3d magnetostatic analysis of magneto-inductive devices for ndt

Transcription

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 1

2 Outline introduction magneto-inductive inspection localized fault (LF) loss of metallic area (LMA) magnetic characterization of ropes design of magneto-inductive devices conclusions 2

3 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 Reduction in the main flux flux changes are revealed by means of flux sensors reduction of main flux local modification of flux path N Local modification of the flux path S 3

4 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 4

5 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 5

6 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) 6

7 Rope Profile LF and LMA signals Narrow gaps LMA Device length higher than width of loss metallic area LF 7

8 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 8

9 metallic ropes 9

10 design virtual prototyping 3D non-linear magnetostatic optimization of permanent magnets width optimization of detector shape electronics... 10

11 magnetic characteristic 11

12 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 12

13 simulations vs. measurements magnetic flux density (T) along device axis (mm) w/o rope 13

14 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. 14

15 simulations vs. measurements magnetic flux density (T) along device axis (mm) w rope 15

16 simulation: relative permeability 16

17 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 17

18 solution 18

19 signals 19

20 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 20

21 detector: sensors LMA and LF sensors are integrated on the same electronic board LF Signal: 20 hall sensor LMA Signal: 6 hall senso LMA probe LF probes LMA probe 21

22 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 22