question on detector ID/frequency technicalities...

NASA-Tom, or others...

In a continuation of trying to understand VLF technology, and how detectors "see" and ID targets (carrying over from the latter portions of the "how widely were U.S. gold coins dispersed" thread), I have a question based on the following paragraph in a paper I'm reading, at this link: [www.gi.alaska.edu]...

The paragraph in question reads:

"Metal detectors can distinguish metal objects from each other based on the ratio of their inductance to their resistivity. This ratio gives rise to a predictable delay in the receive signal at a given frequency. An electronic circuit called a phase demodulator can measure this delay. In order to separate two signals, such as the ground component and the target component of the receive signal, as well as to determine the likely identity of the target, we use two such phase demodulators whose peak response is separated from each other by one fourth of the transmitter period, or ninety degrees. We call these two channels "X" and "Y". A third demodulated signal, we call "G", can be adjusted so that its response to any signal with a fixed phase relationship to the transmitter (such as the ground) can be reduced to zero regardless of the strength of the signal."


While trying to understand this paragraph, a question arises. Is a normal "one ID number for a given target" type of detector like, say, an F75, giving you the RATIO of inductance and resistivity mentioned in the first sentence of the paragraph, while a Minelab FBS machine's "two ID numbers for a given target" are actually the "inductance" and "resistivity" values of the target, INSTEAD of the RATIO between these two values? It would seem to me to be possible that the "FE" (ferrous) number might be the "resistivity," while the "CO" (conductivity) number might be "inductance..."

Steve

I have not 'reverse engineered' a Minelab unit. I speculate that the smart screen (or X-Y coordinate axis) is simply based upon the ferrous phase shift element/component vs the conductive phase shift component. It may be looked upon as a 'ratio' or 'percentage'........ yet, it truly is a phase-shift angle. Yes..... FBS simply has them separated out. I have yet to see 'value-added' benefit for the ferrous component....... especially in nail infested environments whereby ... multiple targets are underneath the coil at any given time. I KNOW there is iron under the coil..... but only want to know IF a non-ferrous item is in co-locate.

Now...... a pulse induction unit looks upon a 'percentage' of decay. How quickly (or slowly) an object will hysteresis-decay.

The target ID number the F75 displays isn't actually the ratio of any numbers, it is a ficticious value derived from the X and R signal levels. Its scaling is chosen to suit the 00 to 99 range of the display, and it is 'stretched' and 'squashed' over various ranges to give it greater utility. Compare it with the the Tek T2, which obviously processes the same raw data, but displays a different 'spread' of target ID vaues - a wider iron range, but with all the copper/bronze/silver coins covering a narrow range. I think the T2 has a more linear/true representation of things, the F75 engineers changed it to make it a better coinshooter.

You lost me from Hello as my technical abilities are limited at best however:

Unfortunately so many junk targets mimic good ones especially when you get into the gold rings category one never knows what is under the plug he is digging until its dug but those that learn their unit well surely have a better idea no matter what unit he is using..

Dan --

Agreed. Knowing your unit is key.

Pimento -- I understand what you are saying about F-series machines (and the T2, along with most others, I'd assume) "spreading out" the values over a scale and all, but I guess my thinking with the "ratio" thought was that if a highly conductive object causes large phase shift, and a less conductive/more resistive object like iron causes a small phase shift, and then machines are attempting to "read" these phase shifts and subsequently ID a target (this is my understanding at this point), then I could see how two objects of different metallic composition -- though their respective resistive and conductive phase shifts are different between the two objects -- could in the end be ID'd as the same number on a "single number" machine. I know it is much more complex than this, but I could see how this might happen. For instance, a small chunk of iron might show a high resistive component, and thus the machine IDs it as iron; but, if the object were a washer -- something that had that "closed loop" characteristic and so apparently does a better job of generating "eddy currents" when electro-magnetized by the transmit coil, then the resistive value could "appear" lower and the conductive value appear higher -- thus a mis-ID as a higher-conductive, good target. I'm just speculating, but thought the "ratio" type of idea might come into play.

Having said that, there is a particular type of small washer or grommet, with a large hole in the middle and a very narrow outer loop -- much like a small thin ring -- that exists, I'm not sure of the metallic composition. Anyway, the outer metal loop is less than 1/16" in width, and the outside diameter of this object is smaller than a dime. Anyway, I find these often, as on my Explorer they sound and ID just like a silver dime. While this sort of disproves my theory, in one way, in that I'd have thought the Minelab FBS units and their "ferrous and conductive" information would better ID these than other units, it does still make sense that while this metal is NOT silver, and probably not copper, but instead a lower conductor (brass, maybe?), it apparently still is effective at generating strong "eddy currents" -- leading to the mis-ID as a highly-conductive coin.

I may be way off here; just trying to wrap my head around all of this.

Steve



Edited 1 time(s). Last edit at 09/28/2011 04:49PM by steveg.

I'm not too familiar with the Minelab dual-axis display.One advantage it has is it gives you more information, but how you interpret that info. is the tricky part.
Re. the brass ring/ dime issue, I think it likely that they will appear identical to all VLF detectors.
For further reading to confuse you, I suggest looking on Minelabs website, where there is a very good Bruce Candy article explaining VLF and PI workings.

Pimento --

The Minelab "dual axis" display is simply taking the two numbers, a "FE" number (ferrous) and a "CO" number (conductive), and then plotting them as x,y coordinates on a grid. I don't use the grid as much, but instead display the numbers -- you as a user can choose to display the info in either way. I prefer the "two-number" display, while many Explorer users prefer the display where the numbers are gridded on the matrix (that you referred to). But bottom line is, yes, interpreting the two numbers is the key. One example...an 03-27 reading (3 FE, 27 CO) is often a wheat penny. However, 07-27 is most often a bottle cap. The CO number is the same, but the FE number gives you ability to discriminate between the two types of targets. As such, I usually won't even stop for an 07-27 target, but will always dig an 03-27 or even an 04-27 or 05-27 (clearly, there's more that goes into that dig/no-dig decision than just the numbers). My point is that there are times that having both numbers is helpful, as opposed to just a single-number ID. I had more trouble with differentiating these same two targets with my F70, whereas the two-number approach on the Explorers/E-Tracs makes this easier. Obviously, this all breaks down as targets get deeper -- just as with any other VDI-type machine...

Anyway, I will have to check out the Bruce Candy article.

Steve



Edited 1 time(s). Last edit at 09/28/2011 04:49PM by steveg.

That's what I thought it was, but didn't know if it gave any frequency-related info, as in - 'it reads aa-bb at 5KHz,cc-dd at 25kHz'.
Without reading the eTrac manual, I can't comment, it seems likely that these Fe and CO values are just as 'ficticious' as the single value displays are.

Pimento --

Nope. No "frequency-related" info on E-Tracs/Explorers. Sounds like you are referring to something like, as I understand it, the v3i can do. Not so on the Minelab FBS units. All the "frequency" stuff happens behind the scenes, on these units.

Steve

I'll throw another monkey wrench your way.
These ID numbers, whatever they turn out to be....probably on any given detector
an assigned ID number has to be interpolated by the ID circuit.
Perhaps similar to RMS.
So as one sweeps over a target besides just one number coming up, say, 45,
Something like this may occur: 37,39,42,45,47,49,51,53....but the display only shows 45 or perhaps one digit below and one digit above.
The point being the detector has to settle on one number......

Steve(MS) --

Yeah, I hear you. This is an awfully complex thing to understand, and interpret as a user. However, I still have found all this discussion (here and in the other thread) informative, as I better understand how the detector at least TRIES to both find, and then identify, targets under the coil...

Steve

Re. 'interpolation' :A detector running at 10kHz, say, effectively samples the ground at 10 thousand times per second. So a target will give a signal for at least 25 samples, I would think ( could be 200, even) so some filtering can and is performed. Analogue detectors do it, modern digital ones of course have the option of much more. More precise, steeper cut-off, variable parameters -'on the fly' or varying with user mode, rejection of 'rogue' data, FIR and IIR types, etc,etc.
If you read the F75 / T2 manual, you will see mention of the fact that audio and TID are independant- fast audio, delayed & 'averaged' TID.

Pimento --

Your level of understanding surpasses mine by an order of magnitude!

Steve

The key word in your post is "averaged".

EDIT:
In principal, I suppose your scan rate per second would be correct along with superior electronics.
Yet "in the ground" results tells a different story.
And scanning over a target even in an air test seems to show there is no true complete "lock on" to any one given ID number.
Perhaps the act of a moving coil causes eddy currents to flow differently thus giving different numbers.
The greater the ID scale tends to allow for more varied ID numbers.
One can observe on segmented meters and see that the lock on provides more stable readings provided the target signal isn't
at the break point between any given 2 segments.

Of course as depth increases, then the ID system starts to give even more erratic readings.
Ground minerals and object angle not to mention multiple objects under the coil at any given time.
VID separate from the audio has been out for many years.



Edited 1 time(s). Last edit at 09/29/2011 04:50AM by Steve(MS).

Candy's explanation in simplified form is:

The target ID displays the ratio of X (reactive) to R (resistive) with simplified formula of X/(X+R). If the target is nearly all X with little R, then essentially R goes to zero, the ratio become X/X = 1. The meter displays full scale (1) for target with 90 deg phase shift. But, if there is equal contribution from X and R in the target, then X nearly equals R, so with X=R as an approximation, the ratio becomes X/(X+X), or 1/2. The meter displays half scale for targets with 45 degree phase shift. If the target is nearly all R with very little X, then X ~ 0, so the ratio becomes 0/R = 0 (zero phase). The meter would read near zero with a high R, Low X target response.

But, since the soil reacts to the magnetic field too, you have to deal with non-target X and R responses that affect the receive coil. So, the X response is composed of the sum of target X and soil X, likewise for R. So, the simple ratio X/(X+R) changes to (TargetX + SoilX) / (TargetX + TargetR + SoilX + SoilR).

Now, if you remove the resistive ground signal by ground balancing, then SoilR ~ 0, and you have (TargetX + SoilX) / (TargetX + TargetR + SoilX).

Assume a deep target where TargetX and TargetR are weak or near zero, then ratio is SoilX / SoilX = 1, that is, meter display full scale due to reactive soil alone with a deep target. The point Candy was making is that low conductive targets (small TargetR) tend to read higher on his design MD meter display in mineralized soil.

Also, the ratio of X to R from a target can be compared against known sample target ratios stored in the MD circuit, so that the best fit provides a target ID symbol (e.g., nickel, foil, etc.)

That looks like the George Payne article quoted there. That describes how Discovery machines TID, one of the many different ways.
The actual 'single-figure' ID that detectors would give if they were laboratory instruments would be the phase lag in degrees. This varies from Zero through to -180 degrees. Near-zero would be the ground itself, -90 would be a low conductivity non-ferrous target, -180 something large/good conductor.
If you generate X and R components in the way commonly described, the phase angle is arctan( X / R).
Manufacturers choose more easily interpreted ways of presenting this value. Turning the values +ve, and squashing them into a 00 to 99 range is roughly what the Tek T2 appears to do. Fishers F series take that further by compressing the 0 to -60 range into 00 to 15 (-ish) and letting the -60 to -180 range become 15 to 99. (just my impression). Whites have also done something fairly authentic, mentioned in an earlier post.
I did have a quick look at the eTrac manual, but came away none-the-wiser - It didn't look like their Fe and Co scales related in an obvious way to the R & X commonly decribed.

Good 'technical' thread!

****That looks like the George Payne article quoted there. ****

Yep, my mistake. Thanks.

Interesting, Pimento. I will dig a bit more and see what I can find out about the FE and CO on Minelab FBS machines, and see what I can figure out. I am assuming that X, being the "reactive" component, could also be called "inductive," just a different terminology?

Steve

The FE on the MLs generally isn't as stable as the CO.
Very early on in marketing, especially from a particular place in Florida, it was thought (hyped) that that the Explorer could tell the diff
between a clad and silver dime based on FE readings.
Also there was some milder hype about it being able to tell the diff. between gold rings and tabs....

My thinking is the FE is similar to the percentage/iron reading on the Whites gold detectors.
As with anything else, the FE reading can be used to one's advantage provided he knows its limitations....that is when to "listen" to it or when not to.
IRT the FE, the cursor "bounces" are a product of it.

Since the data is there (is it?) why don't more manufacturers do something similar to what Minelab (E-Trac's) do regarding two numbers - one representative of ferrous I think? It gives so much more information to the searcher.

My V3i is similar in that in pinpoint I can see each of the 3 frequencies strength on the target. This is a Godsend! The order alone gives information on the target. I don't really even need to use the analysis features. e.g., Even with coins at depth you can often narrow down what silver is by the 2.5kHz frequency hitting the hardest.

Steve(MS) -- I fully agree. Knowing when and how much to "listen" to the FE number is huge on the Explorer. It's different, to some degree, with the E-Trac, as it tries to normalize the FE number on good targets to the "12" value...

earthmansurfer -- fascinating stuff with your v3i...and how you are using the different frequencies to determine what is silver, such that when the 2.5 kHz freq. is "hitting hardest," it's more likely to be silver. I still think this is a fascinating machine, from what I am hearing...

Steve

For the price of the high end models, we should have a computer doing all sorts of analysis on the signals.
Perhaps the Whites is the only one to start moving in that direction.
Strangely, regardless of model/brand, we all do rely on the circuits to hopefully give us a clue about target ID.
Think about it....we all have to make a decision like...iron or not iron or very low non-ferrous, nickel category,etc.
Some claim they don't rely on ID...they do yet may not know it....else they would have to dig every metal signal out of the ground......

steveg - When you pinpoint you get a bar going from left to right based on the targets strength. If you hunt in one frequency you only get one bar. I hunt in 3 frequency almost exclusively and see three bars, 2.5 on the bottom, 7.5 in the middle and 22.5 on top. So, on that nice silver DMark I dug I saw the VDI was 91 in one direction and the top 22.5 frequency was wrapping around a little bit. I did a pinpoint and saw the 2.5khZ frequency was by far the dominant one, followed by 7.5. That is a good sign of silver but it could be another metal or large target.

If I got the preceding VDI and 22.5 was dominant (which would be strange), that would tell me something was wrong. A more typical situation would be a deep target with a mid VDI that is dominant in 22.5. If there is any breakup in the signal (due to iron reject set low), if the pinpoint is off center, I am usually confident it's iron. I also run in correlate, which the DFX had set, and that helps a lot with coins in iron or in heavy mineralized ground.

Coins almost always hit with the 7.5kHz frequency dominant as that frequency is dominant in the middle VDI's.

I don't dig much iron, dug two pieces last hunt (6 hours) and they were both round rings that were deep. I hunted 6.5 hours and got 9 coins, didn't dig much trash (aluminum gets me sometimes though there isn't much of it.) The machine helps cut back on trash without costing me coins. I dig a fair amount of signals (two ways) that others probably wouldn't dig and often they are coins. It's like any detector - sound is #1. Watch my video on the V3i in iron (earthmansurfer69), watch the spectragraph in pinpoint to see what I mean regarding the preceding. The analysis features if you want to use them, can really help with gold rings but I am not interested in low VDI targets, the rings are so few and far in between that it's usually not worth my time.

The machine is mostly turn and go for me these days. It just took a few weeks to get the hang of it.

Is it possible the FE number is tied also to the freqs choosen? The Whites DFX uses 3 and 15 Khz, these freq If you use either one of those separately they spread the number to allow you to better define certain targets. I believe it was Andy Sisbish that said the FE number was merely another plot on the graph..... that to me made so sense without a reason. I do however use the smartscreen over the digital just because of those jumping numbers. The smartscreen allows me to see them PUT TOGETHER so to speak. Much like the XS and other machines the Con numbers are the most stable.... but dont always tell the true story. I believe that additional FE digit is whats made the ML so popular. Go thread.... even thou i flunked math lol. Nice to peak in on some experts discussion.