Why Are Top-performing Dry Land Pulse Induction Detectors So Frickin' Heavy?

[Gold Catcher]

One thing to remember is that it is not just about weight but also about ergonomics. The GPZ weighs 1.5 lbs more than the SDC but I can swing the GPZ much easier/longer than the SDC. The 6000 shows how weight and ergonomics can be dramatically improved and my guess is that the next GPZ will follow this trend.

GC

[Geotech]

19 hours ago, jasong said:

I dont know enough about detector electronics design to know what exactly is required for design specs. But from a pure physics perspective there is another way to achieve a lighter coil and higher db/dt, and that is to use less windings (larger gauge) and a higher voltage, low internal resistance power source that is capable of supplying large currents. The larger the current the larger the B, and the less windings the faster B can change (dB), meaning dB/dt increases. 

Low inductance coils should be lighter since less windings, even with larger gauge wire, if one keeps the B field the same as before though. Plus they decay quicker. 

On the TX side, fewer turns & lower inductance is a benefit. But in a mono coil that same winding is also the RX coil and the induced target signal is proportional to turns. As it turns out, in a traditional PI detector the 300uH coils we use is roughly the right balance between TX field strength, RX signal strength, and decay speed. However, there are other non-traditional methods where 300uH is not the right answer. In some cases more turns are better, in some cases fewer.

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When I was researching coilguns, the way to do this was to pulse discharge high voltage capacitors through a coil with relatively few windings. One coil I wound only had 5 windings, but many thousands of amps of current.

Coin shrinkers work the same way, except for the side effect that the coil literally explodes every time. I figure this to be a good way to optimize a PI design: increase the cap discharge energy until the coil explodes, then back off slightly. 😉

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I'm sure there is some reason they don't do this on a smaller and safer scale with pulse detectors. But I'd be curious why since it seems like you could get lighter coil, higher sensitivity, and faster rise and decay times versus the low current, high inductance coils we use. Voltages/currents too high to be considered safe? Or some detector specific engineering reasons? FCC regs on radiation from the coil? Oversaturstion of the soil? I'm curious if a detector engineer could explain, as there has to be a good reason. 

Realistically, a 300uH coil can be very lightweight. Most VLF detectors have TX coils with higher inductance, and then add an RX coil. Many PI coils are heavy because they were made heavy, not because they needed to be.

To answer your last question, yes, there are ways to design a system with low inductance & high current. It requires using a low drive voltage off a buck regulator but also using a high-voltage kick-start circuit to achieve a reasonable rise time. Similar to what the GPZ does. This would obsolete the massive stock of 300uH coils we currently have, which is a reason companies & customers are reluctant to any changes.

To the original question: PI weight is dominated by batteries and the TX circuit. Pulsing amps of current in a total-loss system means things get hot, and heat requires a way to dissipate. Ergo big parts and heat sinks. Again, there are other ways to do PI that might get around this problem. Also, if you open up a CTX there is probably a half-pound of toroid ferrites inside for EMI suppression. I would assume the same in some of the Minelab PIs.

[jasong]

1 hour ago, Geotech said:

To answer your last question, yes, there are ways to design a system with low inductance & high current.

Thanks, I was hoping you'd respond, informative answer as always!

I have one more question that is probably stupid: when eddy currents are induced in the target, during this period of change, there must also be radiated EM occurring during the transitory (rising/falling) periods in addition to the mag field that detectors couple with. Is this portion of energy converted to radiated EM not useful to detect? Or maybe this part is too small and is attenuated entirely by the ground first and not possible to detect at surface?

Reason I'm wondering is because in air, radiated EM decays much slower than magnetic fields. They also have an E field component in addition to B field, so more options are opened up for detection maybe. And for instance, with interformetry and other noise reduction techniques we can detect EM radiation from across the universe with some high degree of accuracy (for instance, with radio telescope arrays). So as long as some portion of the radiated EM escapes from the ground, now metal detectors are not restricted to simply a coil waved as close to the surface of the ground as possible and certain other design ideas could be implemented instead which don't make a person's arm fall off.

I hesitate to ask this for potentially obvious reasons that get into bearing similarities to other..."devices".  But still, I'm curious.

[Geotech]

I'm certain there is EM energy getting radiated because we measure it when we get detectors FCC-certified. But it's primarily from the circuitry, not the coil or a target, and it's usually in the VHF & UHF bands and comes from switching noise.

I suppose it's theoretically possible that fast transient eddies (as from a PI) could produce enough high energy EM radiation to be detectable. But detecting that radiation then requires a proper antenna which is going to be about as large as a TV aerial (the FCC testing labs often use a log-periodic array that looks suspiciously like a TV aerial).

BTW, GPR does pretty much what you propose, except that they use a proper antenna and frequency for dealing with EM radiation. And then they respond to both scattering and dielectric absorption, meaning that a lot of non-metals will give it fits. There are also combo metal detector + GPR instruments used in land mine detection, Minelab has been involved in this effort.

A few years ago there was a fellow on TNet who was claiming to be developing a new method of detection. I speculated that he was trying to pick up EM radiation just like you suggest. No entirely sure, and then he up & kicked the dust and left even the people who were loosely involved in the project not knowing what he was doing. I wrote a letter to his widow but never got a reply.

[jasong]

Thanks again. In my head something like a really simplified, watered down GPR was what I was thinking about in a way. Simplified insomuch as no imaging would be generated, instead it'd just be an indicator that "something" was there, much like a PI, but breaking out of the "coil on a stick" paradigm we have. And dig everything.

Getting into arrays of antennas, much like radio telescopes lets one perform many magnitudes more levels of noise cancelation than with signal processing alone, such that extremely tiny signals might theoretically be detected. Scaling sounds like a problem for recreational users, but for more serious exploration I wonder if there are potential applications.

Anyways, just something I've wanted to ask for years. I was wondering if it might be scaled smaller and be used to get rid of the detector weight problem all together, but sounds like not. Thanks again for the input!

 

[Lanny]

If I only had to hunt nuggets on relatively flat ground, I probably wouldn't mind the weight as much, but going up and down mountains all day long, not a fan of the weight at all. Switched a long time ago to external, light-weight battery packs to try to help, but still zero fun while working mountain sides and steep gulches/canyons.

So, the main issue of weight that started this thread (that really applies to my situation) and why PI's have to be so heavy, I'll be all over a nice, lighter reliable PI/Hybrid when it comes out.

All the best,

Lanny