Metal Detector with Change in Inductance?

[Pitt123]

We had a new metal detector installed on a conveyor at our site. The detector consists fo a loop of wire above the belt with the loop then attached to a detection module. The idea is for the detector module to be able to detect when there is some amount of metal on the conveyor.

I'm trying to figure out how in principle this works. The coil of wire I guess is esentillay a large inductor, with a particular inductatnce. When metal passes under this coil does it act as a core and change the inductance of the coil? Would the detector then look for a change in inductance or change in current to detect metal? It looks like this would be similar to how car detection works at traffic lights.

 

[charlie b]

Here is my guess. There will be current normally flowing through the coil. That will create a magnetic field within and around the coil. The presence of a metal object in the vicinity (presuming it is a magnetically permeable metal) will disrupt the magnetic field. That will briefly alter the current flowing in the loop. The detection electronics looks for that change.

It is similar, I suppose, to the manner in which a submarine can be detected from an airplane. But I thought that the traffic signal system detected the presence of a car by sensing its weight, as the car drives onto a sensor plate. Are you sure that is a magnetic thing?

Welcome to the forum.

 

[ELA]

Similar to some traffic loop detectors the coil will be part of an tuned oscillator circuit. When conductive metal passes near the coil it detunes the oscillator. The change in frequency of oscillation is then detected.

 

[gar]

091027-1530 EST

All the vehicle sensors embedded in the road in our town are coil type. We also have video cameras at some corners for vehicle sensing and then no coil sensors. For traffic counting they generally use a rubber tube and sense a pressure change. We have no force or weighing type sensors.

A coil sensor can be in a tuned circuit like ELA mentioned, or it can be one leg in a bridge circuit. In a bridge circuit one can detect either an inductive or resistive change. So the bridge circuit can sense non-ferrous metals. Also note that non-ferrous materials have some minor effect on inductance, but it is much less than for ferrous materials.

Many years ago we built a gage for detecting the orientation of an oil seal for installation in a Ford automatic transmission at the Sharonville Ohio plant. The shape of the steel core inside the rubber molding was unsymmetrical and thus exhibited a different inductive change for each side. We used two coils in the sensor and these two in adjacent legs of a bridge circuit. The one coil was simply a compensator for the other.

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[weressl]

Here is my guess. There will be current normally flowing through the coil. That will create a magnetic field within and around the coil. The presence of a metal object in the vicinity (presuming it is a magnetically permeable metal) will disrupt the magnetic field. That will briefly alter the current flowing in the loop. The detection electronics looks for that change.

It is similar, I suppose, to the manner in which a submarine can be detected from an airplane. But I thought that the traffic signal system detected the presence of a car by sensing its weight, as the car drives onto a sensor plate. Are you sure that is a magnetic thing?

Welcome to the forum.

Does not have to be magnetic otherwise it would not work for all metals. The magnetic field by the wireloop induces current in the conductive element that in turn creates a magnetic field that induces a current in the loop, opposite of the main current. That creates the difference in reading.

 

[charlie b]

That's why I called it a guess. Thanks, Lazlo.

 

[gar]

091027-1736 EST

Here are some experimental values for a 500 turn coil at 1 kHz.

Air core only ------ 6.25 MH Q = 3
1/2 Al alloy rod ---- 6.28 MH Q = 2.4
7/16 brass rod ---- 6.40 MH Q = 2.6
3/8 allen wrench -- 13.3 MH Q = 2.2

Note the very small change in inductance for the brass or aluminum rod (non-ferrous material).
The Q value is a measure of the resistive losses. Whereas the inductance is the reactive component.

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[Pitt123]

Does not have to be magnetic otherwise it would not work for all metals. The magnetic field by the wireloop induces current in the conductive element that in turn creates a magnetic field that induces a current in the loop, opposite of the main current. That creates the difference in reading.

Are you saying that the current flowing through the coil creates a magnetic field (AC current) and that this alternating magnetic field induces a voltage and thus current onto the metal object that is passing under the coil. This metal object will then create its own magnetic field and thus induce a votage and current back onto the coil. The current that is induced will be opposite of the origonal current and thus be detected by the detector?