LVDTLinear Variable Displacement

Transducers/Transformers

John RamirezDarwin Valenzuela

March 14th, 2007

Outline

Definition and Uses (4) Variety and Type (3) Underlying Principle (4) Manufacturers/Cost (1)

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

Definition – Why use a LVDT? Friction – Free Operation

NO mechanical contact between core and coil (usually) Infinite Mechanical Life

Infinite Resolution Electromagnetic coupling

Limited only by electrical noise Low risk of damage

Most LVDT’s have open bore holes Null Point Repeatability

Zero displacement can be measured Single Axis Sensitivity

Effects of other axes are not felt on the axis of interest Environmentally Robust

Stable/Strong sensors – good for structural engineering tests!!!

Uses Automation Machinery Civil/Structural Engineering Power Generation Manufacturing Metal Stamping/Forming OEM Pulp and Paper Industrial Valves R & D and Tests Automotive Racing

Source:http://www.rdpe.com/ex/tips.htm

LVDT accessories tips

Uses (cont.) Civil/Structural Engineering Examples

Displacement measurement of imbedded concrete anchors tested for tensile, compression, bending strength and crack growth in concrete

Deformation and creep of concrete wall used for retaining wall in large gas pipe installation

Dynamic measurement of fatigue in large structural components used in suspension bridges

Down-hole application: measuring displacement (creep) of bedrock

Type of LVDT’s DC vs. AC Operated

DC Operated Ease of installation Simpler data conditioning Operate from dry cell batteries (remote locations) Lower System Cost

AC Operated Smaller than DC More accurate than DC Operate well at high temperatures

Type of LVDT’s (cont.) Armature Types

Unguided Armature Fits loosely in bore hole LVDT body and armature are separately mounted – must ensure

alignment Frictionless movement Suitability

Short-range high speed applications High number of cycles

Captive (Guided) Armature Restrained and guided by a low-friction bearing assembly Suitability

Longer working range Alignment is a potential problem

Spring Extended Armature Restrained and guided by a low-friction bearing assembly (again!) Internal spring pushes armature to max. extension

Maintains reliable contact with body to be measured Suitability

Static – slow moving application (joint-opening in pavement slabs)

Type of LVDT’s (cont.)Generic Schematic:

Source: http://www.daytronic.com/Products/trans/lvdt/default.htm#UNG

Examples:

LVDT Components

Signal conditioning circuitry

Primary coil

Secondary coil

Secondary coil

Bore shaft

Ferrous core

Source: http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/lvdt_primer.pdf

Cross section of a DC-LVDT

Epoxy encapsulation

Stainless steel end caps

High density glass filled coil forms

Magnetic shielding

Underlying Principle Electromagnetic Induction:

Li

Where: L= inductance

= magnetic flux

= electric currenti

Underlying Principle Electromagnetic Induction:

Primary Coil (RED) is connected to power source Secondary Coils (BLUE) are connected in parallel but with

opposing polarity Primary coil’s magnetic field (BLACK) induces a current in the

secondary coils Ferro-Metallic core (BROWN) manipulates primary’s magnetic

field

Underlying Principle In the null position, the magnetic field generates currents of equal

magnitude in both secondary coils. When the core is moved, there will be more magnetic flux in one

coil than the other resulting in different currents and therefore different voltages

This variation in voltages is linearly proportional to displacement Null position

Displaced

Source: http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/lvdt_primer.pdf

Manufacturers/Cost Manufacturers:

RDP group: http://www.rdpelectrosense.com/displacement/

lvdt/menu-lvdt.htm Macro Sensors:

http://www.macrosensors.com/ms-lvdt_products.html

Honeywell Sensing & Control: http://www.sensotec.com/lvdt.asp Costs:

Model Type Stroke Price

LAT 100-0.5 AC Unguided Armature 0.5 ± inch $270.00

 LD200-10 AC Unguided Armature 0.5 ± inch  $225.00    

LAT 100-1 AC Unguided Armature 1.0 ± inch $305.00

LAT 101-0.5 Spring Return Armature 0.5 ± inch $410.00

LAT 101-1 Spring Return Armature 1.0 ± inch $470.00

LAT 102-0.5 Captive Guided Armature 0.5 ± inch $410.00

LAT 102-1 Captive Guided Armature 1.0 ± inch $440.00

Cited Sources Macro Sensors

http://www.macrosensors.com/ms-lvdt_faq-tutorial.html

Daytronic Corporation http://www.daytronic.com/Products/

trans/lvdt/default.htm RDPE Group

Source:http://www.rdpe.com/ex/tips.htm