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MINELAB

A

Anonymous

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OK BRUCE CANDY!!
Come on in!! Eric FOSTER has - why don't you also do the same??
You market a very successful product range - goodoh!! Please just share some of your thoughts - not enough to embarrass you commercially, but wenough to direct the thinking of many like me - genuine tinkerers. Your credibility would be magnified many times if you did so.
And everyone could grow.
g.
 
Hi Graeme,
It could well be that Bruce Candy and other detector designers would like to post on a forum such as this, but they are prevented from doing so by company policy. I am a free agent so I can please myself, but I'm sure it would be very different if I was an employee of a company.
Eric.
 
Eric, Your comments accepted. I learned through 30+ years as a Police Officer that you can always turn a smile into a snarl, but you can never turn a snarl into a smile.
And I can only but wonder how could amyone possibly hobble himself through commercial considerations. Free thinking is surely the absolute. Anything else is prostitution.
g.
 
SD's PATENTS AND THEORY
Wow, this forum is a buzz of activity. Graham I don't think you'll get Bruce on. Its about time I made another contribution. A great deal of discussion relating to the SD is misinformed or worse, blatantly incorrect. I don't think many people really understand the theory behind the multi pulse machine or will acknowledge the fact that it constitutes one of the biggest landmarks in P.I. signal processing. Bruce Candy's patent 5576624 should be compulsary reading.
On goes the teachers hat I'll try a simple explanation. Bruce's breakthrough came from the empirical observation that different pulse lengths produced different remenant ferrite decays in mineralised ground. Take two altenating pulses one twice as long as the other. The ferrite decay from the long pulse will be exactly twice the length of the short pulse. It has done exactly the same thing but at half the speed. Conversely, THE SIGNAL FROM A METAL OBJECT WILL DECAY IN EXACTLY THE SAME WAY FOR EITHER THE LONG OR THE SHORT PULSE.
Knowing these variables and relationships gives us the potential of finding eg. a weak gold signal which might PREVIOUSLY BE HIDDEN IN THE FERRITE DECAY.
In other words the gold response may be hidden in the decay of the longer pulse but suddenly manifests itself in the shorter ferrite decay of the short pulse.
How do we process? We might multiply the short pulse result by 2 and subtract if from the results of the longer pulse. If the result does not equal 0 then we have probably detected metal in the target volume of the shorter pulse. The multiplication by 2 in this example is the "time constant" Bruce talks about in patent 5576624 but doesn't elaborate on. Intellectually it is a very elegant solution to P I ground balance problems and the implication is that the more pulse length results we have the greater the chance of finding weak metal targets in a basically ferrite volume.
Most (all??) other P I ground balance attempts like Corbyn or Eric's Gold Scan are "one dimensional" in that they continuously subtract one stored ground balance vector from the incoming signal and as such have very limited dynamic range. In practice JC the shorter pulses are mostly used to find the "goodies" in the SD series. The longer one is usually only a reference point. Graeme why are you trying to retro the far more complex 2200 the earlier machines are far simpler and are entirely analogue? ALL Minelab coils use litz. I present this in the interests of understanding.
Yours Jim S
 
I agree. Even though I work for Analog Devices I can participate all I want on these forums as long as the topics are not work-related. But if I were to discuss the internal design of our ADC's on a public forum I might get into some trouble.
- Carl
 
Hi Jim S,
Ok! You're not Bruce Candy, but I welcome you as his messenger - and interpretor!
A belief I have is that the more open an innovative thinker is to sharing his findings, albeit with due regard to commercial undertakings, then generally, the more valuable the contrabution proves to be. I champion Bill Johnson of Audio Research in this regard - in consequence I enjoy (grasping here for some modesty) fabulous home-built valve sound.
I have struggled at length to understand Bruce Candy and 05576624. And no, I only glanced in the direction of the SD2200 (when the Tx pattern was posted) - just to see where the foremost thinking may have gone. I admit my limitations don't extend beyond analogue function - and even that I struggle with. Its something akin to answering "How long is a piece of string?" I've just got to sit back and unravel the knots before I can attempt to answer. Isn't that what we are all engrossed in by visiting this forum?
Thankyou for your commentary. You have advanced my appreciations tremendously. I am sure that I will have questions bubble-up in future. I ask you retain your teachers hat and be available to assist.
Regards,
g.
 
Hi Jim,
One thing bothers me; in my experience, altering the width of the transmitter pulse does change the shape of the decay curve of a non-ferrous object. If you start with a 50uS pulse and a copper penny as a target then wind the TX pulse up to 250uS width, the shape of the decay waveform changes and lengthens. The reason is that the 50uS pulse is short compared to the object time constant and therefore the full eddy current range is not excited. Increase it to 250uS and the field is there long enough to fully excite the object, giving it its true decay time which becomes exponential after one time constant. This is something that is easily set up on the bench and observed.
Eric.
 
Hi Jim
Well here I go again repeating myself, Eric must get tired of this, but to sum up, you got ferrous, ground and goodie signal in both pulses.
The coil inductance, coil resistance, etc all go into the equation as to the specific geometry of these two transmit pulses. Sawtooth waveforms instead of nice square waves.
These items also effect the geometry of the return signal, fast or slow amps (wrong amp) get their bite on the signal and add some random noise and bit of distortion (whoops). Course you got to make some decisions (compromises) on battery life and weight of a product someone is going to buy (cost)(cheaper amp).
Of course all this is in addition to way the metals actually react. So a nice theorical equation in a patent is good proof to argue about in court, but then implementing this dream can be another story. All sounds good on paper, but in practice always another story.
More decisions about what is small ferrous or what are ground changes. Make it work for objects close and far. Quite a challege to do perfectly, still a few things left to clean up.
Still believe you will get the biggest return signal for a large gold nugget or silver dollar with the longer pulse (up to a point, and then the same). Could be wrong depending on what has been built. But the detector has to work for large objects too. In fact the detector needs to have about an 80 to 100 db dynamic range. (My opinion).
I use litz wire and can tell the difference for a very large (52 inch octagon coil)(nothing magic about the octagon, just could get the 45 degree CPVC joints) over 18 gauge teflon wire. The litz is equavilent 17 gauge. Single coil. (no it is not the difference in gauge or resistance). It is faster. And therefore I get a slighty greater difference signal.
About 10% difference in range, (many yards) for large objects (buddy's Jeep Grand Cherokee). This is an air test.
JC
 
'JC,
You are starting to worry me. This is a forum for thinking people - people who are fascinated or absorbed in the science of detector technology. You appear as a thinker, but why the hell do you keep on about Courts????? I know about Courts, but if you want to talk this way, then let's go elsewhere.
g.
 
Graeme,
<"This is a forum for thinking people - people who are fascinated or absorbed in the science of detector technology.">
This forum was set up to help, and inform all uses of PI detectors, not just those absorbed in the "science of detector technology". We welcome your technical input, but it was not set up to hash, and re-hash, what or who
 
For some reason it did not show up in my post when I hit enter.
Please follow the link below.
 
Hi Graeme,
Bill has a good point. When you look at the recent posts it is confined to just a handful of us with deeper technical interests. We ought to widen the discussion out to cater for the less technical user who still wants a good knowledge of how PI
 
Hi, Just to let you know, there are some of us who are enjoying this technical discussion immensley. I have learned a lot. I've noticed that several posts start out with a general question about a brand name detector and end up a technical discussion about PI in general. Its probably hard to have a discussion about PI detectors without it naturally becoming technical. I for one appreciate the technical side of the school and hope it doesn't stop. Thanks, Charles
 
Bill,
Are you banishing me to Eric's forum?
I know where Erics Forum is. There have been 14 postings there in the month of May - none by participants to this forum.
I know where Carl's forum is. There have been 2 postings there in May.
The heading to this forum currently counts 392 postings for May.
I entered this forum on the 8 May under my true name and e-mail address. I have posted 33 times - in several of which I have attempted to encourage posting by others.
I believed that forums were essentially self-defining. I don't see that my sentence which apparently got up your nose as limiting, defining, excluding, or dictating to any person in any manner.
But you want to define for me what my input should be. Are you going to do this for everyone?
You charge me with hashing and rehashing and pursuing a battle of wits. I posted with solely one objective - not to preach - but to learn. And that I have done in great measure.
Charge me with over-enthusiasm, and I will plead guilty.
g.
p.s. Carl, thankyou for the e-mail.
 
I am certainly enjoying and learning, from the "in depth technical discussion" as well as the practical information. This "school" has become my first stop when I go on line.
Enjoy Hunting,
Jim Madar
 
No one is banishing anyone.
You said your opinion, I said mine. Let it lay at that.
Let the school continue. PI anyone ?
Mr. Bill
 
I know little about electronics other than simple bridge circuits, diodes and transistors but sort of can catch the gist of almost anything that is well written and I have gotten a bunch out of this entire discussion. I even bragged you guys up over on the BBS Forum so others could take a gander...
Thanks Eric for staying with this... Guvner..
 
Thought part of the posts for the last months had to do improving on what has already been done.
But right enough this needs to be backed off technically. Obviously, many have never built any detector from scratch. Electronic knowledge is limited.
But improvement over what has already been done or making custom units is going to take a bit of techinical knowledge.
JC
 
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