test targets

[bbsailor]

I'm not for using a lot of current but I see it different. Chart from reply #7. Target size shifts the amplitude vs distance curves for the different size coils vertically. If a target was just detectable at 25mm with the 300mm coil increasing the current 25% should double the detection distance. The percent improvement is highly dependent on target size and the initial detection distance. For detection distance greater than the coil diameter the improvement is small. For an increase in coil current the percent increase in detection distance for a target cut from an aluminum can side 5x5mm should be a lot higher than a 25x25mm target.

For an increase in current the increase in detection distance might be close to the same for the different size targets but the percent increase is not.

Green and all interested in this topic,

Don't forget to include the full stimulation of the target from your analysis. If the time constant (TC) of the TX turn off current or the TX pulse discharge TC curve is 5 times faster than the target TC, you will fully charge that target and any faster discharge TC will not cause any better stimulation. The TX discharge is calculated by dividing the damping resistor value into the coil inductance.

A 3 uS target takes 5 TCs or 15 uS to fully discharge into the noise level. So, a PI set delay of 10 uS still allows some residual target signal to be detected. The 3 uS target would need a 0.6 uS coil discharge TC to fully stimulate that target (3/5 = 0.6). Now let's translate that into the coil discharge TC. A 300 uH coil would need a 500 ohm effective damping resistance to meet this discharge requirement. Why not actual damping value versus effective damping resistance value. In a mono PI coil the op amp input resistor is effectively in parallel with the damping resistor value as long as the clamping diodes are conducting. A 680 ohm damping resistor in parallel with a 1000 ohm input resistor creates an effective 404.76 value which is a little under the optimal value of 500 ohms. Do this with a 2 uS target and you need an effective 750 ohms damping value. But you would need to sample well before 10 uS to detect the target.

There is a range of target TCs in this 2 to 3 uS range that operate in this critical target TC zone where the fine tuning of coil inductance value, effective damping resistance value and ultimately the full stimulation of the target allows an objective analysis of target detection distance.

This full target stimulation concept of 5 times faster than the target TC should be one more variable to consider when analyzing target size and distance from the coil.

This was a valuable concept that Eric Foster introduced many years ago that has helped me in understanding effective PI performance and coil design for certain target types, sizes and TCs.

Joseph J. Rogowski

[Qiaozhi]

I'm not for using a lot of current but I see it different. Chart from reply #7. Target size shifts the amplitude vs distance curves for the different size coils vertically. If a target was just detectable at 25mm with the 300mm coil increasing the current 25% should double the detection distance. The percent improvement is highly dependent on target size and the initial detection distance. For detection distance greater than the coil diameter the improvement is small. For an increase in coil current the percent increase in detection distance for a target cut from an aluminum can side 5x5mm should be a lot higher than a 25x25mm target.

For an increase in current the increase in detection distance might be close to the same for the different size targets but the percent increase is not.

I see what you're getting at. But you also need to consider the secondary field generated by eddy currents in the target. In the case of a 5x5mm target, this is acting like an extremely small coil, so that the detector [RX] coil is much further away than the target "coil" diameter. In fact, the same applies to the 25x25mm target. Even if the target is placed at the very centre of the coil, it will still be one coil radius away from the windings.

[green]

The LTSpice simulation in post #10 demonstrates how the magnetic flux density decreases at distance along the z-axis of the coil, depending on coil current, diameter, and number of turns. It does not take account of what happens when (from Lenz's Law) a secondary magnetic field is generated by eddy currents in the target that interferes with the primary field. However, the simulation results are clear evidence of why simply increasing TX power, or increasing amplification of the RX signal, produces such a small improvement.

Calculation of the return signal for a particular target is far more complicated.

I'm not for using a lot of current but I see it different. Chart from reply #7. Target size shifts the amplitude vs distance curves for the different size coils vertically. If a target was just detectable at 25mm with the 300mm coil increasing the current 25% should double the detection distance. The percent improvement is highly dependent on target size and the initial detection distance. For detection distance greater than the coil diameter the improvement is small. For an increase in coil current the percent increase in detection distance for a target cut from an aluminum can side 5x5mm should be a lot higher than a 25x25mm target.

For an increase in current the increase in detection distance might be close to the same for the different size targets but the percent increase is not.

[Qiaozhi]

The LTSpice simulation in post #10 demonstrates how the magnetic flux density decreases at distance along the z-axis of the coil, depending on coil current, diameter, and number of turns. It does not take account of what happens when (from Lenz's Law) a secondary magnetic field is generated by eddy currents in the target that interferes with the primary field. However, the simulation results are clear evidence of why simply increasing TX power, or increasing amplification of the RX signal, produces such a small improvement.

Calculation of the return signal for a particular target is far more complicated.

[Koala]

I made two squares awhile back to test a VLF. I found a small error in size made a big difference to its detectablity. I did consider using a paper punch to make some round targets to compare and get consistency In the end I didn't bother as the detector was working good enough for me. Might be an idea for identical target testing in different countries

[Skippy]

Green: Not yet, too much else on. I have it on my mind, every time I see a drinks can in the street (everywhere I go) I look at it with scrutinising eyes--- is it something different? As a result I've brought home over a dozen different cans, from small to large, native and foreign types. Some feel different on the 'squeeze test', but irritatingly, they're all the same thickness - every one is from 0.095 to 0.105mm (including paint/plastic liner). I have a hunch where I may be able to locate older (maybe thicker) cans, I'll make a visit there this weekend.

Baum: I've tended to avoid using the tops of cans, as not only are they much thicker, they may be a different alloy, as they aren't extruded heavily (the main body is a very special alloy that's very strong yet extrudes without problems, cracking especially) so don't compare well.

[baum7154]

The theoretical analysis also assumes the target is a point-sized one. If you were analysing it correctly, you should take into account the finite size of the target - the kind of item you will find 12 inches from an 8 inch coil is not going to be tiny, more like U.S silver dollar size.

I've failed to find any significantly different thickness drinks can metal, it's all 0.100 - 0.105mm, even the small 'energy drink' size cans. I've made a new set of square targets from a different 0.105mm can, I'll try and fire up my in-bits detector and re-measure.

-----------------------------

Hello Skippy,

Your comment regarding the Point-sized target I believe is 'on target' and the reason a few years ago I posted some testing using small cylindrical slugs of .125" aluminum TIG welding wire. The thought then was to get a compact target of a known volume based upon the specific gravity of aluminum versus the SG of gold. The aluminum slug was very close in detection distance to a volumetrically equivalent compact piece of gold. If I remember correctly aluminum was 0.7 the SG of gold and was easily fabricated by measuring weight.
AL target weight= 0.7 * nugget weight to emulate a given small compact nugget.

The aluminum cans I have used for years to fabricate flat targets have all nominally measured .004" or 0.1mm thickness at the mid-point of the can wall. The aluminum does tend to thicken near the top and bottom ends of the sidewalls. The tops and bottoms of the cans are considerably thicker causing much easier detection if targets are made of these parts of the cans.


Dan

[green]

I don't know why it's labelled in seconds, but I assume 100msec equals 100mm distance.
The fact that the bottom line contains (a*a + z*z) is the reason the 6th power is never reached. There's always the 'a' term stopping it reaching the 1/(z^3). George is just showing the inverse cubic effect at distance, you have to square it to get the round-trip inverse 6th power.
The theoretical analysis also assumes the target is a point-sized one. If you were analysing it correctly, you should take into account the finite size of the target - the kind of item you will find 12 inches from an 8 inch coil is not going to be tiny, more like U.S silver dollar size.

I've failed to find any significantly different thickness drinks can metal, it's all 0.100 - 0.105mm, even the small 'energy drink' size cans. I've made a new set of square targets from a different 0.105mm can, I'll try and fire up my in-bits detector and re-measure.

The good thing about the linear relationship in size vs TC is that you can make up standard TC value test samples, eg 2,4,6,8,10 usec by carefully cutting the correct size square.

Wondering if you have tested the new set of square can targets.

[Qiaozhi]

How to convert the time scale to distance? I charted your curve log scales and it didn't get to the 6th law. I'm missing something.

I meant to say the 3rd Law, as this doesn't show what happens during the return journey.
The x-axis is unfortunately labelled as time, but it's really showing distance in meters. There may be a sneaky way of changing the label on the x-axis from time to meters, but I haven't look into that very closely. If you move the mouse cursor to the x-axis and left-click, a dialog box pops up where you can change "time" to "meters", but the change is not allowed.

I don't know why it's labelled in seconds, but I assume 100msec equals 100mm distance.

Yes, that's correct.

[green]

I don't know why it's labelled in seconds, but I assume 100msec equals 100mm distance.
The fact that the bottom line contains (a*a + z*z) is the reason the 6th power is never reached. There's always the 'a' term stopping it reaching the 1/(z^3). George is just showing the inverse cubic effect at distance, you have to square it to get the round-trip inverse 6th power.
The theoretical analysis also assumes the target is a point-sized one. If you were analysing it correctly, you should take into account the finite size of the target - the kind of item you will find 12 inches from an 8 inch coil is not going to be tiny, more like U.S silver dollar size.

I've failed to find any significantly different thickness drinks can metal, it's all 0.100 - 0.105mm, even the small 'energy drink' size cans. I've made a new set of square targets from a different 0.105mm can, I'll try and fire up my in-bits detector and re-measure.

The good thing about the linear relationship in size vs TC is that you can make up standard TC value test samples, eg 2,4,6,8,10 usec by carefully cutting the correct size square.

Thanks. The slope does follow the 3rd law after 200ms(200mm?)

[Skippy]

I don't know why it's labelled in seconds, but I assume 100msec equals 100mm distance.
The fact that the bottom line contains (a*a + z*z) is the reason the 6th power is never reached. There's always the 'a' term stopping it reaching the 1/(z^3). George is just showing the inverse cubic effect at distance, you have to square it to get the round-trip inverse 6th power.
The theoretical analysis also assumes the target is a point-sized one. If you were analysing it correctly, you should take into account the finite size of the target - the kind of item you will find 12 inches from an 8 inch coil is not going to be tiny, more like U.S silver dollar size.

I've failed to find any significantly different thickness drinks can metal, it's all 0.100 - 0.105mm, even the small 'energy drink' size cans. I've made a new set of square targets from a different 0.105mm can, I'll try and fire up my in-bits detector and re-measure.

The good thing about the linear relationship in size vs TC is that you can make up standard TC value test samples, eg 2,4,6,8,10 usec by carefully cutting the correct size square.

[green]

Here's an LTSpice simulation that shows how the magnetic flux density falls off along the z-axis of the coil.



The equation is:



where:
B = magnetic flux density (Tesla)
(vacuum permeability)
N = number of turns on coil
a = coil radius (m)
I = coil current (A)
z = distance along z-axis of coil

The LTSpice files are attached.
Note that the 6th law doesn't really kick-in until the distance is equal to the coil radius. Before that, the drop-off of B is less severe.

How to convert the time scale to distance? I charted your curve log scales and it didn't get to the 6th law. I'm missing something.

[Qiaozhi]

It's a bit subjective, theoretically it NEVER reaches the inverse 6th power slope, even 10 metres away it's probably 5.99 th power, so at which point you decide it's 'close enough' is up to you....when it's beyond 5.75 th power?

I've done quite a few target strength vs distance measurements, but measurements taken beyond 350mm from a "200mm" coil lack accuracy, so I can't say how close to 6th power they were, but they were evidently heading that way.

Here's an LTSpice simulation that shows how the magnetic flux density falls off along the z-axis of the coil.



The equation is:



where:
B = magnetic flux density (Tesla)
(vacuum permeability)
N = number of turns on coil
a = coil radius (m)
I = coil current (A)
z = distance along z-axis of coil

The LTSpice files are attached.
Note that the 6th law doesn't really kick-in until the distance is equal to the coil radius. Before that, the drop-off of B is less severe.

[Skippy]

I've sorted out the details of my experiment with the square drinks-can aluminium samples.
Metal was from the middle section of a normal 400 ml (or thereabouts) can. Measured thickness was 0.105 mm, which is a very common value for cans here in the U.K.
Squares from 6mm up to 25mm were measured, readings were as follows:

Square size (mm) / corner freq(KHz) / time-constant (usec)
25 / 24.5 / 6.50
22 / 26 / 6.12
20 / 29 / 5.49
18 / 32 / 4.97
15 / 37 / 4.30
12.5 / 47 / 3.39
10 / 62 / 2.57
8 / 80 / 2.00
6 / 92 / 1.74

Errors in the time-constant vary, but +/- 0.25 usec is probable.
Plotting them, assuming that 0mm / 0 usec is a real reading shows a good linear variation. See attached graph.

[Skippy]

It's a bit subjective, theoretically it NEVER reaches the inverse 6th power slope, even 10 metres away it's probably 5.99 th power, so at which point you decide it's 'close enough' is up to you....when it's beyond 5.75 th power?
The mathematics of it can be found in one of the 'how metal detectors work' articles by Bruce Candy on ML's website (though I believe there's an error in the maths somewhere). There's other online articles that explain the inverse cube field strength drop off effect, which is just 'doubled up' in detector operation, as it's transmit AND receive.

I've done quite a few target strength vs distance measurements, but measurements taken beyond 350mm from a "200mm" coil lack accuracy, so I can't say how close to 6th power they were, but they were evidently heading that way.