OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009

Aldo Canova (1), Fabio Degasperi (2), Francesco Ficili (4), Michele Forzan (3), Bruno Vusini (1) (1) Dipartimento di Ingegneria Elettrica - Politecnico di Torino (Italy) (2) Laboratorio Tecnologico Impianti a Fune (Latif) – Ravina di Trento, Trento (Italy) (3) Dipartimento di Ingegneria Elettrica - Università di Padova (Italy) (4) AMC Instruments – Spin off del Politecnico di Torino (Italy)

Experimental and numerical characterisation of ferromagnetic ropes and non-destructive testing devices

Summary

The paper mainly deals with the magnetic characterisation of ferromagnetic ropes. The knowledge of the magnetic characteristic is useful in the design of non- destructive testing equipments with particular reference to the design of magnetic circuit in order to reach the required rope magnetic behaviour point. The paper is divided in two main section. In the first part a description of the experimental procedure and of the provided setup devoted to the measurement of the non linear magnetic characteristic of different rope manufactures and different typology is presented. In the second part of the paper the magnetic model of the ropes is adopted for the simulation of a non-destructive device under working conditions. The simulation are provided with a non linear three dimensional numerical code based on Finite Integration Technique. Finally the numerical results are compared with some experimental tests under working conditions.

1 Introduction In addition to satisfying the requirements for the eligibility of magneto-inductive devices for control of rope for people transport equipment, defined by relevant national and European directives, one of the major performances required from a magneto inductive device is to provide an LF or LMA [1] signal (LF: Localized Fault and LMA Loss Metallic Area) with high signal-noise ratio. There are several factors that can influence the performance but it was found that an increase in this provision requires a high level of magnetic saturation of the rope. It is obvious that the increase of magnetic saturation can be reached by increasing the size of the magnetic parts of the , but we must take into account also the user need of a device as light as possible. Is therefore necessary to make an optimal design of magnetic structure that minimizes the weight maximizing the magnetic saturation within the rope. In building a magnetic model that satisfactory approximates the magnetic detector, in many cases must be taken into account the three-dimensional geometry and magnetic characteristics ( B-H curve) of different materials. Usually the magnetisation performances of a device is deduced from the measurements of the internal magnetic induction along its main axis in the absence of the rope (no-load test).

1 Innovative ropes and rope applications

In this case it simply must have the characteristics (first curve) of the soft magnetic materials, usually steel, with which the detector yokes are made and the properties of the permanent (residual induction and coercive field) that generate the magnetic flux. If one want to evaluate the actual degree of saturation of the rope, measurement is usually not possible and one should proceed with the simulation of the instrument in the presence of rope (load test). The problem for this simulation is related to the difficulty of having the magnetic characteristics of the ropes. Although in this case are known the material characteristics of the raw material, the machining alter the magnetic characteristics and therefore is necessary to have the measurement of the B-H curve on a sample of rope made. This work will be presented with an experimental activities on a series of ropes that made it possible to have the real magnetic characteristics. With a full three-dimensional model, nonlinear equipped with all first magnetization curves of different materials, including rope, one can design any magneto-inductive detectors for any section or rope type.

2 Magnetic rope characterisation The magnetic characteristic has been measured for different types of ropes used for people transport systems, in the range 20 – 53 (mm) diameter. The measurement of the B-H characteristic of a magnetic material has been obtained as interpolation of the vertexes of several symmetric cycles. The measurement has been done establishing a H, with a controlled magnitude and direction, in the region where the magnetic material is placed. The value of the induction in the sample can be deduced by measuring the variation of the flux linked with a probe coil due to a known variation of the magnetizing field intensity. A toroidal configuration of the inductor, like the one shown in Fig.1, is the easiest way to set up a magnetic field where both the magnitude and the direction are controlled. Unfortunately, it was not possible to set up this configuration for the real ropes, because the resulting length of the torus was too much long. The measurement of the induction inside a toroidal configuration has been done for a single wire rod, made of the same material used for the real ropes. The wire rod, 8.1 (mm) diameter, has been bended and welded at the edges, the total length of the ring, l, was 60 (cm). The toroidal inductor has been built by winding 2050 (turns) around a rubber tube. With this configuration, the magnetic field inside the torus H 0 is = ⋅ H0 n I (1) where n is the turn density, N/l where N is the number of turns. The B-H characteristic of the sample rod is presented in Fig.3, while in Fig.4 the magnetic relative permeability µr is presented as a function of the magnetic field (magnetic the relative permeability µr permeability is defined as the ration between -6 the material permeability µ=B/H and absolute permeability of the vacuum µ0=4 π10 (H/m )). The measurement of the B-H characteristic of the real ropes has been done with the magnetic circuit shown in Fig.2. The magnetizing inductor is made by 4 separate coils, built by winding 1000 (turns) of copper wire around a plastic tube, 16 (cm) length, and 6 (cm) diameter. The magnetizing inductor made by the 4 coils is a ‘long’

2 OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009 inductor: the total length of the system is 64 (cm), so the ratio between the length and the diameter is around 10. The long inductor assumption means that only the tangential component of the magnetic field should exist and the field magnitude in the centre of the system can be computed by (1), with n now equal to 6250 (turns/meter), when the inductor is empty. To measure the magnetic induction, a probe coil has been turned around the rope and connected to an electromagnetic flowmeter (Yokogawa, Electronic Fluxmeter, Type 3254), then the rope has been placed inside the magnetizing inductors and bended to create a closed magnetic circuit.

The maximum current in the coils is limited to 16 A, so the maximum field H0MAX is 100 (kA/m). When the rope is present in the magnetizing system, the effective field H is different from H0 because the field magnitude is not constant along the rope axis and consequently also the intrinsic induction J in the rope is not constant.

Fig. 1. Schematic of the experimental set up for Fig.2. Schematic of the experimental set up for the BH characterization of the simple wire rod. the BH characterization of the ropes.

2 350 1.8 300 1.6 1.4 250 1.2 200 R

1 µ

B B [T] 150 0.8 0.6 100 0.4 50 0.2 0 0 0.0E+0 5.0E+3 1.0E+4 1.5E+4 2.0E+4 2.5E+4 3.0E+4 3.5E+4 0 5000 10000 15000 20000 25000 30000 35000 40000 H0 [A/m] H0 [A/m] Fig.4. Relative permeability of the sample wire Fig.3. B-H characteristic of the sample wire rod. rod.

The field H can be decomposed into two components, H = H sol + H irr where Hsol = H 0. From: rot H = G (2) div J div H = − (3) µ 0 where G is the current density and J is intrinsic induction, the components of the H field can be expressed as:

3 Innovative ropes and rope applications

div J rot H = 0 div H = div H = − (4) irr irr µ 0 = = = div Hsol 0 rot Hsol rot H G (5) Hirr , the demagnetizing component of the field, cannot be computed but in very simple geometric configuration and consequently only H0 is taken into account as magnetic field for the measurement of the B-H characteristic. The effect of the demagnetizing component depends on the cross section of the rope and it is higher for the biggest ropes. It is worth to notice that this component does not arise in the toroidal configuration. To take into account the effect of the Hirr , the measured value of the induction B or the magnetic bias J have been modified into B’ and J’ according to the methodology described in the following. The magnetic circuit has been studied in analogy with the electrical network theory; using the following definitions:

Φtotal , the measured value of the flux, i.e. the total flux linked with the probe coil,

Φmet the flux in the metal cross-section of the rope,

Φair the part of the linked flux in the air,

Scoil cross-section of the probe coil,

Smet cross-section of the metal part of the rope,

α filling factor of the coil ( Smet /Scoil ); dl dℜ = reluctance of an infinitesimal part of the metal part of the met µ ⋅ µ ⋅ 0 r Smet magnetic circuit, dl dℜ = reluctance of an infinitesimal part in air of the magnetic circuit. air µ ⋅ 0 Sair

The actual flux in the metal, Φmet , can be computed as: dℜ Φ = Φ air (6) met tot ℜ + ℜ d air d met 1−α From (6), using the coefficient f = and Φ = B'⋅S , the modified induction α met met intensity is: Φ f- ⋅ µ ⋅H ⋅S Φ - µ ⋅H ⋅ (S − S ) B'= tot 0 0 met = tot 0 0 coil met (7) Smet Smet = −µ ⋅ The magnetic bias J’ is defined as: J' B ' 0 H 0 and: Φ - µ ⋅H ⋅S J'= tot 0 0 coil (8) Smet In table I the main characteristics of the ropes are presented while in Fig.5 the characteristics B’-H are shown for 5 different kind of ropes.

4 OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009

2.2 2 1.8 1.6 1.4 1.2 1 0.8 Radaelli 158 Radaelli 473 0.6 Reverse Lay 487 Reverse Lay 1374

Induction [Equivalent] [T] [Equivalent] Induction 0.4 Closed 1861 0.2 0 0.0E+0 5.0E+3 1.0E+4 1.5E+4 2.0E+4 2.5E+4 3.0E+4 3.5E+4 Magnetic Field H0 [A/m]

Fig. 5. B’ H characteristic for different ropes The effect of the demagnetizing field is more relevant as the cross section of the rope increases; for this reason the B’–H characteristic of the ‘Redaelli 158’ rode is the closest to measured B–H characteristic of the sample wire rod. All the ropes show different values of the saturating intrinsic field, J’ SAT . This effect is probably due to different residual stresses in the ropes due to the different ways they have been stranded. The values of the saturation J’ SAT are presented for the different ropes in table I.

Rope Name Redaelli Redaelli Ercole Ercole Closed 158 (mm 2) 473 (mm2) 497 (mm 2) 1374 (mm 2) 1861 (mm 2) spiroidal, stranded, fiber metallic core, metallic core, double stranded, fiber core, reverse lay, reverse lay, Rope Type external layer core, Seale type Warrington- external external with Z-formed Seale type stranded stranded wires Diameter (mm) 20 34 31 52 53 12(6+1)+30 1+6+12+18 6(12+6/6+1) 12(6+1)+24 Construction 6(9+9+1)+PPC +24+18+12+6+ +24+30+32 +PPC +18+ 12+6+1 1 +37 Number of wires 114 186 145 175 160 Lay Z/Z Z/Z - Z/S Z Metallic cross section (mm 2) 158 473 497 1374.3 1861

Mass (kg/m) 1.43 4.28 4.35 11.6 15.4 External wire diameter (mm) 1.61 2.18 2 3.46 h=4.20 Average Total Load (kN) 310 925 974 2800 3510 Saturated Magnetization 1.9 2.1 2.1 1.93 1.92 J’ SAT (T) Table 1. Rope characteristics

5 Innovative ropes and rope applications

The measure magnetic induction inside the rope was made by the prototypes appropriately made at the Latif of Trento. The prototype consists of two lengths facing, whose surfaces have been facing worked to make them smooth as possible (Fig. 6). The two lengths of rope are placed inside the detector and separated by a small air gap. The air gap allows you to insert a probe of static magnetic field for the measurement of axial component of the magnetic field inside the air gap that gives a value of magnetic induction close to those reached inside the rope in operation. Two hypotheses have been adopted in this test. The first case concerns the air gap which was considered negligible compared with other air gap in the magnetic circuit (for example between the rope and bushes of the instrument). The second is that the length of the rope prototype is finite, for this reason has been adopted a piece of rope long enough (at least 3-4 times the length of the instrument).

Fig. 6 Rope prototypes for the measurement of magnetic induction inside the rope under operative conditions: zoom of the planar surfaces between each the magnetic flux density is measured

3 Magnetic rope characterisation The performance evaluation of magnetic inductive detectors can be performed by studies of magnetic fields with numerical methods [2],[3]. Given the three- dimensional geometry of the problem is appropriate to use 3D models, although sometimes one can get preliminary results by two-dimensional simulations. In these cases a 3D numerical field problem has been solved and in the present paper it is performed by using the FIT technique ([4], [5]); the Maxwell equations and the constitutive relations are transformed in a discrete domain, by placing the electrical quantities on a grid G and the magnetic quantities on a dual grid G’. A magneto-static not linear formulation is adopted and particular care is devoted to find a satisfied non linear convergence due to the high saturation level reached in the rope. In Fig.7 is reported a possible scheme of virtual detectors designed through the help of the 3D numerical solution and the real device.

6 OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009

Fig. 7 Numerical model adopted in the 3D magnetic field simulation and the corresponding real detector

Through the simulation, it is possible to obtain some important information on magnetic flux density levels in different device parts (in the absence and presence of rope) and inside the ropes. Fig. 8 and Fig. 9 show the trends of magnetic flux density along the main axis of the instrument in the absence of the rope compared with the measurement of magnetic field for two different devices characterised by different permanent size (Device 1 has half of the permanent magnet size of the Device2).

0.20

0.15

0.10

0.05

0.00 B B [T]

-0.05 Simulation Experimental -0.10

-0.15

-0.20 -100 -80 -60 -40 -20 0 20 40 60 80 100 Axial Position [mm]

Fig. 8: Magnetic flux density trend inside the detector versus axial position without rope for the Device1

7 Innovative ropes and rope applications

0.20

0.15

0.10

0.05

0.00 B B [T] Simulation -0.05 Experimental -0.10

-0.15

-0.20 -100 -80 -60 -40 -20 0 20 40 60 80 100 Axial Position [mm]

Fig. 9: Magnetic flux density trend inside the detector versus axial position without rope for the Dev. 2

Regarding the magnetic performances of the device under working conditions in Fig. 10 is reported the calculated magnetic flux density distribution along the rope axis compared with a measured value in the gap of the two rope parts (with a gap of 1.5 mm). The measured value inside the gap is lower to the value can be obtained inside the rope but it can provide an important indicator of the degree of saturation reached in the rope.

2.0

1.5

1.0

0.5

B B [T] 0.0 Simulation without gap Simulation with gap of 1.5 (mm) -0.5 Experimental with gap of 1.5 (mm)

-1.0

-1.5 -100 -80 -60 -40 -20 0 20 40 60 80 100 Axial Position [mm]

Fig. 10: Magnetic flux density behaviour along the rope axis for the Device 2

It is important to note that the saturation of the rope is the level of magnetic induction that can be reached inside the rope, but a better indicator is the value of the relative magnetic permeability: lower this value is, the more the rope is magnetically saturated.

8 OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009

In particular the main goal is to obtain a lower value of the relative permeability in the central part of the rope where the LF magnetic probes (winding or hall sensors crowns) are placed to measure the leakage magnetic flux. It may have some relevance also the axial extension of the saturation zone. Of course this values cannot experimental measured but through the numerical model and the proper rope magnetic characterisation it is possible to have a reliable estimation. In particular in Fig. 11 it is possible to observe the improvement in terms of magnetic saturation (lower permeability) obtained for the two analysed detectors. Finally, another important information given by the numerical simulation is the magnetic field intensity (H) reached inside the rope (see Fig. 12).

400

Device 1 350 Device 2 300

250

200 H [A/m] H 150

100

50

0 -100 -80 -60 -40 -20 0 20 40 60 80 100 Axial Position [mm]

Fig. 11: Relative magnetic permeably inside the rope versus axial position

70000

60000 Device 1 50000 Device 2

40000

30000 H [A/m] H 20000

10000

0

-10000 -100 -80 -60 -40 -20 0 20 40 60 80 100 Axial Position [mm]

Fig. 12: Magnetic field intensity inside the rope versus axial position

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4 Conclusions The magnetic structure sizing of magneto inductive detectors can be today provided by powerful numerical calculation software through which one can optimize the structure with the goal of maximizing the saturation in the wire ropes in order to get a good LF signal-type with particular reference to the internal faults. To do this, the first part of the job is dedicated to an experimental research for the magnetic characterization of some different wire rope types. The data obtained show that, compared with normal steel, the machining on the ropes greatly reduces their magnetic characteristics (reduction of the relative magnetic permeability). In the second part of the work is presented the model and some studies of magnetic field that are made and to assess the magnetic induction and magnetic field inside a detector in the absence and in presence of the rope. The obtained results show a very good agreement among numerical and experimental results that confirm the validity of the proposed design approach. The proposed model is now under development in order to simulate other important device performances as: simulation of rope faults, optimised design of Loss Metallic Area (LMA) device, sensitivity to the distance between sensors and ropes and simulation of magnetic noise (e.g. due to the rope movement) in the region where the magnetic sensors are placed.

Reference [1] H.R. Weischedel, (1999), “Electromagnetic Wire Rope Inspection: Signal Generation, Filtering, and Computer-Aided Rope Evaluation”, The Nondestructive Testing of Rope. Krakow, Poland: (O.I.P.E.E.C.) International Organization for the Study of the Endurance of Wire Rope. [2] A. Canova, B. Vusini, (2008), “Magnetic analysis of non-destructive testing detectors for ferromagnetic ropes”, COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, Vol. 27 No. 4, pp. 869-878, DOI 10.1108/03321640810878289 [3] V. Cacciatore, A. Canova, A. Vallan and B. Vusini, (2007), “Experience and technologies in NDT of ropes”. KEY ENGINEERING MATERIALS, vol. 347, pp. 627- 632. [4] T. Weiland, (1977), “A Discretization Method for the Solution of Maxwell’s, Equations for Six Component Fields ”, Electronics and Communication, (AEU), Vol.31, pp.116-120. [5] www.cst.com

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