FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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2.1 Scanning Bridge and Immersion Tank
The 3-a*es scanning bridge provides control over the movement of
the transducer(s) within a volume of 72" (183 cm) by 48" (122 cm)
by 40" (1O2 cm), corresponding to the size of the stainless—steel
immersion tank. Each axis is controlled by a leadscrew driven by
a 200-step-per-revolution stepper motor, and has a linear
resolution of 0.0005" (0.01 mm). The scanning bridge and
immersion tank are shown in Figure 2.
Also included as part of this subsystem is a stainless—steel,
stepper-motor-driven, drop-in turntable unit for the inspection
of cylindrical testpieces (see Figure 3)«
2.2 Scanner Controller
The microprocessor-based scanner controller converts high-level
commands from the host, such as "move" and "scar,", to the lowlevel
control functions required by the stepper motors driving
the scanning bridge and turntable. This frees the host computer
to perform data acquisition and processing.
Any two of the four possible axes X, Y, Z or T (for turntable)
may be selected for a scan, with the step size and velocity
specified independently for each. While the scan pattern used is
fixed, the size of the scan and the sampling density i.e., the
distance between ultrasonic samples along the scan axis, are
programmable.
Although normally controlled by the host computer, the scanner
controller may also be used as a stand-alone unit by connecting a
computer terminal to one of its RS-232 serial ports and entering
commands directly to initialize parameters and begin a scan.
2.3 Ultrasonic Subsystem
Figure 4 shows a block diagram of the ultrasonics subsystem. It
can utilize up to 8 transducers in transceive mode, or 8 pairs in
transmit-receive mode. Under the control of the slave processor,
each transmit transducer is pulsed in sequence while an eightchannel
time-division multiplexer directs the received echo
through the signal processing circuitry. Either a linear or a
logarithmic amplifier is used to increase the amplitude of the
ultrasonic signal, while an adjustable attenuator is available to
provide control of the signal level in discrete logarithmic
steps. Separate gating is supplied for each of the 8 ultrasonic
channels, permitting independent control of the gate delay and
width for each transducer. Additional signal processing can be