PI TX Settling

[Ferric Toes]

Increased Tx time to 16msec flat. Charted US quarter and 1 troy ounce 99.9% pure copper coin. Straight line lin log after 100usec. Why doesn't it continue straight below 100usec with a 16msec Tx? To find the TC the decay needs to be charted lin log. Maybe a scope linear trace would be just as good or better for looking at signal loss during target sample time. What would be a good time full scale? I'm thinking 50usec would cover most target sample times. Would 20usec or 100usec be better?

By 'below' I assume you mean for times shorter than 100uS. This is because you are in the 'skin effect', 'early time', part of the decay curve. the eddy currents have not yet had time to diffuse to the core of the coin, due to it's high conductivity. At 100uS the diffusion ceases and then we have a single exponential; hence the straight line from that point on.

For a good explanation of this process, download the paper 'Applications of Transient Electromagnetic Techniques' Technical note TN-7 from the Geonics Ltd website. Page 6 for 'Target Responses'. Although this is for geophysical prospecting, the same scaled down principles apply for PI metal detectors.

Eric.

[green]

Increased Tx time to 16msec flat. Charted US quarter and 1 troy ounce 99.9% pure copper coin. Straight line lin log after 100usec. Why doesn't it continue straight below 100usec with a 16msec Tx? To find the TC the decay needs to be charted lin log. Maybe a scope linear trace would be just as good or better for looking at signal loss during target sample time. What would be a good time full scale? I'm thinking 50usec would cover most target sample times. Would 20usec or 100usec be better?

[Carl-NC]

I was surprised too; plus it reads lower than a quarter. Maybe it is a fake. Could it be that the alloys changed between 1889 and the 1960's? I know that some of our coins did.

US silver coins were always the same alloy up until 1964. The Kennedy half had a 40% alloy from 65-67. I recently bought (intentionally) two Chinese counterfeit silver dollars, but I haven't tested them yet.

[crane]

Regarding on-time eddy currents cancelling off-time eddy currents.

There are two schools here. One transmits a pulse where the current rises quickly and then flat tops for a significant period before switch-off. If all goes well the target is just simply submersed in a constant uniform field for the latter part of the pulse with any switch-on induced eddy currents settled out.

The magnetic step at switch-off then causes eddy currents to circulate in the skin. These decay very quickly inducing eddy currents deeper in the target and this process continues until it reaches the center or runs out of steam. This is the onion ring or smoke ring effect.


The other school transmits a triangular current waveform where the current rises until switch-off. The target current is shown in the attached pictures and makes it easier to see what happens in this case.

Note that the current rises rapidly in targets with short TCs and then remains constant while ever the TX current is still rising. The long and medium TC target current rises slowly as shown.

Introducing any resistance into the TX circuit would obviously cause the design to become inefficient, especially on long TC targets if the target current begins to fall before switch-off. This TX method relies on the current in short TC targets being held steady and rising in long TC targets. It relies on transmitting a linear current ramp.

One figure is from US5506506. This patent also discusses the relationship of the received signal during and after the pulse. Targets with short TCs relative to the pulse length give a different amplitude response during and after the pulse than long TC targets.

[Ferric Toes]

A lot of people suggest that flat-topping the TX current allows the target to fully "charge" or fully "energize." This is a slight misnomer, what they really means is it allows the target to fully "discharge" or fully "de-energize." The turn-on dB/dt energizes target eddies, the flat-topped TX current allows those eddies to die out.

The flat topping also gives a constant end of pulse field strength when you vary the width of the TX pulse. This was important in my MVM for getting accurate plots of viscous ground decay with TX widths between 30uS and 300uS.

Eric.

[daverave]

what would be the average tc of say 18/22 ct medium size gold rings ????

[Ferric Toes]

After you bought the Hocking, I decided I wanted one. Took a while but I got one on eBay for a decent price just before I left White's. In the move it got packed up and misplaced.

I'm surprised the silver dollar reads lower than a half. They have identical alloys (90% Ag 10% Cu) but the dollar is thicker and still less than 5 skin depths.

I was surprised too; plus it reads lower than a quarter. Maybe it is a fake. Could it be that the alloys changed between 1889 and the 1960's? I know that some of our coins did.

[Carl-NC]

The idea that sustaining a pulse "energizes" anything is the wrong mindset. Eddy currents are induced by a change in the magnetic field (B):

The initial value of B does not matter, only its derivative (change) does. So a change from B = 1 to B = 0 in 1us induces the same eddy current as a change from B = 0 to B = 1 in 1us, but of the opposite sign. In order to measure the eddy, you hold B at its final value (B = 0 in the first case, B = 1 in the second case) so that dB/dt=0 during measurement.

A lot of people suggest that flat-topping the TX current allows the target to fully "charge" or fully "energize." This is a slight misnomer, what they really means is it allows the target to fully "discharge" or fully "de-energize." The turn-on dB/dt energizes target eddies, the flat-topped TX current allows those eddies to die out.

[Carl-NC]

I have a Hocking Conductivity meter which is an induction device and has an annealed copper calibration disc on the front panel which reads 100 when the instrument is calibrated. The reading for a different metal is as a percentage of the copper reading.

A US 1889 silver dollar reads 84. A 1964 half dollar 92. A silver quarter 90. A copper cent 52. A dime 52., and the humble nickel 5.5. By comparison, a Whites Electronics commemorative 1oz medallion 0.999 fine silver reads 101. The lower figures for the silver coins are due to small quantities of other metals alloyed to make the coins harder than the base metal, and wear better. Pure silver should read 105.

Having TX pulse widths of a millisecond or so is not a problem in itself, and I have completed a few designs for the professional market where this length and more, are used when looking for large conductive objects.

Eric.

After you bought the Hocking, I decided I wanted one. Took a while but I got one on eBay for a decent price just before I left White's. In the move it got packed up and misplaced.

I'm surprised the silver dollar reads lower than a half. They have identical alloys (90% Ag 10% Cu) but the dollar is thicker and still less than 5 skin depths.

[Brian Deese]

Yes, it will be good to do some practical experiments to verify what is happening. I am going to repeat an experiment I did many years ago by having one TX, an RX preamp, and looking at the whole decay directly on a scope. I will have a pot, or switch, in the timing circuit that controls the TX width with a range say of 50uS to 500uS. I will also have a constant current TX that flat tops in 10uS for all widths. This is something I did not do previously but we need to keep the applied field constant so that the only variable is the pulse width. From previously, I remember seeing the waveform for a good conductive object (English copper penny or US quarter) 'fill out' as the width was increased until you reached a point when there was no increase in mid-time amplitude and then that was deemed to be the best TX width for that object. It was also good for anything smaller or less conductive except that you are wasting energy with an overlong pulse for that object.
I don't have one, but if anyone has a storage scope then you can hold the short TX width response and super-impose on it the following long TX responses. It is an easy experiment, but to be valid you need the flat top current. Otherwise the current will increase for each longer pulse width with the effect that you have two variables.

Eric.

Today I tried to duplicate this. I set the series resistance of my coil so the current flat tops at 10uS. I started with a 20uS Tx width,no target and saved the waveform on my scope. That wave form persist's on the screen in a blue trace. Then I placed a large brass cylinder inside the coil and captured an image of 20uS,50uS,100uS,and 150uS Tx widths. I made an album,and put the images in it here. http://www.geotech1.com/forums/album.php?albumid=62 The results were once the coil current is flat topped, increasing the Tx width has no effect. But one thing you can see in the pictures is the brass cylinder pulled the response back. You can see the baseline blue trace out front of the red trace. A steel waste basket sent it the other way!

[green]

I recorded some ground decay curves with a flat and ramp Tx awhile back. The ramp Tx had a steeper slope. Recorded some today with a flat and ramp 510usec Tx and flat and ramp 160usec Tx(Tx time I've been using). The flat and ramp TX slopes at 510usec were the same at -1.16. The amplifier out amplitude for the 510usec flat Tx is about 1/2 the 510usec ramp Tx. The peak current is 1/2 so they would be about the same if peak current was the same. The slope is steeper for the 160usec ramp Tx(-1.35) compared to the 160usec flat Tx(-1.2). The amplifier signal at 10usec delay with the flat Tx is 1/2 the ramp Tx(flat is 1/2 the peak current) but crosses near 500usec.

[Teleno]

I cannot see how a short pulse can energise a large good conductor except for the skin effect currents.

The idea that sustaining a pulse "energizes" anything is the wrong mindset. Eddy currents are induced by a change in the magnetic field (B):

The initial value of B does not matter, only its derivative (change) does. So a change from B = 1 to B = 0 in 1us induces the same eddy current as a change from B = 0 to B = 1 in 1us, but of the opposite sign. In order to measure the eddy, you hold B at its final value (B = 0 in the first case, B = 1 in the second case) so that dB/dt=0 during measurement.


Top graph: B
Bottom graph: eddies

[Ferric Toes]

I will look it up and post tomorrow. It is suppertime here and wife is calling. I must say that this is an interesting thread.

Eric.

[green]

I have a Hocking Conductivity meter which is an induction device and has an annealed copper calibration disc on the front panel which reads 100 when the instrument is calibrated. The reading for a different metal is as a percentage of the copper reading.

A US 1889 silver dollar reads 84. A 1964 half dollar 92. A silver quarter 90. A copper cent 52. A dime 52., and the humble nickel 5.5. By comparison, a Whites Electronics commemorative 1oz medallion 0.999 fine silver reads 101. The lower figures for the silver coins are due to small quantities of other metals alloyed to make the coins harder than the base metal, and wear better. Pure silver should read 105.

Having TX pulse widths of a millisecond or so is not a problem in itself, and I have completed a few designs for the professional market where this length and more, are used when looking for large conductive objects.

Eric.

How to convert Hocking Conductivity meter reading to target time constant? Do you see a problem with the way I measured TC for the US quarter and copper coin decay_4?

[Ferric Toes]

Laying awake at night and thinking about Telenos' short TX pulse and sampling in the on-time, I came to the conclusion that it could work for large and small conductors. In the cold light of day, I have come to the conclusion that it couldn't work. Basically the jury is still out, but I cannot see how a short pulse can energise a large good conductor except for the skin effect currents. Sampling within the pulse does not improve this. I'll have another think tonight.

Eric.