DEEPER PI DETECTION DEPTH

[Davor]

Because it must dissipate all the magnetic energy from space around it. That's easy. On the other hand, you do not have to employ high voltages, just go for lower L and higher currents. Less fuss. I think I've found an ideal pulse that comes from the least suspect source. It is just a concept at the moment.

Lo and behold

[Tinkerer]

Peak coil current.



Then we need to define a new term "Ampere-turns per us." Still, if a VLF can hit 1A-T/us then a PI can easily hit 20A-T/us.

Agreed, on the "Ampere-turn per us"

The problem with the traditional PI, is the waiting time to the first sample. This is a place that can be improved.

Once we move the first sample closer to switch off, sampling while the coil current is still present and decaying, we find that the reactive signal is still present also.

Tinkerer

[Tinkerer]

Because it must dissipate all the magnetic energy from space around it. That's easy. On the other hand, you do not have to employ high voltages, just go for lower L and higher currents. Less fuss. I think I've found an ideal pulse that comes from the least suspect source. It is just a concept at the moment.

Lo and behold

If you can get the transient time down to micro seconds instead of milliseconds, you are in business.

Tinkerer

[Aziz]

Because it must dissipate all the magnetic energy from space around it. That's easy. On the other hand, you do not have to employ high voltages, just go for lower L and higher currents. Less fuss. I think I've found an ideal pulse that comes from the least suspect source. It is just a concept at the moment.

Lo and behold

Hi Davor,

unfortunately not an accurate answer. It doesn't prove the fact however.

It's all about the impedances:
(We are neglecting the parasitic capacitance over the TX coil and the VLF mode at the moment)
The on-time impedance (on-time TX time constant) and the off-time impedance (off-time TX time constant).
Remember, when you switch off a coil, which maintains a current flow, this current must be kept to flow further (as long as possible). Or the magnetic field at switch-off time must be kept as long as possible.

The physics law has no other possibility to react to different impedance.
I=U/Z
Conduct this further and you know, why there is a high flyback voltage in a PI detector.

Aziz

[Carl-NC]

We will see later, why the flyback period in a PI generates a high voltage pulse. The fact U = -L*dI/dt isn't the real proof. But we aren't so far yet.

L*di/dt is a simplification. The actual equation is

The problem with the traditional PI, is the waiting time to the first sample. This is a place that can be improved.

Depending on what you want to detect, waiting might be the right answer. See above equation.

[Aziz]

Hi Carl,

can you elaborate the variables in the formula?
(Just think, I'm stupid.)

Aziz

[Carl-NC]

I=current @ turn-off
Cp = total capacitance
w0 = 1/sqrt(LCp)

This is the classic solution to a critically damped system.

[Aziz]

I=current @ turn-off
Cp = total capacitance
w0 = 1/sqrt(LCp)

This is the classic solution to a critically damped system.

Thanks, it's more clear now.

[Aziz]

I=current @ turn-off
Cp = total capacitance
w0 = 1/sqrt(LCp)

This is the classic solution to a critically damped system.

What's happened to the damping resistor Rd?

Aziz

[Davor]

Remember, when you switch off a coil, which maintains a current flow, this current must be kept to flow further (as long as possible).

Or not - depends on your excitation pulse. In all cases a Tx coil must be terminated with low impedance or even better a short circuit. Current or no current. I'd say we agree on that.

Personally I'd go with step voltage and high L - it can't go any lower Z than that. The equivalent Z of voltage source is a short circuit.

The diagram above is a design that I played with to see the effects of a single coil arrangement, and it is not exactly a ripoff of any existing rig, just my musings. Anyway, pulse duration is just under 20us, cosine shape, and the principle is completely scalable. So, it could have been 100V, and my pulse would again be just under 20us, but with 4A at peak. Funny thing is that Power supply does not suffer any shocks, and power consumption is a few mA. Flyback is under 1us. I'd say it could rock.

...

I think I must review a bit for clarity. I tend to write complicated.
To obtain a transient response you need a voltage pulse, and you need a low Z termination. Depending on initial conditions you will end up with or without residual DC component. In case of single coil arrangement you don't have a choice - you must end up with zero DC. To do that you need a single pulse, which in turn gives not one but two sharp voltage transitions. One is too many because it carries no useful information, in fact it interferes with detection. That is the actual price of single coil operation.

[Carl-NC]

What's happened to the damping resistor Rd?

It is the reason why we end up with that equation, instead of a different equation. In any case,

[Aziz]

Hi Davor,

show us your TX circuit and we show you, where it actually flaws.
(Which means, there isn't a "free lunch".)

To achieve a short decay time, we have to increase the TX-off time impedance. This can be done with impedance transformer too (converter).

Who remembers the split driven TX coil (auto transformer and center-tapped TX coil)? This is such a impedance transformer.
Low Z on-time (driven) - High Z off-time (damped).

Aziz

[moodz]

Peak coil current.



Then we need to define a new term "Ampere-turns per us." Still, if a VLF can hit 1A-T/us then a PI can easily hit 20A-T/us.

The main trick is not actually passing a high bandwidth pulse through the coil .... and I think alot here miss the point .... is exposing the target to a step function magnetic field transition.
The generation of a "stimulated resonance" at a single frequency is trivial ... see circuit below ... that particular one generates 10 amps peak to peak in the coil ....but draws only 350 ma ( with no coil loading ).

The next generation of detectors have a series of planar and spatial coil arrangements each driven by stimulated power resonance drivers. By arranging the oscillator amplitude phase and frequency the summation magnetic field ****at the target ***** generates the required field transition ... the step transition speed is determined by the the highest frequency in the combined fields and what you might call group delay caused by the ground being within the field path however this can be compensated by adjustment of the params of each oscillator.
The summation field is produced F0 + F3 + F5 + F7 etc .....( for a simple square wave field transition ) ... this is well known in signal processing fields.

moodz

[Aziz]

BTW,

Heinrich Hertz would have his 155. birthday today. It's a good time to remember the resonance systems.

The fact, that the impedance of the TX coil get's into a minimum ( X(LC)=0, the resonant tank capacitor reactance X(C) is subtracting the coils inductive reactance X(L) to zero ), resonance systems can really be ***** kicking exciting to the targets. All the spectral energy of the coil current pulse is focussed in a single frequency.

Well, if you can handle the high ground mineralization, you will have the best performing detector in the world.

No matter what you do - only dI/dt matters.
Aziz

[Davor]

Hence, the step voltage excitation rules. Unfortunately only for dual coil systems.

I must rush now, and I don't want to post the actual schematic for the time being because of some stability issues - it is sub optimal in current design - works fine only with narrow tolerances - not at all to my liking. I'll play with it in a few hours. To make it worth a discussion here I'll create it using ideal parts e.g. voltage controlled voltage sources switches etc. I think you'll like it. Point is that with a coil as a king the rest of electronics is irrelevant - it must be made to behave.