high resolution faithfully reproduces the form and position of the small echo. Even if the
echo were comparable to the background noise, its time-delay,
using numerical cross-correlation analysis. Clearly, high digitizer resolution is crucial for the
detection of small flaw echoes.
During linear scans along the fast axis, ultrasonic triggers occur at a regular 1 kHz rate. The
digitizer must not miss any of these triggers otherwise correspondence between captured
waveforms and transducer position will be lost.
Linear scans along the fast-axis take (500 mm / 0.1 mm) / 1 kHz = 5 seconds. Initiation of the
next fast-axis scan is under programmatic control. However, for mechanical reasons,
repositioning of the slow-axis motor must take at least 0.5 seconds.
The 14-bit digitizer is capable of transferring data through the PCI bus at sustained rates of up
to 100 MegaBytes per second. Consequently, the digitizer is fully capable of capturing 50
ultrasonic waveforms and transferring them through the PCI bus to PC RAM in ample time to
prepare for the next 1 kHz trigger. The 14-bit digitizer easily meets this performance
benchmark under a single-tasking Operating System, such as MS-DOS.
A problem occurs, however, with operation under MS Windows. Multi-tasking Windows is
not a Real Time Operating System (RT O/S). Consequently, the amount of time during which
a given task or process is interrupted while Windows services other tasks is indeterminate. As
a result, no repetitive waveform capture performance can be guaranteed under Windows.
Guaranteed, reliable performance is paramount during the system’s fast-axis scan, where not
even a single trigger can be missed.
The solution to the requirement for reliable performance under Windows is ultra-deep onboard
acquisition memory. The engineer can then operate the digitizer in Multiple Record
mode. In this mode, successively acquired waveforms are stacked in on-board acquisition
memory. Between acquisitions, the digitizer is re-armed almost instantaneously by the
hardware with no CPU intervention required. Consequently, once initiated, Multiple Record
mode operates in a completely reliable fashion that is not compromised by the multi-tasking
Windows environment.
The digitizer will require enough on-board acquisition memory to hold data from an entire
fast-axis scan. In order to determine the amount of memory required, we must first calculate
the number of samples in a single 100
Record Length = 100
= 10,000 S = 10 kS´ 10,000 Samples/Record = 50,000,000 Samples.PCI Bus Mastering. Using this method, no CPU´ 50,000,000 Samples / (100 MegaBytes/second) = 1 second
Since the position step size is 0.1 mm and since the fast axis length is 500 mm, there are 5,000
position steps in one linear fast axis scan. The digitizer must capture one 10,000-sample
record per position step. The on-board acquisition memory must therefore be at least:
5,000 Records
The CompuScope 14100 is available with up to 1 GigaSample of on-board acquisition
memory. The appropriate choice for this requirement is 64 MegaSamples.
Between successive fast-axis scans, the system will download all of the data from the
previous fast-axis scan to PC RAM. The digitizer is capable of rapidly transferring data
through the PCI bus using a method called
mediation is required during the data transfer and the digitizer can achieve sustained transfer
rates of up to 100 MegaBytes/second. Since there are 2 Bytes per 14-bit sample, the transfer
of all data from a fast-axis scan will take at least:
2 Bytes/Sample
Consequently, the data transfer will not drastically delay the onset of the next fast-axis scan,
since the positioning system already requires 0.5 seconds of mechanical stabilization time. If
the data transfer process is briefly interrupted by Windows, the transfer time increases slightly
but no data are lost and, once reactivated, the transfer process simply picks up where it left
off.
The engineer wrote a Windows 2000 based application in C using a Software Development
Kit. This kit provides convenient, easy-to-use sample programs that serve as a starting point
for a custom Windows application. Since the digitizer card is a PCI plug-n-play device, lowlevel
configuration details are handled completely by Windows. No low-level hardware
programming is required. The Windows application sets up the scan of the part under test,
controls the positioning motors, and then calls C sub-routines to acquire and download data
from the digitizer.
Today’s high-performance PC-based digitizers provide the high sampling speed, high vertical
resolution, deep acquisition memory and fast data transfer that allow easy construction of
automated, low-cost NDT inspection systems.