LVDT Linear Variable Displacement Transducers/Transformers

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LVDTLinear Variable Displacement
Transducers/Transformers

• John Ramirez
• Darwin Valenzuela
• March 14th, 2007

2
Outline

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

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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 LVDTs have capabilities to measure up to
  20 inches

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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 LVDTs 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!!!

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Uses

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

LVDT accessories tips
Sourcehttp//www.rdpe.com/ex/tips.htm
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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

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Type of LVDTs

• 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

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Type of LVDTs (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)

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Type of LVDTs (cont.)
Generic Schematic
Examples
Source http//www.daytronic.com/Products/trans/l
vdt/default.htmUNG
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LVDT Components
Ferrous core
Epoxy encapsulation
Primary coil
Secondary coil
Bore shaft
Magnetic shielding
Stainless steel end caps
Secondary coil
Signal conditioning circuitry
High density glass filled coil forms
Cross section of a DC-LVDT
Source http//www.macrosensors.com/lvdt_macro_se
nsors/lvdt_tutorial/lvdt_primer.pdf
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Underlying Principle

• Electromagnetic Induction

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Underlying Principle

• Electromagnetic Induction
• Primary Coil (RED) is connected to power source
• Secondary Coils (BLUE) are connected in parallel
  but with opposing polarity
• Primary coils magnetic field (BLACK) induces a
  current in the secondary coils
• Ferro-Metallic core (BROWN) manipulates primarys
  magnetic field

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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_se
nsors/lvdt_tutorial/lvdt_primer.pdf
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Manufacturers/Cost

• Manufacturers
• RDP group
• http//www.rdpelectrosense.com/displacement/lvdt/m
  enu-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

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Cited Sources

• Macro Sensors
• http//www.macrosensors.com/ms-lvdt_faq-tutorial.h
  tml
• Daytronic Corporation
• http//www.daytronic.com/Products/trans/lvdt/defau
  lt.htm
• RDPE Group
• Sourcehttp//www.rdpe.com/ex/tips.htm