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Magneto-elastic (EM) Sensors

Magneto-elastic (EM) Sensors. Thomas Janicke Spring 2008 CEE498kuc. What are EM sensors used for?. Non-contact measurement of an element’s force and stress Provides real time health monitoring of structural elements Can be installed during construction or retrofitted later on.

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Magneto-elastic (EM) Sensors

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  1. Magneto-elastic (EM) Sensors Thomas Janicke Spring 2008 CEE498kuc

  2. What are EM sensors used for? • Non-contact measurement of an element’s force and stress • Provides real time health monitoring of structural elements • Can be installed during construction or retrofitted later on

  3. How do they work? • In general, EM sensors measure magnetic properties of a structural element • Pulsed or periodic magnetic field is applied by primary coil and retrieved by secondary • The relative permeability of a magneto-elastic element depends on the mechanical (or thermal) stress it is subject to

  4. How do they work? • Change in magnetic flux detects the change in permeability • Through laboratory calibration, correlation between magnetic properties and state of stress is attained

  5. Calibration unit • Stress correlation is very sensitive • Only works for specific size from specific manufacturer • Magnetic and dimensional properties must be exact

  6. Data Acquisition • Each sensor is optimized individually • Multi-location, real-time, wireless system

  7. Current applications… • Bridge cables • Bridge hangars • Post-tensioning cables • Pre-stressing tendons • Temporary construction bracing wires

  8. Typical Products from Smart Structures Overall Min diameter: ~0.20 in Max diameter: ~8.9 in ~0.67 in diameter (inner) ~3.5 in length Tendons ~5.6 in diameter (inner) ~13.5 in length Bridge cables

  9. Nanjing Second Bridge, China • 4000ft long cable stay bridge • 2000ft center span • Prestressed concrete box girders with HDPE coated, grouped twisted strands • EM sensors fabricated and calibrated on-site

  10. Other possibilities… • MDL or magneto-restrictive delay line technology has been proposed as a more sensitive alternative to EM sensors • This along with the magneto-impedance effect have been proposed to measure torque as well • None of these have made it to the market yet for economic reasons

  11. Disadvantages • Only really applicable to ferromagnetic materials in tension • Limits on size • Proven in short term, but still relatively new • Requires a detailed database for specific manufacturer material and fabrication properties

  12. References • http://www.smart-structures.com/ • MAGNETIC EFFECTS IN PHYSICAL SENSOR DESIGN AND DEVELOPMENT, E. Hristoforou, Journal of Optoelectronics and Advanced Materials Vol. 4, No. 2, June 2002, p. 245 – 260, http://joam.infim.ro/JOAM/pdf4_2/Hristoforou.pdf • United States Patent Number 5,297,439

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