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