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LVDT Linear Variable Displacement Transducers/Transformers

LVDT Linear Variable Displacement Transducers/Transformers. John Ramirez Darwin Valenzuela March 14 th , 2007. Outline. Definition and Uses (4) Variety and Type (3) Underlying Principle (4) Manufacturers/Cost (1). Definition – What is a LVDT?. Electromechanical transducer

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LVDT Linear Variable Displacement Transducers/Transformers

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  1. 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)

  3. 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

  4. 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!!!

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

  6. 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

  7. 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

  8. 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)

  9. Type of LVDT’s (cont.) Generic Schematic: Examples: Source: http://www.daytronic.com/Products/trans/lvdt/default.htm#UNG

  10. LVDT Components Ferrous core Epoxy encapsulation Primary coil Secondary coil Bore shaft Magnetic shielding Stainless steel end caps Secondary coil High density glass filled coil forms Signal conditioning circuitry Cross section of a DC-LVDT Source: http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/lvdt_primer.pdf

  11. Underlying Principle • Electromagnetic Induction:

  12. 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

  13. 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

  14. 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:

  15. 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

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