2010 Lesson 1 - Flyback

[Eric Foster]

A common misconception is that the voltage flyback pulse has a direct bearing on the signal from a metal target. PI theory shows that it doesn't. The voltage pulse occurs simply because the coil becomes almost open circuit at the end of the TX drive and the collapsing magnetic field tries to maintain the current at the level it was just before cut off. This voltage rises till it exceeds the avalanche voltage of the Mosfet and then stays at this level until the magnetic energy has dissipated such that the voltage falls below the avalanche rating, and then the remaining energy is absorbed by the damping resistor. If you insert a small resistor in the ground end of the coil, the coil current waveform can be observed, which exactly matches the magnetic field waveform. There is no spike, but just a smooth ramp down of the current and field. Now, the ramp down is important in that it must be significantly faster than the target time constant, otherwise signal will be lost. Theory states that provided the ramp down is faster than 1/10th of the object TC, then it is as if the field were removed instantaneously. So, objects with a short TC need a fast ramp down and objects with a long TC can get away with a slower ramp down. Also, for maximum signal you have to factor in the duration of the TX pulse. This too has to be significantly different to the object TC to achieve maximum signal, in this case longer. Again the factor of 10 comes in, but having it 5x longer will still get you around 90% of the signal. Looking at the collapsing magnetic field from the target

 

[bbsailor]

A common misconception is that the voltage flyback pulse has a direct bearing on the signal from a metal target. PI theory shows that it doesn't. The voltage pulse occurs simply because the coil becomes almost open circuit at the end of the TX drive and the collapsing magnetic field tries to maintain the current at the level it was just before cut off. This voltage rises till it exceeds the avalanche voltage of the Mosfet and then stays at this level until the magnetic energy has dissipated such that the voltage falls below the avalanche rating, and then the remaining energy is absorbed by the damping resistor. If you insert a small resistor in the ground end of the coil, the coil current waveform can be observed, which exactly matches the magnetic field waveform. There is no spike, but just a smooth ramp down of the current and field. Now, the ramp down is important in that it must be significantly faster than the target time constant, otherwise signal will be lost. Theory states that provided the ramp down is faster than 1/10th of the object TC, then it is as if the field were removed instantaneously. So, objects with a short TC need a fast ramp down and objects with a long TC can get away with a slower ramp down. Also, for maximum signal you have to factor in the duration of the TX pulse. This too has to be significantly different to the object TC to achieve maximum signal, in this case longer. Again the factor of 10 comes in, but having it 5x longer will still get you around 90% of the signal. Looking at the collapsing magnetic field from the target

 

[grumpyolman]

Could somebody do a translation of what Eric said. I believe it because he said it. But, I don't know what it means. Jim

 

[Reg]

Hi Jim,

Eric is simply trying to explain what is important and what is not if a person were to look at various signals that occur on a PI. What he is saying is, when the pulse in the coil is shut off quickly, the current will try to do whatever it can to continue to flow at the same rate. Since the FET acts like a switch cutting off the current, the switch will act like an open circuit or a very high value resistor. The only way the current can continue to flow at the same rate is the voltage will now have to increase dramatically to keep the current flowing. So, we end up with a large voltage spike.

What is also important is this large voltage spike may look impressive but it really doesn't have anything directly to do with the signal we get back from a buried object. Now, indirectly this spike does have an influence but only because we have to wait for the spike to dissipate before we can examine the signal. So, what has to happen if we want to try to detect objects such as small gold, we have to get rid of that same spike as quickly as we can.

The next part of what Eric is saying is targets have what is called a time constant (TC) which means, the signal from a target will only exist for a period of time and that time is referred to as the target's TC. Now, this signal from an object commonly refered to as a target, will start out large and will decay or reduce to 0V at a certain exponential rate. In very simple terms, this target signal starts out at a level dependent upon the target itself and the pulse and will begin to drop down in voltage at a rate determined by the conductivity of the object. Low conductive objects such as small gold have a very fast time constant which means the signal comes and goes very quickly. To be able to detect such objects, we have to get the spike out of the way soon enough that when we sample, some of the signal from a small target is still there. Just how much signal is left that can be analyzed is determined by the relationship of the time constant of the target and when we can sample.

So, in essence, what has to happen if a person is designing a PI to look for small gold is the spike of the coil has to go away fast enough that the signal from the object is still there. This requires careful design of the coil and other circuitry to try to eliminate any reason that may cause the spike to decay longer.

I hope this helps.

Reg

 

[Ism]

Nice explanation Eric, however a couple waveform pics to demonstrate would help.

I have read a lot of articles on the topic of coil construction by Reg and others. I have been experimenting with coil construction. So after reading your lesson, it dawned on me...Does the damping resistor also reduce the coils effective gain?

Thanks

Randy

 

[Ism]

Could somebody do a translation of what Eric said. I believe it because he said it. But, I don't know what it means. Jim

If you pass a coil through a magnetic field it will induce current in the windings. Taking voltage away after each pulse, induces a current "fly-back" because the magnetic field is collapsing on the coil. This is how I grasp, hope its accurate enough to pass muster here.

 

[Eric Foster]

Hi Ism,

I hope to deal with this subject further with waveforms and maths. However, I was in Ireland when I wrote the post, away from my research notes and pictures. Now I am back in the UK and snowbound, unable to get to my workshop where the material is. As soon as I can, I will be pleased to do as you suggest.

Eric.

 

[TURNMASTER]

Holy cow. I came in to find out if I really want a PI, or at least learn alittle about them before I buy my next machine. I just may have to pay attention

Jeff

 

[bklein]

Given the concepts given so far, I would say that the ideal design would have a transmit coil separate from the receive coil.
The receive coil and circuitry would somehow be isolated from influence by the transmit pulse but then become active as soon as
possible thereafter. We then would be working with a signal level that could be better characterized and compared against known
signatures and differentiated from those. Using a single coil is a simpler implementation but we sacrifice having to work with receive signals
of less than the +/-.6V diode drop we use to limit our input receive range to. As some targets are said to have faster but shorter response to
the transmit pulse we risk missing them as this influence to the coil could be while there is still higher than .6V amplitude of the flyback
pulse. So when we mess with coil design we can use less turns for faster flyback decay - but we don't have then enough turns to get
good receive amplitude. In addition, many designs seem to use a signal sampling method that compares a first sample to a later sample.
The gotcha is where the first sample is set to sample the output. If later it risks missing fast but shorter responses. If too soon it could
miss data entirely and just be sampling the .6V clamp voltage multiplied by 1000 or whatever the first amplifier gain is... Anything
wrong so far?

Barry

 

[The Quest of Captain Kurt]

Good Morning Eric Foster,

I was considering purchasing a Whites TDI, and while attempting to find a forum where folks would be comparing different metal detectors; I found a mention of your new GoldScan 5C.
I understand that you were an integral part in the development of Whites TDI, and perhaps Minelab's PIs. Since I'm a guy that likes to tinker a bit, I practically decided that the TDI was
for me.

Please tell me if you think the GoldScan 5C is better or is an all around more advanced machine than the Whites TDI for prospecting in California's gold country, which like Australia,
can often be greatly mineralized with iron, red with rust and black with magnetite. I'm mostly looking for a detector that has a very short delay time and can locate small gold, perhaps
as small as a BB or a pepper corn and bigger, at best depth possible and good ground leveling. Also, I'm looking for a detector that is a real pleasure to use in the field, preferably one
with a long battery life, that can be recharged.

Please advise me of the caveats, and give me your pros and cons. And what your opinion is.

Thank you.
Your Capt.

 

[Flyguy]

Eric, this is really interesting... can't say that I understand it all but being in the market for a PI machine, do the Minelab multi-voltage (strength) and Multi Perod Sensing features give their PI detectors any advantage? Thanks, Flyguy

 

[Reg]

Hi Capt,

There are a lot of similarities between the TDI and the GS 5 C, with the GS 5C having a few more controls. From a depth capability standpoint, the two detectors are really quite close in overall capabilities.

The TDI was designed to hunt in the areas you mentioned. Since a PI doesn't respond to many of the hotrocks that plague a VLF, it can easily become the detector of choice in many gold producing areas simply because of ease of use.

As for the size of gold one can find, PI's do not find gold as small as VLF's as a general rule. There will be exceptions and some small gold will be found, but overall, a VLF will do better on the really small stuff. Now, keep in mind that no two pieces of small gold will respond the same, so there is no exact size that is safe to say will be detected. I have nuggets weighing 8 grains (about 1/2 gram) that very few PI's will detect at all and those PI's that can detect it have been modified specifically to find such small gold, and I have nuggets in the 2 grain range that can be detected with a reasonable signal by many PI's. So, it all depends upon the gold as to whether a small nugget will be detected or not.

As a safe rule of thumb, most nuggets 2 to 3 grains and larger usually can be detected while smaller sizes may or may not create any signal at all. Again, there will be exceptions both larger and smaller.

The nice thing about the TDI and the GS 5C is both generally ignore magnetite. One might lay the coil down on pure black sand and get a strange response, but raise the coil a little and normally the black sand will not cause any signal. This is what makes a PI a preferred choice in many areas.

Both the TDI and GS 5C use Li Ion batteries with a reasonably long run time, maybe up to 8 hours or so. The batteries are rechargeable. The TDI does come with 2 batteries, which is an advantage of sorts.

Both the TDI and the GS 5 can use ML PI compatible coils as well as coils specifically designed for the two detectors. A Minelab (ML) PI can't use TDI or GS 5 coils.

Reg