NASA CP Basic Principles Book
Indice ROCarneval


Operating and Maintenance Instructions

adaptaed from:

Hard spot inspection trolley • PLAMAT-M • 18201
Operating and Maintenance Instructions • V2.0
Fundamental principles

2.3 Incremental permeability

The measuring principle is based on the eddy current measurement. The small high frequent field of the eddy current coil will be superimposed by the low frequent magnetic field with high field strength produced by the yoke. So the hysteresis curve will be overlapped with a lot of small reversible loops (Figure 7, top). The slope of these inner loops µ∆ = B/H dependent on the actual permeability in each point of the hysteresis loop. So the impedance of the eddy current measuring coil is changing while running through the hysteresis loop. Figure 7, bottom, shows the behavior of the incremental permeability µ∆ in dependency of the low frequent exciting field.

Histereses loop & Incremental permeability
Figure 7: Ferromagnetic hysteresis (top) and incremental permeability µA (bottom)

The following micro-magnetic parameters are considered for the incremental permeability analysis:

DZmax [Ohm] maximum of incremental permeability
DZmean [Ohm] averaged incremental permeability
DZr [Ohm] incremental permeability at the remnant point
Ucdz [V] voltage at maximum incremental permeability
DU75dz [V] curve-widening at 75% signal peak
DU50dz [V] curve-widening at 50% signal peak
DU25dz [V] curve-widening at 25% signal peak
Rem [%] asymmetry of the incremental permeability curve

Incremental permeability
Figure 8: Incremental permeability

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