Ground Balance Theory

[Dave J.]

Referring to Eric's post #400, the reason the Tiva Tuff viscosity drops in the presence of the external magnetic field is because Tiva Tuff is impoverished with respect to SP particles. The external field pushes the behavior of the SP particles closest to the SD boundary into SD behavior by pushing the BH curve harder into the saturation region.

--Dave J.

[Aziz]

Pity, this thread is stagnating. It is very obvious to see, that some market players don't want to see some progress here.
(I didn't have a tin plated hat on. *LOL*)
But there is an another forum you can look for.
Aziz

[Ferric Toes]

Viscosity Anomalies

Having done a considerable number of measurements on magnetic soils, and latterly with greater precision, a few questions emerge regarding the behaviour of different samples. There are also small differences in slope between some samples which are noticed at early times and this could be due to differences in grain size distribution.

I reproduce a page from my notes here to stimulate further thinking, as the fact that there are differences may impact GB filter design.

Eric.

[Ferric Toes]

Long time has gone since I last posted something truly stupid here, so, here it is:
This writing is about one interesting design approach to two sample substraction method, I tried it on prototype with great success. Typical approach, like used in GS detector is two channel, two integrator approach, then substraction is done on integrator outputs. Modified version is with two complete circuits, motion filter included, and substraction done at filter outputs. Highly not recommended however, very high precision and quality\matched components are needed to match response of both channels. I ended up with something else, varying amplifier gain during second pulse to achieve GB adjustment, short description goes like this: Detector is bipolar pulsing (same polarity pulses on bifilar coil, so field is bipolar), no need for EF canceling pulse. Amplifier is differential 2 stage (4 op-amps used) , conveniently AC coupled, differential output too, followed by one single ended integrator. Now, two pulses are used, second one for GB, (exact width, delay and gain change subjected to some experimentation). Timing circuit switches integrator input from one amplifier output during first pulse, to opposite (inverted) one during second pulse, to enable substraction, but amplifier gain is changed after first pulse (to allow settling at new gain), so GB is adjusted by varying (increasing) gain during second pulse. All this in reverse order during reverse polarity pulse. Integrator output will go in one direction for objects below preset TC, in another for longer TC, and remain unchanged for one particular TC (that of ground signal, this is the point of ground balancing). Timing is not complicated at all, only two chips. To disable GB\disc, second pulse is simply disconnected in timing circuit. Not only this achieve nicely adjustable GB, but have some potential with similar design using large bifilar coil and longer, high power pulses. Now GB is not important, but some amount of “discrimination” can be achieved using same method, based on individually adjustable object TC and size criteria. Fortunately, large objects are more predictable in behavior. Aside, bipolar pulsing is very suitable for completely static operation, nice for large coils. This so far works just fine, I hope will have more time soon (my current contract expire end september ) to finish complete design, and publish schematics and all, until then, any opinion or idea on this subject?

You do yourself a injustice as it is a good and practical idea.

I have used a similar scheme for some time i.e. two stage preamp with second stage as a balanced cross coupled differential output with overall gain of 400. two samples in each channel for EF rejection and GB, summing into two matched single time constant integrators, then subtracted in a differential input following stage with gain adjustment in GB channel.

Eric.

[maikl]

I have no idea but I can make the detector, when you post a scheme... try it on the field and comment on the advantages and disadvantages...
Best regards.

[Tepco]

Single integrator ground balancing

Long time has gone since I last posted something truly stupid here, so, here it is:
This writing is about one interesting design approach to two sample substraction method, I tried it on prototype with great success. Typical approach, like used in GS detector is two channel, two integrator approach, then substraction is done on integrator outputs. Modified version is with two complete circuits, motion filter included, and substraction done at filter outputs. Highly not recommended however, very high precision and quality\matched components are needed to match response of both channels. I ended up with something else, varying amplifier gain during second pulse to achieve GB adjustment, short description goes like this: Detector is bipolar pulsing (same polarity pulses on bifilar coil, so field is bipolar), no need for EF canceling pulse. Amplifier is differential 2 stage (4 op-amps used) , conveniently AC coupled, differential output too, followed by one single ended integrator. Now, two pulses are used, second one for GB, (exact width, delay and gain change subjected to some experimentation). Timing circuit switches integrator input from one amplifier output during first pulse, to opposite (inverted) one during second pulse, to enable substraction, but amplifier gain is changed after first pulse (to allow settling at new gain), so GB is adjusted by varying (increasing) gain during second pulse. All this in reverse order during reverse polarity pulse. Integrator output will go in one direction for objects below preset TC, in another for longer TC, and remain unchanged for one particular TC (that of ground signal, this is the point of ground balancing). Timing is not complicated at all, only two chips. To disable GB\disc, second pulse is simply disconnected in timing circuit. Not only this achieve nicely adjustable GB, but have some potential with similar design using large bifilar coil and longer, high power pulses. Now GB is not important, but some amount of “discrimination” can be achieved using same method, based on individually adjustable object TC and size criteria. Fortunately, large objects are more predictable in behavior. Aside, bipolar pulsing is very suitable for completely static operation, nice for large coils. This so far works just fine, I hope will have more time soon (my current contract expire end september ) to finish complete design, and publish schematics and all, until then, any opinion or idea on this subject?

[Monolith]

I added an integrator to the circuit. I am getting a change with the clay at coil off. About 5 mv change at 5 to 10 usec. Less at 40 to 80 usec and less at 80 to 160 usec. The change is opposite polarity of targets, coins and nail. Not what I expected. Any suggestions what I might try or what I might doing wrong. Maybe post should be in ground theory on the bench. Either I don't understand theory which is true or I'm doing something wrong in hardware. I'm building one part of the circuit at a time and trying to understand whats happening.

During ON time, that is, at any time that there is a current running in the TX coil, the ferrous response is opposite to the non ferrous response. This also applies to the TX coil current decay.

One way to differentiate ferrous and non ferrous targets with a mono coil PI, is to observe the target during the TX current decay, by prolonging this decay so as to have enough time for sampling.

While current is running in the TX coil; with a clad coin that has a ferrous core, the response of the core being opposite to the outer metal response, the 2 responses sometimes cancel each other partially.
When turning the ferrous core coin on edge, the magnetic response is enhanced and the non ferrous response eddy currents are diminished because of the small area of exposure of the coil magnetic field.

It would be helpful if you would show the circuit as far as you are using it, with the results.

For testing targets with a specific TC, it is good to use a ON time of 5 TC and a near linear TX ON current curve. The coil curve and the target curve are superposed. A good way to separate the 2 curves is to use an IB coil arrangement which greatly reduces the TX coil response due to the TC of the TX coil and due to the eddy currents within the TX coil winding.

[green]

I plotted some decay curves in excel. Not sure if they are correct or make sense. The amplitude for the first sample starts at 100. One plot doubles the integration time each sample. The other plot integrates equal time each sample. I think the plots show where the holes are if you do ground balance. The sampling is done in binary steps. I'm thinking the curves are valid for any clock frequency. Four counts equals 5 usec, 8 usec, 10 usec etc.

[green]

I added an integrator to the circuit. I am getting a change with the clay at coil off. About 5 mv change at 5 to 10 usec. Less at 40 to 80 usec and less at 80 to 160 usec. The change is opposite polarity of targets, coins and nail. Not what I expected. Any suggestions what I might try or what I might doing wrong. Maybe post should be in ground theory on the bench. Either I don't understand theory which is true or I'm doing something wrong in hardware. I'm building one part of the circuit at a time and trying to understand whats happening.

[green]

Interesting graphs.

Instead of looking at the trigger voltage, looking at the coil current during ON time, will give much more information. The TC of the target becomes apparent and you can see the difference in eddy current amplitude with different coil TC and target TC.

Thanks for the reply. I've been wondering if I could discriminate ferrous/nonferrous looking at on and off times with a PI. I was trying different targets, coins and nails and it was looking promising when I tried a nickel. Flat was ok. On edge looked like nail. Then I noticed it was a Canadian nickel. Clay from yard causes a large signal during on time and would have to be ground balanced. I'm wondering if other grounds decay with a TC during coil charge time and what amplitude difference I might see between on and off times with severe ground conditions. Tried different coil TC by adding resistance in series with coil. Changed signal TC.

[Monolith]

Interesting graphs.

Instead of looking at the trigger voltage, looking at the coil current during ON time, will give much more information. The TC of the target becomes apparent and you can see the difference in eddy current amplitude with different coil TC and target TC.

[green]

Plotted a nickel and a quarter during on and off time. The slope for the nickel is close, the quarter not. Forgot to divide by 5 for the ground TC above post.TC 60 usec for the clay and lava during on time.

[green]

I've been playing with a PI circuit with a DD coil looking at coil on and off signals. I tried a piece of landscaping lava rock and some clay from the yard. I get a signal during on time, not much if any during off time. Exponential, 300usec TC. Should I expect the on slope to be different than the off slope(power)? Are my samples not typical? Excel plots, target - reference.

[Tepco]

For delays <20uS the decay appears to have another faster curve superimposed on it. This should become more apparent when I run a modified timer to observe signals at 5, 10, and 15uS where the discrepancies can be seen. Till now, I thought that it could be an instrumentation error, but as I see it with the faster coil, it may well be real. This has implications for those trying sub 10uS sampling, as the amplitude will rise even faster than it does with the fundamental decay.

Eric

[ATTACH]25626[/ATTACH]

This is interesting, I noticed something similar. By all chance, there are actually 3 exponents. First, usually fastest one is dictated by coil and changes only with coil, another one, lasting to about 10uS is target related. Varying with shape and material, in not quite logical and predictable way. Also, strongly dependent on pulse width, gradually fading out at very short pulse excitation. After that, 15us or so later, everything is quite regular single exponent. How this can affect real detector is less important, very fast setup is needed for this effect to be visible. I'm using old and obsolete LM733 (NE592) video amplifier, in fully differential connection, but at least no speed limit and recovery issues with it, measurement after only few hundred nS is possible.


Another interesting effect I noticed is related to flyback release time and shape. Mostly related to “wet sand effect”, I can be wrong with this, but here is my conclusion, may be relevant to soil samples too, so please correct me if i'm wrong. I never figured out what is exactly going on with this wet sand stuff, salt water alone, or sand alone produce no response, but in right combination (and right sand type, ordinary sand for construction work will not respond, something grain size related probably), problem appears at short delays. Now, effect that looks like very short TC object happens. Only, problem is by all chance not initiated by TX pulse, independent on width, but by flyback cycle dV\dT in relatively low conductivity medium. Slower rate of change produce less effect, exactly opposite of what fast coil designer is trying to achieve. Not only longer sampling delay, but intentionally slower coil seems to help here.


I played with this effect, and figured out that intentionally underdamped coil, with flyback release in form of one complete ringdown sine cycle significantly reduce response. With flat spiral coil, each cycle is only 1.5-2uS, so delay is reduced only by 3-4uS, still fast enough, but more stability with same delay compared to “optimally” dumped coil. Even, at some empirical adjustment, consisting of 5-6 cycles of exponential ringdown, large, some 500g ferrite is completely ignored by detector, everything else being unaffected?!? Unfortunately, not soil sample or red brick, so nothing about “passive” ground balance. Still this may affect response from low conductivity soil samples, also may be interesting for underwater detectors. Any idea, what i'm missed, my measurement error or something else, these effects are real. Devil is in the waveforms , positively.

[Paul99]

...my answers will be brief and clear.

Em noise, powerline and ground noise do however limit the performance of ML detectors in many areas such that the maximum detection capability is not realisable.

...

Eric.

Eric, my understanding is that increased emi, powerline and ground noise amplitude affect the depth capability of all high performance detectors. Could I ask you to make one or two points more clear for me please....

Am I right in understanding your statement that "Em noise, powerline and ground noise do however limit the performance of ML detectors in many areas such that the maximum detection capability is not realisable" to infer that it is possible to design a usable detector that will realise full detection depth in the presence of emi, powerline and ground noise?

And are you aware of any detector that is unaffected by emi and powerline noise that has depth performance comparable to the Minelab GPX series of detectors when working under powerlines?

The reason I ask is because there are many thousands of km of powerlines throughout Australian gold bearing areas, and your statement that powerline noise will "limit the performance of ML detectors in many areas such that the maximum detection capability is not realisable" infers that ML machines are not the ideal machine for working these areas. However, my experience is that other makes of detectors in the hands of experienced users have demonstrated less depth capability under powerlines than ML detectors achieve.

Your advice on a more appropriate detector for these areas would be appreciated.