PI induction current direction question

[wyndham]

I understand that the TX pulse is an alternating current pulse and that this change induces the eddy current in the target that in turn produces the return magnetic field that the RX gets the target current from. This is my loose understanding.
While following up several threads, I found a link to a MIT professor doing an intro to how a induction fields works. His demo and blackboard explanation in simplified manor showed that a clockwise induction produced a magnetic field perpendicular to the plane of the wire and toward the observer (since this was on a blackboard)but a counter clockwise current produced a field away from the observer.
The question that occurred to me is if this were to apply to a target from the PI TX coil, the eddy current would alternate with each pulse and only half the time would the magnetic field produced by the eddy current radiate toward the RX coil. I may well be mixing several basic laws here and drawing a bogus conclusion but it seems from my limited POV to maybe be correct(yes/no/???) .
There have been many post on several different forums about some detectors having a blank spot where the target is not seen by the detector. ML 4000 seems to have some of these traits at times. Could any of this indicate that either the frequency or the pulse rate or the alternating magnetic field ,could be nulling the RX signal.
If anyone wishes to indulge my inexperienced question, thanks ahead of time. Wyndham

 

[Carl-NC]

Hi Wyndham,

Alternating magnetic fields do have a "direction" in the sense of polarity, just like a magnet has a north and south pole. But with induction the field direction doesn't matter, either direction creates induction in the same way. Just like either pole of a bar magnet will stick to the refrigerator. So a target field with a direction "away" from the receive coil is the same as saying it's a negative field "toward" the receive coil.

Regarding blind spots on the 4000, I'm not very familiar with this detector but I think it uses two or more pulse widths that respond differently for small/large nuggets. I'll guess that for a very particular size nugget, the two responses can actually cancel.

- Carl

 

[mikebg]

Hi Windham,

I'm trying to ignore Carl's postings in this forum, because he ignores my postings in his forum. Read the thread "Targets frequency response" in Carl's forum because all this questions and answers are visible in frequency domain. They can be plotted in complex plane and to show graphically how should a metal detector work (how should be designed to obtain best results).

Each target and surrounding medium (matrix) opposes to change of magnetic field in his volume. They use for this purpose his conductivity and permeability. Conductivity opposes with eddy currents, however permeability opposes with orientation of magnetic domains in his crystal structure.

Direction of magnetic field is not important for eddy currents because the process is linear - they always decay to zero. The direction is important for magnetic domains because they viscously change his orientation and some of them remains oriented - they have memory. The memory of magnetic domains is used for signal recording (magnetic tape, hard disc, ferromagnetic bead).

The change of magnetic induction B generates signal in metal detectors. Browse in this forum the thread "OZ ironstone plots". You will find a hysteresis cycle of magnetic material showing how unidirectional TX current generates smaller signal than bidirectional. The change Bm-Br is smaller than the change Bs - (-Bs) or the change Br-(-Bs).

Yours Mike

 

[wyndham]

Thanks to both, I should have started this endeavor 40 yrs ago. Thanks again Wyndham

 

[Carl-NC]

Hi Mike,

Sorry, I don't mean to ignore you or anyone! I've been traveling a lot over the past month, so my forum time is extremely limited. Where a thread already has healthy discussions, I tend to listen more than talk.

- Carl

 

[mikebg]

Thanks, Carl, that you answered to my yoke seriously. I have used (by habit) an ancient type emoticon to show the end of yoke. I am glad that you have time to see what happens in the forums and even to give answers. However, IMNHO, it is time you to ask questions. After many years marvellous work for metal detecting world, after collecting lot of metal detecting machines, you can ask the worlds brain for example: "How should a metal detector work?" and then "Why until now it not works so?" or "How should be designed a metal detector to achieve theoretical possible limits in depth and target ID?" Let metal detecting world think and give answers. The answers from engineers and scientists will help CDG (commercial design group) to create absolute perfect machines. The theoretical limits should be reached.
Recently in your forum happened something strange in a thread started by me. Few people observed these strange things in postings and asked why, but no reasonable answers. Because I estimate highly all participants in the forums, I will explain this in private message to you. See the box in your forum.
Regards and bow to the ground for your merits.