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Magneto-elastic (EM) Sensors Thomas Janicke Spring 2008 CEE498kuc.

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Presentation on theme: "Magneto-elastic (EM) Sensors Thomas Janicke Spring 2008 CEE498kuc."— Presentation transcript:

1 Magneto-elastic (EM) Sensors Thomas Janicke Spring 2008 CEE498kuc

2 What are EM sensors used for? Non-contact measurement of an elements 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 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?

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 ~5.6 in diameter (inner) ~13.5 in length Bridge cables ~0.67 in diameter (inner) ~3.5 in length Tendons Overall Min diameter: ~0.20 in Max diameter: ~8.9 in

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 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

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