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

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

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 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

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