MAGPI V4 -- CCPI MONOCOIL

[moodz]

Is it possible to combine VLF+PI mode in alternating cycles. Because repetition rate is so high you could have 50% VLF 50% PI of the total time but is it possible to switch in and out the capacitor to achieve this?

Now re-reading your question more carefully ... the solution above is alternating bursts ... to do it cycle by cycle ( as in waveform ) would require maybe quite a bit more circuitry as the current flows and voltages are quite different in the coil for PI mode compared to sine wave and cant be changed "instantaneously".

Not ruling out the possiblity though might be able to do something if I fiddle with the phasing.

moodz

[eclipse]

Ideally if the switch is mosfet based there will be less drawbacks. Perhaps something like

[moodz]

Thanks Eclipse .... that was along the lines of the switch I am going to try.

[moodz]

Before I go much further with the electronic switch ... it turns out that I can use a plain old mechanical switch to change modes from PI to VLF so I did that because I am very keen to see what this circuit will do with a real DD coil. ( on the RX side )

So I have connected the DD coil ... Its a big one from Minelab ( BIG THANKS TO MINELAB FOR DONATING THE COIL !!!!!!! )

The TX coil is 0.5 ohms and 450 uH ... the RX coil is about 18 ohms and about 850 uH.

The voltage supply is 5.4 volts at 1 amp and this gives 2 amps peak to peak in the TX coil.

The circuit is working in PI mode.

Below are the TX flyback and the RX voltage .... can plainly see targets just using the CRO .... with amplification and processing this is going to be a great detector.

moodz.

[moodz]

and in VLF mode ... can plainly see ferrous and non ferrous response.

The Pic shows TX current lighter blue( 6 amps peak to peak )

RX shows voltage darker blue.

moodz

[eclipse]

Some amazing thing you got working there! Good job!

[moodz]

Video of dynamic seamless switching from PI to VLF mode.https://youtu.be/aU787CJVopA

[green]

...slaving over a hot soldering iron all day has resulted in the intial results for the transmit circuit and so far it is matching or exceeding the simulation results.

with moderate settings ( to avoid smoke )

In PI mode ...

0.5 ohm TX coil / 300 uH ( your fairly standard PI TX coil )

1 amp peak to peak with 3 microsecond transitions and power supply = 2.5 volts @ 500 ma = 1.25 watts consumption.

Yellow trace = gate drive
Pink trace is flyback at TX coil
Blue trace is 0.1 ohm current sense resistor.

Notice that tilt correction is applied ( very simple solution ).

next I will be testing the VLF mode



...

Have you determined an acceptable tilt and a way to display tilt. Easy to display with simulation, not sure how with real circuit. 1amp/sec tilt, 100uA/100usec your example is 10uv tilt with the .1 ohm current sense resistor.

[moodz]

Have you determined an acceptable tilt and a way to display tilt. Easy to display with simulation, not sure how with real circuit. 1amp/sec tilt, 100uA/100usec your example is 10uv tilt with the .1 ohm current sense resistor.

I transferred the sim circuit to a real circuit and used values that resulted in approximately flat tilt. I am still testing what types of targets / ground etc might make the tilt worse or better.
However one thing is very clear ... all the target action takes place during the fast field transition at the pulse edges as the current in the coil reverses.

What needs to be considered is that the unlike a convention PI ( where the magnetic field grows to a value then falls to zero suddenly ) the magnetic field in this case is reversing with a sudden transition through zero. This is not the same conditions from the target/ground point of view as a convetional PI where the field sits at zero for some time.

The other factor is that if the tilt on the positive current pulses is ramping up ( H field increasing ) then the opposite is happening on the negative current pulse ( H field decreasing ) and if you sample consecutively ... reversing sign .. the net effect of tilt is zero because the two ramps cancel.

...and this seems to concur with the real circuit where ( regardless of the ramp ) the target response remains consistent because the fast current transition is the same regardless of tilt ramp.

In the Pic below the top ramp is increasing H field and the bottom ramp is decreasing H field .... sampling the ramps gives an average of zero for tilt because the H field slopes cancel.