New advanced PI project

[Altra]

Thanks for the calculator link! It looks very handy.

[Tinkerer]

Calculating the capacitance with the misc_El calculator for an inductance of about 3120uH and frequency of 156250Hz gives about 330pf.
330pf capacitance looks like a big problem.
How does it go again? “Every problem gives you an opportunity” bla, bla, bla, I want to see you solve this problem ha, ha.

We have seen that to discharge the coil within a usable time, we need a different method than just a simple damping resistor.
Besides, burning up all that good energy in heath is just plain waste, unless you use it to warm your coffee.
So, how do we do it?
Electricity is all about magnetic fields and electric fields. We use the di/dt (rate of change over time) of the magnetic field to induce eddy currents in the target.
Changing the about 30 Ampere-turns of magnetic field to 0A within 1.6us gives a rate of change of about 18.75A /us. This is what kicks the target.

[Tinkerer]

I was playing with the circuit today, so I took advantage of taking a screenshot of the Flyback and the damped RX input.

[Tinkerer]

So, how do we discharge the coil in a short time?
To achieve this, we convert the magnetic field into an electrical field. We charge this pesky 330pf capacitance to 1000V. In other words, we change the current into voltage. We can do that very fast. To convert the 285mA into 1000V takes 1.6us with this coil.
285mA to 0A in 1.6us.
So now, what do we do with this energy stored in the capacitance? We discharge this energy through the coil again. We convert the electric field again into a magnetic field. This time in the opposite direction. With 1000V the impedance of the coil is easily overcome. Thus, instead of having the “Flyback as a useless artifact” like in the traditional PI, we have the Flyback as a means for charging a high inductance coil very fast.
The coil is charged to 285mA current in 1.6us.
0A to 285mA in 1.6us
Once we send the current through the coil in the clockwise direction. Then we send the current in the counter-clockwise direction.
Notice that the switching from clockwise to counter-clockwise happens at the time when the current in the coil is 0A. Hmm, we have a little problem there. I said that it is continuous constant current. Yet the current goes to 0A and stops there for a nanosecond or two.
Sorry for the inaccuracy of my statement.
Pay attention in which direction the current flows in the coil, because the eddy currents in the target will flow in the opposite direction. Important.

[Monolith]

What you describe, would make a sinewave of 150kHz. Is that it?

[eclipse]

You run this type of PI 5-10khz normally as its always working /no dead time/ you switch polarity and so you have doubled flyback response on each polarity switch and you get the flyback response shown there in the previous picture.

[green]

eclipse, thank you for the feedback.
Yes indeed, the original idea square current wave came from Deemon.
Altra's and King's designs are quite different from mine.
Recently Moodz also posted a square wave TX design. Different too.

My TX design has been running for a few years. As I am 79 years old now I decided to post it now, before it is too late.

I plan to explain in detail how it works so that everybody can use it.

Hi Tinkerer

I have tried unipolar, bipolar and CC bipolar. All three have worked but I haven't been able to get bipolar or CC bipolar (more complicated) to be better than simple unipolar. Interested in what your circuit looks like. I'm in the process of moving so it will be awhile before I can try your circuit. I'm 80 years old and have been thinking posting my circuit before it is to late but would like to try yours first.

[Tinkerer]

What you describe, would make a sinewave of 150kHz. Is that it?

Right.
The SFR.
The self resonant frequency of the coil.
Now we make it into a square wave.
At the peak of the sine wave, when the current is 285mA, we insert a DC current of 285mA for 50, 100, 200 or whatever time we need to be able to receive the target return signal.
Positive peak and negative peak.

So, there we have our square wave.
Yes, OK, it is not exactly square. If we look closely at it on paper, we see that the vertical parts are not exactly vertical.
For this coil the vertical parts take 3.2us to change from minus peak to plus peak.
Another way to look at that is to be like the Slew Rate of an op-amp.
These are the coil design specifications we decide on, at the time we chose the purpose of the detector.


The important part is the "flattop". The time where there is no change in the coil current.

Zero rate of change in the coil current.

[Tinkerer]

You run this type of PI 5-10khz normally as its always working /no dead time/ you switch polarity and so you have doubled flyback response on each polarity switch and you get the flyback response shown there in the previous picture.

Here is a better picture of the Flyback, TX in yellow and RX in red and green.
Unfortunately I can only show you one of the TX Flybacks, because I have only one high voltage scope probes.

[green]

The important part is the "flattop". The time where there is no change in the coil current.

Zero rate of change in the coil current.

Does the rate of change need to be zero or is there an acceptable rate of change? Maybe 1A/second, more or less?

[Tinkerer]

The important part is the "flattop". The time where there is no change in the coil current.

Zero rate of change in the coil current.

Does the rate of change need to be zero or is there an acceptable rate of change? Maybe 1A/second, more or less?

You can still get it to work with even more than that, but you will have to make adjustments in the RX.

[willybayot]

The important part is the "flattop". The time where there is no change in the coil current.

Zero rate of change in the coil current.

Does the rate of change need to be zero or is there an acceptable rate of change? Maybe 1A/second, more or less?

If there is any rate of coil current change, will there be adverse Eddy currents generated during the signal reading period?

[Tinkerer]

If there is any rate of coil current change, will there be adverse Eddy currents generated during the signal reading period?

Even a small rate of change in the coil current will generate eddy currents in the ground. We want to avoid the ground eddy currents that happen at the same time we read the target responses.
How much rate of change is too much? 1mA / ms? 1mA/ms does not sound like a lot, but we must not forget that this is multiplied by the number of turns in the coil.

[ivconic]

...My TX design has been running for a few years. As I am 79 years old now I decided to post it now, before it is too late...

Full schematic, possible pcb and short video demonstration would do.

[Tinkerer]

Traditional PI Flattop coil current

The important part is the "flattop". The time where there is no change in the coil current.


Let?s have a closer look at the ?flattop?.

Here is the simulation of a traditional PI TX.

Flattop coil current
We see the current ramp up to maximum and then stay flat. This is what we call the ?Flattop?

You are welcome to try the simulation yourself and change the variables.

L2, L3, L4, L5, are targets of a TC of 1us, 5us, 10us, 100us, 200us.