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Quantifying the Detection Capability of Metal Detectors using Metal Spheres
Abstract
Plots of maximum detection distance as a function of target radius, for a set of metal balls, have been used as a measure of the sensitivity of metal detectors. This simple idea allows detection capability at a given distance to be quantified. One of the advantages of the sphere is that it is easily modelled mathematically. We compare results of detector measurements with theory for some of the metals tested. In particular, both theory and experiment support the use of permeable steel balls as standard test targets.
... The parameter measurements were carried out to a great extent at the JRC Ispra along the lines of [7,8] and will be possibility to recognize the truth (TP, TN) or to miss the truth with a false indication (FN, FP). ...
... A first 'hands on' adoption of the ASME [6] published in detail within a JRC report. As described in [7,8], maximum detection distances were measured for different steel balls and ITOP targets in air and in soil. From the in-air values we can derive a rough forecast of detection performance in cooperative soil while the specific conditions in uncooperative soil might even reverse these results. ...
Because the demining process is always connected with a danger for human beings, it is necessary to make sure the proper functioning of mine searching equipment in detecting mines. Detecting a hidden target by penetrating physical interaction of rays or waves is similar to non-destructive testing. The non-destructive testing profession developed procedures to check the reliability of testing. These procedures, like the performance demonstration where the successful detections are statistically evaluated against false call rates and their implementation in an industrial standard (ASME section XI appendix VIII) are used as a template. A first adoption to demining was accomplished in the prescription for blind trials in the CEN workshop agreement CWA 14747: 2003 for test and evaluation of metal detectors. Within an ITEP project a number of blind trials were carried out to learn about the necessary statistical basis of those trials to achieve true, reproducible and repeatable results to give guidance for the selection and improvement of metal decretors. The special focus in this first investigation was on the influence of the human factor due to the different degree of experience of the operators and the influence of the soil.
... One reason that metal balls are useful targets to use is that predicting the response of such targets in the fields of circular coils is readily theoretically calculable. Theoretical models and comparison of their results with experiment are reported elsewhere [2], [3]. ...
Because the demining process is always connected with a danger for human beings, it is necessary to make sure the proper functioning of mine searching equipment in detecting mines. Detecting a hidden target by penetrating physical interaction of rays or waves is similar to non-destructive testing. The non-destructive testing profession developed procedures to check the reliability of testing. These procedures, like the performance demonstration where the successful detections are statistically evaluated against false call rates and their implementation in an industrial standard (ASME section XI appendix VIII) are used as a template. A first adoption to demining was accomplished in the prescription for blind trials in the CEN workshop agreement CWA 14747: 2003 for test and evaluation of metal detectors. Within an ITEP project a number of blind trials were carried out to learn about the necessary statistical basis of those trials to achieve true, reproducible and repeatable results to give guidance for the selection and improvement of metal detectors. The special focus in this first investigation was on the influence of the human factor due to the different degree of experience of the operators and the influence of the soil.
The document is devoted to eddy-current metal detectors (MD). Its purpose is to collect test knowledge from 1997 to 2006 split in reports during this time.The technical analysis evaluate the main factors influencing on MD performance i.e., overview about test campaigns, CEN Workshop Agreement 2003, reliability test, influencing factors on test results, influence of ground (soil), feature assessment of MD.
This report describes laboratory tests of the following commercial off the shelf metal detectors: −CEIA MIL-D1 and MIL D1 DS −Ebex 421 GC, Ebex 420 H-Solar, Ebex 421 GC/LS −Guartel MD8+ −Foerster MINEX 2FD 4.500.01 −Minelab F3, F1A4 and F1A4 UXO −Schiebel ATMID −SHRIMT– Model 90 −Vallon VMH3, VMH3C UXO The aim of the tests is to provide information to enable users to assess which detector would be best suited to their purpose, to aid manufacturers in development and to aid CEN workshop participants to frame a possible update of the standardized test protocols. The experimental work reported here was conducted by the European Commission’s Joint Research Centre (JRC) with the assistance of staff from GICHD, BAM and Qinetiq, at the JRC’s Ispra site in northern Italy during the period November 2003 to January 2006. Testing was conducted according to the methods of CEN Workshop Agreement 14747:2003, any minor modifications being explained and described. Results for in-air and in soil sensitivity tests are reported, including the effects of speed, temperature, mutual interference, repeatability and drift. Tests of pinpointing and target resolution, and ergonomic and operational aspects are also reported. Very clear differences in performance may be seen between the detectors in essentially all aspects tested. Results for sensitivity and soil compensation are broadly consistent with the results of earlier STEMD field trials in Laos and Mozambique with the following important exception. In the lab, detection capability was measured for small metal objects and results were similar when measured in-soil and in-air with the detector at the same sensitivity. This finding contrasts with the Mozambique field-trial results for real mines and simulants with full-sized mine bodies where in-soil values were often very different from in-air values at the same detector setting. Taken together, these two sets of results imply that only in-soil measurements with realistic targets with full-sized mine bodies can be trusted to give accurate indications of detection depth. Further study is recommended to confirm this finding.
No single detector performed best in all tests, so it is recommended that demining organizations assess the results according to the ERW threat that they deal with and the circumstances in which they work. e.g. for some users sensitivity may be a high priority, others may be more concerned about detector handling and ergonomics. It is recommended that manufacturers use the results to compare their current products and prototypes with the state of the art. It is hoped that it will help them to decide priorities for research and development. The report includes information relevant to the following specific environments and threats: low-metal mines, small UXO items, wide range of target depths, soils with uncooperative magnetic properties, high and low temperatures. The main lesson learnt is that the methods of CWA 14747 in its current form constitute a very thorough test regime and yield a large amount of useful information but they are too lengthy and laborious to perform in their entirety. Recommended follow-on work: Similar testing should be repeated periodically as new metal detectors and new types of electronic mine detector, such as dual sensor ground penetrating radar/metal detectors, become available. Experience gained should be used in an update of CWA 14747:2003 and establishment of test protocols for new types of detectors. Priority should be given to finding ways to shorten or automate the testing whilst still obtaining the critical information.
Test and evaluation trials of metal detectors that have been made in recent years have shown the need for common testing specifications so that the results of such trials are useful to the wider demining community. Stemming from initiatives to promote standardization within Humanitarian Mine Action, a CEN Workshop (an "informal" international standardization process) was established to standardize the testing of metal detectors. This has produced a "CEN Workshop Agreement" that gives principles, guidelines and procedures for detector testing. In particular a method for quantifying detection capability has been agreed.
The EPFL and the VUB have been investigating the response of metal detectors within the framework of humanitarian demining research activities, in particular frequency domain systems. A simple circuit model has bene looked at first, followed by the analysis of a more completed model. As has also been stressed before, this analysis indicates the possibility of identifying some metallic objects. In addition the phase shift of the received signal turns out to be a continuous, monotonically decreasing function of the object size; this leads to the idea of imposing a 'phase threshold' in order to reduce the amount of detected clutter. This discrimination-based approach is less ambitious than object identification, but is likely to be more robust and to work when looking for metallic objects of a certain size, e.g. non minimum-metal mines or UXO. A first series of measurements was therefore carried out using a commercially available, differential tow frequent metal detector, the Foerster MINEX 2FD. The detector's internal signals have been recorded in a laboratory setup along linear scans varying different object parameters for several representative objects. The collection of data as a function of movement enables the possibility of analyzing the data in the complex plane, and makes it possible to exploit global object properties. Some representative results are presented and the limits of such discrimination/identification approaches briefly outlined.
The time-harmonic solution for the anomalous vector potential due to a conducting permeable sphere in the field of a current-carrying loop is used to derive the corresponding step response. The step response is then used to obtain analytical expressions for the voltage induced in a second loop due to a chosen exciting current pulse train. The voltage induced in an actual system of coils is obtained by superposition. The effect of the measurement system is included in the analysis in order to experimentally verify the model. Measured responses of a number of aluminum and steel spheres at various distances from the coils are compared with theoretical predictions. The agreement between the two is generally good.
The secondary magnetic fields are evaluated for the case of a conducting sphere in a relatively poorly conducting medium under the influence of a time varying magnetic field. The sinusoidal and step function responses are both considered. The responses so calculated are thought to be useful in a geophysical prospecting method which utilizes the transient behavior of induced eddy currents in a highly conducting ore zone.
The effect of a conducting permeable spherical particle on the impedance of a solenoid is calculated, and consideration given to maximising the effect for detection purposes. For greatest sensitivity either the resistive change or the reactive change should be optimised, according to the method of detection; the frequency ranges needed to do so are indicated for both magnetic and non-magnetic particles
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