Replies to Minipulse Plus

[KingJL]

Are the loops wired in phase or out of phase, does it make a difference?
Thanks

I don't know which phase is correct... but It does make a difference... before you button up the coil, test it... If you do not get a positive response with a nickel passed over the overlap zone, reverse your rx leads.

[KingJL]

KingJL thanks for your reply, what did you use for shielding ? and did you induction balance the coils ?

I used Scotch copper mesh for shielding,on each of the 2 coils following bbsailor's advice. The coils were balanced by observing the preamp output signal and adjusting the overlap until the tx signal was completely cancelled. Fine tuning of the balance after the overlap had been carefully set was accomplished by a small length of the rx coil lead (usually refered to as an adjustment loop) that I left exposed when building the coil. There is an old geotech thread/post by Reg where he discusses building the dd, which I used as a guide when constructing mine. The instructions were well written and easy to follow.

[6666]

KingJL thanks for your reply, what did you use for shielding ? and did you induction balance the coils ?

[johnandles]

DD coils are made with a slight overlap of the 2 D sections...... I love DD's!

Are the loops wired in phase or out of phase, does it make a difference?
Thanks

[KingJL]

KingJL in what way does your DD discriminate ferrous , thanks.

How did you screen it ?

DD coils are made with a slight overlap of the 2 D sections. The detection zone of the dd is in the overlap area. Ferrous targets provide a negative response in the detection zone and a positive response on either side of the dectection zone. All other targets provide a provide a positive response in the detection zone and a negative response on either side of the zone. So as you are swinging the coil, non-ferrous targets provide an audio beep as the coil crosses the target. A ferrous target provides a beep as you approach the target, goes silent as you cross the target, and provides a beep as you leave the target. I love DD's!

[6666]

I love the dd... it gives me the added advantage of ferrous metal discrimination.

KingJL in what way does your DD discriminate ferrous , thanks.

How did you screen it ?

[green]

http://www.geotech1.com/forums/showt...ircuits-for-PI From mickstv reply #456

With spice I get a gain of about three, same as calculation from reply #459 by King. The (track/hold differential integrator)from above post #1 has a pot in series with output opamp feedback resistor labeled Integration TC. Changing the pot value or sample time changes integrator TC Changing the pot value has a small effect on gain. Still haven't figured the circuit formulas, maybe my spice model isn't correct, but that's what I'm getting.

Scope pictures from spice. Target = 1volt, EF = 0 volts, signal DC coupled to input with a 100 ohm resistor. Fc approximate from scope screen. 1khz sample rate

[Davor]

...Changing the pot value or sample time changes integrator TC Changing the pot value has a small effect on gain...

It sets slew rate.

[green]

http://www.geotech1.com/forums/showt...ircuits-for-PI From mickstv reply #456

With spice I get a gain of about three, same as calculation from reply #459 by King. The (track/hold differential integrator)from above post #1 has a pot in series with output opamp feedback resistor labeled Integration TC. Changing the pot value or sample time changes integrator TC Changing the pot value has a small effect on gain. Still haven't figured the circuit formulas, maybe my spice model isn't correct, but that's what I'm getting.

[Davor]

... I determined the effective bandwidth by using the FFT function of the simulated output.

There is a much more effective way of establishing the bandwidth and its nature in time domain. What you need is a step stimulus that you already have in a form of Ncycles in a sinus voltage source, and you observe the output. It most certainly lags against the stimulus, but the slope and its curvature say a lot. You'll notice that there is a slew rate limited slope, and not a classic LPF slope. It means this integrator will follow input stimulus very accurately, and will not flutter unless screwed up completely.
You can estimate inverse corner frequency (1/f for -3dB point) as 3 x rise time to 85% or 3 x fall time to 15%. You don't have to be extremely accurate on this.
More importantly, you'll notice that this integrator provides more of a constant slope, rather than an exponential one, which indicates a slew rate constraint.
In practice it means that you'll be able to miss some targets if you swing your coil as a lumberjack, but it will still be much faster to respond than a normal LPF response detector, and even more so than the one with overshooting problem.

[Tinkerer]

I normally use stranded tinned wire with teflon coating to make coils.
So let me see if I understand this right. I take a twisted pair of stranded wires and wind a coil and I then connect the two wires together at both ends so effectively they are in parallel ? I can get some cat 5 cable that is exactly that. 4x twisted pairs of stranded wire and a 30m length will be enough for 4 twisted pairs (4 coils). This is why I want to use litz wire. Its made up of 50 thin enamelled wires that are woven (twisted) an the 50 strands are connected at each end to create a coil with a higher q factor. I can get litz wire as used in high end commercial coils locally for $1 per meter.

1) It is not just stranded wire, but tinned stranded wire. The Tin helps confining most of the eddy currents within the single strands.
2) the twisting of the 2 wires makes that there are only short pieces of wire lie parallel to each other and therefore generate little inter-wire capacitance.
3) The insulation and the twisting put more distance between the wire turns and therefore reduce the inter-wire capacitance even more.

This combination of effects comes near the effect of basket weaving, but can also be used when basket weaving to enhance the effect.

[KingJL]

Ok I tried it with a mono coil. It does work and with much reduced noise but very low sensitivity.

If carefully made ( carefully balancing the 2 haves of the figure 8 and carefull layout ) the figure 8 can be very sensitive, but only in the center of 1 of the figure 8 halves.

[KingJL]

Gain is R23/R25, LP cutoff is 1/( 2×3.1416×R20×C13 ), band stop is 1/( 2×3.1416×R24×C13 ).

This of course is modified by the sample width and the filtering of the input and output circuits. My Minipulse Plus implementation with 15 usec samples and additional filtering of the output (a 10k in series ahead of C15 and 100n in parallel with R29 ) results in an effective gain of ~1.75 and a 6 dB cutoff of 11.5 Hz. I determined the effective bandwidth by using the FFT function of the simulated output.

[josewashere]

KingJL

Probably the next best thing after the Litz wire, is fine strand tinned insulated wire. It seems that the eddy currents are much less and the insulation reduces the inter-wire capacitance. Twisted, fine stranded, tinned insulated tween leads in parallel give even better results.

I normally use stranded tinned wire with teflon coating to make coils.
So let me see if I understand this right. I take a twisted pair of stranded wires and wind a coil and I then connect the two wires together at both ends so effectively they are in parallel ? I can get some cat 5 cable that is exactly that. 4x twisted pairs of stranded wire and a 30m length will be enough for 4 twisted pairs (4 coils). This is why I want to use litz wire. Its made up of 50 thin enamelled wires that are woven (twisted) an the 50 strands are connected at each end to create a coil with a higher q factor. I can get litz wire as used in high end commercial coils locally for $1 per meter.

[josewashere]

In the figure 8, only the rx coil is formed into the 8
( or a facimile thereof ), the tx coil circumscribes the entire rx figure 8.

Ok I tried it with a mono coil. It does work and with much reduced noise but very low sensitivity.