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Hi all, I think you guys are great and there is a vast amount of knowledge out here. I have a coil question for a BFO (ETI561). Yes I known old tech but I like playing around with them. But anyway my question is would it make any difference if I kept the coil of wire flat (vertical) instead of wrapping it into a round bundle. I could use plastic strips to glue the coil to to keep it flat until I set it into a mold of some kind. If I was to use carbon fiber strips (which super glue loves to adhere to) would the carbon fiber effect the operation of the coil? I tried to include pictures but I don’t think it came out right. I guess I need help how to include picture in my post also. Also have 2 question on shielding the coil, the instruction say to use aluminum strips to wrap most of the way around the coil I have some 1inch wide copper tape will that work as well, be better or worse? I think I read somewhere about a gap between the coil of wire and the shield if I was to wrap the coil in some plastic spiral loom wrap before I apply the shield would that be better, worse, or make no difference? Sorry for the long post and so many question still learning. Thanks to all. Rob.
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Your coil looks good, but keep in mind that the search oscillator should be around 110Khz. I'm not sure what your search loop inductance would end up being if you leave it flat as apposed to bundle coil, but it should be OK. However I would go with bundle coil as its what the project suggests. Not sure about the copper tape for this particular project. Good Idea to put electronics in metal box. -
The coil you show (solenoidal) is generally the most efficient way to create magnetic fields. In a BFO, it might add a couple percent to the sensitivity. The carbon fiber should be fine. A small gap between the coils wires & shielding helps, but the most critical is the gap between the beginning & end of the shield -- don't completely encircle the coil.Comment
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Hi Carl-NCOriginally posted by Carl-NC View Post
I’m intrigued by your comment “The coil you show (solenoidal) is generally the most efficient way to create magnetic fields.” Is the comment based on anecdotal evidence or based on electromagnetic theory that a solenoidal coil will produce a more powerful fields compared to a say a bundle coil for the same size. Any technical reference would be greatly appreciated.
AuTitchComment
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You can calculate the B-field for 2 extremes, the solenoidal and the flat spiral, and find that the solenoidal is more efficient. In a flat spiral, flux created by outer windings cuts through the inner windings and reduces efficiency. Same effect for inner-on-outer. Scramble-wound coils are somewhere in between and due to their random nature don't lend themselves to calculated solutions. But they have slight efficiency degradations for the same reason. It's not a big difference, and scramble-winding is so super-easy that almost no one goes to the trouble of making a more perfect coil.Comment
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I did some reading up on the solenoidal coil since you brought that up. The only thing is as you put it being scrambled because my winding is in no means wound nice and neat and all in perfect order and in a row with each other, wouldn't that still be scrambled up and not so much of a solenoidal coil. The only difference is I haven't pulled the wires up into a bundle yet.Comment
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Since the wires are already randomly wound, just bundle the wires, make life easy for yourself, don't try to make coil too fancy. There will be no appreciable difference in performance.
Stick to the no. of turns and all will be alright.Comment
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I first try to make working circuit, with first coil. then I experiment with other coils.Comment
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According to some quick calculations, the bundled coil achieves a selected inductance with fewer turns than either the flat spiral or single-layer solenoid. For example, an 8" ~300uH bundled coil using 0.56mm thick wire requires 23 turns, the solenoid 28 turns, and the flat spiral 29 turns. Intuitively that's what you would expect.Comment
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I was already planning on bundling the coil up but some very interesting info has came up here. I have made a flat spiral coil using some speaker wire for PI detector I built and it works very good and there is a noticeable difference using the flat spiral coil over some of the other bundled coils I have made for it. I would say maybe about a 2 inch difference on picking up a quarter size coin using the flat spiral over a bundled one. It intrigued me on the solenoidal coil and I will have to do some experiments with them. Thanks for all the comments.Comment
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http://www.geotech1.com/forums/showt...255#post214255Originally posted by RobN View Post
Couple tests I tried with different coils. Didn't see a big difference between the coils. Tried a bundle coil when I first started playing with coils, didn't seem as good as the spiral coils. Not sure why, maybe because of using AWG28 solid magnet wire. Interested in what you learn with your experiments.Comment
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hi green.
why in the pi detector. When I increase the size of the coil. the detection distance does not increase much or nearly the same as that of a coil at least the size. In particular coil of size 1m.
A lack of electrical voltage in the coil or a problem in the receiverComment
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What coil sizes are you comparing? Is the coil inductance the same? What is the target distance you are comparing at? Including a chart calculated with HyperPhysics. Increasing coil diameter probably increases noise so detection distance difference probably not as much as charted. Think chart is correct, maybe someone that knows more about metal detectors could correct my thinking or add some thoughts.Originally posted by sbalio1 View PostComment
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Eric Foster posted a good rule of thumb for target types/sizes and coil sizes. It works like this. If you can detect a target at exactly the maximum distance equal to the coil radius, that is the maximum distance for that target and increasing the coil size will make the detecting distance less for only that same target. Increasing the coil size until the maximum detecting distance equals the coils radius is a way to optimize but only for that target. However, larger coils also are affected by more noise.
All targets have a time constant. The maximum eddy currents stimulated in a target will die out in 5 time constants. If a coin has a 100 uS (micro Second) total decay time, it's time constant (TC) will be one fifth or a 20 uS TC.
Now, assume that you have a small gold nugget with a 2 uS TC and you wave it very close to the coil what do you hear? It depends on the size of your coil, and most importantly the minimum delay setting of your PI machine design. At a 10 uS delay the eddy currents in the 2 uS target will have died out by the time the RX circuit is turned on. Here is where minimizing the delay below 10 uS will allow you to detect smaller targets. Once you start to detect this target, the next optimization is selecting the best coil size for that target.
When comparing coil performance you need to know what the TC of the coil is and how long your TX pulse is to know the current level going through the coil. A 300 uH coil that has a total TX circuit resistance of 3 ohms has a charging TC of 100 uS. The total resistance is the coil resistance plus the MOSFET on resistance and any other resistance in series with the coil. In one TC or 100 uS the current raises to 63.2% of maximum current. 200 uS will raise the current to 86.5% and 300 uS will raise the current to 95.1% of maximum. Current field density is based on the coil area and whether you are looking at near field power projection typically within the coil radius detection distance or far field projection beyond that.
Now you need to add the Target TC variable and PI minimum delay time to your analysis. As you can see, comparing coils is a balancing act. Different target shapes may offer a different detecting distance depending on their orientations relative to the coil axis. That is why some research papers compare coils sizes using ball bearing shaped targets to eliminate target orientation from the variables that need to be accurately measured.
Magnetic fields tend to follow the inverse cube rule. The magnetic field on a target at twice the distance from the coil is about 8 times weaker (1 over the 2 to the third power). But there is more!! There is now the other trip back from the stimulated target to the coil which is another 8 times weaker. The total weakening is now 1 over 2 to the sixth power or 64 times weaker. There are some additional variables between near field and far field measurements which can be affected by noise levels and op amp saturation recovery time. Integrating many RX samples is another way to overcome noise and make the PI detector more sensitive to a select range of target TCs.
One subtle thing to consider is where on the current charge curve the TX current turns off. If you turn off the current when the current rise curve is more vertical than horizontal, you will cancel out some of the TX power by the quick current change at this point in the current charge curve. Typically you want to turn off the current at 3 TCs or about 95% of the charge curve where the curve is more horizontal than vertical.
Trying to form a good mental model of these things will allow you to better reverse engineer your coil sizes for particular target types/sizes and optimize the TX PPS rate, delay and RX gain to optimize for a range of desired target characteristics.
I hope this helps?
Joseph J. RogowskiComment
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bbsailor Thank you
I will follow the subject to understand how the distance is measured in relation to the radius of the coil.Because I didn't understand it correctlyComment
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