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

11. 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

12. 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

13. 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,
United States Patent Number 5,297,439