Anurag Arya Archit Rastogi Ashish Tiwari Anubhav Singh Avishank Singh Ashutosh Yadav Arham Jain LVDT Linear Variable D ifferential Transducers

 Definition  Components  Structure  Types  Operations  Advantages  Uses  Summary CONTENTS

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

LVDT L inear V ariable D ifferential T ransformer  Transformer: AC Input / AC Output  Differential: Natural Null Point in Middle  Variable: Movable Core, Fixed Coil  Linear: Measures Linear Position

LVDT COMPONENTS Signal conditioning circuitry Primary coil Secondary coil Bore shaft Ferromagnetic core Cross section of a LVDT Stainless steel end caps High density glass filled coil forms Magnetic shielding

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. 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. What Is An LVDT?

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. What Is An LVDT?

Types of LVDT’s  Unguided Armature  Captive Armature  Spring-extended Armature

Unguided Armature Measured Object Armature fits loosely in the bore Armature must be attached to the specimen Body must be separately supported & properly aligned Elevation/Cross-Section View

Unguided Armature  There is no wear on the LVDT because no contact is made between armature and bore.  LVDT does not restrict the resolution of measured data (“infinite resolution”). Elevation/Cross-Section View Measured Motion

Captive Armature Body must be separately supported Measured Object Armature must be attached to the specimen Armature is both guided and restrained by a low friction assembly

Captive Armature Advantages compared to unguided armature: ► Better for longer working ranges ► Preferred when misalignment may occur

Spring-Extended Armature Elevation/Cross-Section View Measured Object 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.

OPERATiON  The LVDT's primary winding, P, is energized by a constant amplitude AC source.  The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2. Figure illustrates what happens when the LVDT's core is in different axial positions

OPERATiON  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. Figure illustrates what happens when the LVDT's core is in null positions

 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). OPERATiON Figure illustrates what happens when the LVDT's core is in different axial positions

 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. HOW DOES LVDT WORKS MAGNITUDE OF DIFFERENTIAL AC OUTPUT

HOW DOES LVDT WORKS  The phase angle of this AC output voltage, E out, 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. PHASE ANGLE OF OUTPUT RELATIVE TO PRIMARY  This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry.

 FRICTION FREE  In normal use, there is not any mechanical contact between the LVDT's core and its coil assembly. There is no rubbing, dragging, or other source of friction. This feature is particularly useful in materials testing and vibration displacement measurements.  INFINITE RESOLUTION  Since an LVDT operates by using electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This resolution may be circumscribed by the LVDT signal conditioner’s signal-to-noise ratio. WHY TO USE LVDT

 SINGLE AXIS SENSIVITY  A LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position.  SEPERABLE CORE AND COIL  Because the only interaction between an LVDT's core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. WHY TO USE LVDT

 ENVIRONMENT ROBUST  The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields. WHY TO USE LVDT

Uses  Automation Machinery  Optimize productivity of goods  Civil/Structural Engineering  Measure soil strength  Manufacturing  Control weight & thickness of pills  Metal Stamping/Forming  OEM Original Equipment Manufacturer  Automatic part inspection  Pulp and Paper  R & D and Tests  Power Generation LVDT accessories tips

SUMMARY  LVDT is electromechanical transformer used to measure displacement.  LVDT’s are robust equipment for measuring deflection.  There are three types of LVDT: unguided armature, captive armature, and spring-extended armature.  AC LVDT’s require separate signal conditioning equipment, while DC LVDT’s include signal conditioning equipment on the device.  AC LVDT’s cost less than DC, but the entire measurement system must be considered.

Thank You en.wikipedia.org/wiki/Linear_variable_differential_transformer nees.buffalo.edu/pdfs/lvdt.pdf