Getting ready to build the MPP

[green]

Green - I cannot remember if you've posted this already, but do you have a [simple] table of time constants for different types of target?

Not yet. Other than a couple thin wire rings I've made a US nickel with a TC of 10usec is the longest exponential decay I've measured. Foil,small nuggets and pieces of aluminum can are usually shorter TC's. I'll look at the data and see if I can put something together.

[green]

Reply #281 I stated total gain 400. True for target sample. Total gain 7200 for GEB sample to eliminate ground signal. Increased integrator gain

[Qiaozhi]

Not yet. Other than a couple thin wire rings I've made a US nickel with a TC of 10usec is the longest exponential decay I've measured. Foil,small nuggets and pieces of aluminum can are usually shorter TC's. I'll look at the data and see if I can put something together.

That would be interesting. I'm sure the metal detector manufacturers must have this information, but I cannot find anything in the public domain.

[green]

That would be interesting. I'm sure the metal detector manufacturers must have this information, but I cannot find anything in the public domain.

Some pictures from other threads.(gold chain) need to scan down to see ruler for size, exponential decay TC=3.8usec.(coins, al slugs, cu coin ring) US nickel TC=10usec, wire ring TC=24usec exponential decay, the rest not. (steel balls) not exponential decay. (aluminum balls) not exponential decay. (ferrous) most, exponential decay after 10usec. (lead bullets) smaller, exponential decay, larger not. (gold nuggets_2) exponential decay, orientation effects TC. I don't remember having Tx on time or shape effecting TC for targets with exponential decay, maybe it can. Tx on time and shape does effect target decay for targets that don't have an exponential decay. If we are going to make a simple chart, need to define what the chart should look like. I think most larger targets don't have an exponential decay.

[WM6]

Hi green
How you calculate/conclude on TC?
Par example, as I understand TC of target No. 7 is about 10 not 1.9 as stated.
Can you explain a little more?

[green]

Hi green
How you calculate/conclude on TC?
Par example, as I understand TC of target No. 7 is about 10 not 1.9 as stated.
Can you explain a little more?

One of two ways. Time between amplitude of 3 and 8 for 1 TC or time between 5 and 100/3(3 TC 1/20 or 5%), hope it's right. I usually draw a line on the chart parallel with the target in Excel where I can move it and get a closer reading.

[WM6]

One of two ways. Time between amplitude of 3 and 8 for 1 TC or time between 5 and 100/3(3 TC 1/20 or 5%), hope it's right.

Thanks. For my understanding it is time between switch off and lower point of (back EMF) voltage drop. So I should correct myself for target No. 7 (gold nugget 2) - this way TC is about 6 not 10. OK, probably I will never touch some field in theory.

[green]

Thanks. For my understanding it is time between switch off and lower point of (back EMF) voltage drop. So I should correct myself for target No. 7 (gold nugget 2) - this way TC is about 6 not 10. OK, probably I will never touch some field in theory.

One time constant is the time for the signal to drop to 37% of it's value. 100 to 37, 8 to 3, 1 t0 .37. The signal should have an exponential decay, straight line on a lin-log chart.

[KRinAZ]

That would be interesting. I'm sure the metal detector manufacturers must have this information, but I cannot find anything in the public domain.

This is interesting - I like what green has measured and established.
I have a growing interest in the establishing of a TC for various metals, been thinking along the lines of unit of time per unit of weight for specific metals, using the radioactive decay model as a guide - I'm thinking this is what green is doing - I think it would be nice to make a standardized chart of signal decay for various metals and alloys per unit of weight e.g. for say 1 gram of the given material, how many microseconds to decay to .37 target signal strength.

[baum7154]

OK, I've build yet another 8" round SSBW (self-shielding basketweave) coil and I'm going to stick with it as I'm finally happy with the coil - I finally got it as tight as I wanted - to improve the magnetic field's strength, quality, and consistency. It uses exactly the same length of wire as the last one so has the same resistance, just wound tighter, and gave me a wonderful 20% increase in inductance (& presumably magnetic field strength) - without sacrificing even 1uS in speed, very nice. Previous coil was 300uH.
Here are it's spec's:
7 basketweave turns
360 uH
3.0 ohms (due to the small gauge 26AWG wire)
2455 ohms critical damping
Flyback spike recovered at 4uS after end of TX pulse - by monitoring the coil, and also at TP3.
Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet, anyone have suggestions on a good higher voltage replacement? 800 volts would be nice...

Also since this coil has has relatively high resistance & about the same ohms as current limiting R3 (3.3 ohms) I removed & jumpered R3.

This coil should be very good for trying out the timings I'm going to experiment with. I am almost done putting the MPP on a pole setup, so more soon...

[ATTACH]33301[/ATTACH]

----------------------------------------------
Hi Kyle, did you keep the original 1K input resistor on the input to the preamp? If so the actual 'in circuit' critical damping resistance with the 2455 ohm coil resistor in parallel with that 1k input resistor is 710.5 ohms. Working the formula back for the srf/critical damping puts the SRF of this 360uh coil at about 628.2 kHz in circuit with the feed line.

Regards,

Dan

[KRinAZ]

Hi Dan, yes, I kept the 1k resistor in place, & thx for pointing that out - I guess it makes sense when thinking about why I don't detect a piece of lead smaller than about .3 gram - the coil just isn't fast enough. I think one reason it's not faster is the wire resistance - I think I need to use a significantly larger wire (smaller AWG number) to get the resistance down (& impedance also) to well below 1 ohm. I think the SRF would go up a lot and the spike recovery would be at least 1 or 2 uS quicker. Any thoughts on an optimum wire size? - I'm running 26AWG stranded teflon coated now and it's too small/resistance is too high. I would like to get the right gauge wire and wind another coil exactly like this one (it's nice & tight). I'm able to adjust my MPP min main delay to match - it is capable of sub uS delay timing if I ever get a coil that fast...

[KRinAZ]

Thinking of building & measuring fast coils - I think I'm going to get another MPP board & just build the power sources [needed] and just the TX circuit - to purely observe the coil with no RX circuit influence - I'll do that before the next coil build. Also thinking of fast coil sampling - anyone read the KB article about the MPF technology on Minelab's new SDC 2300? It's linked to on the product page and quite an interesting read!

[greylourie]

On the tx side, it all revolves around speed to switch off. You could go ahead and use long tx pulse, and use some means to reduce the turn-off / flyback time period. But if you are targeting a particular size range of metal bits, then just how long does your tx need to be ? How much power, and what duration ? Maybe increasing PPS improves the odds too, in other ways. I have heard posters here mention S/N ratio is better.

Its funny they mention 3kHz in that article. The TDI range of detectors run at a similar rate. Companies are constrained, they have to produce product that will fulfill a wide range of expectations. A hobbyist with knowledge of the size and TC of what he is looking for, and the depth - can try to tailor tx power and duration.

Eric Foster provides some good examples.
http://www.geotech1.com/forums/showt...d-deepstar-and
Here he mentions how later version evolved, reducing pulse width and upping PPS but still getting the goods. These were high power P.I. All sizes of jewelry and more, at great depth.

http://www.findmall.com/read.php?34,...978#msg-171978
Here he mentions the details of tx width/repetition rate of lower powered P.I (Goldquest 10 000 PPS max), and also sample width. He points out the type/size of targets that could be detected. And mentions that copper and silver coins wont be found at great range due to the tx width, and sampling regime.

I think your idea to build a tx experiment board is a good one.

[Qiaozhi]

http://www.findmall.com/read.php?34,...978#msg-171978
Here he mentions the details of tx width/repetition rate of lower powered P.I (Goldquest 10 000 PPS max), and also sample width. He points out the type/size of targets that could be detected. And mentions that copper and silver coins wont be found at great range due to the tx width, and sampling regime.

It appears from the above link, to a post made by Eric Foster, that he uses the magic number of 100,000. In other words, the product of the TX pulse rate and the sample pulse width needs to equal 100,000.
For example:
Goldquest SS: 10,000 pps x 10us sample = 100,000
Textile Detector: 100,000 pps x 1us sample = 100,000
EF's 5us unit: 20,000 pps x 5us sample = 100,000

You could also try reducing the coil inductance as a means of speeding up the decay time.

[greylourie]

Some more inspiration.

http://www.geotech1.com/forums/showt...128#post165128