The ultrasonic subsystem is based on a single trans-
ducer operating in the pulse-echo technique. The ultra-
sonic pulse-echo technique has proven to be a reliable
method for locating cracks and other internal defects. In
this mode of operation, the same transducer generates
the ultrasonic waves and receives the echoes from the part
under test.
In this work, both the transducer and the sample under
test are immersed in liquid in order to facilitate the trans-
mission of the ultrasonic waves travelling through them.
The block diagram is also depicted in Fig. 1 (dashed line).
For convenience and affordability, water was used as liquid
during the tests. However, to avoid corrosion problems a
corrosion inhibitor solution should be used instead of. In
addition, ferrous P/M parts were used in the experiments
although the ultrasonic inspection system is valid to in-
spect any P/M part type.
The ultrasonic subsystem is composed of three main
blocks: the ultrasonic transducer, the pulser and the recei-
ver. The pulser will be in charge of supplying the electric
voltage spikes to the ultrasonic transducer in order to gen-
erate the ultrasonic pulses. The pulse repetition frequency
is externally controlled by a computer. Electrical signals
provided by the ultrasonic transducer at reception are gen-
erally very weak and need first to be amplified by the re-
ceiver block before being acquired and processed by the
computer.
A commercially available focused immersion ultrasonic
transducer for non-destructive testing has been chosen
after the evaluation of several alternatives. The transducer
is a V324-SU model from Panametrics-NDT with a center
frequency of 25 MHz, a nominal element size diameter of
6 mm and spherically focused at 50.8 mm.
The pulser block generates short and large amplitude
electric pulses of controlled energy which are converted
into short ultrasonic pulses when applied to the ultrasonic
transducer. The pulser has a very low impedance output to
better drive the ultrasonic transducer and a maximum
negative spike voltage output of 300 V. Pulse repetition
frequency has been selected to 5 kHz and can be externally
controlled.
In the receiver block, the generally weak electrical sig-
nals provided by the ultrasonic transducer are amplified.
The receiver block is composed of Time Gain Controlled
Amplifier (TGCA), a voltage ramp generator and a logarith-
mic amplifier. The TGCA is based on the AD8331 integrated
circuit of Analog Devices which includes an ultralow noise
preamplifier and a variable gain amplifier. The gain of the
TGCA is controlled by the voltage ramp generator and in-
creases with the elapsed time, thus compensating the
attenuation of the ultrasonic waves over the travelled dis-
tance. The logarithmic amplifier is based on the AD8310
integrated circuit of Analog Devices, an ultra-fast
responding and highly versatile logarithmic amplifier.