T.O. 33B-1-1
4-37
SECTION IV
TEST SYSTEMS AND SUBSYSTEMS
4.4
TEST SYSTEMS.
4.4.1
Eddy Current Systems.
Eddy current systems generally consist of three subsystems. One is the probe or probe subsystem. Second is the eddy
current instrument. The third is the accessory subsystem. Scanners and recorders are included in items considered
accessories.
4.4.2
Probes (Coil Assemblies) - General.
4.4.2.1
Purpose.
Eddy current probes consist of one or more coils designed to induce eddy currents into a part being inspected and detect
changes within the eddy current field. A fundamental consideration in selecting an eddy current probe is its intended
use. A small diameter probe or narrow encircling coil will provide increased resolution of small defects. A larger
probe or wider encircling coil will provide better averaging of bulk properties with a loss in sensitivity to small defects.
Also the probe or coil must match the impedance range of the eddy current instrument with which it is to be used.
4.4.2.2
Classification of Probes.
Eddy current probes and coils can be classified by mode of operation, application, or geometry.
4.4.2.3
Mode of Operation.
There are three general modes of operation for eddy current coil assemblies; absolute, differential, or driver/receiver
(also called reflection). Absolute probes consist of a single coil and interrogate the area immediately adjacent to the
coil. They may have other discrete electrical elements such as capacitors included in the probe housing for matching to
specific equipment requirements. Differential probes, on the other hand, consist of two or more coils and operate by
comparing the response of one coil to the response of another coil. Normally, one coil is used for interrogating the area
of interest on the part while another coil is responding to an adjacent area on the same part or an area on a known good
part being used as a reference standard. The usual equipment connections to the differential probe allow subtraction of
the response of the reference coil from the response of the interrogating coil. Driver/receiver probes can have a wide
variety of configurations but all have a driver coil physically separate from one or more receiver coils. The driver coil
is used to induce the eddy current flow in the part. A common configuration for the receiver coils is for one receiver
coil to be adjacent to the part inspected and the other to be removed from the part (but still within the probe housing
and near the driver coil). The eddy current instrument is adjusted for zero output for this condition. Then as the area
interrogated by the first coil changes, the eddy current instrument output changes in a manner that can be related to the
change in the area inspected.
4.4.2.4
Method of Application.
Eddy current probes can also be classified by the method of application. The most common application is the contact or
surface probe used for flat or relatively flat surfaces of a part. Eddy current probes used to encircle a part are called
encircling coils. Eddy current probes completely encircled by the part are called ID coils or bobbin coils. Through
transmission probes, which utilize a coil on each side of a part (a sheet of aluminum for instance) is another method of
application. All of these probe applications can be operated in absolute or differential modes (Figure 4-34 through
Figure 4-36).
