T.O. 33B-1-1
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4.4.2.9
Probe Design Considerations.
Eddy current probes have several conflicting requirements. First, they must be a reasonable match to the electrical
impedance requirements of the instrument to which they are connected. Also, the coils need to be sized for the flaw
size to be detected with smaller flaws requiring smaller coils.
4.4.3
Eddy Current Instruments.
4.4.3.1
Functions.
The eddy current test instrument performs three basic functions. First, it generates the alternating current that induces
the eddy current flow in the part to be inspected. Second, it processes the responses to the induced eddy current flow.
Third, it displays the responses in a manner to aid interpretation.
4.4.3.2
Current Generators.
The current generator is usually a variable frequency oscillator operated at a single frequency for any given inspection.
Useful frequencies can extend from less than 50 Hz to over 6 MHz. Newer instruments have provisions of providing
multiple frequencies to the test coil(s), either sequentially or simultaneously.
4.4.3.3
Processing.
The processing function of the eddy current instrument includes a number of subfunctions. Most instruments include
some form of a balancing or compensating circuit which is adjusted to provide essentially a zero output for non-flaw
conditions. The simplest form of balancing circuit is a bridge circuit which is discussed more fully in subsequent
paragraphs. The signal from the bridge circuit is amplified before proceeding to the detector and/or analysis circuitry.
Signals can be analyzed for their amplitude and phase contents. The output from the analysis circuits may be further
filtered to assist interpretation before display.
4.4.3.4
Display Methods.
The primary display method of most eddy current devices is either one dimensional, such as a meter, or two
dimensional, such as a CRT or an LCD screen. The outputs can also be transferred to X-Y recorders, strip chart
recorders, magnetic storage media or even computers to both generate inspection records as well as aid in the analysis
of the eddy current signals.
4.4.3.5
General Eddy Current Instrumentation Requirements.
Eddy current instrumentation is the core of an eddy current system, whether the system is a simple instrument/coil
combination or a fully automated scanning inspection station. In order to assure reliable operation, the instrumentation
must have the capabilities described below.
4.4.3.6
Sensitivity.
The sensitivity to find the size and types of flaws to be detected.
4.4.3.7
Low Noise.
The noise should be low enough so that the signal from the smallest flaw to be found (or smallest calibration flaw) is at
least three times the noise level of the instrumentation.
4.4.3.8
Response Time.
The response time of the circuitry must be fast enough to process and display signals at the required scanning rate.
4.4.3.9
Selectivity.
The instrumentation should be immune to external sources of electromagnetic interference.
4.4.3.10
Stability.
The instrumentation should be drift free during the required testing period.
