PICKINI V4 - an easy to build, self adjusting PI detector

[F117]

So, it's more about resolution in measuring pulse width

Correct. That is why we want to measure the pulse width as close to the steady state voltage as possible.
If you slice the pulse too close to steady state, the results will become unstable.

Only the part of the flyback pulse where it returns to steady state is interesting.
That is why both offset and gain of the opamp are optimized to output only that small portion of the flyback pulse.
The further you slice the pulse from steady state, the less sensitive it will be.

[SaltyDog]

Only the part of the flyback pulse where it returns to steady state is interesting.

I agree the part close to the steady state is interesting for "sensitivity", but I think the earlier part of the pulse, once it get's below the diode voltage, is more useful for detecting the type of metal (see graph again) ..
I intend to look at that part as well with a different slice level .... that way we get the best of both worlds ... sensitivity to small targets, and discrimination ..

So once your kit arrives (.....dang Covid-19) I am going to work on that part with my FPGA code ..

Cheers

[SaltyDog]

Ok, I have had some success with this, I can now discriminate between gold and ferrous, with code running on my FPGA.

I have used a MPP board to provide the front end, I have connected my fpga to the op-amp output, via an A.D.C. And I am looking at the slope just after the op-amp comes out of saturation...

Ferrous materials give a decrease in slope, and gold gives a slight increase, I have still some work to make it more sensitive, but as it stands, does discriminate well..

I have 9 bits of resolution to the slope at present..

When I get the Pickini board, I will finish the design..

[6666]

Ok, I have had some success with this, I can now discriminate between gold and ferrous, with code running on my FPGA.

I have used a MPP board to provide the front end, I have connected my fpga to the op-amp output, via an A.D.C. And I am looking at the slope just after the op-amp comes out of saturation...

Ferrous materials give a decrease in slope, and gold gives a slight increase, I have still some work to make it more sensitive, but as it stands, does discriminate well..

I have 9 bits of resolution to the slope at present..

When I get the Pickini board, I will finish the design..

Sounds good

[SaltyDog]

Sounds good

Thanks...

Just to be clear, I have 9 bits resolution for the analog side, 12 bits measuring time over the slope..
I won't be able to get it more sensitive to discrimination until I can accurately control the op-amp offset, hence the need to wait for the Pickini..

[F117]

I won't be able to get it more sensitive to discrimination until I can accurately control the op-amp offset, hence the need to wait for the Pickini..

Just to be clear:
The opamp offset is controlled by the PIC DAC output which has only 5 bits of resolution. This calibrates the DC steady state level to about 4V at the input of the internal comparator.
The comparator slice level ( the other input of the internal analog comparator) is controlled by a 10bit PWM output.

[waltr]

Ok, I have had some success with this, I can now discriminate between gold and ferrous, with code running on my FPGA.

I have used a MPP board to provide the front end, I have connected my fpga to the op-amp output, via an A.D.C. And I am looking at the slope just after the op-amp comes out of saturation...

Ferrous materials give a decrease in slope, and gold gives a slight increase, I have still some work to make it more sensitive, but as it stands, does discriminate well..

I have 9 bits of resolution to the slope at present..

When I get the Pickini board, I will finish the design..

Yes, sounds good.How are you measuring the 'slope'?
What gain are you running on the pre-amp? Did you drop gain to 7 as you posted above?
Would like to see timing of sampling and flow chart of processing.

What is you ferrous target? How you tried a rusty nail are different angles?

[SaltyDog]

Just to be clear:
The opamp offset is controlled by the PIC DAC output which has only 5 bits of resolution. This calibrates the DC steady state level to about 4V at the input of the internal comparator.
The comparator slice level ( the other input of the internal analog comparator) is controlled by a 10bit PWM output.

Hi Bernard,

You misunderstand ... currently my FPGA stuff is running with with an MPP board, I am just using it's front-end op-amp.
I am using that board to try this stuff on, since your board has not arrived due to COVID-19.

Your resolutions have nothing to do with what I am implementing in an FPGA ..

What I have achieved is discrimination because I have looked at the early part of the slope ... discrimination info is lost once you slice at the tail end of the slope .

My intention is to do "both" early slice for discrimination and late slice for sensitivity .. hope that makes sense ..

[SaltyDog]

Yes, sounds good.How are you measuring the 'slope'?
What gain are you running on the pre-amp? Did you drop gain to 7 as you posted above?
Would like to see timing of sampling and flow chart of processing.

What is you ferrous target? How you tried a rusty nail are different angles?

HI, I am measuring the slope by taking multiple slices early in the discharge curve (I can take as many as I like with the FPGA as it's only a line of code .. )

Running standard gain at the moment (>1000) as I am using stock MPP board. When Pickini board arrives, I will play around with the gain as well as the offset level ..
(The offset level I will drive via an DAC which will be the output of a PID control loop ..to keep it steady ..)

I will adjust the offset to make sure my ADC window is in the area of interest on the slope, so I will sample early for discrimination, then late in the slope for sensitivity ..

Yes, rusty nail works at any angle ... I can't do any in-depth testing till I get the hardware .. I will do the best I can with my current set-up.

PS: I also have a custom micro-controller in the FPGA that links all the hardware together and spits out data via a USB port ...

[SaltyDog]

UPDATE my friends ..

I have managed to detect my gold wedding band (worn very thin) at 200mm AND discriminate that it is gold. (All on a fairly standard coil)
I can now if I want, eliminate everything except gold .. AND still detect it at depth, which I could not do yesterday .. yahoo ...

I have managed this with judicious use of technology including FIR and IIR filters, and dual detection slopes ..
(one for sensitivity and one for discrimination)

I am still using the MPP board for this ... only connection is the pre-amp output (it is a dual pre-amp design) ignoring the rest of the circuit, and also using a Lattice ICE40HX8k FPGA.
I havn't even got offset control at the moment, that could make it even better in the field ..

All in all an excellent day ...

Bernard .. thank you for your excellent idea's, but I think I have crashed your thread enough, time to start my own, as this is a quite different beast now.

[6666]

Well done

[dbanner]

Well done satlydog.

This is my machine which detects only gold and ignores all other metals. Notice the dual slopes on either side of the centre sample point which is curved at the top. I am looking to apply for a patent for my device, though it suspiciously resembles but not to be mistaken with another device which is in common everyday use. This is entirely different beast.

A little humor to break the monotony of the quarantine, spirits are sagging over here.

[MartyJ1963]

@SaltyDog,,,,,Well done! I've been following this thread since it began in 2016 and Bernard's Pickini is near the top of the list as one of my favourite Homemade PCB's,,,looking forward to seeing forth coming results with your experiments,,,Good luck,,, eagerly awaiting! Regards, Marty

[SaltyDog]

Well done satlydog.

This is my machine which detects only gold and ignores all other metals. Notice the dual slopes on either side of the centre sample point which is curved at the top. I am looking to apply for a patent for my device, though it suspiciously resembles but not to be mistaken with another device which is in common everyday use. This is entirely different beast.

A little humor to break the monotony of the quarantine, spirits are sagging over here.

Hehehehehe .... Hahahahahah ...

[SaltyDog]

Further update:

I have managed to extract a metal Id number for each metal type.

Gold and silver and stainless steel come under one Id,
Iron is another,
Lead is another

I haven't tried any other metals just yet ..

I will be mapping these to tones for in the field, as I want minimum controls, I envisage only a threshold adjustment, and maybe a pin-point button..

Cheers