scanning systems are used for automated data acquisition and imaging.
They typically integrate a ultrasonic instrumentation, a scanning
bridge, and computer controls. The signal strength and/or the
time-of-flight of the signal is measured for every point in the
scan plan. The value of the data is plotted using colors or shades
of gray to produce detailed images of the surface or internal
features of a component. Systems are usually capable of displaying
the data in A-, B- and C-scan modes simultaneously. With any ultrasonic
scanning system there are two factors to consider:
how to generate and receive the ultrasound.
how to scan the transducer(s) with respect to the part being
most common ultrasonic scanning systems involve the use of an
immersion tank as shown in the image above. The ultrasonic transducer
and the part are placed under water so that consistent coupling
is maintained by the water path as the transducer or part is moved
within the tank. However, scanning systems come in a large variety
of configurations to meet specific inspection needs. In the image
to the right, an engineer aligns the heads of a squirter system
that uses a through-transmission technique to inspect aircraft
composite structures. In this system, the ultrasound travels through
columns of forced water which are scanned about the part with
a robotic system. A variation of the squirter system is the "Dripless
Bubbler" scanning system, which is discussed below.
It is often desirable to eliminate the need for the water coupling
and a number of state-of-the-art UT scanning systems have done
this. Laser ultrasonic systems use laser beams to generate the
ultrasound and collect the resulting signals in an noncontact
mode. Advances in transducer technology has lead to the development
of an inspection technique known as air-coupled ultrasonic inspection.
These systems are capable of sending ultrasonic energy through
air and getting enough energy into the part to have a useable
signal. These system typically use a through-transmission technique
since reflected energy from discontinuities are too weak to detect.
The second major consideration is how to scan the transducer(s)
with respect to the part being inspected. When the sample being
inspected has a flat surface, a simple raster-scan can be performed.
If the sample is cylindrical, a turntable can be used to turn
the sample while the transducer is held stationary or scanned
in the axial direction of the cylinder. When the sample is irregular
shaped, scanning becomes more difficult. As illustrated in the
beam modeling animation, curved surface can steer, focus and defocus
the ultrasonic beam. For inspection applications involving parts
having complex curvatures, scanning systems capable of performing
contour following are usually necessary.