Machine-integrated Magnetic Collector Design and Testing

by Erik de Brun [ GICHD Consultant ] and Stephen Ahnert [ GICHD Consultant ] - view pdf

The Geneva International Centre for Humanitarian Demining led a test program to evaluate a machine-integrated magnetic collection system. Promising results suggest it could speed up manual follow-up activities and provide valuable data during technical survey operations.

In 2011 and 2012, the Geneva International Centre for Humanitarian Demining (GICHD) led a test program to evaluate the feasibility and effectiveness of a mechanical demining, machine-integrated magnetic collector designed to collect ferrous metal debris during flailing operations. The purposes of this integration and test effort were to determine if

Figure 1. DOK-ING MV-4 utilized during testing.
All graphics courtesy of the authors/GICHD.Figure 1. DOK-ING MV-4 utilized during testing.
All graphics courtesy of the authors/GICHD.
Together, GICHD and DOK-ING designed a magnetic collection system and integrated it with an MV-4 flail. In March 2012, the authors, along with other team members from the Swedish Explosive Ordnance Disposal and Demining Center (SWEDEC) and DOK-ING, conducted functional and statistical testing in Zagreb, Croatia. During functional testing, the setup and configuration of the magnetic collection system was optimized and subsequently utilized for statistical testing. The statistical testing results were very promising, with 44% (240 of 544) of the seeded ferrous debris recovered during the first pass of the machine and 34% (102 of 304) of the remaining debris recovered on the second pass. In the end, 68% (371 of 544) of the seeded debris was collected. Although the testing was only conducted in one set of conditions and utilized seeded debris, the collection percentages are sufficiently high to suggest that a machine-integrated magnetic collector could dramatically reduce the amount of ferrous material remaining in the field following flailing operations. If results hold in field conditions, this methodology could dramatically speed up manual follow-up activities and provide valuable data during technical survey operations.

Introduction

Mechanical demining systems can greatly increase the effectiveness, safety and efficiency of mine-clearance operations. They clear or release large areas more quickly and safely than manual demining alone. In most cases, national standards require some form of manual follow-up after machine clearance, which can range from visual inspection to full manual clearance requiring the removal of all metal debris. When 100% metal-free clearance is required or when operating in areas heavily contaminated with ferrous material, follow-up manual clearance can be painstakingly slow because every metal detector indication must be investigated.

GICHD recognizes that, combined with mechanical tools or as stand-alone assets, magnets can increase manual clearance productivity by removing ferrous metal debris from the clearance area. In addition, the collection of metal debris can provide invaluable information about the type and location of contamination during technical survey and clearance operations. Ideally, magnet-equipped machines would collect a large percentage of the metal contamination in a given area, increasing overall operational efficiency.

GICHD previously tested a combined flail and magnet system using a Bozena 5 that towed a permanent magnet. An operational assessment was conducted in Azerbaijan between January and March 2010. The towed magnet picked up some ferrous debris, and recovery effectiveness was very low overall. A full report on the testing can be obtained from GICHD.1 Based on that testing’s results, several improvements to the magnetic collector design and configuration were hypothesized, and DOK-ING was contracted to assist with design and construction of a revised magnetic collector that would be integrated directly with the machine flail head. This article documents the testing that GICHD conducted at DOK-ING’s manufacturing facility in Zagreb, Croatia, in March 2012.

Materials and Location

The following testing equipment was used:

DOK-ING MV-4. Two separate MV-4 machines with flail attachments were utilized during testing.

Magnetic roller. A magnetic roller was one component of the magnetic collection system. Measuring 220 mm in diameter and 1,740 mm wide, it was installed directly behind the flail head (Figure 2). On each roller’s side, teeth ensured that it rotated as the machine advanced. The roller height relative to the flail was adjustable. The roller contained 242 neodymium permanent magnets (each 42 mm by 40 mm by 6 mm) spaced evenly, adhered directly to the base metal roller and covered with an abrasion-resistant rubber. Field strength of the magnets was 0.17 Tesla on the dorsal and ventral faces, and 0.34 Tesla on the lateral faces.

Figure 2. Magnetic roller attachment.

Figure 2. Magnetic roller attachment.

Magnetic sheet. Another component of the magnetic collection system was a magnetic sheet (Figure 3) that was mounted behind the flail head in place of the chain guard. The sheet was 1,740 mm wide by 500 mm tall with magnets present in the lower two-thirds. The sheet contained 175 neodymium magnets evenly spaced in a 5-by-35 grid covered with an abrasion-resistant rubber coating, yielding an overall field strength of 0.2 Tesla at the sheet surface.

Figure 3. Magnetic sheet attachment.

Figure 3. Magnetic sheet attachment.

Magnetic upper catch. In addition to the magnetic roller and sheet, a magnetic catch was installed along the front edge of the flail shroud, above the flail head (Figure 4). This upper catch was designed to capture magnetic debris thrown forward by the flail hammers. The magnetic catch was constructed similarly to the sheet but contained only a single row of magnets.Figure 4. Magnetic upper catch attachment.Figure 4. Magnetic upper catch attachment.

Ferrous debris. Various types of ferrous debris (Figure 5) were used to seed the test lane. The debris elements were selected to reflect the size and shape of ferrous debris that would typically be recovered during actual clearance operations. Table 1 lists the different types of material used during the testing.

Testing was performed in a prepared lane at DOK-ING’s main production facility in Zagreb. The test lane was approximately 45 m long, 4 m wide, 0.5 m deep and filled with relatively fine riverbed sand (Figure 6).

With the weather clear, temperatures ranged between 18 C and 22 C during the test period. The sand was dry throughout the tests and was not compacted beyond the compression provided by the MV-4 tracks. Rakes were used between tests to level the sand as necessary, and a bulldozer periodically leveled the lane.

Figure 5. Ferrous debris utilized during testing.

Figure 5. Ferrous debris utilized during testing.

Figure 6. Test lane and close-up of soil.

Figure 6. Test lane and close-up of soil.

Testing Procedures

The testing was divided into two separate phases: functional/experimental testing and statistical testing. During the functional tests, the setup and configuration of the magnetic collection system was varied in order to identify the most effective arrangement. Each setup was tested using different seeding materials, flail rotational speeds, machine speeds and working depths in order to identify the effects of these variables on the effectiveness of the different configurations. Once the most effective configuration was identified, the focus shifted to statistical testing. The statistical testing focused on generating a consistent, statistically significant data set from which debris-recovery percentages could be estimated.

Functional tests. A number of functional tests were performed to evaluate and optimize the magnetic collection system’s performance.

Statistical tests. Based on the results of the functional testing, the following magnetic collection system and machine configuration (Figure 7) was used for all of the statistical tests:

The test lane was divided into four boxes, each approximately 7 m long, with a gap of approximately 4 m between each area. Each box was seeded with a specific set of ferrous debris (Table 2). With 68 seeded targets in each of the four test boxes, there was a total of 272 seeded items for each test. Within each test box, debris was randomly seeded within a strip approximately 1.5 meters wide in the test lane’s center. The debris was buried to varying depths up to 15 cm. The statistical test was performed twice. During the first test, the seeded debris was painted green; during the second test, the seeded debris was painted yellow so that any remaining debris from the first test that was collected during the second test could be identified and excluded from the results.

After completing each box in the first test, the flail was removed so that captured debris could be removed and recorded. After completing the initial pass through the four test boxes, displaced soil was pushed back into the flail track with rakes. In order to see what percentage of the remaining debris each test box could recover, this process was repeated without any additional reseeding or manual clearance. A third pass was also performed without stopping after each box.

Before the second test, a hand-held metal detector and shovels were used to find and remove as much of the remaining debris as possible. This manual-collection effort reduced the amount of contamination for subsequent tests and identified the approximate depth of the debris not recovered by the magnets.

The second statistical test procedure was very similar to the first test except that four passes were performed. During the third and fourth passes, the flail path was shifted slightly to the right and left, respectively, in order to process areas where soil was pushed out to the sides during the first and second passes.

ID Description OD ID Thickness/Length Mass
1 Large Washer 28.0 mm 6.7 mm 2.0 mm 8.6 g
2 Medium Washer 20.0 mm 10.5 mm 2.0 mm 3.1 g
3 Small Washer 15.0 mm 3.0 mm 2.0 mm 2.6 g
4 Large Nail 3.4 mm   78.0 mm 5.7 g
5 Small Nail 2.8 mm   58.0 mm 3.1 g
6 Wire 3.0 mm   100–150 mm 7.5 g
7 Medium Slug 24.0 mm   15.0 mm 55 g
8 Small Slug 16.0 mm   15.0 mm 21g
9 Large Slug >30.0 mm   5–15 mm 36–382 g
Table 1. Characteristics of seeded ferrous debris.

 

ID Description Qty
1 Large Washer 12
2 Medium Washer 12
3 Small Washer 12
4 Large Nail 12
5 Small Nail 12
6 Wire 12
7 Medium Slug 6
8 Small Slug 2
Total 68
Table 2. Seeded debris in each test box (type and quantity).

Results of Functional Tests

The functional testing’s main purpose was investigating each component of the magnetic collection system and determining the optimal configuration for the system as a whole. Initial testing with surface-laid debris showed that the debris is easily captured yet cannot be easily dislodged if it comes into contact with one of the magnetic collectors. Testing of the magnetic roller showed that collection was much more effective if the roller was set as low as possible (centerline of the roller was approximately 5 cm above the flail skids), allowing the roller to plow through the soil deposited just behind the flail head. As the machine advanced, the roller would push a large mound of soil ahead of it, causing flailed soil to be pushed back into the path of the upward-moving flail hammers. Forward soil ejection from the top of the flail shield increased dramatically compared to previous tests, and a substantial amount of soil flowed over the top of the roller (Figure 8). As a result of the soil flow over the roller, the recovery percentage was dramatically higher than previous tests (30–50% recovery), and additional passes through the same test area continued recovering substantial debris.

The magnetic sheet alone was not very effective (capturing up to 20% of the debris), but the collection effectiveness was increased dramatically when placed just behind the roller due to the amount of soil contact. In addition to the magnetic collection system configurations, many operational variables, including fail speed and machine speed, were also investigated.

Based on testing, the optimal magnetic collection system configuration consisted of the magnetic roller placed in its lowest position, the magnetic sheet positioned directly behind the roller and the upper catch placed at the front of the flail shield (Figure 9). All subsequent statistical testing utilized this configuration. Figure 8. Increased soil turbulence with magnetic roller in lowest position.Figure 8. Increased soil turbulence with magnetic roller in lowest position.

Results of Statistical Tests

The optimized magnetic collection system configuration (Figure 10) utilized during the statistical testing proved quite effective. During the two combined statistical tests, 44% (240 of 544) of the seeded debris was recovered on the first pass, and 34% (102 of 304) of the remaining debris was recovered on the second pass. The collection effectiveness decreased significantly to 8% (17 of 202) of the remaining debris for the third pass. Figure 11 shows the percentage of available debris recovered during each pass, separated by debris type. In general, a similar debris percentage was recovered on each pass, regardless of debris type.

Figure 9. Optimal configuration of the magnetic collection system.

Figure 9. Optimal configuration of the magnetic collection system.

Figure 10. Statistical test run.Figure 10. Statistical test run.In addition to the quantity of each debris type, the recovery location (roller, sheet or catch) of the debris was also recorded and analyzed. Figure 12 shows the breakdown of recovery location, separated by debris type. For the lighter types (washers, nails, wires), the roller collected the majority of the debris (50% on the roller, 26% on the upper catch and 24% on the sheet). However, for the larger, heavier debris types (medium and small slugs), the percentages shifted dramatically with 34% collected on the roller, 65% on the upper catch and 2% on the sheet. One potential explanation for this difference is that a direct hit from one of the upward-swinging flail hammers could impart enough momentum to free a slug from the surrounding soil and send it to the upper catch, whereas the smaller debris types are less likely to encounter direct hits from the flail hammers and are slowed more dramatically by the surrounding soil due to their shape and smaller inertia.

In general, all three components of the statistical test configuration contributed significantly to the overall recovery effectiveness, which suggests that placing magnets in multiple locations around the flail head yields higher collection percentages.

Figure 11. Percentage of remaining debris collected by pass.
Figure 11. Percentage of remaining debris collected by pass.

Figure 12. Location of breakdown of collected debris.
Figure 12. Location of breakdown of collected debris.

Following the completion of the statistical testing, a purely qualitative test was performed in a topsoil area contaminated with ferrous material adjacent to an industrial warehouse and machine shop. A section approximately 2 m in length was flailed to a depth of 15 cm. As seen in Figure 13, several handfuls of metal debris, ranging from small particles to large chunks, were collected. The result, while purely qualitative in nature, suggests that the configuration may be effective in soil conditions other than dry, loose sand. It also shows that magnets are effective at capturing ferrous debris covered with substantial oxidation and other surface contamination conditions likely to be found in the field.

Figure 13. Qualitative topsoil test and collected debris.

Figure 13. Qualitative topsoil test and collected debris.

Discussion

The testing showed that machine-integrated permanent magnets can be effective in collecting ferrous debris (during testing, more than 40% of seeded debris was collected on the first pass). Although the testing was conducted in dry, loose sand using seeded debris, the collection percentages are sufficiently high to suggest that machine-integrated magnets could dramatically reduce the amount of ferrous material remaining in the field following flailing operations. Reducing the number of metal-detector indications during manual follow-up can significantly increase deminer speed, which improves the overall efficiency of clearance operations. The results also suggest that machine-integrated magnets can provide beneficial data on minefield contamination when used during technical survey operations.

Soil/magnet contact. The testing showed that the action of the flail hammers tended to deposit metal debris in the loose soil behind the flail and the majority of the debris remained below the surface of the flailed soil. Since permanent magnets do not typically have sufficient strength to pull material through a substantial amount of soil, magnetic configurations passing over the top of the loose soil recover only a small fraction of the debris. Because of this, magnetic collectors pulled behind machines have very low effectiveness. In order to increase collection effectiveness, raising the percentage of the soil that comes into direct contact with the magnetic surface is necessary. With the magnet geometries available during this test period, the most effective method involved placing the roller in its lowest position. The resulting configuration caused soil to flow over the roller and dramatically increased the amount of soil thrown up toward the sheet and the upper catch, which substantially raised the percentage of soil and debris that came into direct contact with the magnetic surfaces.

Debris removal. Once the debris adhered to the magnets, removal was relatively time-consuming. The magnets did not include any provision for wholesale removal of the debris, so pieces were removed individually by hand. While this was acceptable for testing, during actual clearance operations in heavily contaminated areas, metal debris accumulation may be so rapid that the magnets must be cleared at frequent intervals to the point where area processing speed would be adversely affected by time-consuming debris removal.

Conclusion

The results of the testing suggest that machine-integrated permanent magnets can be effective at capturing ferrous debris during flailing operations. However, after observing the movement of the debris-filled soil during testing, the test configuration could clearly be further optimized to improve debris collection. The flail shroud could be designed to efficiently guide the soil deposited behind the flail head to the magnetic collection area. A ramped surface immediately behind the flail head (in place of the roller) would allow soil to be thrown upward and funneled into channels, maximizing its exposure to magnetic surfaces. A larger upper catch would further improve collection effectiveness. In addition, any integrated magnetic collector must include provisions to easily clear debris from the collection surfaces.

Once the magnetic collection system is redesigned, additional testing in a controlled environment (such as SWEDEC) and a representative field environment (such as an actual minefield or known battle area) is recommended. The focus for these tests should be

With additional input from field testing, machine-integrated magnetic debris collection could dramatically speed up manual follow-up activities and provide valuable data during technical survey operations. c


Biographies

Erik de BrunErik de Brun is a partner and co-founder of Ripple Design. He is involved in the design, journal, testing, and manufacturing of mechanical demining equipment as well as the management of demining operations. Prior to founding Ripple Design, de Brun designed and tested armored vehicles with BAE Systems and V-22 Osprey flight-control software with Boeing Rotorcraft. He holds a Master of Science in mechanical engineering and applied mechanics from the University of Pennsylvania (U.S.) and a Bachelor of Science in mechanical and aerospace engineering from Princeton University (U.S.).


Stephen AhnertStephen Ahnert is a partner and co-founder of Ripple Design. He has extensive experience in mechanical design and analysis spanning a variety of different industries, including demining and commercial nuclear power. Prior to founding Ripple Design, Ahnert designed and tested products for high volume manufacturing and worked in the nuclear power industry performing combine thermal-structural analysis and tool design. He holds a Bachelor of Science in mechanical engineering from Princeton University (U.S.).



Contact Information

Erik de Brun
Principal Engineer & Partner
Ripple Design (GICHD Consultant)
444 North 4th St., Suite 102
Philadelphia, PA 19123 / USA
Tel: +1 267 872 5768
Email: erik.debrun@rippledesign.com
Website: http://rippledesign.com

Stephen Ahnert
Principal Engineer & Partner
Ripple Design (GICHD Consultant)
Tel: +1 703 309 9117
Email: stephen.ahnert@rippledesign.com

 

Endnotes

  1. ANAMA trial of Bozena-5 magnetic collector used with the flail, 2010. Geneva International Centre for Humanitarian Demining (GICHD).

 

TOP OF PAGE