3d magnetostatic analysis of magneto-inductive devices for ndt

F. Freschi3d magnetostatic analysis of magneto-inductive devices for ndt8th EMF conference 2009 1

3d magnetostatic analysis of magneto-inductive devices for ndt

Aldo Canova(1)(2), Fabio Freschi(1) and Bruno Vusini(2)

(1) Department of Electrical Engineering, Politecnico di Torino

(2) AMC Instruments s.r.l. – Spin off of Politecnico di Torino

[email protected]

http://www.polito.it/cadema


Outline

introduction

magneto-inductive inspection

localized fault (LF)

loss of metallic area (LMA)

magnetic characterization of ropes

design of magneto-inductive devices

conclusions


magneto inductive inspection

permanent magnets are placed as close as possible to the rope, thus obtaining the rope saturation. defects cause changes in the magnetic circuit

\N S

N S

Reduction in the main flux

Local modification of the flux path

flux changes are revealed by means of flux sensors

reduction of main flux

local modification of flux path


magneto-inductive inspection

magneto inductive instruments are classified depending on the way the flux is measured:

LF (Localized Fault): the leakage flux is measured

LMA (Loss of Metallic Area): the main flux is measured


LF devices

most used technique to identify damages in a metallic rope

Working principle:

Rope defects cause a flaw of the magnetic flux

LF probes measure the radial flux component

☺Very sensitive to the external broken wires

☹ Reduced sensitivity to the internal defects

☹ No quantitative information about damages


LMA devices

defects cause a variation of the rope area

estimation of area decrease is obtained by measuring the main magnetic flux

☺independent form position of broken wires (external or internal)

☺quantitative information

☺suitable to detect gradual changes of the rope section due to corrosion

☹ reduced sensitivity for very closed broken wires

☹ strong influence of external leakage fluxes (end effects)


Rope Profile

LMA

LF

Device length higher than width of loss metallic

area

Narrow gaps

LF and LMA signals


design of magneto-inductive devices

objective:

design a device able to perform both LF and LMA analysis

rope must be saturated but with “medium” permeability in order to avoid flux shunting in LF

problems:

very difficult to perform measurements inside ropes

unusual magnetic characteristic (due to mechanical stresses)

complex rope geometry

a single device suitable for different rope diameters


metallic ropes


design

virtual prototyping

3D non-linear magnetostatic

optimization of permanent magnets width

optimization of detector shape

electronics...


magnetic characteristic


simulation setup

the main performance indicator is the magnetic saturation of the rope which depends from the magnetic flux density at “no magnetic load” conditions (device without rope)

without rope it is possible to provide a comparison between experiments and simulations


simulations vs. measurements

magnetic flux density (T) along device axis (mm) w/o rope


results

measurements under working conditions (with rope) are difficult to perform

a suitable rope prototype consisting on two pieces facing each other has been realized

two lengths of are separated by a small air gap.

hall probe is located in between

The axial component of magnetic flux density is closed to those reached inside the rope under test.


simulations vs. measurements

magnetic flux density (T) along device axis (mm) w rope


simulation: relative permeability


problem

device must be suitable for many rope diameters

reluctance of the magnetic circuit change drastically

hall probes for LMA detection are prone to saturation


solution


signals


detector

Dimensions: 18 cm x 16 cm x

8 cm (without centering

system)

Weight: 7.2 kg (without

centering system)

Rope diameter: from 12 to 26

mm


detector: sensors

LMA and LF sensors are

integrated on the same

electronic board

LF Signal: 20 hall sensor

LMA Signal: 6 hall senso

LF probes

LMA probe

LMA probe


conclusions

the goodness of the magneti-inductive technique is linked to the magnetic behaviour of the rope under test

the magnetic characteristic of the rope which is usually an unknown information

the experimental characterization of ropes puts in evidence that the rope material is far from “soft magnetic material” and requires high magnetic field for reaching the desired saturation level

virtual prototyping allows a fast and reliable optimized design of M-I devices

3d magnetostatic analysis of magneto-inductive devices for ndt

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