PI MD

[green]

Getting ready to start MD build. A block diagram of what I'm thinking. Was trying to figure out how to change sound to identify targets. Have been using a voltmeter looking at integrator out on the bench, works good. Thinking it might work on the ground. Use sound and motion to find the target and the meter reading to identify(coil on for ferrous non ferrous)(coil off for short and long time constant targets). Wondering if gains and frequency cutoff make sense or anything else that might need changing or adding. A cheap multi meter could be used. 2volts FS gives 330nv/digit at the coil. Could be used to check battery volts.

[green]

Finally got the circuits wired and working on the bench. Three separate circuits, (coil driver and power supplies), (control logic), (preamp, integrators and post amplifiers). Have been using +battery for analog common with a -regulator for the -rail(digital and analog) with a separate battery and a +regulator for the +rail(digital and analog). Since I'm using an IB coil I tried something from the Surf master II thread. -battery for -rail(digital and analog) +regulator for +digital rail and a different +regulator for +analog rail with a TLE2426 rail splitter across the analog supply for analog common. Added the coil on integrator switch control signal to control logic(starts integrating about 10usec after coil on, stops integrating at coil turn off), better if stopped integrating 1/2usec before coil turn off(will change). Coil on integrator out signal polarity for ferrous targets including ground, hot rock and brick opposite non ferrous targets. Was hoping to put circuits in a box and try on the ground but I had a bad connection on the control card loading the digital power supply. Finally got it working. After looking at the solder joints to the SMD chips with a magnifying glass I don't think it would be reliable on the ground. Need to order some SMD adaptors and rewire. The GEB circuit works but I keep changing what to change to adjust GEB. Now I'm thinking target delay adjustable 7 to 15usec, target sample adjustable 8 to 20usec(GEB adjust), GEB delay 1usec, GEB sample 100usec. It's a learning experiment. A simple PI with GEB and maybe ferrous indication. I think it will depend on signal to noise ratio if it makes a metal detector.

[6666]

Good work,

[green]

Been playing with ferrous indication. Integrating the last 135usec of a 160usec coil on signal. IB coil(two 200mm round coils, opposite phase, surronded be a race track coil). Tx pulse, constant rate(6250 amps/second). Looking at either of the round coils ferrous targets cause integrator out signal to change opposite polarity non ferrous targets. Some ferrous targets did cause a small non ferrous indication at certain coil locations. Didn't find any non ferrous targets causing a ferrous indication. Short time constant non ferrous targets caused little or no indication. Some targets other than the obvious that caused a ferrous indication, QuikSteel(steel-reinforced epoxy putty), Two different bottles of pink finger nail polish, Kendall Monoject Needle 22Gx1inch, plastic two blade razor, 1/4x20x3/4 stainless bolt. The bolt had less signal than the razor or needle. The needle and the razor had a stronger pull against a magnet. Haven't found a way to eliminate ground signal like can be done during coil off and still identify ferrous targets. I think it works good on the bench but the change in target signal needs to be greater the change in ground signal for it to work in the field. Hoping the figure 8 Rx coil and non motion helps make it usable. Maybe there is another method that solves the ground signal and indicates ferrous targets. I could chart some target signal vs distance if anyone is interested. Don't know what it means. I have a quart bag with some hot ground from California and another from the back yard. The one from California gives a signal about 2x the other during coil on and about 5x during the coil off sample. Both cancel at the same GEB settings.

[Chet]

Hi Green


I like your ferrous / nonferrous scheme.
Could you operate in normal motion mode with both receiver coils searching for a target? Then switch to a pinpoint mode when a target is located using a trigger switch or a thumb switch? In pinpoint mode have one coil hovering over the target for maximum signal strength. The other coil configured to receive and integrate the adjacent ground signal.
This provides two signal channels; (target + ground) and (ground). Then combine/subtract the two signal channels to balance/remove the ground signal.
In your block diagram you have an oOo coil. But in the text you describe that you are using an “IB coil (two 200mm round coils, opposite phase, surrounded by a race track coil)”. Will the “(coil on for ferrous / nonferrous)” discrimination work with a true oOo coil configuration?


Have a good day,
Chet

[green]

Hi Green


I like your ferrous / nonferrous scheme.
Could you operate in normal motion mode with both receiver coils searching for a target? Then switch to a pinpoint mode when a target is located using a trigger switch or a thumb switch? In pinpoint mode have one coil hovering over the target for maximum signal strength. The other coil configured to receive and integrate the adjacent ground signal.
This provides two signal channels; (target + ground) and (ground). Then combine/subtract the two signal channels to balance/remove the ground signal.
In your block diagram you have an oOo coil. But in the text you describe that you are using an “IB coil (two 200mm round coils, opposite phase, surrounded by a race track coil)”. Will the “(coil on for ferrous / nonferrous)” discrimination work with a true oOo coil configuration?


Have a good day,
Chet

Hi Chet
The last stage is capacitor coupled so the mean output should be zero volts. The block diagram shows a gain of 20 but it's closer to 10 with the filter roll off. The threshold pot sets a + and- level so either coil would indicate a target. Normal operation is motion. Once the target is found, the volt meter change in reading indicates a (ferrous or non ferrous target) or a (short or long time constant target if GEB is enabled) depending which direct coupled integrator it's switched to in non motion operation.
My error in calling the coil o0o. I've tried to find the correct name. It's like a Bigfoot but with round coils. I've seen it called a figure 8, but then I've seen the two receive coils without the race track coil Tx called a figure 8. So far I like it best.
Mostly it's a learning exercise, it keeps changing. Hope to end up with something that works. Not sure about the true o0o coil configuration.

[green]

Took the circuit and coil outside on the ground. Coil not on a pole yet. Looks like the ground signal change is going to be more than alot of the target signals to discriminate ferrous using the coil on integrator. I'll try the circuit again when the detector is finished but it didn't look promising. Works good on the bench. The figure 8 Rx coil does cancel ground but a small tip caused a large signal change.

[Chet]

Hi Green
My biggest problem with coils has been poor shielding. They are really sensitive to ground capacitance. My most stable coil was shielded with Scotch #24 tinned copper braid. But it added more than 100pf of capacitance to the coil. I wound a self-shielding basket weave coil. It still had problems when close or touching the ground. It worked better when it was a kept at a constant height above the ground but it was annoying to use it that way. I am winding a new 18” DOD coil which will use conductive paint for a shield.
Have a good day,
Chet

[green]

Hi Green
My biggest problem with coils has been poor shielding. They are really sensitive to ground capacitance. My most stable coil was shielded with Scotch #24 tinned copper braid. But it added more than 100pf of capacitance to the coil. I wound a self-shielding basket weave coil. It still had problems when close or touching the ground. It worked better when it was a kept at a constant height above the ground but it was annoying to use it that way. I am winding a new 18” DOD coil which will use conductive paint for a shield.
Have a good day,
Chet

Hi Chet
How do you know if it's ground capacitance or ground signal causing the problem? I've seen waving the hand near the coil to test for capacitance, maybe there is a better way. When I tried the circuit yesterday(coil on integrator) tipping the coil one way near the ground caused the signal to go negative, tipping the other way caused a positive reading so I'm thinking it was ground signal not capacitance. GEB seemed to work when looking at the coil off integrator when enabled. Had a signal change with GEB disabled.

[Chet]

Hi Green


I compare my coils with commercial coils on the same ground with the same detector. The ground balance was on for all tests. The ones with poor shielding didn’t seem to ground balance as well as the commercial coils. I haven’t constructed any figure 8 or similar multiple coil configurations. So maybe tipping the coil higher at one edge may be a characteristic problem? Or it could be more ground capacitance on one coil verses less ground capacitance on the other is the problem?


Have a good day,
Chet

[green]

Put the detector on a pole. I think ferrous indication would work if there is no ground signal. My ground in the yard is not as severe as some ground I have from California. Coil, RX two 8 inch round coils(figure eight) Tx oval coil surrounding Rx. Voltmeter connected to coil on integrator for ferrous indication saturates(greater than 2000) with coil tilted about 7 degrees and one end on the ground. Reads about 200 when connected to coil off integrator when tilted 7 degrees with one end on the ground(GEB off). No change with GEB on. Reads about 200 with a US nickel at 5 inches(GEB 0n).

[green]

Added a level detector and threshold pot. Led lights if integrator out signal greater than threshold. Including a test with GEB off and GEB on(target sample time adjusted to cancel ground signal). Threshold was adjusted for at least 5sec between no target noise indication. Target was lowered on a pendulum(peak velocity about 1 meter/sec) until the led flashed at least 5 crossings without missing. Don't know how it compares with other detectors, think I've seen the can detected at greater distances. Rx(two eight inch round coils, figure eight). Tx(oval coil surrounding Rx). A quart zip lock bag with some dirt from California was used to adjust GEB(no indication with bag touching one of the figure eight coils), detected at about 8 inches over one of the figure eight coils with GEB off.

[green]

Added a level detector and threshold pot. Led lights if integrator out signal greater than threshold. Including a test with GEB off and GEB on(target sample time adjusted to cancel ground signal). Threshold was adjusted for at least 5sec between no target noise indication. Target was lowered on a pendulum(peak velocity about 1 meter/sec) until the led flashed at least 5 crossings without missing. Don't know how it compares with other detectors, think I've seen the can detected at greater distances. Rx(two eight inch round coils, figure eight). Tx(oval coil surrounding Rx). A quart zip lock bag with some dirt from California was used to adjust GEB(no indication with bag touching one of the figure eight coils), detected at about 8 inches over one of the figure eight coils with GEB off.

Tested some nuggets for distance to see how they compared with the .25x.25 inch can side and if a different size coil would be better. Included a picture from another thread and the can side test. Nugget test with GEB on. Nuggets were tested flat, on edge would be different. Looking at the pictures I got the nugget TC's from a different test picture.

[green]

Might have a chance to test the detector on a salt water beach. Want to try some tests to learn something about salt acting as a target. Including a possible test procedure. I want to try it in my back yard first. Don't have a chance to test in salt water very often so if anyone has other test suggestions they would be appreciated. Including a diagram of the detector now, similar to what I was thinking in first post. Looking at the test chart I need to add a column for target delay time. Any others and what should be entered in test conditions?

[6666]

Bring home a big tub of wet sand.