Acoustic Emission Testing
Digital AE System Benefits
Matrix Inspection currently employs two types of digital AE system. The first (DSP-4) is a four channel plug-in replacement card for the existing multi-channel “Spartan-AT” systems. The second (DISP) is an all-new system based on the advance “PCI-DSP” four channel processing board. The main difference between the two is that the ‘PCI-DSP’ card uses the PCI data bus allowing greater throughput of data.
Both variants have the great advantage of being fully compatible with the widely used ‘analog’ systems. This means that the data sets produced by the DSP/DISP boards are analyzed with well-established software and procedures. There are 20 years of testing experience behind the ‘Monpac Plus’ procedure and 15 years or so behind current thermal ‘cooldown’ testing procedures. It would be foolish to abandon this body of experience just for the sake of “going digital.”
This last point is very important because the current AE procedures are empirical. That is to say, they are based on large bodies of test data recorded with a certain type of instrument and sensor. To deviate from the ‘standard’ data set without extensive testing with a different type of AE system is to invalidate the procedure. This is why Matrix chose to stay with the de-facto standard and maintain a fully compatible line of instrumentation.
In addition to compatibility with 20 years of testing history, the new boards offer new features and will take us into the next generation of AE procedures. Matrix is participating in development of new technology for testing of in-service equipment. This will be wave based and require the additional features of the ‘DISP’ system. Trials on this technology are due to start in September this year but there is much work to do before it can replace the established practicesThe main benefits of the digital systems, at present, are ease of use, stability and ability to measure some additional waveform features. The boards are more compact and lighter, something our technicians appreciate. There is no need for frequent maintenance and calibration. An additional convenience is that frequency bands (filters) are changed through software settings instead of removing and replacing plug-in filter boards.
Both types of board have a wide dynamic range and low noise characteristics. This, coupled with the “Smart Threshold” feature produces about 6dB (double) more sensitivity for tank bottom assessment work and does a much better job in optimizing sensitivity for on-line monitoring and cooldown testing than a human operator. The main specifications and features of the two digital systems used by Matrix are summarized in the following sections.
The DSP-4 uses advanced digital signal processing technology to process incoming AE signals and extract wave features at high speed. The principal specifications are as follows:
 | 16 bit, 10MHz A/D converters for high resolution and a wide (better than 82dB) dynamic range. |
| 4 high pass and 4 low pass filter selections per channel. |
| Compatible with existing Spartan-AT hardware and can co-exist in the same unit. |
| Instantaneous, on-board (DSP) feature extraction for AE data processing. |
| Higher measurement accuracy and virtually zero drift. |
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“Smart Threshold” algorithm for continuous
optimization of test sensitivity under changing background noise conditions.
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Based on the personal computer 32 bit bus standard, this board may be plugged into a standard PC or a purpose built chassis with built-in PC. The PCI bus allows very fast data throughput (up to 132 Megabytes/sec) and facilitates implementation of advanced signal processing features and waveform recording at very high data rates.
| 16 bit, 10MHz A/D converters for high resolution and a wide (better than 82dB) dynamic range. |
| 4 high pass and 4 low pass filter selections per channel. |
| Dual path processing with instantaneous, on-board (32 bit DSP) feature extraction for AE data processing and simultaneous waveform recording. |
| Higher measurement accuracy and virtually zero drift. |
| Smart Threshold” algorithm for continuous optimization of test sensitivity under changing background noise conditions. |
In terms of defect detection in process equipment, the digital technology has few benefits today. Most of the advances developed over the last few years involve feature-based software and analytical techniques including pattern recognition processing. These techniques use feature extraction technology common to both the analog and digital systems. Viewing waveforms is interesting but the only way to analyze them objectively is to extract their features and treat them statistically. Once this is done, you can’t tell whether the data was recorded by a digital or analog system.
The (software) switchable filters and ‘Smart Threshold’ feature of the Matrix systems have benefits in cases where optimizing the test sensitivity is important such as on-line monitoring and storage tank floor assessment. New, frequency domain, features have interesting possibilities for analysis and filtering but an experience base must be built before they can be used with confidence.
As stated earlier, Matrix is participating in a four-year development program and has sponsored university research work to improve AE testing of process equipment. The goal is to precisely locate defects and determine fitness for service directly from the AE test data. This technology relies on determining the nature of an AE source from the wave properties. In this case the capabilities of the ‘DISP’ system are essential, in fact the new procedure and software is based exclusively on the ‘DISP’ system.
Digital AE technology has many benefits but it is important to separate the “bells and whistles” from the reality of AE testing today. Our procedures are founded on many years of experience and correlation of AE data with inspection findings. Our philosophy is to remain at the forefront of AE technology by building on a body of successful tests that go back 20 years. The real benefits of digital AE are only beginning to emerge and remain substantially in the future as far as large scale testing of process equipment is concerned. By investing in the next generation equipment now and participating in development of the new practices and procedures, Matrix Inspection will be in a position to offer the real benefits of digital technology as soon as they emerge from the laboratory.
Last updated:
September 06, 2002