The most basic distinction between probes can be made based on their mode of operation.
This includes:
(a) Absolute eddy current probes.
(b) Differential eddy current probes.
Absolute Eddy Current MeasureAbsolute eddy current probes consist of a single coil or its equivalent. A winding separated into two or more sections, would still be considered absolute if it performs as such. In this type of probe, the impedance or the induced voltage in the coil is measured directly (their absolute values rather than changes in impedance or induced voltage). FIG. 3.2 and FIG. 3.3 show absolute eddy current probes.
FIG. 3.2. Single-coil absolute arrangement.
FIG. 3.3. Double-coil absolute arrangement.
In the single coil absolute arrangement, it will test only the area under coil and does not compare itself with a reference standard (external reference). As was observed in Fig. 3.3 for double coils, the secondary coil has the indicating device connected across the coil and is not connected to an AC source. Normally the secondary coil is located inside the primary coil and the two coils are referred to as a double coil.
When double coils are used the primary coil generates or induces eddy currents into the article. The eddy currents in turn, generate a magnetic field that reacts against the field of the primary coil and also induces a current into the secondary coil. Changes in eddy current flow are reflected as changes in the current induced in the secondary coil. Thus the indicating device presents the change in eddy current flow. The double coil absolute arrangement is also known by names such as, driver pickup probe, driver driven probe, pitch catch probe and, more commonly, a reflection probe.
Differential eddy current measure
Differential eddy current probes consist of a pair of coils connected in opposition so that a net measured impedance or induced voltage is cancelled out when both coils experience identical conditions. The coils can sense only changes in the material under test, therefore differential eddy current probes are used to react to changes in test materials while cancelling out noise and any unwanted signals that affect both coils. FIG. 3.4 shows a typical single coil self comparison differential arrangement and FIG. 3.5 shows a typical single coil external reference differential arrangement. FIG. 3.6 shows a typical double coil self comparison differential arrangement and FIG. 3.7 shows a typical double coil external reference differential arrangement.
FIG. 3.4. Single coil self comparison differential arrangement.
FIG. 3.5. Single coil external comparison differential arrangement.
FIG. 3.6. Double coil self comparison differential arrangement.
FIG. 3.7. Double coil external comparison differential arrangement.
Comparison between absolute and differential probes
Absolute probes Differential probes
1. Sensitive to both sudden and gradual changes in properties and dimensions.
1. Not sensitive to gradual changes in properties of dimensions (may not sense long gradual flaws).
2. Combined signals are usually easy to interpret.
2. Signals could be difficult to interpret.
3. Show total length of flaws. 3. Detect only ends of long flaws.
4. Sensitive to drift from temperature changes.
4. Not Sensitive to drift from temperature changes
5. Sensitive to probe wobble. 5. Less Sensitive to probe wobble 3.1.3 Types of probe
The eddy current probes can have a variety of forms. The choice of the type depends upon the test situation. Following are the three major types of probe mainly used in eddy current testing:
(a) Internal (bobbin type) probe.
(b) Encircling probe.
(c) Surface probe.
Internal probe
Internal probes consist of circular coils used to test the interior of tubes or circular holes.
FIG. 3.8 illustrates a type of coil which can be inserted into a tube to inspect discontinuities on the inner circumference of the tube. As with the encircling coil, the internal coil induces currents that encircle the entire circumference of the tube so that the entire section surrounding the coil is inspected.
As the currents induced in the material are strongest near the coil, the internal coil is more sensitive to defects lying on or near the inner surface of the tube.
FIG. 3.8. Internal coil.
Encircling probes
Encircling probes are similar in structure to internal probes except for the fact that the test material is passed inside the coils. They are primarily used to inspect the outside surface of
induces eddy currents in the bar that encircle the entire circumference of the tube or rod so that the entire section under the coil is inspected at any one instance.
The width of the coil is a function of the application. Wide coils cover large areas, so they respond mostly to bulk effects, e.g. conductivity, whereas narrow coils sense small areas and so are more responsive to small changes such as those produced by discontinuities. The magnetic field of the coil extends slightly beyond the ends of the coil.
FIG. 3.9. Encircling coil.
Surface probes
Surface probes are one of the most widely used eddy current probes for inspecting surfaces, flat or contoured for defects or material properties. Defects can either be surface or sub-surface. These are also called probe coils. FIG. 3.10 shows a typical surface probe. The surface probe may be hand held or mounted in automated scanning equipment. The coil mounted in the end of the probe is provided with a protective coating of epoxy to serve as a wear surface. The magnetic field produced by a coil is approximately of the size of the coil.
Other variations of surface probe designs are pancake probe, flat probe, horse shoe or gap probe, spring loaded probe spinning probe and pencil probe.
FIG. 3.10. A typical surface probe.
3.2. Eddy current distribution relative to coil position