TDI ground balancing scheme, twice over

[porkluvr]

Eric Foster's CS4 uses a CD4017 to divide the input signal by ten. With three clocks
used to excite the TX coil, that leaves seven clock (time slots). Primary and
secondary samples are spread among the remaining seven slots.

By adding a flip flop and an analog switch you can force the CD4017 count to go as
high as eighteen (but the count must be an even number, divisible by two). By adding
an inverter it is possible to count to twenty, but that is not where I am going
because it gets hairy when you try to implement a twenty count in the scheme I
describe below.

I am attempting a design with two channels working at the same time. Channel 1 (CH1)
counts to 18 then resets, and CH2 counts to 16 then resets. Using two different diode
OR gates allows for having each channel to excite the transmitter for differing
numbers of clocks.

I have figured out a way to interleave the two channels so that either CH1 and CH2 can
output between one and nine cycles (inclusive) before the other channel takes over.

That means, you can interleave the transmit signals with 1:1, 5:3, 8:5, 3:5, 9:1 ratios
(and etc., you get the picture). Or, use either channel alone. The interleave ratio in my
simulation screen-shot picture is 1:4.

What I am attempting to do is replicate the TDI ground cancelling scheme twice over,
with each channel timed differently so that the detection holes in the two channels
will not overlap.

I do not have the math skills to analyze my circuit to determine where either
detection hole will be, and that is why I am here.
Can anybody help me with the math, or logic (or whatever) to try and ascertain that
the two detection holes will not overlap? My hope is that if a good target is missed by one
channel, it might still be detected by the other channel.

I almost hate to divulge my idea because some other rascal might be able to bring this
beast to fruition before I can (I'm like a slow leak) - but here it is. Each channel has separate drive circuitry
and different values of series resistance can be used in each channel to limit the two coils'
maximum current. You could (or should) have different coil tc's for each channel.

You could use a monocoil, no problem. One possibility for this design is to drop
the TDI ground cancelling scheme, instead, using a dual or center-tapped coil and
transmitting each channel with opposite polarity. For that matter, a OoO coil
might be employed, but that discussion will be left for another day.

As I understand the TDI's ground cancelling scheme (thank you Carl, for the info)
ground cancelling is accomplished by taking four samples, TGT, GB, EF1, EF2. The
receiver's post amplifiers are arranged to implement the following formula:
A(TGT-EF1)-B(GB-EF2), where A and B represent differing levels of gain (and B>A).

Now, again, how does the timing get analyzed to determine where detection holes will fall?
I suppose the math is over my head so that I may just as well rely on trial and error
(which I can be very good at) but I need to ask, just the same. Calculus strains my brain but
we all need to broaden our horizons, right? I'm still trying to "proof" my circuit before committing
it to PCB but most of the layout is done (just hoping I won't feel the need to do any extensive rework).


Thanks for any and all replies.

[Qiaozhi]

Isn't there a basic flaw in this idea, or have I missed something?

With the TDI ground balancing scheme there will always be a unavoidable "hole" in the target response. This "hole" must be there because this is where the ground signal appears. Unfortunately though, any desired target that happens to have the same TC as the ground matrix will also be eliminated.

Now, if you create two separate channels with their own ground samples, this will simply create two "holes" in the target response. I agree that any desired target which falls into the hole of channel 1 (for example) will be allowed through in channel 2 ... BUT, so will the ground signal that is supposedly rejected in channel 1. So how can you distinguish between the two, since the the ground response and any target with the same TC are inextricably linked?

[Orbit]

I do not understand so that no holes signal of targets crossed the ? possible hole patched




in several ways , but then i think that no passing by targets ! or maybe I'm wrong ?

[Orbit]

http://www.youtube.com/watch?v=lq7Ua...ndrrakFCfpDWw#
also the smaller the holes undergoing very little targets in my approach the two channels .

[Qiaozhi]

http://www.youtube.com/watch?v=lq7Ua...ndrrakFCfpDWw#
also the smaller the holes undergoing very little targets in my approach the two channels .

Setting up the TDI ground balance system to reject ferrous targets only works for small nails ... AND they have to be all the same size and shape. Anything with a different TC will be outside the "hole".

You can think of the ground balance system as a reject notch that cancels the signal for targets of a specific conductivity, which is fine if your mineralized soil is fairly homogenous.

[Orbit]

If I understand we have coin diameter for example 1cm iron and coin of bronze of the same diameter iron coin gives the difference in tone compared to coin 1 cm bronzed the bronze no double ton ,iron double ton and if target larger like for example pliers should not fall into in hole and Of course
no sounds of a double ton she is out of the hole Do you think so? thank you !

[green]

Don't know if this makes sense or not. The chart has 4 TC curves and a ground curve with a 1/t slope of -1. To reject ground, sample the ground curve twice. A straight line between the two points defines the target TC hole. The first sample for the lines I added is 10usec but could be anywhere as long as the two samples fall on the ground curve. To have a TC hole at 10usec the first sample would have to be sooner than 10usec. If the two samples were 2usec and 10usec the TC hole would be 5usec

[porkluvr]

Isn't there a basic flaw in this idea, or have I missed something?

With the TDI ground balancing scheme there will always be a unavoidable "hole" in the target response. This "hole" must be there because this is where the ground signal appears. Unfortunately though, any desired target that happens to have the same TC as the ground matrix will also be eliminated.

Now, if you create two separate channels with their own ground samples, this will simply create two "holes" in the target response. I agree that any desired target which falls into the hole of channel 1 (for example) will be allowed through in channel 2 ... BUT, so will the ground signal that is supposedly rejected in channel 1. So how can you distinguish between the two, since the the ground response and any target with the same TC are inextricably linked?

Dang Qiaozhi, you have busted my bubble.

Have you missed something? Probably not, but maybe I did. What I failed to mention is that I would welcome critique of my overall method... so lay it on me. My understanding of the TDI GB concept is obviously a little shakey.

So what you're saying is that when the TDI's GB method is applied, no matter how the sample timing is implemented, ground tc determines which target(s) may go undetected and thank you for pointing out that basic flaw in my scheme. I won't expect a miracle.

I had read in some other threads where (as I recall) it was suggested that there could be some way to insure "complete" coverage (my words) by parallel processing two channels operating with slightly different sample timings. That's what got me thinking I needed to go to the trouble to kludge two extra sample pulses between each transmitted pulse. I already had the basic complex waveform scheme worked out on paper, but without the ef samples.

Well, all is not lost. There should still be an advantage by having two different coil tc's and two discrete primary sample delays operating in a parallel pipeline.
It appears that for "complete" coverage I will need to drop the TDI scheme and have the two channels transmitting with alternating polarity.

[Qiaozhi]

If I understand we have coin diameter for example 1cm iron and coin of bronze of the same diameter iron coin gives the difference in tone compared to coin 1 cm bronzed the bronze no double ton ,iron double ton and if target larger like for example pliers should not fall into in hole and Of course
no sounds of a double ton she is out of the hole Do you think so? thank you !

I think a lot is getting lost in translation, but I assume you're referring to the ability of the TDI to generate 2 tones. A high tone for low conductivity items, and a low tone for high conductivity. These tones occur either side of the hole, and anything in the hole gets rejected.
However, since all this is common knowledge, I'm not sure how this relates to porkluvr's original post.

[Qiaozhi]

Dang Qiaozhi, you have busted my bubble.

Have you missed something? Probably not, but maybe I did. What I failed to mention is that I would welcome critique of my overall method... so lay it on me. My understanding of the TDI GB concept is obviously a little shakey.

So what you're saying is that when the TDI's GB method is applied, no matter how the sample timing is implemented, ground tc determines which target(s) may go undetected and thank you for pointing out that basic flaw in my scheme. I won't expect a miracle.

I had read in some other threads where (as I recall) it was suggested that there could be some way to insure "complete" coverage (my words) by parallel processing two channels operating with slightly different sample timings. That's what got me thinking I needed to go to the trouble to kludge two extra sample pulses between each transmitted pulse. I already had the basic complex waveform scheme worked out on paper, but without the ef samples.

Well, all is not lost. There should still be an advantage by having two different coil tc's and two discrete primary sample delays operating in a parallel pipeline.
It appears that for "complete" coverage I will need to drop the TDI scheme and have the two channels transmitting with alternating polarity.

I've also heard this idea of two ground balance samples being used to overcome the hole problem, but no-one has elaborated any further than that. Perhaps we're both missing something.

But, having said that, there are no ground-balancing PI detectors on the market without an annoying hole in the response.

[porkluvr]

I've also heard this idea of two ground balance samples being used to overcome the hole problem, but no-one has elaborated any further than that. Perhaps we're both missing something.

But, having said that, there are no ground-balancing PI detectors on the market without an annoying hole in the response.

Wouldn't transmitting with (equal but opposite) alternating polarity pulses fill the bill? Or, does it...?

edit: but you did say "on the market".

[porkluvr]

Don't know if this makes sense or not. The chart has 4 TC curves and a ground curve with a 1/t slope of -1. To reject ground, sample the ground curve twice. A straight line between the two points defines the target TC hole. The first sample for the lines I added is 10usec but could be anywhere as long as the two samples fall on the ground curve. To have a TC hole at 10usec the first sample would have to be sooner than 10usec. If the two samples were 2usec and 10usec the TC hole would be 5usec

Green, you lost me, too many blue and bluish lines (I get drawn in by what looks like an optical illusion).

I'll keep trying to figure out your meaning.

Incidentally, I do not really know how to determine if a target response falls on or off a line using analog means, (but let me say just for completeness, I don't know how to do it digitally, either). Maybe I'd better ponder that notion.

[green]

I've also heard this idea of two ground balance samples being used to overcome the hole problem, but no-one has elaborated any further than that. Perhaps we're both missing something.

But, having said that, there are no ground-balancing PI detectors on the market without an annoying hole in the response.

I just trying to learn PI detectors. Don't know how the TDI works, so maybe I'm thinking some different. Gave an example in reply #7 how I think it might work with two different sampling channels. Probably doesn't if no one does it. Each channel eliminates ground, but not all of the target signal eliminated on the other channel.

I apologize for the graph not being clear. I'll try to come with a better way to explain it.

[Bill512]

I've also heard this idea of two ground balance samples being used to overcome the hole problem, but no-one has elaborated any further than that. Perhaps we're both missing something.

But, having said that, there are no ground-balancing PI detectors on the market without an annoying hole in the response.

George...maybe the Minelab SD2000: two independent transmit ,receive and ground balance channels.

[Qiaozhi]

Wouldn't transmitting with (equal but opposite) alternating polarity pulses fill the bill? Or, does it...?

edit: but you did say "on the market".

Bipolar transmitters allow you to dispense with the Earth Field sample, as the Earth's magnetic field (and even external noise) will always be the same polarity, whereas the bipolar pulses (by definition) will be of opposite polarity. Therefore, if you invert one of the samples (e.g. the negative pulse sample) and add it to the positive pulse sample, the Earth field is automatically eliminated. If you wave a fridge magnet near to a PI with bipolar pulsing there is absolutely zero response, unlike a unipolar transmitter.