 Issue 7.3, December 2003 |
Issue 7.3, December 2003 |
Standardized Testing of Metal Detectors (Cont)
Detection Capability in Soil
Many soils found throughout the world have electromagnetic properties that can cause problems for metal detectors. It often happens that the most important aspect of a detector to a user is its capability to reject noise signals from the soil and still have a good capability for detecting metal. Therefore, tests have been specified to measure in-soil detection capability. The most useful test can be made when some device has been implemented to change the depth of a target within the soil. This enables the minimum target detection curves to be repeated in soil. The results can then be compared to the in-air curves, to show any degradation caused by the soil.
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| Figure 5: The effect of soil on detection capability. |
Many detectors have advanced “ground compensation” functions for rejecting soil signals; others simply require the sensitivity to be reduced when used on certain soils. The tests specified show how these adjustments affect the detection capability. Figure 5 shows the results of such a test.
In addition to this type of in-soil test, specifications are given for the type of test on targets buried at a fixed depth that will be more familiar to most users of metal detectors in demining.
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| Figure 6: Detection reliability test in progress. c/o C. Müller, BAM |
The in-soil tests that tend to be given most importance by detector end-users are the in-field detection tests in which operators try to detect realistic mine targets (often disarmed real mines) in terrain that is representative of areas to be cleared of mines. Because of the amount of uncertainty introduced by the less-controlled conditions of such tests, they are usually statistical, using large numbers of test targets. CWA 14747 gives guidelines and specifications to make such “detection reliability” tests standardized, so that the results of a trial are useful to the wider demining community. Figure 6 shows an in-field test in progress.
Operational Performance Tests
In addition to the many tests focussing on the detection capability, specifications are given for tests of location accuracy, for the ability to characterize target shape and the capability to resolve adjacent targets. Tests are given to measure the effect of specific soils and other media encountered in the field.
One aspect of metal detector performance that is often important is its ability to operate near a large, linear, metal structure such as a rail. A test is included to measure the minimum operating distance in this situation.
Specific tests to determine whether particular interference sources affect a detector, or to what extent detectors interfere with one another are given.
Ergonomic and Operational Aspects
Although the major part of the CWA is taken up with measurements of the detection performance in one way or another, part of the document is devoted to other aspects of the evaluation of detectors that are important to users. Among these aspects is the robustness of the detector. Any equipment used in demining must be sufficiently robust to endure rough handling for many years of operation without breaking. The weight and balance of detectors are also important concerns; users want operators to be able to use them for long periods of time. Detectors need to be easy to use and the way that they should be used needs to be understandable to deminers. Guidelines are therefore given in CWA 14747 for the analysis of ergonomic and operational qualities of a detector.
Characterization of Soil
The electromagnetic properties of soils that affect metal detectors are the complex (frequency-dependent) magnetic susceptibility and (to a lesser extent) the electrical conductivity. Unfortunately, a simple scale of soil “noisiness” based on these properties is not yet established. Making strict comparisons between the metal detector test results obtained on different soils is therefore difficult. However, guidelines have been produced that begin to create a classification of soils based on their properties.
Application of the CWA Tests
Different parts of CWA 14747 are intended to be used by research and development (R&D) laboratories, manufacturers, operators of test and evaluation facilities, organizations needing to procure metal detectors, mine action centres (MACs) and metal detector operators in the field.
The order of the testing in the CWA follows a logic that begins with tests of the basic operating performance. These tests are conducted in the most controlled conditions, for which targets are in air, not soil. To achieve such controlled conditions requires equipment and facilities that are usually not available in field environments, so many of these tests need to be performed by specialized laboratories. Analogous tests, however, are specified for less controlled conditions. Next, the CWA describes tests on targets in soil—again as controlled as possible. Tests then follow that may be feasibly performed in the field with a minimum of equipment.
Few users of the document will wish to, or be able to, perform all of the tests specified. A user in the field under MAC control, for example, may perform the detection reliability test, some of the tests of operational performance characteristics and some of the basic in-air and in-soil sensitivity measurements. However, the value of testing is greatly increased if a laboratory has already performed controlled tests to determine, for example, whether the sensitivity of the detector under test varies with operating temperature.
Users of the CWA who wish to conduct a trial of various metal detectors using the tests specified may also wish to conduct a pre-trial assessment to exclude detectors at the beginning that clearly do not meet their requirements. Such a pre-trial assessment would include one or more of the tests specified in the CWA, with acceptance levels set by the users according to their own requirements. The basic in-air sensitivity measurement could be used, for example, with a minimum acceptance level for the maximum detection height. In order to help different users get the maximum benefit from using the CWA, a number of categories of testing have been established.
One of the IMAS (IMAS 03.40)13 deals with the test and evaluation of mine action equipment. IMAS 03.40 defines two types of testing trial; a consumer report trial (in which equipment is tested against general requirements); and an acceptance trial (in which equipment is tested against specific—usually local—requirements).
Plans for Future Work
The first version of CWA 14747 was issued in June 2003 and has been presented to UNMAS and the GICHD with a view to its being included, or at least referred to, within the IMAS system.
A project is planned to verify how well the specified tests work, as well as to publicize the CWA. This would involve performing trials using the CWA. Feedback would then be obtained on what improvements could be made. CW07 will then be reconvened to produce any possible revision to the CWA that may be required.
The work of CW07 has stimulated research into some of the problems encountered. For example:
Some of this work has influenced the content of CWA 14747:2003. Some of the work is ongoing but should provide evidence for future revisions.
*All graphics courtesy of the authors unless otherwise noted.
Acknowledgements
The authors are very grateful for the efforts of all of the members of CEN Workshop 7: Humanitarian Mine Action—Testing and Evaluation—Metal Detectors; this article describes their collective work.
References
Contact Information
Dr. T. J. Bloodworth, Dr. A.J. Sieber
European Commission Joint Research Centre
Via Enrico Fermi
21020 Ispra (VA)
Italy
Tel: +39 0332 789131
Fax: +39 0332 785469
E-mail: thomas.bloodworth@jrc.it
alois.sieber@jrc.it