Ground Balance Theory

[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.

[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]

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.

[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]

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.

[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.

[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?

[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.

[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.

[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.

[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

[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.

[Davor]

This one makes me wonder...

Many of the Earth minerals are ferromagnetic to some extent. The nuclear fission and radioactive decay reactions cause the Earth to be positively charged and in thirst of electrons. Einstein's photoelectric effect aids to this by knocking electrons into, eventually, ionosphere, leaving the Earth crust even more positively charged. The charges are distributed on the surface of the Earth so as the Earth rotates, the current flow builds up the magnetic field of the Earth. This is also helped by ferromagnetic minerals, and there you have it: the Earth magnetism.

Too often we hear about a changing polarity of Earth magnetism due to some bogus ideas of the magnetic iron core ... heated well past the Curie temperature, and thus by no means magnetic.

According to this, the only way for a magnetic pole to change direction is by rotating a whole crust against the axis of rotation.

[Tepco]

Wow! From where you get this?

[Davor]

By thinking too much. Somehow it does make sense

You know... no pain - no gain
No brain - no pain.

Or in Croatian, blago onom 'ko rano poludi - čitav mu život prođe u veselju.
(blessed is the one 'who goes crazy early - his whole life pass in joy)