Target response tester

[Skippy]

I've thought about the thick lead disc vs. the 1 ounce coin ....

the circulating currents in the flat coin will be predominantly in the one flat plane; they will be that way initially, and will stay that way as they decay, hence the uniform 'slope'. The bulky lead disc may start off with currents circulating mainly in the plane, because of the way the detectors' coil / magnetic field pass through the sample. But it seems likely that they may not stay circulating that way, and could easily end up flowing in loops in a perpendicular plane, ie. through the bulk thickness. The currents will still decay (as the lead has electrical resistance)... but the magnetic field they generate will not be picked up (or picked up much less) because their plane is perpendicular to the detectors coil. It's the "coin-on-edge effect", on a microscopic scale.
This is just a thought .... I'm not sure what particular experiments you could try, to understand the behaviour. Would a second coil, at right-angles to the flat disc/ 1 ounce coin give the answer? A drawing would help explain ... the second coil is on the same plane as the flat coin, but oriented so it sees the coin as 'on-edge'. Edit , attached pic.

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And regarding your response tester: I was thinking of other ways to clean up the noise on your charts.
It's not hard to implement a simple ( or not-so-simple ) digital filter in Excel.
A simple one to try is: Filtered sample value = (0.25 x Previous sample value + 0.50 x Sample value + 0.25 x next sample value)

Example: if you have samples s₃, s₄, s₅ etc chronologically
then your new samples would be: S₄ = 0.25 x s₃ + 0.5 x s₄ + 0.25 x s₅
You could create dummy values s₀ = s₁, and s_last+1 = s_last to make the first/last S data points less duff.


( did you try the low-pass filter on the 'reference current' input of the log-amp, by the way?)

[green]

Are we going to move back to the original "detection depth" thread, now you seem happy with your log-amp setup?

http://www.geotech1.com/forums/showthread.php?24020-detection-depth/page6

This thread can still run, Eric is working on some projects, including a logamp.

Sounds good to me. I am interested in what Eric gets with his log amp.

Trials with slightly different pulse shapes and pulse durations exhibit a calculated scatter in the power law ranging from -0.8 to -1.4.'

The pulse used to obtain -0.99 was 370uS wide of which 320uS was cc and a switch off time of 2uS. From Eric's reply #84.

I have gotten power law ranging from -1 to -1.4. Wondering what would cause ground to be less than -1. The two grounds I have chart the same slope if the Tx pulse is the same. Longer TC targets with skin effect chart straight line decay on log X log Y charts with slopes less than -1 out to near there TC where they have straight line decay on linear X log Y charts.

Looking at added chart, near there TC might be the wrong words.

Haven't tried different switch off times. Could that be causing a big slope change?

I am happy with the log amp, wish the preamp had less noise.

[Skippy]

Are we going to move back to the original "detection depth" thread, now you seem happy with your log-amp setup?

http://www.geotech1.com/forums/showthread.php?24020-detection-depth/page6

This thread can still run, Eric is working on some projects, including a logamp.

[green]

Some ground comparison charts using the log amp. Two bags of California ground, one measures about 1.25 inches thick laying flat and the other about 1 inch thick recorded with 8inch fig8 Rx coil. Some ground from the back yard compared with some California ground in 35mm film canisters with the 1.5 inch fig8 Rx coil. The recordings were done in sequence at bottom of chart to check repeatability. The Alabama and California ground chart the same slope but it would be hard to tell if they GEB at the same settings. Recordings are amplifier out, integrator in. The integrator filters the signal a lot, can easily detect a signal change 2 decades below the 0 line(25mv). Both did GB at the same settings with my bench circuit before I zapped it. Coil size and sample size didn't seem to effect decay slope.

[green]

Thanks.
I will continue to read your posts and try understanding all that is happening.

I do have some additional data on my GEB issue. Busy last night and tonight dancing so hope to document this tomorrow and post in my HH@ thread.

I worked in a test lab for over 50 years measuring things. Charting this data is about half, can I do it and the other half to learn something. Since I didn't know anything about metal detectors when I started I have a lot to learn. I think the tests tell me something or at least generate more questions. Not sure how useful they are.

check your PM

[waltr]

Thanks.
I will continue to read your posts and try understanding all that is happening.

I do have some additional data on my GEB issue. Busy last night and tonight dancing so hope to document this tomorrow and post in my HH@ thread.

[green]

green,
How are you actually getting the CC TX in the last graph?
This should be impossible due to any inductor will have a Tau on the current rise, current Lags in an inductor.
Or is this just due to the time scale of the scope trace, the rise is not seen since it is rises in useconds. If this is true then are you limiting the current rise in thegraph before this?

I have been reading all your TC posts but haven't figured out what they mean.

Current is in a control loop. Constant current commands .5 amp(.25v to control loop) but it takes about 20usec to get the current to .5amps where it levels off at .5amp. Tx width adjustable from 50usec to 50000usec.

Or is this just due to the time scale of the scope trace, the rise is not seen since it is rises in useconds. If this is true then are you limiting the current rise in thegraph before this?

Yes, can't see 20usec rise with 5000usec/div. The scope is measuring the volts across a .5 ohm feedback resistor in series with mosfet source with scope trigger sensing Tx off. Yes, constant rate command to the control loop, is charging a capacitor with an adjustable current source. Capacitor charge adjustable from 10vsec to 10000v/sec.

Just recording Rx amplifier out after Tx off. Target response. Fairly simple to display decay on scope with linear X linear Y. Easier to see decay with log X or log Y scales. Target detection distance vs signal strength charts better with log Y scale. Hopefully to help determine optimum sample and delay times for one thing.

[waltr]

green,
How are you actually getting the CC TX in the last graph?
This should be impossible due to any inductor will have a Tau on the current rise, current Lags in an inductor.
Or is this just due to the time scale of the scope trace, the rise is not seen since it is rises in useconds. If this is true then are you limiting the current rise in thegraph before this?

I have been reading all your TC posts but haven't figured out what they mean.

[green]

I think you are in uncharted water if you use pulse shapes other than rectangular cc. A paper by Dabas, Jolivet and Tabbagh, 'Magnetic susceptibility and viscosity of soils in a a weak time varying field', state 'Between 5 and 180uS a best fitting power law of -0.99 has been derived taking into account the finite rising time and decay time of the transmitter pulse. Trials with slightly different pulse shapes and pulse durations exhibit a calculated scatter in the power law ranging from -0.8 to -1.4.' The pulse used to obtain -0.99 was 370uS wide of which 320uS was cc and a switch off time of 2uS.

Eric.

Including a chart posted earlier showing current profile for my tester. The constant current has a 20usec ramp up to constant current. Would that be considered constant current or would the time to get to constant current need to be a lot shorter? I added a constant rate pulse to simulate a low resistance coil where Tx width was less than the coil time constant. I have two reasons for charting the ground decay. #1, how accurate can I chart the decay. #2, what is the decay slope and what variables effect the decay. The log amplifier has made the decay trace cleaner and I see things that I didn't see before. Tx width and shape effect decay slope. Wanting to see what decay slope you get when you add the log amplifier to your circuit.

The chart for the US quarters TRT_7.png where done with method I used before adding the log amplifier

[Ferric Toes]

Was looking at ground chart TRT_21 reply #82. Noticed 160usec Tx has a straight line decay back to <6usec. The 16msec Tx is less steep back to a little over 10usec where it has a slope similar to the 160usec Tx. Any thoughts why this would be?

I think you are in uncharted water if you use pulse shapes other than rectangular cc. A paper by Dabas, Jolivet and Tabbagh, 'Magnetic susceptibility and viscosity of soils in a a weak time varying field', state 'Between 5 and 180uS a best fitting power law of -0.99 has been derived taking into account the finite rising time and decay time of the transmitter pulse. Trials with slightly different pulse shapes and pulse durations exhibit a calculated scatter in the power law ranging from -0.8 to -1.4.' The pulse used to obtain -0.99 was 370uS wide of which 320uS was cc and a switch off time of 2uS.

Eric.

[green]

Good to hear you are getting better. I'm happy with the LOG114 amplifier, can get 4decades with capacitor discharging into input. My PI amplifier noise is limiting me to maybe 2 octaves over 2decades.I always get a ground decay slope greater than -1 with my normal Tx width's. Got -1 with 16msec Tx awhile back with the method of charting data I was using. Tried today with 160usec and 16msec cr and cc TX. Close to -1 with 16msec Tx. Interested in what you get for ground when you finish your test setup.

Was looking at ground chart TRT_21 reply #82. Noticed 160usec Tx has a straight line decay back to <6usec. The 16msec Tx is less steep back to a little over 10usec where it has a slope similar to the 160usec Tx. Any thoughts why this would be?

[green]

My experiments have stalled this past week as I contracted a bad dose of 'flu and couldn't do a thing. I feel a bit better this morning but it will likely be a few days yet before I can do anything practical. Anyway, reading the good posts coming in keeps me up to speed.

Eric.

Good to hear you are getting better. I'm happy with the LOG114 amplifier, can get 4decades with capacitor discharging into input. My PI amplifier noise is limiting me to maybe 2 octaves over 2decades.I always get a ground decay slope greater than -1 with my normal Tx width's. Got -1 with 16msec Tx awhile back with the method of charting data I was using. Tried today with 160usec and 16msec cr and cc TX. Close to -1 with 16msec Tx. Interested in what you get for ground when you finish your test setup.

[6666]

My experiments have stalled this past week as I contracted a bad dose of 'flu and couldn't do a thing. I feel a bit better this morning but it will likely be a few days yet before I can do anything practical. Anyway, reading the good posts coming in keeps me up to speed.

Eric.

Thats no good Eric, try zinc tablets they always help me get over that damn flu and colds.

[Ferric Toes]

My experiments have stalled this past week as I contracted a bad dose of 'flu and couldn't do a thing. I feel a bit better this morning but it will likely be a few days yet before I can do anything practical. Anyway, reading the good posts coming in keeps me up to speed.

Eric.

[green]

Tried storing scope data with average sample and normal sample(clad quarter). I was thinking it only effected the scope screen. Averaging helped copied data to Excel also. I see I forgot to label pictures. First is normal sample, second is 2 averages, third is 4 averages and forth is 8 averages. Looks like 2 averages cleans up some of the noise and is probably as good or better than the others. Average trace good to maybe 300mv below zero where the noise would cause the signal to go below zero volts.