### EddyCentre Model control

#### Model Types

The EddyCentre modelling software models the electromagnetic properties of work-pieces, defects, probes and their fields. In almost all cases, this involves numerical approximation of analytic expressions of these properties. The EddyCentre Model control is used to control these numerical approximations, to set grid size and precision parameters.

There is an EcModel control associated with every EcProbe control and every EcChannel control. The EcModel control is an invisible control and comes in three types:

• Simple Probe Model type
• Channel Model type
• Ferrite-cored Probe Model type

The three EcModel control types are discussed in more detail below.

##### Simple Probe type

A 'simple probe' is a probe that consists of a single coil without a ferrite core. One of the electromagnet properties of such a probe is its free-space inductance, L0, i.e. if the probe were placed in air away from any other materials and the inductance of the probe were measured. The free-space impedance Z0 is related to the free-space inductance L0 by
L0 = Z0/omega,
where omega = 2 Pi x frequency.

L0 is a known function of the coil geometry that can be approximated numerically. The precision of this approximation is managed by the EcModel control.

##### Channel

The Channel type of EcModel control is used to manage the way that every channel in an Inspection is modelled. It controls the precision of all field calculations, their interaction with any defects present and will approximate the impedance change in the probe if desired. The Channel type model works only for non-ferrite cored probes. If the probe has a ferrite core, the Ferrite Probe model must be used. This is described in the next section.

The channel model assumes that the defect volume is divided up into equal volume cells and then models the interaction between each of these cells and the field generated by the probe's coil, its 'incident field'. The perturbed field created by this interaction, called the 'scattered field', is then calculated. The scattered field can then be used to model the impedance change in the coil due to the presence of this field.

Not all of these calculations will be carried out for each channel as some will not have a defect or the user will be interested in only the incident or just the scattered fields, etc. Only the incident electric field in the defect region needs to be modelled in order to calculate the impedance change in the coil. However, it is possible to model the fields in the other regions so that they can be viewed. If a raster scan is being modelled EcModel control makes sure that the incident electric field in the defect region has sufficient extent and resolution for the scan and defect.

If there is a defect present, the number of cells in each of the co-ordinate directions determines the volumetric size of each of the defect volumetric cells or sub-regions. Usually, using more cells provides a more accurate approximation; however, the more cells used also makes the overall problem larger and slower to calculate. Because the model assumes the presence of a single electric dipole at the centre of the volume, the overall dipole distribution will not be modelled well if the cells become extremely long (relative to their width) or extremely flat.

The properties pages of the EcModel control are used to set all the modelling parameters.

##### Ferrite Probe
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