walk-through metal detector

[pal114525]

Hi,

I am designing a walk-through metal detector using two separate coils namely transmit and the receive coil. It is a single zone metal detector.
I couldn't figure out how the coil arrangements would be and how would the coils be inductively balanced.

Would you please explain me?

Thanks.

 

[chemelec]

What is your proposed Schematic for this Metal Detector?

How Small of Metal do you expect to detect and at What Distance?

 

[pal114525]

Hi,

There is a Wien Bridge Oscillator in my design that would generate the desired frequency in the range of (3-30 KHz) of oscillation and would be transmitted through the transmit coil.
It would go through the medium (which is the target metal) and would generate eddy current in the metal target. This eddy current would induce an emf in the receiver coil.
The transmit coil would be placed in one side of the door frame and the receiver coil would be placed on the other side of the door frame. The distance between the transmit coil and the receiver coil is approx. 1 meter.

would you be kind to explain me the following:

1. How the coil arrangement would be for both the coils, the transmit coil and the receive coil?
2. Does it need any feedback/bucking coil for inductive balance?

Thanks.

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Hi,

Please find the attachment here.

Thanks.

 

[betwixt]

Firstly, the oscillator is fine but you will need quite a powerful amplifier after it to produce enough magnetic coupling to travel 1m through air.

Your principle is basically to use anything metalic between the coils like it was a core in a transformer. It might work but it would have low sensitivity and may not be able to detect small object or ones made from some metals.

The method of detecting the inductance of a single coil you proposed in another thread will probably work far better. You might also consider 'zones' using several smaller but similar detectors stacked on top of each other so each can be made more sensitive by covering a smaller height range.

Brian.

 

[pal114525]

Hi,

Thanks for your valuable feedback.

There are two ways to improve the signal strength after the oscillator:
1. Using a suitable current amplifier after the oscillator
2. Increasing the no. of turns of the transmit coil

Would you please explain me which one is the best suitable option for walk-through metal detector?

Thanks.

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Hi,

If increasing the signal strength is best suitable by increasing the no. of turns of the transmit coil,

would you please explain me,

1. How can we design the transmit coil for a walk-through metal detector?
2. What should be the optimum turns ratio of transmit coil and receive coil for a walk-through metal detector?
3. Does it require a feedback/bucking coil for inductive balance in case of walk-through metal detector?

Thanks.

 

[betwixt]

There will be an optimal number of turns on the transmit and receive coils, adding more turns will not make it more sensitive. Adding turns increases the inductance and depending on what frequency you use it could make it even harder to drive.

If your coil design is fixed, the only way to recover more from the receiver is to increase the current in the transmit coil or amplify the voltage picked up by the receive coil.

Brian.

 

[chuckey]

I would be tempted to use a single signal source mounted half way up the door and two receiving coils on the other side of the frame, so you can balance the two received signals for zero metal.
As said you need lots of turns and current in the transmit coil.
Frank

 

[pal114525]

Hi,

Thanks for your valuable feedback.
If I understand it correctly, if there is any metal, then only there would be eddy current generated in the target metal which will further induce an emf in the receive coil.
For, NO metal situation, there is no eddy current generated and no emf would be induced in the receiver coil.

Would you be kind to explain me,

1. Why are two receiver coils required, then?
2. If it is required, then what would be the no. of turns for a walk-through metal detector?

Thanks.

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Hi,

Would you be kind to explain me the following point:

How can we be ensured that the receiver coil will pick up only the magnetic field that has been generated by the eddy current in the target metal and the receiver coil would not pick up the magnetic field generated by the transmitter coil?

Thanks.

 

[betwixt]

1. Why are two receiver coils required, then?
2. If it is required, then what would be the no. of turns for a walk-through metal detector?

1. the idea is to subtract the signals from one coil from the signal on the other. If the system is balanced (no metalic object present) the signals cancel so the receiver does not pick up the transmitter directly. If something unbalances the signals, the difference tells you something is present.

2. It depends on the frequency you use. Higher frequencies need fewer turns, lower frequencies need more. You generate most magnetic field when the coil is resonant at the frequency you are using.

Hint: 1/(2*pi*sqrt(L*C)) where f=frequency and L=coil inductance, C = parallel capacitance.

Brian.

 

[pal114525]

Hi,

Thanks for your valuable feedback.
Trusting that two receiver coils are required for a walk-through metal detector ( single zone).

Would you be kind to explain me the following point:

How can we be ensured that when we use two receiver coils for walk-through metal detector to cancel out the transmitted field in NO metal situation, it will not cancel out the induced emf in the receiver coils generated by the eddy current in the target metal, if there is any metal target?

A circuit level description would be highly appreciated.

Thanks.

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Hi,

Would be kind to explain me the following point:

If we use a tune circuit at the receiver to tune to the frequency of the eddy current generated field, then, one receiver coil may do the job. In this case, two receiver coils are not required. One receiver coil is enough to pick up the magnetic field generated by the eddy current in the metal target.

Thanks.

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Hi,

Thanks for your valuable feedback.

would you be kind to explain me?

If the transmitted field frequency is f, then the field generated by the eddy current would have the same frequency or it would have a different frequency?

Thanks.

 

[chuckey]

Unless the position of the metal is symetrical , i.e. directly on the centreline , the effect of the metal will be different on the two receiver coils so some imbalance will occur.
Frank

 

[pal114525]

Hi,

Thanks for your valuable feedback.
would you be kind to explain me the following point:

1. Will the two receiver coils be symmetrical in shape and size for a walk-through metal detector?

Thanks.

 

[betwixt]

Yes, ideally they should be identical so they pick up equal amounts of signal from the transmit coil.

If we use a tune circuit at the receiver to tune to the frequency of the eddy current generated field, then, one receiver coil may do the job. In this case, two receiver coils are not required. One receiver coil is enough to pick up the magnetic field generated by the eddy current in the metal target.

The eddy current will be at the same frequency as the magnetic field. Consider this: If you use one coil, it will still pick up a variation in output as some metals are placed between the coils but it may be only (say) 1mV in 1000mV = 0.001%. If you use two coils and cancel their static outputs, the same variation will be 2mV in 0V, two thousand times bigger! In theory, an object passing EXACTLY central between the coils could be missed but the chances are low and if it is considered a possibility, use multiple pairs of coils to narrow the detection aperture even further.

I have never worked on security gate systems but I would imagine they use a mix of technologies. Almost certainly they 'scan' at several frequencies to make it easier to detect some shapes and they may use magnetometers as well.

Brian.

 

[pal114525]

Hi,

Thanks for your valuable feedback.
I have attached here the receiver circuit for a walk-through metal detector (single zone). In this circuit, I've used L1 and C1 which forms the resonant circuit to ensure maximum pick-up sensitivity.

Would you be kind to explain me the following point:
1. How can I connect another receiver coil to cancel out the their static outputs?

Thanks.

- - - Updated - - -

Hi,

Thanks for your valuable feedback.
I have attached here the receiver circuit for a walk-through metal detector (single zone). In this circuit, I've used L1 and C1 which forms the resonant circuit to ensure maximum pick-up sensitivity.

Would you be kind to explain me the following point:
1. How can I connect another receiver coil to cancel out the their static outputs?

Thanks.

 

[betwixt]

I think you may need a lot more amplification. Your circuit may work if someone rides a bicycle through the detector but it wouldn't pick up small objects!

To cancel the static signal, just make the coils as identical as possible and connect them in series with the wires from one reversed with respect to the other. If properly balanced, the polarity of voltage they pick up will be reversed and cancel out.

Brian.

 

[pal114525]

Hi,

Thanks a lot for your valuable feedback.

With this explanation, this is understood that in a walk-through metal detector if the two coils are connected in series with wires from one reversed with respect to the other, then the transmitted field will be cancelled out by this arrangement.

Would you be kind to explain me the following point:

1. How can we be ensured that the Eddy current generated magnetic field will not be cancelled out if the two identical receiver coil connected in series with wires from one reversed with respect to the other?

Thanks.

- - - Updated - - -

Hi,

Thanks a lot for your valuable feedback.

With this explanation, this is understood that in a walk-through metal detector, if the two coils are connected in series with wires from one reversed with respect to the other, then the transmitted field will be cancelled out by this arrangement.

Would you be kind to explain me the following point:

1. How can we be ensured that the Eddy current generated magnetic field will not be cancelled if the two identical receiver coil connected in series with wires from one reversed with respect to the other?

Thanks.

 

[betwixt]

The receiver coils should be physically apart, for example one at foot to waist height and one from waist to head height. The transmitter coil would go around the whole structure so it works like a transformer with one primary (transmit side) and two secondaries (receive side) one above the other. The chances of something disturbing the field and it being cancelled are very small, the object would have to be symetrical and placed exactly in the middle or one coil would see more change than the other.

Brian.

 

[Audioguru]

What are you trying to detect? A cannon, gun, sword, knife or what else? Some modern guns are ceramic and are not detected.
The underwires in my wife's bra set off a metal detector. The stainless steel wires holding together the hernias I had in my groin also set them off.

 

[D.A.(Tony)Stewart]

UNless you are planning to use high gain Metal detectors which may be problematic on threshold vs orientation and height, the coil diameter must be bigger than the spacing between coils, and floor to max height. Just like RFID coils used in stores or magnetic detectors in airports.

The metal provides a conducting path for the H field and thus gain and sensitivity depends on detection requirements ( cell p hone, Lighters, detonator remote control)

Choose single coils or centre tapped differential on driver, detector amps, tuned by LC resonance on both sides, for best results..... Typical freq are 50k to 500k depending on SRF of coils.

Eddy currents will create a phase shift with apparent impedance of transfer function, if one wants to get really sensitive whereas coupling factor from 0 to 1% of medium just requires amplitude detection of H field induced.

 

[chuckey]

Digging a bit deeper, you need to put Faraday screens around your receiver coils else the change of capacitance when a liquid filled human approaches the coils asymetricaly. I would think that you need a gain of more then 1000 between coil and output device (not 2). You will also need some amplitude balancing control as one coil will be more sensitive then the other. So connect the coils up for maximum output from your amplifier. Reverse the connections to one coil, hopefully the output will fall be 60 dB ( 1000), if it does not try to connect a high value pot across one coil, it will damp it a bit and its output will fall, swing the value of the pot, you should go through a nul. If you don't, put the pot across the other coil.
Make sure that your amplifier has very low gain at 50 HZ, else you will pick up mains hum which will give a big output all the time.
Frank