How a COIN looks to the Electronics. TECHNICAL

[Cody]

A use coin can be analyzed and represented as a simple L/R circuit. L is the inductance and R the resistance. They are considered to be in series with a sequence of long and short pulses as the source which is the TX of the detector. The long and short pulses have both high and low frequency components. This is difficult to visualize but can be seen with Fourier analysis. In short however a square wave contains multiple frequencies. The Explorer is interested in 28 of those dominant frequencies.

The ratio of L/R is the time constant. A coin has a dominant time constant which is called the frequency of the metal. Using pulsed time domain analysis a high, low and medium time constant can be evaluated and compared to the predicted time constant of a coin. Some % of variations from the predicted time constants indicate if the target is presented as a coin or rejected as trash.

Iron also has an L/R ration but has permeability. The magnetic properties of the metal and permeability will generate and very short time constant. We can see this if we observe Lenz

 

[Cody]

Ok, this is the last technical post for now but I need to get this off my chest. We know how a coin looks to the electronics but how about the iron?

Iron looks to electronics different, as we can imagine, than a coin. A piece of iron in the soil minding its own business has the magnetic domain in random. The iron has inductance and resistance also bothering no one. A pulse of energy causes the magnetic domains to align with the electromagnetic field. Eddy current flows and self and energy is stored in the eddy current generated electromagnetic filed. We would have a very interesting situation here if not for the resistance/impedance of the metal and Lenz

 

[Cody]

Let me get this in while it is still hot off the press.

Conductivity and depth of detection is often confused as to how they are related. A silver dollar has not more conductivity than a silver dime. If we look the atomic structure of metals it is the same for silver no matter how much silver there is in the sample other than the number of electrons available to carry current. A silver dollar has more capacity to conduct eddy current than a silver dime. When an electromagnetic filed is used to create eddy current flow in a metal the term used is conductance. The conductance of a silver dollar is greater than the conductance of a dime although the conductivity is the same.

If we take a sample that is 1x1 inch and see how far we can detect it from the coil and compare that to a sample that is 1X1 inch of the same metal we will detect the large square target at a greater distance. We can detect a target with a larger surface area at a greater distance. So surface area is a major factor in detection depth and the ration of L/R and permeability is how a target is identified.

Consider a nickel and why it is a problem. If we use silver as an example the conductivity is constant other than the slight change due to temperature. Nickel on the other hand it is an alloy and if we look at the conductivity it has wide variations. Also, a nickel will corrode while silver tarnishes for the most part or wears down. A nickel will produce a wide variation in time constants. It is more difficult to predict if a non-ferrous target is a nickel.

Jewelry is almost always precious metals alloyed with other metals for hardness. It is kind of like mixing dirty water with clean. We don

 

[Captain Kirk]

Cody,

#1 THE EFFECTS OF THE CONFIGURATION OF THE TARGET

An interesting thing about a 1"x1" piece of silver is that you can remove the majority of the material inside the outer perimeter (leaving a square ring) and still detect it at the about same depth. The reason is because of "skin effect". As the frequency is increased, then the eddy current is forced further to the outside perimeter of the conductor leaving very little current flowing in the interior region (See #A below). That is the reason why Litz wire was developed. Litz wire uses multiple parallel strands of smaller gauge wire wound in such a manner that the overall high frequency resistance of the wire can be greatly reduced using much less metal than if a single strand were used. But remember that Litz wire is much more expensive than a single strand wire.

Another thought is to consider a sterling silver ring. You will find that you can take the silver in the 1"x1" plate and turn it into a 1" circular ring (with a very thin but very wide band) and produce much higher conductivity at the higher frequencies than the 1"x1" plate.


THE USE OF LOW FREQUENCY TO DETECT GOLD

Gold has considerably lower conductivity than does either silver or copper. When higher frequencies are used, then greater voltage is developed (by the detector's magnetic field) in order to generate eddy currents. But, the eddy current is forced towards the outer perimeter making the effective resistance increase dramatically. This effect reduces the eddy current and the effect on the magnetic field. This makes the gold target harder to see. This MAY BE (but I could be out in left field on this one) the reason why the Explorer uses some long duration pulses (for gold targets) and short duration (for silver targets).


#3 CONSIDER THE RESPONSE TO A FERRITE TARGET

How would you explain the effect of running a piece of ferrite material under the coil. The ferrite has essentially zero conductivity but extremely high permeability? The eddy current would be essentially zero. Where would you expect it to hit on the Smart Screen and why?



#A THE MECHANISM OF SKIN EFFECT

As I have frequently stated, a perfect conductor plane (zero resistivity material) will not allow a changing magnetic field to pass through it perpendicular to the plane. The math and physics for this are very complex, but the net result is that all of the current will be forced to the outside perimeter of the conductive plate.


Keep up the good work,
Glenn

 

[Cody]

Ferrite does not support eddy currents so it is not necessary for the ferrite to have conductivity. ( We don't need eddy current flow in wood to conduct a magnetic field.) The magnetic dipoles will align in the direction of an applied magnetic field. Unless the applied magnetic filed is intense enough to permanently aligned the dipoles they will return to random alignment. The pulses of energy from the detector are selected so that no permanent change of the physical structure of the ferrite, permanent alignment of dipoles, takes place. The target volume consisting of the salts, slightly conductive, concentration and types of ferrite constitute a target with a time constant. I think this is about 1us which is very large. So the display is to the left of the screen in the IM-15 area. However, we might have soil minerals that have very little iron oxides so the soil is slightly conductive or a lot of iron oxides with very little conductive salts so the target soil volume can be anywhere from 0/31 to 31/31.

Now how about pure ferrite? There will be no indication at all as the detector will be balanced to pure ferrite. But we don

 

[Cody]

Boy, these are real good. I am printing many of them for my folder on detectors.

 

[Captain Kirk]

Cody,

I have no complaints about your posts. Sorry if I offended you. I was just looking for you opinion on a couple of matters.

My point about the ferrite material (used to make low loss cores for high frequency transformers) is that the permeability is very high and the conductivity very low. Therefore, it will hit over on the left edge, but at the very bottom of the screen because of essentially zero conductivity (no eddy currents in the ferrite). It should produce a low pitched growl in either CONDUCTIVE or FERROUS modes. My main point is that you do not have to have eddy currents to detect a target. The ferrite will greatly distort the magnetic field and increase the coupling coefficient to the receiver coil.

What are you thoughts about the configuration about silver target?

HH,
Glenn

 

[Cody]

I must be the worst when it comes to expressing myself. You did not and have never offended me. I greatly enjoy your posts and look forward to them. Let me look at the post. I know from experience, I was a radar technician back in the late sixties, and know at high frequencies current starts to be "pushed" away from the center of the conductor. In a way it is as if the center is a very small conductor with many other conductors consisting of layers around the center. The current will move to higher rings of conduction with an increase in frequency. We can cut out the center the conductor and the conduction is not changed to an ultra high frequency. I have taken pulltab tested them by cutting the ring and twisting them in different shapes. I also took a gold ring and cut it and behold could not detect the right just by current the ring. If I bend the ring straight then I can detect it like it is a nail. My assumption is the shape has a dramatic way of altering the flow of eddy currents.

With the top end of the Explorer at 100khz I did not expect this. I thought we needed to get up to the MegHz or even to Gig. This is an area that I think about but have not really come to the point where I think I have it understood well enough to be comfortable with it.