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automatic Balance - Eric?

A

Anonymous

Guest
Hello Eric,
can you please explain the circuit "bridge feedback network", performing the automatic balance in the PPD1? Can you post the schematic?
thank you
Walter
 
I am also curious as to how you did this. I have done electrical nulling on vlf and it is a pain, there is a patent out there called Nulling of the Imbalance or something like that which talks about this subject.
JC
 
US patent 5729143 titled "Metal Detector with Nulling of Imbalance" give some information on this subject.
Abstract- A metal detector includes a receive coil and a transmit coil connected in an inductive bridge. To overcome imbalances in the bridge due for instance to misaligement of the coils or the presence of mineralization in the medium which is being examined, the metal detector automatically .....
Hope this helps.
Anytime IB coils are needed to do metal detection I automatically return back to a past life where it appeared to me to automatically limit what performance could be obtained. Looking at signals during turn-on in a PI is one of these. It is neat, but in the end if I am limited in power and gain and end up back at vlf depth performance, its hard to get too excited. Something like this is needed for vlf to make any big steps forward (my silly opinion) and this patent is for vlf. Maybe needed for PI too????
JC
 
A method for active loop nulling is described in patent US 4,628,265 Johnson et al.; which, by the way, used to be one of Candy's favorite patents. (G'morning, Bruce! Sorry circumstances don't allow you to make posts here.)
Prior to the mid-1980's, there was quite a bit of interest in active loop nulling.
I don't happen to know how Eric did it, or what the primary reasons for it were.
Concerning what else could be said about this subject matter, my lip is zipped for now.
--Dave J.
 
Just after I made the post about keeping my lip zipped, I saw JC's post "nulling of imbalance", wherein he seems to know where this trail leads.
Go for it, JC!
(dang, that guy's good, ain't he?)
-- Dave J.
 
Sorry about blabing it.
Been there, done that, it worked, it sucked, ain't going to do it again. I had yet another way of doing this, for a walkthrough metal detector. Lots of power availible for electonic parts, lots of room, 32 bit fast motorola processor, so lots of room to move. Also had to do 12 channels, six on each side.
These things never pan out as good as they do on paper either. It gets you that much further, but it has its limits too, and then you hit em.
JC
 
Because of my business interests, there are certain things I can't say in public. However, if someone else happens to know the same thing and then blabs it, especially here where everyone can see it, I'm quite delighted. It's the relatively open exchange of information on this forum that in a few short weeks is allowing us to rewrite the book on how PI is done.
So please blab anything and everything you can, JC. Your contributions to this forum have been invaluable.
--Dave J.
 
Hi Dave,
Thanks for the good words. Wish I knew more about all this cause you're right, I'm not restricted by what I say. Can't you tell?
Still have alot of respect for you, and Eric, and as I've stated before even Bruce Candy believe it or not. Of course any engineer who has been abused, and beaten up over the years by CEOs or whoever, has compassion for fellow engineers, technicians, etc. Still got a soft spot for those who actually do the work and the technical thinking, knowing how painful it can be.
Could this be where the sarcasism comes from?
Maybe some charge too much for their detectors, (maybe they don't) but if they sold em for $300, alot of other companies could lose alot of their sells (or something else).
Like your bluesky thinking, that's the kinda stuff real ideas usually come from. Try to keep posting, but got to get back to work here too.
JC
 
Hi JC, Walter and all,
One of the features of the PPD1 is that the balanced coil part of it is not needed until you get a signal. The centre coil of the stack is a normal TX/RX, and the receiver channel connected to this behaves as a conventional PI giving audio output for ferrous and non-ferrous. All the time there is no signal, the discriminator channel holds the discriminator coil balance in a tight feedback loop. The output of this channel is displayed on a centre zero meter which is rock steady until a signal is received. Once the signal on the conventional channel exceeds a certain threshold, the tight feedback is released and the sampler takes a peek at which way the balance has gone. A meter then displays this as a left or right indication. Good conductors give a big deflection to the right; poor conductors hardly move the needle at all. Ferrous material always deflects to the left. There are two problems with this scheme which need to be solved and hopefully, with the expertise on this forum, perhaps we can do it. More later, plus some schematic info on the balance arrangement.
Eric.
 
You're welcome.
The sarcasm is born from the frustration of knowing what can be done, and being unable to find a company which will pay me (or apparently anybody else, for that matter)to do it.
What has been the response of American metal detector companies to what we've been doing here the last three weeks? Beats me. I've got no contacts at Minelab, no inside dope, but I have twenty bucks that says they understand what they're looking at (possibly better than we do), and that they're already slinging solder and making a mad scramble to get some more patents filed. Would anyone like to bet against me on that?
In a few short years, Minelab went from nothing, to the undisputed technological leader in the industry. This happened in plain view: any company which thinks that technology is important has an example to follow and even improve on. That removes all excuses, doesn't it?
Sorry, there I go getting sarcastic again.
The last three weeks on this forum have been a whole different world than the one I'm used to operating in. You can't imagine how much fun it's been.
--Dave J.
 
Not quite the same subject, but the PI CAN be redone.
A couple of years ago I inadvertantly discovered a way of getting a PI to discriminate, AND IT WORKED!!!
If you look VERY closely at the dv/dt of the LOWER portion of the decay from the coil, then you'll see that it changes, one way for ferrous materials, and the other for NON ferrous.
Sampling this decay using a Digital scope, with a delay on the trigger, made the near vertical edge of the initial pulse, move one way or the other, depending on the nature of the target, i.e. iron or gold. this movement is VERY pronounced.
A nifty bit of circuitry to emulate this, would entail a delay in the signal path, but the trigger would be immediate.
Sample using a 14 bit A to D, store the results in RAM, then finally analyse using a micro.
You'll find that the depth this system will pick up a
 
I wouldn't embrace your phrase "abused & beaten up" etc. On the whole, this industry has treated me fairly well, allowing for the fact that all human enterprises are pretty badly flawed, and I ain't no saint myself. On most of those occasions where someone shot me in the foot, they were shooting themselves in the foot and accidentally happened to hit us both. And there have been times when managers have cut me more slack that perhaps I deserved.
For my first 11 years at Fisher, I was given pretty free reign when it came to technology, which helped to make up for some of the other areas in which engineering management was sorely deficient. Overall we did all right for a bunch of amateurs.
--Dave J.
 
Hi guys,
My discriminator uses the same waveforms but different circuitry. The signal from the Dual D coil was amplified and applied to a sample and hold circuit.
The circuit is a two op-amp instrument grade difference amplifier. The inputs are very high impedance and the CMRR is very high.
The non inverting input uses an analog switch U1a and a capacitor C1 to make a sample and hold detector. A half monostable C3 and R5 fires the analog switch from the + edge of the TX pulse for about 2uS.
The inverting input also has a capacitor C2 to ground. This capacitor is connected to the non inverting input via a second analog switch U1b.
During operation, U1b is closed. The inverting and non inverting inputs are the same and there is no output from the amplifier.
The regular "R" signal is demodulated seperately as per a normal PI. When the "R" signal reaches a certain threshold, a voltage comparator circuit opens analog switch U1b.
The result is that the non inverting input is demodulating the "X" input and the inverting input is now storing the last value of "X" before the target caused "R" to hold the signal. This last value of "X" was due to the ground signal.
The difference amplifier now provides a zero to positive signal or a zero to negative signal as you go over the target. I am in the process of improving this circuit so as to provide a more linear operation than using an "R" threshold switch.
I have found that the signal is not all "X". This can cause some low conductivity targets to read as ferrous. To fix this, sum in a small amount of -R from the PI' regular "R" demodulator.
Now all we need to do is to add the PI ground balance circuit and things will get to be very interesting. All the best, Dave. * * *
OK Let me try to attach the schematic.
 
I haven't worked in Electronics for a lot of years now and only in the last 2 years began playing with the soldering iron again. It used to be nice to have others around to toss ideas around with and to help when you get stuck. I miss that now but you guys are so far ahead of me I hesitate to talk about my simple little pittely old problems, I wouldn't want to waste your time <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)"> Right now my pi circuit is working on the bench but I'm having trouble with the audio. I thought that was going to be the easy part <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT=";)">
Good luck all,
Charles
 
I wasn't sure I understood the description of the test setup.
I was left wondering:
1. what made the timing independent of the
target distance from the loop?
2. was high-conductivity nonferrous tested?
3. what did ferrite do?
4. was this indirectly measuring flyback
duration, something like the Periscope?
Clarification would be welcomed by those of us who didn't get it the first time around. Thanks.
--Dave J.
 
Hi there.
I've been toying with the idea of sampling the
TX-off voltage with a 22 bit A/D and doing a
fourier analysis on it. This should pick up
any signals like this that may have been missed
in the past.
Has anyone ever connected a spectrum analyser to
a PI coil?
The TX on/off waveforms from Eric's PPD1 look
very rich in information and look like they
could provide better discrimination than any IB.
I wonder if the TX-on waveform information is
exactly what the square wave driven multi-
frequency IB's are in effect using.
Cheers
Malcolm
 
Here in the UK, any new ideas discussed on this
formum or disclosed in any way could not be
patented.
Cheers
Malcolm
 
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