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Some units have a dynamic ground balance in disc: two posts by DJ, and One by George :biggrin:
Posted by: vlad
Date: April 27, 2018 12:18AM
(((If nothing else call up the factories and ask--an adjustable disc balance adds depth)))
Dave: There is no one ground balancing system that’s best for everyone under all conditions. Some experienced users prefer manual balancing because of the control it gives them, but it takes some skill. Some tracking systems out there just plain aren’t very good—I won’t mention any names. In the early 1990s, Minelab introduced single-frequency gold machines with a ground tracking system that was good enough to be worthwhile. Its fondness for tuning out targets and losing ground balance in the process was a bit of a nuisance, but it wasn’t a fatal flaw, and it opened up prospecting to people who couldn’t get the hang of manual ground balancing. My mission at Tesoro was to come up with a better ground tracking system, which led to the Lobo ST. It wasn’t perfect, but it was much more resistant to tuning out targets and much faster to reacquire ground balance. The mission at White’s was to develop a ground tracking system that was as least as good as that on the Lobo ST without infringing Tesoro’s IP (intellectual property) rights. The new tracking system took most of a year to develop and get working well, but the result was slightly superior to that in the Lobo ST.

Having designed the two best ground tracking systems in the industry, I am painfully aware of their limitations. They are fairly resistant to tracking out a target if you keep your sweep wide, but it’s just instinctive to narrow your sweep when checking out a target. Now the system becomes destabilized and no two sweeps sound the same, while the target itself disappears and the ground gets noisy. I decided that working here in El Paso, I wasn’t going to put ground tracking into a product until I knew how to solve the problems of ground tracking and get it right.

One manufacturer’s advertising has tried to convince people that a ground tracking system is always balanced to the ground under the searchcoil, therefore avoiding the loss of depth that results from changing ground conditions. That’s just plain false. Ground tracking systems respond to historical ground data and therefore are balanced to what the ground was (for instance) a few feet or tens of feet away and not to what’s under the searchcoil right now, if the ground conditions are changing.

So what we’ve done in El Paso is “ground grab.” The system is constantly collecting ground data. When ground starts to get a bit noisier than you think it should be, you tap the button, preferably while bobbing the searchcoil, and bam! You’ve ground balanced. It won’t tune out a target—you can loiter over the target to check it out.

When we first introduced the T2, a lot of customers asked, “But where’s the automatic tracking?” We replied, “Don’t worry, with ground grab you don’t need it,” and they discovered it was true. Within about 6 months the whole issue of “where’s the ground tracking?” went away. Customers ask us for a lot of things, but ground tracking is something they rarely ask us for.

Chris: Many of your more recent machines have been multipurpose type units designed to be used for coin shooting as well as nugget detecting. Certainly with the interest level of a $1,700+ per ounce gold market, there is a demand for these units. However, what do you see as the future for the no compromise, gold-only type of higher kHz VLF optimized for extreme detecting of small gold? (I have a number of friends who are enthusiastic devotees of the units you have designed in the past that fall into this category.)

Searching in All Metals, ground balance is a necessity to get decent depth in most soils. The only machine we manufacture with an "autotune" (first derivative response) all metals mode that doesn't have an explicit means of ground balancing, is the Teknetics Delta-- which also has a fixed-phase all metals mode based on motion discrimination for knocking out the ground signal in places where the first derivative all metals system is too noisy on the ground.

I mostly agree with 5900, searching in disc mode for shallow and medium depth targets, in most ground, there's very little difference in performance between fixed-phase ground balance and variable ground balance circuits/algorithms. In public parks and tot-lotting you darn well better not be digging targets 10 inches (25 cm) deep even if the machine can find targets that deep. I've known people swinging the lowly BH Tracker 4 (air test about 6 inches) who could clean up in parks and schoolyards better than most people swinging $1,000 machines because the people with the fancy machines were trying to run at high sensitivity and fighting electrical interference and iron masking.
Second Post:
Regarding the issue of "carrying ground balance over to disc mode from all metals", a general rule of thumb is this:

1. The older machines that were mostly analog circuits without very much if anything being done in a microcomputer, ground balance doesn't carry over to disc. This is because to do it right requires additional circuitry and in most cases the engineer figured the extra cost didn't buy enough extra performance to make it worth the trouble.

2. Most of the newer designs do most of the signal processing in a microcomputer, and carrying ground balance over from all metals to disc mode adds little or no manufacturing cost and not much additional software engineering work. So on machines that have some form of variable ground balancing, it usually carries over to disc mode.

There are a few exceptions to the above rules of thumb, but in general they hold true.

George Payne:

Fixed (preset) vs Adjustable GB; and coil design.

A pure ground is a soil condition that reacts like it was pure ferrite. In other words a perfect magnetic condition where no electrical conduction (eddy currents) takes place. We can think of this as a soil that produces a signal in the detector with zero phase shift relative to the transmitted signal. This is considered our reference signal of zero phase to which all other signals can be referenced to. Of course the only real life object that produces this type of signal is pure ferrite. So ferrite becomes our reference target and produces what we call a pure "X" reactive signal.

Of course real ground conditions do not behave like pure ferrite. When subjected to a detectors magnetic field small currents begin to flow in the soil. This will cause the soil signal to be displaced slightly from that of pure ferrite. We call this difference a phase shift and define it to have an angle in degrees negative relative to pure ferrite. In addition, this phase shift produces a new signal in the detector which we call the "R" component signal. We can carry this analysis one more step. Using Trigonometry the ratio of the X signal to the R signal can be shown to be the actual measured phase of the ground.

All grounds have varying amounts of magnetic and conductive properties. Therefore, the ratio of the X or magnetic signal and R, the conductive signal, will vary from one location to another. However, the phase produced by this characteristic will always be negative relative to zero, the phase of pure ferrite.

From my experience most grounds produce a phase that falls somewhere between zero (ferrite) and a -5 degrees. Some highly magnetic soils can have a phase that is quite low, but it can never be zero. Once the phase exceeds several degrees the ground characteristics begin to fall into an area where it becomes more saline. This doesn't mean that its not magnetic. Its just that the R or conductive component of the ground becomes stronger in relation to the magnetic portion. Thus the phase becomes greater.

The manual ground adjustment works in this manner: When you position the “Ground Adjust” control to the phase of the target, in this case the ground, any up or down motion of the coil does not produce a corresponding change in the audio volume. For example, when you position the control to zero phase, and then move a piece of ferrite around near the coil, the audio volume will not change. In other words you have balanced out to the ferrite. However, if you now lower the coil to real ground the audio will increase in volume. Of course this indicates that you are not balanced to the ground. As you begin to turn the control counter clock wise the ground adjust control phase changes from zero to a more negative amount. Once you have reached the point of “ground balance” the control and ground phases match. Of course as the coil is moved to various locations the ground phase changes slightly and you must readjust the control for a neutral reaction. As you can see there is no one control phase position that matches every condition since the ground phase varies from one location to another.

The introduction of the Motion detector solved this problem.....sort of. In a Motion detector design you can calibrate the “fixed” ground adjust control phase to approximately +0.5 degrees and set the audio threshold for silent operation. If that is done the detector will appear not to respond to the ground. In reality it is responding. Its just that you don’t hear it since all ground reactions cause the audio to decrease in volume.

And since the audio is already silent you don’t hear anything. Remember I said that all real targets, which includes the ground, have a phase between zero and some negative value. The preset ground control phase of +0.5 degrees is in a location where no real targets ever exist. Therefore, you never have a condition where you are balanced to anything, least of all the ground. As you move the coil over the ground, the internal detector signals are continually being driven negative. Any weak positive target signal is easily over-ridden by the huge negative ground signal. Of course, if the target is close enough to the coil its positive signal can override the negative ground signal and you will hear the reaction in the audio. The greater the phase and strength of the negative ground signal the more it will mask the positive target signals. A manual ground balance design would avoid this since the operator can adjust the control for a (near) neutral reaction on the ground.

For fixed machines the phase error between the internal “ground preset balance” and the actual ground condition can be much more than “slight”. The internal preset is calibrated for +0.5 degrees. This is in an area where a real ground phase never occurs. The actual ground phase may be -2 or -3 degrees “negative“. That’s a huge difference, maybe 2.5 to 3.5 degrees. This much phase error will in effect cutoff several inches of detection depth.

Re: Some units have a dynamic ground balance in disc: two posts by DJ, and One by George :biggrin:
Posted by: Mike Hillis
Date: April 27, 2018 08:28AM
Thanks Vlad.

HH
Mike



Currently own and use:
*****Fisher F75 LTD w/DST ***** Whites V3 ***** Teknetics G2+LTD *****GoldBug Pro*****Minelab E-Trac*****Teknetics ETPro *****Teknetics Omega 8500***** Tesoro Golden Sabre II *****Tesoro Compadre *****Rutus Alter71*****Nokta Impact*****

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Very Good Reading! Thanks Vlad:thumbup:N/T
Posted by: John-Edmonton
Date: April 27, 2018 08:32AM

(This message does not contain any text.)


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As I think back the first auto ground unit was the RB-10 (Red Baron)
Posted by: vlad
Date: April 27, 2018 09:57AM
Circa 1982--auto set but not tracking. (but they pulled them back in for a modification--range was not wide enough.)
https://static.secure.website/wscfus/6268851/uploads/BHrb10sheet.pdf
George Payne developed an auto ground for the Tek Mk-1 and mounted it on his personal unit but it never went into production because it would have added $100 to the price.
When he made the Treasure Baron Cointrax it had one touch balance: hold loop about 2" above ground, push & release the right inner cursor key-no pumping, you heard a beep, it was done. (1/2 second)
Arguably still the fastest "gun in the west," had tracking and inhibit too--and adjustable autotune for stat (something the F-75 could add to the chip.)
(On the Mk-1 you can see the pointer move right or left-if its in disc.- as you adjust the ground pot.)

Not sure about today but preset ground units of the past
had a GB trimmer internally that could be adjusted to change ground reference point. Its a good thing to know.
A problem with preset ground balanced detectors is just the opposite in near mineral free soil:.preset detectors come ground balanced way too positive



Edited 2 time(s). Last edit at 04/27/2018 10:24AM by vlad.

I've read that before, but it's always good to re-post it. Good read. Thanx! N/t
Posted by: Tom_in_CA
Date: April 27, 2018 04:44PM
n/t

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Re: Some units have a dynamic ground balance in disc: two posts by DJ, and One by George :biggrin:
Posted by: Elton
Date: April 27, 2018 06:49PM
A nice post Vlad... Explains a lot.... Your knowledge and memory are outstanding.



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Re: As I think back the first auto ground unit was the RB-10 (Red Baron)
Posted by: Tom_in_CA
Date: April 27, 2018 08:44PM
Some day, 200 yrs. from now, someone's going to take all yours & Monte's posts, from all the various md'ing forums , and collect them to a single link. It will be a very good time-capsule on the detecting technology of the mid 1960s through the mid 1980s.
I distinctly recall that "lightening fast evolution" period of time, from about 1975 to 1985, when all that was happening. We went through machines like the 77b and 66TR. Then it was VLF/TR (5000d, groundhog, etc...). Then it was Red Baron SPD, 6000D , and 6db. Then all of the sudden it was Teknetics with their TID, and whites quickly followed suite by adding a very crude TID to their 6000's.

In perhaps 7 yrs. time, we had seen things go from the dinosaur age, to the rocket ship age. Very exciting times. If you had a machine that was a mere 2 yrs. old, you were painfully spanked, haha

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