PI SAMPLE DELAY QUESTION

[deemon]

Both TC's need to be considered. The TC of the coil and the TC of the target.

By the way - even the presence of the transmitter coil isn't important here . We can make a simple "mental experiment" , using a permanent magnet instead of the coil . Just imagine that we bring the magnet to the target , wait for some time and then remove the magnet very quickly by any kind of mechanism ( or simply blow it ) . So , when we bring the magnet - the metal target would "resist" ( according to Lenz's law ) , because the eddy current in the target must produce its own opposite field , that can compensate the external field . If the target material is a kind of superconductor , for example , this eddy current would never dissipate and its field might perfectly and permanently compensate the external field , so the field in the target body would never rise from zero . In another words we can say that external magnetic field never can penetrate into a superconductive target . But with the real metal target ( with big enough TC , of course ) we'll see another thing - eddy currents must dissipate exponentially , and with the same exponential law the field will penetrate into the target , so after some time ( about 2-3*TC ) the field inside the target will be almost equal to the initial field of our magnet ( at the same distance ) . With a small TC target ( and slowly mowing magnet ) all this dissipating process would occur too , but the eddy current must be too weak to measure it . But the field would penetrate into the target anyhow .

And if we quickly remove the magnet at this moment ( or later ) - then , according to the same Lenz's law , the target will resist again .... when we brought the magnet closely it was trying to "suppress" its field , and now - it is trying to "restore" the collapsing field . But how can it do this ? Only by its eddy current that must be the same magnitude but the opposite polarity . And now - it's the most interesting thing - we can notice the situation when the magnet is already absent ( it's far away now ) , but the target is "trying to be a magnet" for some time ( this time is nothing but target TC ) . So if we have a proper sensor near the target ( Hall sensor , for instance ) - it would feel this dying target field ... and this exponentially decaying response ( delayed field ) it what our PI detectors does feel . But if we remove our magnet from the target before the moment of "target saturation" ( before all eddy currents has been dissipated ) - this decaying process must be started not from the full field magnitude , but from the value equal to the input field value minus the eddy current opposite field ( that haven't dissipated at that moment , as I told before ) - so this target reply cannot have its full strength , and this is why I say that "the target was not properly charged"

[baum7154]

With HER208 i get 12 cycles/20uS on Pin 6 from the NE5534 without damping resistor. So I have around 600kHz. Dampingresistor with your formula should be 556 Ohm. Critical dumping with network is 761 Ohm. One good sideeffect from the HER208 is that the curve on Pin 6 /5534 is cleaner now, finding a clear Frequencysetting of the mirage is now easier.

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Excellent result! That is what I was hoping for so now your coil operates much closer to it's true Self Resonant Frequency.

Great work mschmahl!

[Tinkerer]

By the way - even the presence of the transmitter coil isn't important here . We can make a simple "mental experiment" , using a permanent magnet instead of the coil . Just imagine that we bring the magnet to the target , wait for some time and then remove the magnet very quickly by any kind of mechanism ( or simply blow it ) . So , when we bring the magnet - the metal target would "resist" ( according to Lenz's law ) , because the eddy current in the target must produce its own opposite field , that can compensate the external field . If the target material is a kind of superconductor , for example , this eddy current would never dissipate and its field might perfectly and permanently compensate the external field , so the field in the target body would never rise from zero . In another words we can say that external magnetic field never can penetrate into a superconductive target . But with the real metal target ( with big enough TC , of course ) we'll see another thing - eddy currents must dissipate exponentially , and with the same exponential law the field will penetrate into the target , so after some time ( about 2-3*TC ) the field inside the target will be almost equal to the initial field of our magnet ( at the same distance ) . With a small TC target ( and slowly mowing magnet ) all this dissipating process would occur too , but the eddy current must be too weak to measure it . But the field would penetrate into the target anyhow .

And if we quickly remove the magnet at this moment ( or later ) - then , according to the same Lenz's law , the target will resist again .... when we brought the magnet closely it was trying to "suppress" its field , and now - it is trying to "restore" the collapsing field . But how can it do this ? Only by its eddy current that must be the same magnitude but the opposite polarity . And now - it's the most interesting thing - we can notice the situation when the magnet is already absent ( it's far away now ) , but the target is "trying to be a magnet" for some time ( this time is nothing but target TC ) . So if we have a proper sensor near the target ( Hall sensor , for instance ) - it would feel this dying target field ... and this exponentially decaying response ( delayed field ) it what our PI detectors does feel . But if we remove our magnet from the target before the moment of "target saturation" ( before all eddy currents has been dissipated ) - this decaying process must be started not from the full field magnitude , but from the value equal to the input field value minus the eddy current opposite field ( that haven't dissipated at that moment , as I told before ) - so this target reply cannot have its full strength , and this is why I say that "the target was not properly charged"

Now you are taking into account the third magnetic field: The Earth's field, which actually might be the most important one. Considering we have the Earth's magnetic field of about 0.5 Gauss with the field lines parallel. With a magnet or a coil, we have a dipole magnetic field which is within the Earth's field. We can orient this dipole any which way we want, but every direction we align it will have a specific influence on the surrounding Earth's field. It will distort the surrounding field. The distortion has a rate of change. The rate of change and the field strength generate the eddy currents in the target.
When we remove our field, the earth field returns to it's original alignment. Again we have a rate of change, this time in the opposite direction.

Thus, if we want to look a bit deeper, we must look at our cycle as starting with an existing magnetic field.

[Nupi]

By the way - even the presence of the transmitter coil isn't important here . We can make a simple "mental experiment" , using a permanent magnet instead of the coil . Just imagine that we bring the magnet to the target , wait for some time and then remove the magnet very quickly by any kind of mechanism ( or simply blow it ) . So , when we bring the magnet - the metal target would "resist" ( according to Lenz's law ) , because the eddy current in the target must produce its own opposite field , that can compensate the external field . If the target material is a kind of superconductor , for example , this eddy current would never dissipate and its field might perfectly and permanently compensate the external field , so the field in the target body would never rise from zero . In another words we can say that external magnetic field never can penetrate into a superconductive target . But with the real metal target ( with big enough TC , of course ) we'll see another thing - eddy currents must dissipate exponentially , and with the same exponential law the field will penetrate into the target , so after some time ( about 2-3*TC ) the field inside the target will be almost equal to the initial field of our magnet ( at the same distance ) . With a small TC target ( and slowly mowing magnet ) all this dissipating process would occur too , but the eddy current must be too weak to measure it . But the field would penetrate into the target anyhow .

And if we quickly remove the magnet at this moment ( or later ) - then , according to the same Lenz's law , the target will resist again .... when we brought the magnet closely it was trying to "suppress" its field , and now - it is trying to "restore" the collapsing field . But how can it do this ? Only by its eddy current that must be the same magnitude but the opposite polarity . And now - it's the most interesting thing - we can notice the situation when the magnet is already absent ( it's far away now ) , but the target is "trying to be a magnet" for some time ( this time is nothing but target TC ) . So if we have a proper sensor near the target ( Hall sensor , for instance ) - it would feel this dying target field ... and this exponentially decaying response ( delayed field ) it what our PI detectors does feel . But if we remove our magnet from the target before the moment of "target saturation" ( before all eddy currents has been dissipated ) - this decaying process must be started not from the full field magnitude , but from the value equal to the input field value minus the eddy current opposite field ( that haven't dissipated at that moment , as I told before ) - so this target reply cannot have its full strength , and this is why I say that "the target was not properly charged"

You're on the right track

[deemon]

Now you are taking into account the third magnetic field: The Earth's field, which actually might be the most important one. Considering we have the Earth's magnetic field of about 0.5 Gauss with the field lines parallel. With a magnet or a coil, we have a dipole magnetic field which is within the Earth's field. We can orient this dipole any which way we want, but every direction we align it will have a specific influence on the surrounding Earth's field. It will distort the surrounding field. The distortion has a rate of change. The rate of change and the field strength generate the eddy currents in the target.
When we remove our field, the earth field returns to it's original alignment. Again we have a rate of change, this time in the opposite direction.

Thus, if we want to look a bit deeper, we must look at our cycle as starting with an existing magnetic field.

But don't forget that the Earth's field is constant ( or very slowly changing ) , and the target lays still in the ground , so this field cannot produce any eddy current in the target , being completely useless in colored metal target detecting The only "theoretical" case when this field can have an influence - that if we have a ferrous target or ground . Because it possibly can "shift" the target magnetization from zero , so we should get a different reaction to positive and negative transmitter pulses , due to nonlinearity of the magnetization curve . But when we use a classic PI circuit with unipolar pulses - we cannot notice this , so it's better to use a bipolar pulse technology , and perform some experiments . As for me , I never tried this , maybe in future I'll have a time ...

[daverave]

Dave,

The nature of large copper coins puts their TC in the 100us range so that a TX pulse of about 300us fully charges the mass of the coin's metal to release eddy currents when the TX current is suddenly turned off. The quick discharge of the TX charge current releases the eddy current energy stored during the TX pulse that is detected in the RX window right after the delay time set on the PI delay setting. The optimum value damping resistor will allow you to move the RX sample window as close to the TX turn off point to begin detecting the residual target eddy currents just after the set delay time. If the delay were set at 15us then the RX sample window would detect targets from about 16us to 30us but during the initial 15us delay, low TC targets like gold nuggets, some gold jewlery and some metals will be decaying quickly during the 15us delay time and may not have much energy current during the RX period. Only targets that are on the edge of being detected will be mostly affected by small tweeks in reducing the delay.

The energy in the flyback pulse needs to be damped so that the turn off current and resulting flyback pulse is damped to zero volts as soon as possible. The closer the turn off slope is to 90 degrees vertical the better. Since all coils have some capacitance in the form of the construction techniques, including the coax capacitance, coil wire turn to turn capacitance, shielding capacitance, and MOSFET COSS will store energy and form a resonant frequency that extends the time it takes to fully damp the flyback pulse. That is why it is better to fine tune the coil and damping resistor to the PI machine for optimum results.

The value of the damping resistor (Rd) determines the discharge slope of the turn off current. This also form the discharge TC of the coil. A 300uH coil with a 1000 ohm Rd will have a discharge TC of 300/1000 or 0.3us which is optimum for targets with TCs 5X higher or 1.5us. The optimum turn off pulse TC is 5 X faster than the target TC.

When the MOSFET clamps for a few microseconds to keep the flyback peak at the MOSFET voltage, the time during which the pulse is flat topped (clamped) will cause a delay in the time when the earliest sample can occur.

When you are using a TX pulse in the 3K PPS range, you will be integrating many samples and the RX sensitivity will be somewhat affected by the speed of your sweep. Try speeds of 1.2M sweep width per 2 seconds and then try 1.2M sweep width in 1 second to see if the sweep speed affects sensitivity.

On my high speed PI machines, I get better range on U.S. nickels than on U.S. laminated quarters.

Joseph Rogowski

im just in the process of experimenting with dual field coil...i used your damping setup as in your artical and was able to damp the 12" coil at about 700 ohm but the small coil would not damp with the damping setup so i used a 1k power potentiometer and it would only damp at around 50 ohm which does not sound right being very low resistance.