New PI technology proposals.

[Aziz]

Hi Carl,

I don't like the naming "artifact". It is an Uind = -L*dI(t)/dt (Lenz's and Faraday's rule). So dI(t)/dt in the coil end up to dB(t)/dt where the magnetic field energy will be converted to heat during the flyback process. This seems the fastest way of killing the B field. The target is contributing to this process more or less.

If the induced voltage is not dissipated through the damping resistor, it will ring and eddy currents will still be produced on the targets (the magnetic field energy will then be dissipated on targets and conductive objects nearby the coil). But we are not taking VLF and PI process to the same level. We have the damping process, which we focus on. And I still state, this is the most important process of a PI, which kicks the target on PI method.

Anyway, we are almost describing the same. We have only different names.

Aziz

[Carl-NC]

The rise of the flyback voltage will cause rise of eddy currents in the target (damping is active). But the signals can not be processed at this time (high voltage!).

Again, it is not the flyback voltage causing the eddy currents, it is the di/dt which produces a dB/dt which induces eddy currents. The di/dt also causes the flyback voltage... it's just an artifact. Keep in mind that VLF detectors use a sinusoidal current to create a di/dt (and therefore a dB/dt) and there is no flyback voltage.

As long as the clipping will last, the real "kick of targets" will be delayed until the breakdown voltage is not clipped anymore. ... MOSFET falls short of its avalanche breakdown voltage and begins to decrease. This is the real kick of target. We will have now a higher dI(t)/dt, which will generate target eddy currents.

The real "kick" occurs at maximum di/dt, which coincides with the peak of the flyback. By the time the flyback voltage starts heading back down (whether clipped or not) the peak di/dt is over. We're done kicking the target.

- Carl

[Carl-NC]

Some targets are obviously pulse length dependent and others aren't and the latter can't possibly be represented by a simple L/R. If you did then you would be wrongly assuming they exhibit an exponential decay over their complete decay period and this is definitely incorrect. It is obvious to anyone conducting practical tests and is well documented in patents etc. You might also wish to look at Eric Foster's waveforms and tell him it is all heresay?

I think it's only a factor with some of the high pulse frequency / low coil current designs Eric does.

[Aziz]

Hello friends,

I will add some more comments on previous post for clearness.

Again: The maximum eddy current of target is reached at MOSFET's avalanche breakdown voltage (or at maximum flyback voltage). At this time, we have a constant voltage level over the damping resistor if the MOSFET is clipping the flyback voltage. It will convert the magnetic field energy with constant rate: P=U*U/Rd. This will last, until the flyback voltage is below the breakdown voltage. Then the eddy current level of the target will decrease exponentially (natural decay).


Did you all think about the time constant of the coil? It limits the rate of time for energy conversion (building up magnetic field energy, discharging magnetic field energy by converting to heat). So this rises a very interesting question about the Q of the coil.

Aziz

[Tinkerer]

Charge curve x discharge curve

Spice simulations can tell a lot. However ALL the factors have to be accounted for.
If you have a target that has a decay\discharge curve of, say 100uS, how long did it take to charge this target?

If your Spice model target has no realistic TC, will the spice simulation result be realistic?

Tinkerer

[Aziz]

Hi friends,

I will give you the correct answer, what is exactly "kicking the target".
It is the decay of the flyback voltage itself (current decay through the coil and damping resistor)!

What is happening in detail?

1. MOSFET switches on and a current will start going through the coil (from 0 to some level). The rise time of this current is compared to switch-off very small (small slope dI(t)/dt)) thus generating only small eddy currents in the target. As the sampling time is very very later, the induced eddy currents will have less and less effects.

2. MOSFET will be switched on for some period to achieve a defined coil current, which generates the magnetic field energy E = 0.5*L*I*I. The dI(ti)/dt will decrease the longer the transmit pulse width is lasting.

3. MOSFET switches off. The MOSFET is not switching instantly! It's resistance will increase over some time (ns range). We will have now a higher dI(t)/dt which generates the flyback voltage (increasing steadily to a maximum possible level which is dependent on exposed magnetic energy and coil characteristics (damping resistor, coil capacitance, coil inductance, additional parasitic capacitive load)). The rise of the flyback voltage will cause rise of eddy currents in the target (damping is active). But the signals can not be processed at this time (high voltage!).

4. MOSFET reaches the avalanche breakdown voltage. The flyback voltage is clipped to this breakdown voltage. Now the dI(t)/dt becomes constant! It is not contributing a rise of eddy currents in the target. As long as the clipping will last, the real "kick of targets" will be delayed until the breakdown voltage is not clipped anymore.

5. MOSFET falls short of its avalanche breakdown voltage and begins to decrease. This is the real kick of target. We will have now a higher dI(t)/dt, which will generate target eddy currents. As the target and coil are inductively coupled, the target signal will affect the decay curve.

From the point of target, the voltage clipping (avalanche breakdown voltage of MOSFET) is waste of enery and delays the first sampling time. It may be convenient, to get dI(t)/dt to zero before switching the coil off. Then switch-on target signals will be decayed to zero to have no or less effects for real target kick offs.

The idea of avoiding the avalance breakdown voltage will improve the overall performance, when the high voltage is extended (higher voltage rated MOSFETs) or decoupled from the MOSFET (center-tapped coil). A higher target stimulation will therefore be possible.

Please, make use of SPICE simulations. You can see this process more in detail.

Aziz

[Qiaozhi]

The spike obviously doesn't "kick" the target but the end result can still depend on what occurs during the pulse on time. Some metal targets are pulse length dependent and others aren't so it's incorrect to model all targets as a simple L/R circuit.

Your post has left me a little confused.

In my attempts to investigate the claims (made by Aziz) that a center-tapped coil has advantages over a mono coil, when it comes to settling time, I did not include any models for the target. Also, the latter simulations were extreme simplifications to show the relationship between voltage and current in the search coil. In these particular simulations, I purposely excluded the coil capacitance and damping resistor. It was basically stripped back to the minimum parts required to demonstrate what's going on. Only Aziz's simulations included a target model.

So I'm not sure whether you're actually agreeing with me, or not.

The misunderstanding that some people have about PIs, is what actually "kicks" the target, thus creating eddy currents to flow. As Carl said earlier - the back-emf (or "flyback", if you prefer) voltage is simply an artifact. In reality it is the di/dt (rate of change of current with time) generated by the collapsing magnetic field (when the switch is opened) that creates the "kick". There is often a misconception that the ON-current somehow charges the target. This is incorrect.

[robby_h]

"and is well documented in patents etc."
patents are NOT subjected to sceintific peer review are they?
all that patent atterneys are interested in are the claims
you can patent ANYTHING even if science is totally crap!
by the way whose patents?

Unregistered, if you can't understand what I said then perhaps you could try using plain old common sense? You are actually claiming that all metal targets decay exponentially over their full decay period?

https://www.geotech1.com/thuntings/showpost.php?p=39469&postcount=2

Remember this? It was in answer to what I believe to be your question.

"Also, objects whose time constant is longer than the applied transmitter field duration can exhibit a decay that looks like magnetic mineralisation. Again, you are looking at a sum of different time constants that does not settle down to a single exponential until after 1 TC".

This is correct although it can often take several TCs!

pure heresay! Where's the evidence!

What's wrong D? Not game to post under your usual name?

Do you post on EVERY well known public forum as Robby_H?

"and is well documented in patents etc."
patents are NOT subjected to sceintific peer review are they?
all that patent atterneys are interested in are the claims
you can patent ANYTHING even if science is totally crap!
by the way whose patents?

[robby_h]

Some metal targets are pulse length dependent and others aren't so it's incorrect to model all targets as a simple L/R circuit.

pure heresay! Where's the evidence![/quote]


What's wrong D? Not game to post under your usual name?
The evidence is in the statement itself. Some targets are obviously pulse length dependent and others aren't and the latter can't possibly be represented by a simple L/R. If you did then you would be wrongly assuming they exhibit an exponential decay over their complete decay period and this is definitely incorrect. It is obvious to anyone conducting practical tests and is well documented in patents etc. You might also wish to look at Eric Foster's waveforms and tell him it is all heresay?

pure heresay!

Some metal targets are pulse length dependent and others aren't so it's incorrect to model all targets as a simple L/R circuit.[/QUOTE]

pure heresay! Where's the evidence!

[robby_h]

If I understand your water analogy correctly, I think you are attempting to prove that the back-emf is not a negative voltage. This is incorrect.

If you monitor the voltage across the coil you will see that the amplitude is positive when the switch is closed. But when the switch is opened the collapsing magnetic field generates a large negative voltage (back-emf) in the coil.

Let's take an even simpler example than we did previously. i.e. a 10uH coil (with a 10 ohm DC resistance) connected in series with a switch (10 ohm ON, 100MEG OFF), and connected across a 10V supply - see SPICE netlist.

Plot 1: Switch closes. No surprises here - voltages across coil and switch both settle at half the supply voltage (5V), as we effectively have two 10 ohm resistors in series across a 10V supply.

Plot 2: Switch opens. There is a back-emf generated across the coil of -448V due to the collapsing magnetic field, and +458V appears across the switch. Note that this value is the sum of the back-emf + the power supply voltage (448 + 10). This has to be the case, in order to satisfy Kirchoff's laws.

Plot 3: Now this is the interesting one. This shows what happens to the current in the coil. When the switch was closed, a steady-state current was established in the coil of 500mA. i.e. 10V/20ohms. When the switch opens, the magnetic field collapses, and the current in the coil rapidly decays. At no time does this current become negative. Therefore, from Ohm's Law, if the voltage is negative and the current is positive, then the resistance must be negative. In this case, because we are dealing with a coil, it is actually the inductive reactance that goes negative. This simply means that the coil has now become a generator.

Personally I don't like water analogies when describing electrical circuits. It only seems to be relevant for simple passive circuits.

Qiaozhi,
I agree it is hard to use water flow to describe inductive circuits because the system, in the absence of gas bubbles, is purely hydraulic and very unforgiving but I still think it "works" in this case.

I also agree with: "This simply means that the coil has now become a generator", as it does in my crude analogy, but your plots appear to show what occurs if we switch the positive coil supply, whereas most conventional circuits switch the negative supply and in this case we would need to invert your wave forms.

If we connect the appropriate end of a diode to the fet drain and the other end to a large capacitor then your circuit will charge the cap to a negative high voltage and the conventional circuit will charge the cap to a positive high voltage. Place your finger on the diode/cappy junction and you should feel the accumulated current! A large portion of the peak pulse-on current is transferred to the spike at switch off otherwise an SMPS wouldn't work.

If we stick to the conventional switching circuit then a high voltage negative spike appearing at the fet drain would turn the coil on harder, which doesn't and can't happen, whereas a high voltage positive spike would (and does) attempt to dissipate it's energy back thru the coil and this will only occur once for an idealised coil switching circuit with zero capacitance. This "ideal" circuit wouldn't need a damping resistor so all of the energy would try to dissipate back thru the coil's resistance.
This process though doesn't occur "instantly" and takes time. Current is still rising in the coil when the spike begins to rise and the spike current eventually begins to oppose the coil current and, if all goes well, the result is zero net coil current and this is why we can't see current passing back thru the coil in your plot or with a cro.

Now place a capacitor across the coil and it will charge to the initial spike's peak voltage but this stored energy then discharges back into the coil and this back and forth process repeats itself several times resulting in ringing. So in the real world we place a resistor across the capacitor to assure the process only occurs once. In the turbine analogy, this would be a restricted pipe connecting the turbine output to the input but this also allows some output surge to bypass the turbine, or spike current to bypass the coil which results in a slower decay.

So, any attempt to "get rid" of the spike must also result in a slower collapsing field.

The spike obviously doesn't "kick" the target but the end result can still depend on what occurs during the pulse on time. Some metal targets are pulse length dependent and others aren't so it's incorrect to model all targets as a simple L/R circuit.

[Aziz]

Hi Qiaozhi,

By the way, I asked Aziz, a few post ago, whether he now agreed that there was no "free lunch", and that there was no benefit whatsoever to using this split-coil approach. But there was no reply...
Aziz - are you still there?

I am quite busy at the moment. I am preparing some new analysing methods. The last simulations I made are showing me very very interesting results. The free lunch is still there (with right models, SPICE is showing this). I am checking right now, which free lunch could taste better. It is a lunch of many many composited ingredients. So cooking becames difficult and it takes lots of simulations (experiments).


I recommend all people to start with SPICE simulations. As this demystifies the whole PI process, one can learn really a lot.

I am also checking some old coil ideas which would allow a completely unshielded coil and cable solution and immunity to EMI, ground effects, temperature drift and input amplifier saturation. This idea allows a much higher amplification of the signals.

I am thinking of making a PI test module with an ATtiny2313 (AVR). This would allow me to proof some new ideas. It will be quite difficult to make this without an oscilloscope. So I have to work in the darkness almost.


Aziz

[Qiaozhi]

I only got involved in this discussion because I do not agree that the emf created, on the rx coil, by a collapsing field is what causes a response from a target. Still don't agree with that claim as made in the original post.

This is exactly the same reason why I also got involved. As Carl said earlier: "... it is the di/dt of the turn-off that kicks the target.".

I also do not see a benefit to putting the tx pulse on only 1/2 of the coil.

Neither do I; and this is confirmed by the simulation results.

On another note, while I really like Spice and several of the current simulator programs, I do not think they are ready to model complex inductor relationships. I hope that one day soon they will be. But, I don't think that day has come.

Actually, SPICE is quite capable of handling complex inductor relationships. The problems are more related to the inductor model than the simulator. Because the interactions are so complex, you really have to build your coils first, make all the measurements (such as inductance, DC resistance, interwinding capacitance, coupling coefficients, etc.) and then back-port them into the model. The initial simulations are only used to get you into the right ballpark. At the end of the day, SPICE is just another tool to have in your toolbox.

By the way, I asked Aziz, a few post ago, whether he now agreed that there was no "free lunch", and that there was no benefit whatsoever to using this split-coil approach. But there was no reply...
Aziz - are you still there?