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I need to detect the presence of large metallic objects. The detector will be on the ground. The detectable object will be around 60-70cm away. The detector will need to be protected to support heavy weight. The detector will be on/near/under asphalt and cars might run over the detector. I am not sure about what type of detection will be suitable for this scenario.

  • What possibilities of detection mechanisms exist for such a scenario?
  • What type of detector will be most accurate?
  • Which of the options would be most cost effective?
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  • \$\begingroup\$ @OlinLathrop I'll update my answer with the design specifics. \$\endgroup\$
    – eestein
    Commented Apr 19, 2013 at 14:29
  • \$\begingroup\$ @OlinLathrop I just updated, is that enough? \$\endgroup\$
    – eestein
    Commented Apr 19, 2013 at 14:34
  • \$\begingroup\$ It still seems like a shopping question. If not, what part of the electrical engineering design aspect are you asking about? \$\endgroup\$ Commented Apr 19, 2013 at 14:45
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    \$\begingroup\$ May be, if you share the exact problem statement (in detail), with constraints (budget, cost, space, complexity etc.), people here would be able to suggest better alternatives. Your question has a "range-detection" tag, and if this is for life-size cars, then sonar, laser etc. are the standard range detection techniques. \$\endgroup\$
    – bdutta74
    Commented Apr 19, 2013 at 14:55
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    \$\begingroup\$ I've often seen traffic counting done with portable equipment consisting of a rubber hose placed across the road connecting to some box. I suspect those work by air pressure in the hose activating a switch. Some gas stations have hoses which ring a bell when a car comes in and drives over them. \$\endgroup\$
    – Kaz
    Commented Apr 19, 2013 at 16:00

2 Answers 2

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Modern traffic counting loops are actively driven at higher frequencies( 10's of kHz - don't know the precise frequency) and with different loop topologies with the Quadrupole and higher field patterns being supported. A common misperception is that ferrous materials are required for detection, this is based upon the misperception that the ferrous material increases the linked magnetic fields. With an active sensor (actively driven magnetic fields) the detection is through a decrease of the inductance, because the ferrous material cannot respond and permittivity at these frequencies drops. Of course this also allows non-ferrous materials to be detected.

The proof of this? these loops handily detect bicycles with aluminum rims. This is because of the eddy currents (v. small) induced in the rims.

I would imagine that you may have to have an extensive pattern of loops cut into your asphalt with perhaps multiple detectors to be able to discriminate the placement of the object and resolution.

I think that initially these instruments worked by coupling the earth's mag field into the detection loop but modern systems are designed to have a 3 dimensional field topology close to the pavement surface to ensure the maximum probability of interception. Instead of a sheet of parallel filed lines (from the earth) imagine a lumpy bubble wrap like field surface. Any conductive surface at any orientation is assured of coupling with the field lines.

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  • \$\begingroup\$ Thank you, I found that is inductive loop I'm looking for. Thanks for you explanation. \$\endgroup\$
    – eestein
    Commented Apr 22, 2013 at 13:29
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Now it seems you want to detect large metal objects, like cars, passing over asphalt.

This is a solved problem. There are already commercial sensors used in roadways for exactly this purpose. This is how "smart" traffic lights get the info to be smart.

These things work by burying a large inductive loop just beneath the roadway. The diameter is typically a few feet.

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  • \$\begingroup\$ Do you know the name of this object? This is what I need. \$\endgroup\$
    – eestein
    Commented Apr 19, 2013 at 14:46
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    \$\begingroup\$ @eestein: No, I don't know the common name used for this in road design. The electrical pickup part is called a "coil". \$\endgroup\$ Commented Apr 19, 2013 at 14:48

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