magneto-elastic (em) sensors thomas janicke spring 2008 cee498kuc
TRANSCRIPT
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
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
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?
Calibration unit
Stress correlation is very sensitive Only works for specific size from specific manufacturer Magnetic and dimensional properties must be exact
Data Acquisition
Each sensor is optimized individually Multi-location, real-time, wireless system
Current applications…
Bridge cables Bridge hangars Post-tensioning cables Pre-stressing tendons Temporary construction bracing wires
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
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
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
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
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