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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