PI VLF combo for discrimation

[Ray Huffman]

I submitted this idea to another forum. I wanted to see if anyone else had thought along these lines and maybe discarded it as not doable.
Just had a thought about PI discrimination. Would it be possible to piggyback a vlf type circuit and signal with iron discrimination on the transmit coil of a pi then filter from the pi receiver and then have a seperate circuit for the vlf with all the discrimination circuits. Possibly turning off the vlf during transmit and receive for the PI portion then skip a pi pulse and turn on the vlf. Then use the discrimination to tell you it is iron up to a point that is. Just a thought and way beyond what I could do but I thought I would throw that out there since the forum has kind of slowed down lately. Maybe it is not possible and shows my lack of understanding about all this.
Ray

[Hard Nosed]

Re: PI VLF combo for discrimation..........great idea

Ray,

I was wondering about, what seems to be the same idea some time ago. Asked a similar question a while back on another forum. Here is the link.


http://www.findmall.com/metal/school1/config.pl?noframes;read=3236


I hope that someone does come up such a detector because it would be ideal and I think I would go for one.

David Di

[Carl-NC]

Re: PI VLF combo for discrimation

I think it's possible. If you run the PI at a fairly low rate, say 500 pps, the you have about a full millisecond to burst a sinusoid (or a square wave). You would want several cycles for settling & averaging, so maybe a 10-15kHZ TX frequency would work.

The challenge is in the coils. A VLF RX coil typically has a lot of turns and higher inductance than a mono PI coil. Not sure if this holds for DD PI coils, where perhaps the RX coil can be higher inductance than the TX coil. If so, this eases the problem.

Another challenge is in the VLF filtering. I think that because of the burst nature, you would have to do everything in digital-land, which is probably fine. Or, just look at raw phase like the old TR's.

If done right, this would be useful for ID'ing shallower targets, esp small iron. The PI will still (hopefully) exceed the VLF in depth.

--Carl

[Brian]

Re: PI VLF combo for discrimation

Isn't this just taking us back to the PPD1 by Eric Foster in the early 80's and the Protovale Pulse Analyst both of which had a ferrous I.D. meter operating at the same time as the pulse or have I missed the point ?

[Eric Foster]

Re: PI VLF combo for discrimation

Hi Carl and All,

Yes, a separate RX coil on a PI can have considerably more inductance than the TX coil. The TX coil inductance is determined by the switch on time constant as well as the switch off. For the RX, only the switch off time constant is relevant i.e. the coil inductance looking into the RX amplifier input impedance and its separate damping resistor. I made a "special" beach machine a few years back, that had a TX of 300uH and an RX of 1mH. This had considerably more range than a machine with just the single 300uH coil acting as both TX and RX. The main reason why PI's generally use just one coil is simplicity. Using two coils means that each coil can be better optimised for its separate task. They don't have to be DD's or even balanced; just two concentric coplanar windings work well. However, for discrimination in the TX on time, a balanced coil is necessary.

Eric.

[Ray Huffman]

Re: PI VLF combo for discrimation

Thanks all for the responses. One other thought on the discrimination would be just a switch to momentarily turn on the vlf circuit when you wanted to Id a target, this would obviously require a change in pulse times to allow the on time for discrimination then switch back to the normal pulse sequences for PI detection or in other words turn off the pi and turn on the vlf. I I guess it would sorta be like carrying 2 detectors in one.

[Ray Huffman]

Re: PI VLF combo for discrimation..........great idea

Thanks for the info David. You think you have come up with a new way to do something and 8 other people already thought about it. I went to the link to check out what was said and you are right someone else has already followed that line.
Thanks again

Ray