Hi
i'm trying to experience about metal detectors based on eddy currents.
The basics principle is that something remains after applying a varying magnetic field induced by a current pulse,eddy currents should go on for many tenths of us.
I have built a circuit that sends a current pulse on a 30 turns coil from an existing broken
metal detector.
The steep variation of the current is only in one direction, during the falling edge.
The current decreases from 2A to 0A in about 50us.
This repeats not very often ,say once in 100 ms,eddy currents probably goes zero in such long time.
The test object is a 10 Kg weight lifting barbell plate at only 10 cm.
Checking on a 2nd coil i can see on oscilloscope the pulse of induced voltage at same time of the steep variation of current in 1st coil.
I expect to see also something after the "strong" current variation,but i can't see no difference
depending if the iron plate is present or not.
There is something missing,but i dare ask you before guess too much,
my impression is that it has to do with repetition of pulses,but why?
Do eddy currents grows if induced by close enough pulses ?

Thanks for your opinion.

Diego

On 23/07/2021 21:38, blisca wrote:
>
> Hi
>  i'm trying to experience about metal detectors based on eddy currents.
> The basics principle is that something remains after applying a varying
> magnetic field induced by a current pulse,eddy currents should go on for
> many tenths of us.
> I have built a circuit that sends a current pulse on a 30 turns coil
> from an existing broken
> metal detector.
> The steep variation of the current is only in one direction, during the
> falling edge.
> The current decreases from 2A to 0A in about 50us.
> This repeats not very often ,say once in 100 ms,eddy currents probably
> goes zero in such long time.
> The test object is a 10 Kg weight lifting barbell plate at only 10 cm.
> Checking on a 2nd coil i can see on oscilloscope the pulse of induced
> voltage at same time of the steep variation of current in 1st coil.
> I expect to see also something after the "strong" current variation,but
> i can't see no difference
> depending if the iron plate is present or not.

The signal due to the target object will be very very much smaller than
the signal due to the transmit pulse. I think a typical oscilloscope
might not be able to handle the overload of the transmit pulse without
messing up its behaviour for a long time so that it could never see the
tiny target signal. I think you will need a good electronic (MOSFET?)
switch to disconnect the coil from the receiver during the transmit
pulse, and then the receiver will need very high gain, quite likely more
than an oscilloscope would normally provide, so you may need a
pre-amplifier.

There may be a benefit in averaging the echoes from multiple transmit
pulses, as a way to reduce the effect of interference and noise,
especially if you test it near or in a building containing electrical
wiring and appliances. Really you probably ought to test it outdoors,
far from any building or cable.

Chris Jones <lugnut808@spam.yahoo.com> wrote:

> On 23/07/2021 21:38, blisca wrote:
>>
>> Hi
>> �i'm trying to experience about metal detectors based on eddy
>> currents.
>> The basics principle is that something remains after applying a varying
>> magnetic field induced by a current pulse,eddy currents should go on
>> for many tenths of us.
>> I have built a circuit that sends a current pulse on a 30 turns coil
>> from an existing broken
>> metal detector.
>> The steep variation of the current is only in one direction, during the
>> falling edge.
>> The current decreases from 2A to 0A in about 50us.
>> This repeats not very often ,say once in 100 ms,eddy currents probably
>> goes zero in such long time.
>> The test object is a 10 Kg weight lifting barbell plate at only 10 cm.
>> Checking on a 2nd coil i can see on oscilloscope the pulse of induced
>> voltage at same time of the steep variation of current in 1st coil.
>> I expect to see also something after the "strong" current variation,but
>> i can't see no difference depending if the iron plate is present or
>> not.
>
> The signal due to the target object will be very very much smaller than
> the signal due to the transmit pulse. I think a typical oscilloscope
> might not be able to handle the overload of the transmit pulse without
> messing up its behaviour for a long time so that it could never see the
> tiny target signal. I think you will need a good electronic (MOSFET?)
> switch to disconnect the coil from the receiver during the transmit
> pulse, and then the receiver will need very high gain, quite likely more
> than an oscilloscope would normally provide, so you may need a
> pre-amplifier.
>
> There may be a benefit in averaging the echoes from multiple transmit
> pulses, as a way to reduce the effect of interference and noise,
> especially if you test it near or in a building containing electrical
> wiring and appliances. Really you probably ought to test it outdoors,
> far from any building or cable.

There is a wealth of fascinating information on metal detectors on youtube.
For example, search using the query

https://www.youtube.com/results?search_query=metal+detector

1. You will find two main types, Concentric Coil vs Double D:
Metal Detecting 101 - DD vs Concentric Coils
https://www.youtube.com/watch?v=ZgRtmPxk4Jg

2. You can make your own detector. Here is a Double D version:
DIY DD metal detector 1 - first success
https://www.youtube.com/watch?v=eCRgNr6jj2g

3. This compares the performance of a Concentric Coil and Double D:
11 inch Nexus Concentric search coil vs Minelab 11 inch DD search coil
https://www.youtube.com/watch?v=UB29wo_-FGQ

4. Some detectors can discriminate between different metals:
Metal Detector Discrimination Explained (with a Garret AT Pro)
https://www.youtube.com/watch?v=EcuTsifSgBs

5. The coil frequency affects the results:
Metal detecting frequency tests. Do they make a difference? Some
interesting results
https://www.youtube.com/watch?v=_VlKDhi1MWo

--
The best designs occur in the theta state. - sw

Chris Jones <lugnut808@spam.yahoo.com> wrote:

> On 23/07/2021 21:38, blisca wrote:
>>
>> Hi
>> �i'm trying to experience about metal detectors based on eddy currents.
>> The basics principle is that something remains after applying a varying
>> magnetic field induced by a current pulse,eddy currents should go on for
>> many tenths of us.
>> I have built a circuit that sends a current pulse on a 30 turns coil
>> from an existing broken
>> metal detector.
>> The steep variation of the current is only in one direction, during the
>> falling edge.
>> The current decreases from 2A to 0A in about 50us.
>> This repeats not very often ,say once in 100 ms,eddy currents probably
>> goes zero in such long time.
>> The test object is a 10 Kg weight lifting barbell plate at only 10 cm.
>> Checking on a 2nd coil i can see on oscilloscope the pulse of induced
>> voltage at same time of the steep variation of current in 1st coil.
>> I expect to see also something after the "strong" current variation,but
>> i can't see no difference depending if the iron plate is present or not.
>
> The signal due to the target object will be very very much smaller than
> the signal due to the transmit pulse. I think a typical oscilloscope
> might not be able to handle the overload of the transmit pulse without
> messing up its behaviour for a long time so that it could never see the
> tiny target signal. I think you will need a good electronic (MOSFET?)
> switch to disconnect the coil from the receiver during the transmit
> pulse, and then the receiver will need very high gain, quite likely more
> than an oscilloscope would normally provide, so you may need a
> pre-amplifier.
>
> There may be a benefit in averaging the echoes from multiple transmit
> pulses, as a way to reduce the effect of interference and noise,
> especially if you test it near or in a building containing electrical
> wiring and appliances. Really you probably ought to test it outdoors,
> far from any building or cable.

I did some research and found the technique is called "Pulse Induction", or
PI. It is a very complex tecnique and relies on the detection of microvolt
level signals immediately after a strong transmit pulse. As you mentioned,
it is very sensitive to external low frequency noise, such as atmospheric
noise, 60 Hz radiation from power lines, radiated noise from flourescent
tube lighting circuits, etc.

There is a wealth of information available. Here are some selected
articles:

1. KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf
https://www.minelab.com/
__files/f/11043/KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf

2. Building the Geotech Baracuda (Rev-A).pdf
https://diydetector.co.uk/store/index.php?
controller=attachment&id_attachment=2

3. TA5.pdf
https://users.cecs.anu.edu.au/~Salman.Durrani/_teaching/TA5.pdf

4. Pulse Induction Metal Detector Circuit | element14 | Sensors
https://www.element14.com/community/groups/sensor-
technology/blog/2017/10/28/pulse-induction-metal-detector-circuit

5. TreasureSweeper � For the Metal Detecting Enthusiast
https://www.treasuresweeper.com/

6. Pulse induction or VLF detectors
https://www.youtube.com/watch?v=40DO_zo_nAc

7. DIY Mono Coil for Pulse Induction Metal Detector
https://www.youtube.com/watch?v=AqZEsPaXV-Y

8. Field Coil DIY project for PI Detector
https://www.youtube.com/watch?v=vyY3-dSVbQs

9. Mirage Pulse Induction Metal Detector DIY steps 1, 2 and 3
https://www.youtube.com/watch?v=wlv3OCyoogA

--
The best designs occur in the theta state. - sw

In data luglio 2021 alle ore 08:15:57, Steve Wilson <spamme@not.com> ha scritto:

> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>
>> On 23/07/2021 21:38, blisca wrote:
>>>
>>> Hi
>>> i'm trying to experience about metal detectors based on eddy currents.
>>> The basics principle is that something remains after applying a varying
>>> magnetic field induced by a current pulse,eddy currents should go on for
>>> many tenths of us.
>>> I have built a circuit that sends a current pulse on a 30 turns coil
>>> from an existing broken
>>> metal detector.
>>> The steep variation of the current is only in one direction, during the
>>> falling edge.
>>> The current decreases from 2A to 0A in about 50us.
>>> This repeats not very often ,say once in 100 ms,eddy currents probably
>>> goes zero in such long time.
>>> The test object is a 10 Kg weight lifting barbell plate at only 10 cm.
>>> Checking on a 2nd coil i can see on oscilloscope the pulse of induced
>>> voltage at same time of the steep variation of current in 1st coil.
>>> I expect to see also something after the "strong" current variation,but
>>> i can't see no difference depending if the iron plate is present or not.
>>
>> The signal due to the target object will be very very much smaller than
>> the signal due to the transmit pulse. I think a typical oscilloscope
>> might not be able to handle the overload of the transmit pulse without
>> messing up its behaviour for a long time so that it could never see the
>> tiny target signal. I think you will need a good electronic (MOSFET?)
>> switch to disconnect the coil from the receiver during the transmit
>> pulse, and then the receiver will need very high gain, quite likely more
>> than an oscilloscope would normally provide, so you may need a
>> pre-amplifier.
>>
>> There may be a benefit in averaging the echoes from multiple transmit
>> pulses, as a way to reduce the effect of interference and noise,
>> especially if you test it near or in a building containing electrical
>> wiring and appliances. Really you probably ought to test it outdoors,
>> far from any building or cable.
>
> I did some research and found the technique is called "Pulse Induction", or
> PI. It is a very complex tecnique and relies on the detection of microvolt
> level signals immediately after a strong transmit pulse. As you mentioned,
> it is very sensitive to external low frequency noise, such as atmospheric
> noise, 60 Hz radiation from power lines, radiated noise from flourescent
> tube lighting circuits, etc.
>
> There is a wealth of information available. Here are some selected
> articles:
>
Thanks for your hints,links and for having to do with my poor english ,

going on with patience I just found that effect of eddy currents(or something after current pulses) are visible on oscilloscope(2 mV/div)!
That was possible only arranging the 2 circular coils in a double D fashion,say as olympic rings.
Before reading anything about the double D arrangement, it's advantages became evident:
if the two coils centers are exactly aligned the effect of current pulse appears huge on 2nd coil,say it has positive sign,anything right after this huge pulse is hidden by ringing;
if the coils are side by side the sign of the pulse on 2nd coil changes,there is still big ringing ;
moving again the centers toward alignment,the negative pulse decreases,and when the entering field equals the outgoing field, the pulse disappears,so does the ringing,and effect of eddy currents appears from noise.
In 2021 i just noticed what engineers probably got clear in the 50's, on tube oscilloscopes

blisca <blisca@tiscali.it> wrote:

> In data luglio 2021 alle ore 08:15:57, Steve Wilson <spamme@not.com> ha
> scritto:
>
>> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>>
>>> On 23/07/2021 21:38, blisca wrote:
>>>>
>>>> Hi
>>>> i'm trying to experience about metal detectors based on eddy
>>>> currents.
>>>> The basics principle is that something remains after applying a
>>>> varying magnetic field induced by a current pulse,eddy currents
>>>> should go on for many tenths of us.
>>>> I have built a circuit that sends a current pulse on a 30 turns coil
>>>> from an existing broken
>>>> metal detector.
>>>> The steep variation of the current is only in one direction, during
>>>> the falling edge.
>>>> The current decreases from 2A to 0A in about 50us.
>>>> This repeats not very often ,say once in 100 ms,eddy currents
>>>> probably goes zero in such long time.
>>>> The test object is a 10 Kg weight lifting barbell plate at only 10
>>>> cm. Checking on a 2nd coil i can see on oscilloscope the pulse of
>>>> induced voltage at same time of the steep variation of current in 1st
>>>> coil. I expect to see also something after the "strong" current
>>>> variation,but i can't see no difference depending if the iron plate
>>>> is present or not.
>>>
>>> The signal due to the target object will be very very much smaller
>>> than the signal due to the transmit pulse. I think a typical
>>> oscilloscope might not be able to handle the overload of the transmit
>>> pulse without messing up its behaviour for a long time so that it
>>> could never see the tiny target signal. I think you will need a good
>>> electronic (MOSFET?) switch to disconnect the coil from the receiver
>>> during the transmit pulse, and then the receiver will need very high
>>> gain, quite likely more than an oscilloscope would normally provide,
>>> so you may need a pre-amplifier.
>>>
>>> There may be a benefit in averaging the echoes from multiple transmit
>>> pulses, as a way to reduce the effect of interference and noise,
>>> especially if you test it near or in a building containing electrical
>>> wiring and appliances. Really you probably ought to test it outdoors,
>>> far from any building or cable.
>>
>> I did some research and found the technique is called "Pulse
>> Induction", or PI. It is a very complex tecnique and relies on the
>> detection of microvolt level signals immediately after a strong
>> transmit pulse. As you mentioned, it is very sensitive to external low
>> frequency noise, such as atmospheric noise, 60 Hz radiation from power
>> lines, radiated noise from flourescent tube lighting circuits, etc.
>>
>> There is a wealth of information available. Here are some selected
>> articles:
>>
> Thanks for your hints,links and for having to do with my poor english ,
>
> going on with patience I just found that effect of eddy currents(or
> something after current pulses) are visible on oscilloscope(2 mV/div)!
> That was possible only arranging the 2 circular coils in a double D
> fashion,say as olympic rings. Before reading anything about the double D
> arrangement, it's advantages became evident: if the two coils centers
> are exactly aligned the effect of current pulse appears huge on 2nd
> coil,say it has positive sign,anything right after this huge pulse is
> hidden by ringing; if the coils are side by side the sign of the pulse
> on 2nd coil changes,there is still big ringing ; moving again the
> centers toward alignment,the negative pulse decreases,and when the
> entering field equals the outgoing field, the pulse disappears,so does
> the ringing,and effect of eddy currents appears from noise. In 2021 i
> just noticed what engineers probably got clear in the 50's, on tube
> oscilloscopes

You may have found a new method of PI. None of the references I found
mention double D coils. Please continue updating us on your progress.

--
The best designs occur in the theta state. - sw

In data luglio 2021 alle ore 07:18:49, Steve Wilson <spamme@not.com> ha scritto:

> blisca <blisca@tiscali.it> wrote:
>
>> In data luglio 2021 alle ore 08:15:57, Steve Wilson <spamme@not.com> ha
>> scritto:
>>
>>> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>>>
>>>> On 23/07/2021 21:38, blisca wrote:
>>>>>
>>>>> Hi
>>>>> i'm trying to experience about metal detectors based on eddy
>>>>> currents.
>>>>> The basics principle is that something remains after applying a
>>>>> varying magnetic field induced by a current pulse,eddy currents
>>>>> should go on for many tenths of us.
>>>>> I have built a circuit that sends a current pulse on a 30 turns coil
>>>>> from an existing broken
>>>>> metal detector.
>>>>> The steep variation of the current is only in one direction, during
>>>>> the falling edge.
>>>>> The current decreases from 2A to 0A in about 50us.
>>>>> This repeats not very often ,say once in 100 ms,eddy currents
>>>>> probably goes zero in such long time.
>>>>> The test object is a 10 Kg weight lifting barbell plate at only 10
>>>>> cm. Checking on a 2nd coil i can see on oscilloscope the pulse of
>>>>> induced voltage at same time of the steep variation of current in 1st
>>>>> coil. I expect to see also something after the "strong" current
>>>>> variation,but i can't see no difference depending if the iron plate
>>>>> is present or not.
>>>>
>>>> The signal due to the target object will be very very much smaller
>>>> than the signal due to the transmit pulse. I think a typical
>>>> oscilloscope might not be able to handle the overload of the transmit
>>>> pulse without messing up its behaviour for a long time so that it
>>>> could never see the tiny target signal. I think you will need a good
>>>> electronic (MOSFET?) switch to disconnect the coil from the receiver
>>>> during the transmit pulse, and then the receiver will need very high
>>>> gain, quite likely more than an oscilloscope would normally provide,
>>>> so you may need a pre-amplifier.
>>>>
>>>> There may be a benefit in averaging the echoes from multiple transmit
>>>> pulses, as a way to reduce the effect of interference and noise,
>>>> especially if you test it near or in a building containing electrical
>>>> wiring and appliances. Really you probably ought to test it outdoors,
>>>> far from any building or cable.
>>>
>>> I did some research and found the technique is called "Pulse
>>> Induction", or PI. It is a very complex tecnique and relies on the
>>> detection of microvolt level signals immediately after a strong
>>> transmit pulse. As you mentioned, it is very sensitive to external low
>>> frequency noise, such as atmospheric noise, 60 Hz radiation from power
>>> lines, radiated noise from flourescent tube lighting circuits, etc.
>>>
>>> There is a wealth of information available. Here are some selected
>>> articles:
>>>
>> Thanks for your hints,links and for having to do with my poor english ,
>>
>> going on with patience I just found that effect of eddy currents(or
>> something after current pulses) are visible on oscilloscope(2 mV/div)!
>> That was possible only arranging the 2 circular coils in a double D
>> fashion,say as olympic rings. Before reading anything about the double D
>> arrangement, it's advantages became evident: if the two coils centers
>> are exactly aligned the effect of current pulse appears huge on 2nd
>> coil,say it has positive sign,anything right after this huge pulse is
>> hidden by ringing; if the coils are side by side the sign of the pulse
>> on 2nd coil changes,there is still big ringing ; moving again the
>> centers toward alignment,the negative pulse decreases,and when the
>> entering field equals the outgoing field, the pulse disappears,so does
>> the ringing,and effect of eddy currents appears from noise. In 2021 i
>> just noticed what engineers probably got clear in the 50's, on tube
>> oscilloscopes
>
> You may have found a new method of PI. None of the references I found
> mention double D coils. Please continue updating us on your progress.
>
It sound very strange to me,when i started gathering informations I met soon
lot of references to double D coils.Anyway,I'm not shure that the only reason is
suppression of the exciting magnetic field.
https://bigboyshobbies.net/double-d-coil-vs-concentric-coils