magneto-elastic (em) sensors thomas janicke spring 2008 cee498kuc
TRANSCRIPT
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Magneto-elastic (EM) Sensors
Thomas Janicke
Spring 2008
CEE498kuc
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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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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
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Change in magnetic flux detects the change in permeability
Through laboratory calibration, correlation between magnetic properties and state of stress is attained
How do they work?
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Calibration unit
Stress correlation is very sensitive Only works for specific size from specific manufacturer Magnetic and dimensional properties must be exact
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Data Acquisition
Each sensor is optimized individually Multi-location, real-time, wireless system
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Current applications…
Bridge cables Bridge hangars Post-tensioning cables Pre-stressing tendons Temporary construction bracing wires
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Typical Products from Smart Structures
~5.6 in diameter (inner)~13.5 in lengthBridge cables
~0.67 in diameter (inner)~3.5 in lengthTendons
OverallMin diameter: ~0.20 inMax diameter: ~8.9 in
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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
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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
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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
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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