT output amplitude varies linear with core displacement

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16-11-2012, 06:14 PM

Linear Variable Differential Transducers

.pptx   LVDT.pptx (Size: 854.68 KB / Downloads: 47)

Definition – What is a LVDT?

Electromechanical transducer
Coupled to any type of object/structure
Converts the rectilinear motion of an object into a corresponding electrical signal
Measures Displacement
Precision of LVDT
Movements as small as a few millionths of an inch
Usually measurements are taken on the order of ±12 inches
Some LVDT’s have capabilities to measure up to ±20 inches

What Is An LVDT?

The coils are wound on a hollow coil formed of thermally stable glass and reinforced by polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor. Whole sensor is enclosed and shielded so that no field extends outside it effects it.
The moving element of the an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil's hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore, with no physical contact required between the core and the coil.

Spring-Extended Armature

Like the captive armature, it has a low-friction bearing assembly
Internal spring to continuously push the armature to its fullest possible extension

Spring-Extended Armature

The spring-extended armature is best suited for slow-moving applications.
Attachment between armature and specimen is not required.


If the core is located midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in windings S1 and S2 respectively, are equal.
At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2), is essentially zero.
If the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2).
Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1).


The diagram shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity.
The phase angle of this AC output voltage, Eout, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown graphically in this diagram.

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