Some eddy current equipment provides waveform output on a two dimensional display of small, square spots called
pixels. Light is generated on such a screen by applying a small voltage to the individual pixels. A wave form is created
by energizing the pixels that are needed to shape the appropriate waveform. Since the persistency of a digital display is
controlled by an applied voltage rather than by electron impact with a phosphor coating, the persistency can be
controlled by the operator. In general, the lighted pixel will remain lighted until the operator "erases" them by turning
off the voltage to the pixels.
Impedance Plane Display.
Some eddy current equipment uses the vector point display technique of displaying information on a screen. Signal
phase and amplitude are directly presented for analysis of the eddy current information. The display consists of a point
of light rather than a waveform. Changes in the test article relative to the reference standard will cause the point of
light to move. Movements of the point of light can be analyzed to determine which test variable (conductivity
permeability or dimension) causes the change.
Linear Time Base Display.
Some types of eddy current test equipment use a linear time base display. The display's vertical signal, i.e., the phase
shift, is received from the test coil. The display's horizontal signal, i.e., time, is received from a timing voltage. The
timing voltage is adjusted to the frequency or period of the generator and provides a linear horizontal sweep of the
vertical input voltage. A change in reactance of the test coil result, in a phase change of the voltage across one of the
bridge circuit arms (vertical signal). This phase change is evidenced by a shifting (along the horizontal baseline) of the
waveform. During operation, the timing or sweep voltage is used to adjust the display to show the desired number of
waveform cycles (usually one). Generally control is also included in order to control the horizontal position of the
waveform on the screen.
Recorders are used primarily in testing where the test coil or the test parts are moving relative to one another. Many
newer applications using a test fixture and a mechanical scanner to move an eddy current probe across a specific area of
a part can utilize a recorder to map the flaw indications in a part. Modulation analysis testing is an example of an older
test technique that uses a recorder. A recorder for eddy current applications may be any of several types; however, the
strip chart recorder is probably the most common. Other types of "pen and ink" and thermal recorders, as well as
various "plotters", can also be used. Some of the newer eddy current instruments provide means of storing information
on digital media. This is particularly useful where down time is important, since testing can be accomplished as
rapidly as possible, and the information stored on tape for later analysis. When selecting a recorder for use with a
particular eddy current instrument, several factors must be considered: impedance match between recorder and
instrument; frequency response of recorder; recorder sensitivity (voltage range); and response time.
GENERAL APPLICATIONS FLAW DETECTION.
Requirements For Eddy Current Flaw Detection.
Eddy current techniques are particularly well suited for detection of service-induced cracks in aircraft related materials
in the field. Eddy current equipment is very portable, with many systems utilizing battery power. Eddy current
inspection has greatest application for inspecting small localized areas where possible crack initiation is suspected
rather than for scanning broad area of metal for randomly oriented cracks. In some instances, however it is more
economical to scan relatively large areas with eddy current rather than to strip surface coatings, inspect by another
method, and then refinish.