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International Journal on Advances in Systems and Measurements, vol 5 no 3 & 4, year 2012, http://www.iariajournals.org/systems_and_measurements/ Figure 4. Object test set-up. In the proposed system, pulsed excitation is applied to the coil. Pulsed excitation provides the opportunity to apply an interrogating field with rich frequency components in a single waveform. In the tests detailed in this paper, a pulse repetition frequency of 500Hz was used with a pulse width of 1ms and an applied current of 0.5A – 1.5A. III. SYSTEM EFFICIENCY AND TEST VALIDITY This section explains different experimental setups used during tests to improve the ability and efficiency of the detection system. Also, repeatability is examined to check the validity and to confirm the measurements. A. Different orientation experimental setups To check validity of the proposed system, experiments were carried out to study the reflected signals from objects under different orientations. Figure 6 shows the test set-up for different orientations. The objects were moved past the array dynamically and data were taken while the object was moving. Data were taken with the samples orientated in three directions. The results of the orientation test for the kitchen knife sample are shown in Figure 6. It can be seen from the images that the feature map follows a fairly predictable evolution with the rotation of the object; in the x-direction and y-direction the object appears as a dipole distribution, with the rotation of the distribution correlating to the rotation of the object. In the zdirection, only one end of the “dipole” is presented to the array, so a uni-polar distribution is observed. Figure 5. Orientation Test set-up x-direction y-direction z-direction 20 40 60 80 100 120 140 Figure 6. Different orientation results from kitchen knife sample test. 2012, © Copyright by authors, Published under agreement with <strong>IARIA</strong> - www.iaria.org 5 10 15 20 25 30 35 40 0.8 5 0.6 10 0.4 15 0.2 20 0 25 -0.2 30 -0.4 35 -0.6 40 20 40 60 80 100 120 140 92 35 -0.6 0.05 40 0 20 40 60 80 100 120 140 A similar trend follows for all objects; the object appears as two peaks in the feature map and as the object is rotated, this distribution is rotated from the x-direction image to the ydirection image. The z-direction, though, exhibits a clear unipolar distribution; this is mainly due to the dimensional configuration of the sensor array. B. Real weapon experimental setup In order to verify the new system effectiveness using real threatening items, a series of tests using real handguns was carried out. This section reports some of the results from these tests. Six real handguns are used (borrowed from a police station) and the specifications of these handguns are listed in Table 1. Figure 7a shows the handguns in the sample holder constructed for the tests and there reflected EM images are shown in Figure 7b. The holder was configured to ensure that the samples retained a constant and comparable distance and orientation with respect to the array during each pass through the system. Table 1. SPECIFICATION FOR THE ALL HANDGUNS. Sample Description 1 Small revolver – 0.38” Smith & Wesson – Deactivated. 2 Revolver – 0.38” Enfield service revolver – Deactivated. 3 Large automatic – 9mm Glock G17 – Live. 4 Large automatic – 0.45 Colt M1911 – Replica. 5 Small revolver – Brocock Puma air pistol – Live. 6 Small automatic – 7.65mm Walther – Deactivated. The test samples were representative of a range of handguns which would be of interest for detection. In general, it is unlikely that the deactivated weapons would give a drastically different response to the same live weapon. Although the presence of 0.4 5 10 0.2 15 0 20 -0.2 25 30 -0.4 0.4 0.35 0.3 0.25 0.2 0.15 0.1
International Journal on Advances in Systems and Measurements, vol 5 no 3 & 4, year 2012, http://www.iariajournals.org/systems_and_measurements/ ammunition clearly increases the volume of material within the arch, it would not affect results significantly. Sensor number Sensor number 5 10 15 20 25 30 35 40 5 10 15 20 25 30 Sample #1 Sample #2 Sample #3 Sample #4 Sample #5 Sample #6 Sample #1 - V DC 2 4 Time - s 6 Sample #4 - V DC 0.01 5 10 0.005 15 0 Sensor number 20 25 -0.005 30 35 -0.01 40 x 10 4 -3 3 2 10 Sensor number 5 1 15 0 20 -1 25 -2 30 Sample #2 - V DC 2 4 Time - s 6 Sample #5 - V DC 0.015 5 10 0.01 15 0.005 25 -0.005 30 -0.01 35 -0.015 40 Figure 7. Handgun samples (a) In sample holder, and (b) Reflected EM image results. The first set of tests was carried out in the same way as the previous detection tests (explained in Section II above). Results are shown in Figure 7b and are generally good, as would be expected for the controlled test setup. The one exception is sample 4, the replica gun, which is very difficult to locate in the test results. It had the lowest amplitude response and therefore the poorest signal to noise ratio. It is notable that samples 2 and 3 gave similar results, responding to a type of dipole distribution, indicative of a ferromagnetic object made predominantly from a single type of metal. The simple form of distribution also indicates that there is very little metal in the handle of these objects, and the array saw them as a simple tube/block of metal. Another test setup for the uncontrolled walk-through tests using real handguns is shown in Figure 8a. The handguns were carried in the inside jacket pocket of an individual walking 0 Sensor number 20 0.01 5 10 0.005 15 0 20 -0.005 30 Sample #3 - V DC 2 4 Time - s 6 Sample #1 Sample #2 Sample #3 35 -3 35 35 -0.015 -0.01 40 -4 40 40 -0.02 2 4 6 2 4 6 2 4 6 Sample Time - s#4 Sample Time - s #5 Sample Time - s #6 a) b) Sensor number 25 Sample #6 - V DC 0.04 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 0.02 0.015 0.01 0.005 0 -0.005 -0.01 2012, © Copyright by authors, Published under agreement with <strong>IARIA</strong> - www.iaria.org 93 through the arch. Test results are shown in Figure 8b. The EM images for the objects are clearly compressed along the x-axis compared to the controlled tests, due to the object moving through the arch at a greater speed; however, the actual distributions remain very similar. Sensor number Sensor number 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Sample #1 - V DC 2 4 Time - s 6 Sample #4 - V DC 2 4 Time - s 6 Sensor number x 10 -3 6 5 4 10 2 15 0 20 -2 25 -4 30 -6 35 Sensor number 40 x 10 -3 4 5 3 10 2 15 1 0 20 -1 25 -2 30 -3 35 -4 40 Sample #2 - V DC 2 4 Time - s 6 Sample #5 - V DC 2 4 Time - s 6 0 20 35 -5 40 Figure 8. Walk-through test (a) Test set-up, (b) Results for all hand guns from the walk-through test. Figure 9. EM images for the sample #3 - a) Controlled, b) Uncontrolled tests. x 10 -3 5 Sensor number 5 10 15 25 30 Sample #3 - V DC 2 4 Time - s 6 Sample #1 Sample #2 Sample #3 x 10 5 -3 5 0 Sensor number -5 40 Sample #6 - V DC 2 4 Time - s 6 Sample #4 Sample #5 Sample #6 a) b) b) 10 15 20 25 30 35 5 0 0.02 0.015 0.01 0.005 0 -0.005 -0.01 -0.015 -0.02 x 10 -3 -5
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