Detection distance for a US nickel and quarter

[Carl-NC]

I'm gonna offer a different explanation. Suppose that the TX pulse width is wide enough that the coil current has flat-topped and target reverse eddies have decayed to zero. The TX current then steps to zero, and this step induces a step EMF in the target. The induced EMF is the same regardless of the target (EMF = -dPHI/dt). The EMF produces an eddy current i = EMF/Z (where Z is the "impedance" of the target, which is a combination of metal conductivity, skin depth, and size/shape) and the eddy current produces the reverse magnetic field sensed by the coil as another EMF (the voltage you see with the scope).

Nickels have a higher "impedance" than quarters so you would expect the initial eddy current to be lower, and thus the initial reverse field to be lower. But all this assumes a starting eddy current of zero at the TX step. If the TX pulse is not wide enough, then targets will have a reverse eddy current flowing at the TX turn-off step and this directly subtracts from the t=0+ forward eddy. High tau targets require a much wider TX pulse to kill the reverse eddies so for a given TX pulse width you should see a progressively lower starting RX voltage for progressively higher taus.

A way to test for this is to widen the TX pulse and see if the higher conductors start increasing. Eventually they should surpass the nickel.

[Qiaozhi]

To demonstrate what Carl is saying, please see the attached LTspice simulation results.

The simulation does 4 runs:
1. Series resistor 3R3, TX pulse width 50us, target tau 50us
2. Series resistor 6R86, TX pulse width 250us, target tau 50us
3. Series resistor 3R3, TX pulse width 50us, target tau 10us
4 .Series resistor 6R86, TX pulse width 250us. target tau 10us

Detection Depth Sim: shows all 4 simulation plots together.
Detection Depth Sim - zoomed in: shows a closeup view of all 4 plots.
Detection Depth Sim - flat top: shows the plots for the 50us and 10us TC targets with flat-topping. In this case you can see that the strongest signal comes from the target with a tau of 50us.
Detection Depth Sim - no flat top: shows the plots for the 50us and 10us TC targets without flat-topping. In this case you can see that the strongest signal comes from the target with a tau of 10us.

[Ferric Toes]

I'm struggling with this statement of Eric's:
"with the result that the voltage has to rise to a higher value for the high resistance target than it does for the low resistance one to try and maintain the same current and field"
It is the current that creates the magnetic field, which is measured by the detector coil. So the different voltage on the target is an interesting side-effect, but not one that is being measured. ?

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.

[Carl-NC]

To demonstrate what Carl is saying, please see the attached LTspice simulation results.

Thanks, I was going to add that somewhere there is a thread on this topic with lots of sims, but then my Internet went out.

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.

[green]

Thanks, I was going to add that somewhere there is a thread on this topic with lots of sims, but then my Internet went out.

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.

https://www.geotech1.com/forums/atta...4&d=1588518934 chart from reply #7. Tx is 20us ramp to .5A with constant .5A for 4980us. Should be long enough? Maybe I'm doing something wrong?

[Ferric Toes]

I can see that I will have to fire up one of my later test units where I can vary the flat topped pulse width from 100 - 1000uS and plot the decay for nickel and quarter. Ideally I will look for a low conductivity coin the same size and thickness as the nickel to remove the size variable.

Eric.

[Carl-NC]

https://www.geotech1.com/forums/atta...4&d=1588518934 chart from reply #7. Tx is 20us ramp to .5A with constant .5A for 4980us. Should be long enough? Maybe I'm doing something wrong?

Oh, I see. Yes, 5ms should be long enough. Time to rethink.

[Skippy]

Eric: The 5c nickel IS a low conductivity coin, being Cupronickel. Our British 1 shilling ( or the identical large 5p ) is actually a decent match for the nickel, and if you can find 50% silver and 92.5% silver shillings, you have an experiment in the making.

[green]

I can see that I will have to fire up one of my later test units where I can vary the flat topped pulse width from 100 - 1000uS and plot the decay for nickel and quarter. Ideally I will look for a low conductivity coin the same size and thickness as the nickel to remove the size variable.

Eric.

Interested in what you get. Added targets would be interesting. If you have a nickel and clad quarter I would be interested if your data is similar to mine. Still not sure why the smaller nickel has a larger signal than the larger quarter.
Charted a nickel and quarter again to see if I get similar results. Tx on time, 75us, 150us, 300us and 600us.

I'm guessing the nickel would have a larger signal if it was 1/2 the diameter of the quarter

[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.

I have wondered about area under the curve. If I double peak Tx current target Rx signal doubles. Tx energy increases 4 times, does signal double because Tx out Rx back. I recorded amplifier out, no target and target. Subtracted no target from target and averaged signal from 8us to 108us. Quarter(.239V) nickel(.133V). Maybe my method isn't the best, wonder what you get if you try to measure average.

[Ferric Toes]

I found a 1954 shilling and it has a IACS of 5.4. Diameter 23.53mm, thickness 1.7mm.

A 1982 US quarter has IACS of 42.6, diameter 24.29, and thickness 1.74mm.

By comparison a 1971 nickel has IACS of 5.3, diameter 21.24mm, thickness 1.87mm.

A shilling is certainly a good match for the size of a quarter as Skippy stated.

This different diameter of the shilling and nickel should give an even greater initial signal for the shilling when placed in the same geometrical position in the coil. Signal amplitude is proportional to the cube of the diameter of the coin so we should get 1.36 x the nickel amplitude; if I worked it out right.

Hope to plot some decays shortly.

Eric.

[Skippy]

I've done some tests on my 13 kHz VLF setup ( Fisher F75 based ). I used the three different versions of our British one shilling coin; 925 Sterling silver, 500 fine silver, and Cupro-nickel [ 75% Cu / 25% Ni, the same as US 5 cent ]. The coin is bigger than a 5c, smaller than a 25c US.
I placed the coins 8cm above the sweetspot of the coil, in the same place each test. The demodulated voltages were measured, and a bit of Pythagorus used to calculate the total signal strength.
The results are:
CuproNickel : 275 mV
500 silver : 442 mV
925 silver : 469 mV
As a percentage of the strongest, these values are: 58.6% ; 94.2% ; 100%

The measured time constants and corner freqs for them are:
CuproNickel : 9.5 usec / 16.8 kHz
500 silver : 35 usec / 4.55 kHz
925 Silver : 62 usec / 2.57 kHz

I think it's highly likely there's some skin effect visible on the two silver coins, the 1 : 6.5 ratio of the time-constants doesn't reflect the 1 :15 (very approx) ratio of conductivities.
This is just fer interest, it's something I've not tried measuring before.

[green]

I found a 1954 shilling and it has a IACS of 5.4. Diameter 23.53mm, thickness 1.7mm.

A 1982 US quarter has IACS of 42.6, diameter 24.29, and thickness 1.74mm.

By comparison a 1971 nickel has IACS of 5.3, diameter 21.24mm, thickness 1.87mm.

A shilling is certainly a good match for the size of a quarter as Skippy stated.

This different diameter of the shilling and nickel should give an even greater initial signal for the shilling when placed in the same geometrical position in the coil. Signal amplitude is proportional to the cube of the diameter of the coin so we should get 1.36 x the nickel amplitude; if I worked it out right.

Hope to plot some decays shortly.

Eric.

A test with 1inch square vs 2inch square aluminum foil targets. 1 layer targets have a different decay rate so the 2inch square target is 8 times(9 divisions on the chart) the 1inch square target at around 6us only. The 1inch 2layer target has about the same TC as the 2inch 1 layer square target so the difference is consistent(about 8 divisions)6.4 times not 8 times. Tried with a round target this morning. 1inch and 2inch diameter targets have a different TC so they would be 8times different at one time only the same as the square targets. What is the reason or math why signal amplitude is proportional to the cube of the diameter? Wondering if I'm doing something wrong.

[Ferric Toes]

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.

The next plot is for the GB cupro nickel Shilling. 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. 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. This has the same metallic composition as the Shilling, but due to its smaller diameter, has less amplitude.

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

Eric.

[Skippy]

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