PI metal detector for really small nuggets

[6666]

Have wondered if there is a way to damp the Tx coil to allow earlier sampling. Tried with spice. Better but not much. Would take more circuit, should I expect a bigger difference if I made a real circuit?

Tough question, spice is a great tool, but useing real circuits gives us a real world result with the types and quality of the components that we have access to.

[green]

Made spice simulation bipolar, similar to my unipolar circuit. Need about 3.5us delay with my circuit, spice circuit looks like less than 2us delay would work. Maybe my amplifier or something I'm missing is the problem. Wondering if coil volts not decayed enough could be causing the problem. Coil volts=rate of current change(X signal?). How low should the rate of change of Tx current be when sampling the signal with a PI(separate Tx and Rx)? .015V/300uH=50A/second, could that be a problem? Looks like trying the circuit not critical damped would be worth trying?

[bbsailor]

Made spice simulation bipolar, similar to my unipolar circuit. Need about 3.5us delay with my circuit, spice circuit looks like less than 2us delay would work. Maybe my amplifier or something I'm missing is the problem. Wondering if coil volts not decayed enough could be causing the problem. Coil volts=rate of current change(X signal?). How low should the rate of change of Tx current be when sampling the signal with a PI(separate Tx and Rx)? .015V/300uH=50A/second, could that be a problem? Looks like trying the circuit not critical damped would be worth trying?

Green,
Here is a mental model to consider to solve your fast sampling question. When damping the initial flyback pulse the idea is to quench the flyback oscillations as quickly as possible to sample as soon as possible. The key question is:what causes these oscillations?

Think about it from the perspective of the total capacitance seen at the points where the damping resistor is located. All the energy in the oscillations that you are trying to damp comes from the total capacitance seen at that point and during the time it takes for these oscillations to stop or drop to a low enough value that you can begin to RX sample the target. Low TC targets give up their fully stimulated energy very quickly. The most creative solutions require identifying all the sources of capacitance seen as oscillations after damping and reducing or eliminating as many as you can. Here are some places to look.
1. Lower capacitance coax cable.
2. MOSFET coil driver COSS.
3. Coil winding capacitance.
4. Coil to shield capacitance.
5. Coil geometry, single coil, DD coil, figure 8 coil, etc.
6. Optimum coil size for your desired target size and TC.

I hope these things get you looking for the source of the unwanted capacitance to allow you to sample sooner.

Joseph J. Rogowski

P.S. A final thought. When sampling at very low times, the eddy currents in the coil wire may be detected as a target. Use wire with a lower TC than your smallest desired target. Litz wire with very fine strands and a thick outer insulation with a low dielectric constant is a good choice.

[green]

Made spice simulation bipolar, similar to my unipolar circuit. Need about 3.5us delay with my circuit, spice circuit looks like less than 2us delay would work. Maybe my amplifier or something I'm missing is the problem. Wondering if coil volts not decayed enough could be causing the problem. Coil volts=rate of current change(X signal?). How low should the rate of change of Tx current be when sampling the signal with a PI(separate Tx and Rx)? .015V/300uH=50A/second, could that be a problem? Looks like trying the circuit not critical damped would be worth trying?

Wondering about the X signal(coil current not decayed enough)I did a test this morning with the 133mm fig8 coil unipolar circuit. Adjusted damping for best amplifier out signal. X and R signal are opposite phase for non ferrous targets(nickel). Amplifier signal goes more negative until R signal overrides X signal at about 2.5us after Tx off. Maybe the reason can't sample closer to spice simulation time around 2us delay? Make sense or am I still missing something?

CH2 clipped to Tx coil hot lead insulation not hot lead.

[6666]

Green,
Here is a mental model to consider to solve your fast sampling question. When damping the initial flyback pulse the idea is to quench the flyback oscillations as quickly as possible to sample as soon as possible. The key question is:what causes these oscillations?

Think about it from the perspective of the total capacitance seen at the points where the damping resistor is located. All the energy in the oscillations that you are trying to damp comes from the total capacitance seen at that point and during the time it takes for these oscillations to stop or drop to a low enough value that you can begin to RX sample the target. Low TC targets give up their fully stimulated energy very quickly. The most creative solutions require identifying all the sources of capacitance seen as oscillations after damping and reducing or eliminating as many as you can. Here are some places to look.
1. Lower capacitance coax cable.
2. MOSFET coil driver COSS.
3. Coil winding capacitance.
4. Coil to shield capacitance.
5. Coil geometry, single coil, DD coil, figure 8 coil, etc.
6. Optimum coil size for your desired target size and TC.

I hope these things get you looking for the source of the unwanted capacitance to allow you to sample sooner.

Joseph J. Rogowski

P.S. A final thought. When sampling at very low times, the eddy currents in the coil wire may be detected as a target. Use wire with a lower TC than your smallest desired target. Litz wire with very fine strands and a thick outer insulation with a low dielectric constant is a good choice.

Hello Joseph
just thinking about your point 6

6. Optimum coil size for your desired target size and TC.

Given I prospect for sub gram gold nuggets in heavy mineralized Aussie soil, what theoretical procedure or testing method would you suggest to achieve "optimum coil size ", thanks

[bbsailor]

Hello Joseph
just thinking about your point 6


Given I prospect for sub gram gold nuggets in heavy mineralized Aussie soil, what theoretical procedure or testing method would you suggest to achieve "optimum coil size ", thanks

6666,
The key to optimizing one variable such as coil size is to understand how other variables may interact. In the advancer search box enter Optimizing Target Responses to read more about this.

The first thing you need to know is the Time Constant range for the nuggets you are seeking. Here is an example. If you are seeking a nugget with a 2uS (microsecond) TC, the fully stimulated target will decay to fully decayed energy in 5 Target TC, time constants. A 2uS target will fully decay in 10uS. If you are sampling at a 10uS delay, guess what...no signal left to detect with any size coil. When comparing coil sizes, make sure your delay is fast enough to detect your target. If your nuggets have a target TC of 1uS then you need to have a delay faster than 5uS just to potentially detect that nugget. Generally, smaller coils are more effective in detecting smaller targets. Noisy or mineralized soil is another variable that may need a different coil style to minimize the effect of mineralized soil.

Generally, faster sampling produces a stronger RX signal. By integrating many RX signals you can improve the sensitivity to smaller targets but only as long as the target is within the coil area. Look up the theory behind the concept of a lock-in amplifier to see how integrating many signals improves the signal to noise ratio.

Good research requires knowing how related variables interact, and trying to isolate variables when trying to optimize any one of them by creating good mental models of:
1. Fully stimulating a particular target
2. Sampling soon enough to capture target energy before it fully decays
3. Coil style proven to be effective in particular environments, ground types and targets sought
4. Coil size for targets sought
5. Unique coil design features to overcome limitations such as putting the first stage preamp module in or near the coil housing.

This forum contains a lot of good information to help you more clearly see how these things interact or even see a hole that you can creatively fill.

Joseph J. Rogowski

[green]

A test comparing 133mm and 300mm I.D. coils at 3.5 and 5us delays with #9, #8, #6 and #4 lead shot. Tested lead shot is good for comparing distance for nuggets less than about 2 grains. Adjusted target distance for about 1.5mV change at integrator out.

[6666]

6666,
The key to optimizing one variable such as coil size is to understand how other variables may interact. In the advancer search box enter Optimizing Target Responses to read more about this.

The first thing you need to know is the Time Constant range for the nuggets you are seeking. Here is an example. If you are seeking a nugget with a 2uS (microsecond) TC, the fully stimulated target will decay to fully decayed energy in 5 Target TC, time constants. A 2uS target will fully decay in 10uS. If you are sampling at a 10uS delay, guess what...no signal left to detect with any size coil. When comparing coil sizes, make sure your delay is fast enough to detect your target. If your nuggets have a target TC of 1uS then you need to have a delay faster than 5uS just to potentially detect that nugget. Generally, smaller coils are more effective in detecting smaller targets. Noisy or mineralized soil is another variable that may need a different coil style to minimize the effect of mineralized soil.

Generally, faster sampling produces a stronger RX signal. By integrating many RX signals you can improve the sensitivity to smaller targets but only as long as the target is within the coil area. Look up the theory behind the concept of a lock-in amplifier to see how integrating many signals improves the signal to noise ratio.

Good research requires knowing how related variables interact, and trying to isolate variables when trying to optimize any one of them by creating good mental models of:
1. Fully stimulating a particular target
2. Sampling soon enough to capture target energy before it fully decays
3. Coil style proven to be effective in particular environments, ground types and targets sought
4. Coil size for targets sought
5. Unique coil design features to overcome limitations such as putting the first stage preamp module in or near the coil housing.

This forum contains a lot of good information to help you more clearly see how these things interact or even see a hole that you can creatively fill.

Joseph J. Rogowski

Hi Joseph , Thanks reply.

The path I am going down at the moment is trying to make fast 6 inch and 3 inch coils, and also trying to turn the TX fet off quickly, and making the pre amp faster, all things covered in different threads across the forums.
With the coils, I have been trying different construction methods to reduce the SRF and hence the capacitance.
The object of the exercise is to try and enhance the response to small ~ .06 gram nuggets in our hot Aussie soils, but its difficult to measure small differences in performance.


There is also another phenomenon with Aussie Victorian hot soil, and its what I refer to as " the rate of change of mineralisation over a given coil sweep distance", that is the amount of mineralisation can change a lot over a short distance, and it seems small coils ( 3-6 inch) are more effected by it than say a 12 inch coil, but the mineralisation changes can be measured with a Whites GMT.


Its all good fun, but I dont know if its worth the trouble sometimes.

[bbsailor]

6666,

You stated that you want to reduce the SRF and hence the capacitance. That is the opposite of what really happens. A reduced SRF indicates a higher capacitance. Looking at the SRF of a coil is an easy way to derive the capacitance of the coil construction if you know the coil inductance. The dielectric constant of the wire insulation and the insulation thickness will mostly affect a close wound bundle coil SRF. Read my Making A Fast PI coil article in the projects section to see an easy way to use a signal generator and oscilloscope to see the resonant peak of your coil.

Joseph J. Rogowski

[6666]

6666,

You stated that you want to reduce the SRF and hence the capacitance. That is the opposite of what really happens. A reduced SRF indicates a higher capacitance. Looking at the SRF of a coil is an easy way to derive the capacitance of the coil construction if you know the coil inductance. The dielectric constant of the wire insulation and the insulation thickness will mostly affect a close wound bundle coil SRF. Read my Making A Fast PI coil article in the projects section to see an easy way to use a signal generator and oscilloscope to see the resonant peak of your coil.

Joseph J. Rogowski

Yes my mistake , I meant to say increase SRF, reduce cap, not watching what I said.

[green]

Made a bipolar Tx circuit similar to picture(Tx eight). Different MOSFETS, used what I had. Bipolar doesn't look bad, unipolar circuit needs less damping(spice circuit also). Including spice simulations I've been paying with. Has anyone tried dumping the coil current to zero similar to bipolar2 simulation. Looks faster than critical damped but requires turn off time to repeat to 1 or 2 nano seconds(spice simulation). Anyone have another suggestion for dumping the coil current faster than critical damped.

Forgot to turn on a target, bipolar2.zip

[green]

Made a bipolar Tx circuit similar to picture(Tx eight). Different MOSFETS, used what I had. Bipolar doesn't look bad, unipolar circuit needs less damping(spice circuit also). Including spice simulations I've been paying with. Has anyone tried dumping the coil current to zero similar to bipolar2 simulation. Looks faster than critical damped but requires turn off time to repeat to 1 or 2 nano seconds(spice simulation). Anyone have another suggestion for dumping the coil current faster than critical damped.

Forgot to turn on a target, bipolar2.zip

https://www.geotech1.com/forums/atta...8&d=1610389484 D1 isolates the coil from the M1 during discharge with the unipolar circuit. Any ideas isolating the coil from M1 and M2 during discharge with the bipolar circuit? Real circuit effected more than spice circuit.

[Carl-NC]

https://www.geotech1.com/forums/atta...8&d=1610389484 D1 isolates the coil from the M1 during discharge with the unipolar circuit. Any ideas isolating the coil from M1 and M2 during discharge with the bipolar circuit? Real circuit effected more than spice circuit.

Do the same thing, add series diodes.

[green]

Do the same thing, add series diodes.

Not sure where you are suggesting adding the diodes. I added D3,4,5 and6 in spice. Looked better in spice but wasn't better in real circuit. Used 5EWH06 diodes instead because didn't have enough MUR460 which I used for D1 an2.

[eclipse]

I have the same simulation except d3 and d4 which i dont have and a board design waiting to be finalized. In what way it wasnt better? The coil settle time is not improved?