Detection distance for a US nickel and quarter

[Ferric Toes]

Eric, there's something amiss with two attachments. ( And your inches have become question-marks )

Thanks Skippy. Not sure why that happened on the first two pictures, although there was a glitch when my computer went off page briefly.

Eric.

[green]

Here are some linear plots which show why the detection range is greater for a Nickel than for a Quarter, or even a Dime.
The test setup consists of a Tx with a HER208 series diode to minimise the capacitance seen by the search coil. Pulses are not constant current but are 350us duration and 2A peak. Search coil is 8 inches diameter with coins placed on a platform 1.5 inches from the base of the coil and on axis. Rx is a NE5534 and NE5532 giving a total gain of 500. Waveform is inverted with maximum signal at the -3.25V saturation level. X axis is 10uS - 100uS. Comparisons are taken on the 10uS line. Probe was set to x10. Hence the Y axis values should be multiplied by 10.
The first plot is the response with no coin. [ATTACH]50190[/ATTACH]

The next plot is for the GB cupro nickel Shilling. [ATTACH]50191[/ATTACH] This 1954 coin was set 1.5 inches from the coil centre so that the maximum signal just drove the amplifier near to saturation at the 10uS time and -3V. The TC of the decay is close to that measured by Skippy.

The third plot is the clad Quarter. [ATTACH]50187[/ATTACH] Here we see that the starting amplitude at 10uS is very much less than the Shilling, but continues for a longer time. On expanding the timebase, the waveform finally merges with the noise ripples on the 0V line at400uS, compared to the 65 - 70uS for the Shilling.

Fourthly, the Nickel. [ATTACH]50188[/ATTACH] This has the same metallic composition as the Shilling, but due to its smaller diameter, has less amplitude.

Last a Dime. [ATTACH]50189[/ATTACH] Again, a small coin similar to the Quarter in composition. It also displays a low starting amplitude but long TC.

Eric.

Thanks for the test. Looks like your nickel and quarter were about the same at 20us+ like I got reply 24 https://www.geotech1.com/forums/atta...1&d=1588627927

Tried another test with larger target 2times the smaller. The wire ring time constants are closer than the solid targets. 2in target is 8times higher than the 1in target at about 30us and doesn't vary a lot.

[green]

You are right in that we are not measuring the voltage induced in the target, but the the rate of change of the current with time and the voltage this induces in the now RX coil (assuming a mono). This is explained in a paper I have somewhere, but still packed in a box after last August's move. From memory, two identical solid spheres are described with the only difference being the conductivity. The low conductivity sphere exhibits a high starting voltage as seen as a voltage across a Rx coil, with a fast decay; while the high conductivity sphere starts with a low voltage and a long decay. The area under the curves (energy dissipated) is the same for the two spheres.

This effect is also exhibited by the coil itself, which becomes its own target at switch off. Suddenly open circuiting the coil causes the voltage across it to rise to several hundred volts as it tries to maintain the magnetic field. This energy is partly dissipated in the damping resistor.

Eric.

Don't have two identical spheres different conductivity. Tested a 1inch square and a 2inch square cut from aluminum foil. Wanted to see if area under the decay curve is the same. Extended the curves on the measured chart. Used Excel to make decay curves(similar decay rate that crossed at 3.5us) that I could calculate area under the curve for both targets. Calculated close to same if 1us was the start time. Thinking about doing the nickel and quarter. Quarter doesn't have straight line decay linear X log Y so generating a quarter curve with Excel is slowing me down. Any thoughts appreciated.

Wrong, compared 1 inch square targets with 1 and 2 layers.

[Polymer]

Following this thread with great interest. Did a simple test with my MPP E Ferrite Pinpointer.
It does fit in with what you are delving into, green. Normal silver stuff only gets about half the distance of the gold.

For what its worth:
PI probe 340uH, 68 Ohm resistor in series. cable/probe/shielding @ 256pF.
PPS 2370Hz, Pulse width 31us, sample width 17us.

Distances for solid detection:
Sample delays: 10.5,11,12,15,20,36us
Nickel: 9,9,8,8,7,5cm
Quarter: 6,6,6,6,5,5cm
3.1g Au 9ct.: 9,9,8,8,7,6cm

[green]

Thanks for the replies. Think some of the statements are missing something.

Why is the decay start signal for the nickel greater than the quarter or the Ike dollar? The nickel is smaller, I could guess but then it would be a guess. reply #7

The area under the curves (energy dissipated) is the same for the two spheres. reply #18. Think for targets having the same diameter, area under the decay curve is the same. If target time constants are the same, slope and amplitude will be the same. If time constants are different the lower time constant target will have a higher starting voltage.

Signal amplitude is proportional to the cube of the diameter reply #26. Think average signal amplitude is proportional to cube of diameter. If larger target is twice the diameter the smaller, the average decay signal for the larger target will be 8 times the smaller. If decay slopes are different, the larger target will be 8 times the smaller at one sample time only.

BTW, it's not enough to flat-top the current; it has to remain flat-topped at least 5x longer than the longest target tau you are dealing with. That could be a very long time, by PI standards. reply #19

Think above is true for a PI, don't know for a VLF.

[Ferric Toes]

In the early days of PI development, detecting larger and highly conductive targets was not a problem. Tx pulse widths were longer, as were sample delays and sample pulse widths. Detecting small and thin coins were a problem, and because the focus at the time was surveying archaeological sites in Greece where very small coins abounded, some development time was spent on improving performance in this area. My early PI's for the hobby market could detect an old penny at 15 inches, but struggled to find a cupro nickel sixpence, which is a little bit larger than a Dime. Initial delay times gradually came down from 100uS to 30uS and today 10uS or less is not too difficult. Tx on times have come down too, which has not helped in the detection of good conductors.

Tx width is a factor in energising high conductors, but also the Rx sampling is very important and this is worth exploring more fully. Because of the desire to detect ever smaller objects, both flyspeck size gold nuggets and broken off needle tips in fabrics, ever shorter delay times resulted. Short Tx pulses, and increased repetition rates were also the order of the day. Now we need to go the other way, but maintaining some small object sensitivity at the same time.

Back in the late 1970's, my company was involved in developing a detector for clearing land of large live munitions and one of my technical colleagues suggested using a very long sample pulse so that a large part of the return waveform would be sampled, rather than just a small portion, as would be the case with a short sample pulse. He called it 'total field recovery' (TFR). As far as I remember, it was never tried in practice, but I have often thought about it since. Certainly we used longer sample pulses but because there was, as now in many designs, a second sample pulse for earth's field cancellation, this would have meant that for long signal decays the later portion of the decay would partially cancel the first. This would be true in the case of coins such as US clad Quarters and even more so for silver ones.

The best way to test TFR would be to use a bipolar Tx so that a sample, or samples, could be taken after a short delay from the end of +Tx1 right to the start of -Tx2. This alternate polarity pulsing would also cancel earth's field without any cancellation of the wanted signal. What I would like to know is whether a number of short successive samples which are summed at the integrator would add up to a much larger voltage whose amplitude is proportional to the length of the target decay. Presumably a long single sample would just give an average value?

I don't have Spice to try these ideas, so this could be a little lock-down or self -isolation project to have a go at, for someone who has.

Eric.

[Carl-NC]

The best way to test TFR would be to use a bipolar Tx so that a sample, or samples, could be taken after a short delay from the end of +Tx1 right to the start of -Tx2. This alternate polarity pulsing would also cancel earth's field without any cancellation of the wanted signal. What I would like to know is whether a number of short successive samples which are summed at the integrator would add up to a much larger voltage whose amplitude is proportional to the length of the target decay. Presumably a long single sample would just give an average value?

Funny you posted this. For the last couple of weeks I've been optimizing the timing on a new rev of the Fisher pinpointer. It uses bipolar pulsing and with a short (20us) sample width has the obvious problem of detecting nickels well but quarters not so well. I decided to stretch the sample width out to get the quarters better and expected the nickel response to either degrade or get noisier -- it did not. Quarters got better, nickels stayed exactly the same. This uses a sampling integrator which may explain why. Had I used a SHA-type demod then I think the nickel would have been weaker due to averaging. I did not stretch the sample width all the way to the next TX pulse but will try that.

[green]

Chart I posted awhile back when my bench circuit was working. Integrator out readings, first sample(target sample) and second sample(ground sample + target sample). Ground sample=second sample-first sample.

First two columns for the nickel(first delay, 6us). First sample(10.2us, 62.5mV). Second sample(100us, 42mV). Ground sample -20.5mV. If first sample was 110us with no GB(83mV). 1.33 times signal increase

First two columns for the quarter(first delay, 6us) First sample(10.2us, 20.5mV). Second sample(100us, -41mV). Ground sample -61.5mV. If first sample was 110us with no GB(82mV). 4 times signal increase

Can't do GB if first sample is over 100us?

Hope it makes sense, was getting confused typing it.

[Ferric Toes]

Funny you posted this. For the last couple of weeks I've been optimizing the timing on a new rev of the Fisher pinpointer. It uses bipolar pulsing and with a short (20us) sample width has the obvious problem of detecting nickels well but quarters not so well. I decided to stretch the sample width out to get the quarters better and expected the nickel response to either degrade or get noisier -- it did not. Quarters got better, nickels stayed exactly the same. This uses a sampling integrator which may explain why. Had I used a SHA-type demod then I think the nickel would have been weaker due to averaging. I did not stretch the sample width all the way to the next TX pulse but will try that.

Yes it is worth trying for as long as you can make it. I tried lengthing it on the monopolar board that I used for the earlier plots I posted in this thread. Tx is 350uS and the minimum delay controlled by a pot is 10uS with 20uS sample. Max. delay is 50uS. I put a 22k pot in series with a 10k resistor which is in the timing network for the two sample pulses this would give a range of 40uS - 100uS, so as to see what difference in signal there was with a US Quarter and a similar size UK cupro-nickel Shilling. Unfortunately, the bin in which I keep 10k resistors, had a 100k reel by mistake. This gave me a pair of sample pulses of 200uS width. It seemed to work OK so I did some quick tests and 'lo and behold' it gave considerable improvement in the range of a Quarter and came out a bit better than the shilling, which seemed largely unaffected. Range was considerably improved for a silver dollar too, but looking at the total decay time, it could be better still with greater spacing between samples which currently is 350uS between the end of SA1 to the start of SA2. There is enough space between Tx pulses to accomodate 600uS between samples at the max delay of 50uS and 200uS sample widths.

I am only using an 8 inch coil at the moment, suspended between floor and ceiling as I made a big mistake in having all my new workshop lined, including underfloor, with insulation board which I later found out had a foil surface. Also the electrician fitted LED lights which cause a lot of interference, so I have to switch off the lighting and resort to filament bench lights when doing any testing. These mistakes happened when I was having chemo for lymphoma and was out of the loop for a year.

I will look at this in more detail over the next few days. Is your cache still buried?

Eric.

[Carl-NC]

Hope it makes sense, was getting confused typing it.

Eh, not really.

I will look at this in more detail over the next few days. Is your cache still buried?

No, I dug it up 12 years ago when I moved to the West Coast. Never did bury it again.

[green]

Eh, not really.

110us sample gives 1.33 times the signal as a 10us sample for the nickel. 110us sample gives 4 times the signal as a 10us sample for the quarter. A longer sample might help for the quarter not the nickel. Probably can't do ground balance with a 110us target sample.

[Ferric Toes]

110us sample gives 1.33 times the signal as a 10us sample for the nickel. 110us sample gives 4 times the signal as a 10us sample for the quarter. A longer sample might help for the quarter not the nickel. Probably can't do ground balance with a 110us target sample.

For the application I am working on GB is not required, but better range on higher conductivity coins is needed. With lower conductivity items, the fact that the range is little better than with a short sample pulse, is fine. I am also going to look into the integrator time constants as these may not now be optimum with the long samples.

Eric.

[green]

For the application I am working on GB is not required, but better range on higher conductivity coins is needed. With lower conductivity items, the fact that the range is little better than with a short sample pulse, is fine. I am also going to look into the integrator time constants as these may not now be optimum with the long samples.

Eric.

What determines optimum integrator time constants? Maybe sweep speed, coil size and ?

[Carl-NC]

For the application I am working on GB is not required

Same with me.

What determines optimum integrator time constants? Maybe sweep speed, coil size and ?

Yes, target response speed, which depends on sweep speed.

[waltr]

What determines optimum integrator time constants? Maybe sweep speed, coil size and ?

Yes, target response speed, which depends on sweep speed.

Also, the Pulse/sample rate.