DEEPER PI DETECTION DEPTH

[Tinkerer]

TX-RX- BU-TARGET COUPLING

Could somebody help me getting the K Spice directive right?

The circuit is for the IB coil configuration. K1 L1 L3 0.25 is about right. But I can not get all the other couplings to work at the same time. K2, K3, K4,

Tinkerer

[Aziz]

Exactly -- no one has explained how tau can be used to discriminate conductive metals... maybe iron is more feasible, I'll follow Tinkerer's ideas...

-SB

You should discriminate reactive (X) and resistive (R) response however.
This is feasible and reliable.

Aziz

[simonbaker]

Hi SB,

When will you give up the discrimination?
Aziz

Exactly -- no one has explained how tau can be used to discriminate conductive metals... maybe iron is more feasible, I'll follow Tinkerer's ideas...

-SB

[Tinkerer]

Target response with FE DISCIMINATION

Attached is a screen shot, with the target responses. The blue trace represents an FE target.
Note that the target response of the FE target is 180 degrees phase shifted.

Also attached is the LTSice file, zipped.

This would be the approximate response as seen by the RX coil in an IB configuration.

Tinkerer

[Tinkerer]

Concrete TX proposal

Here is a concrete proposal for TX parameters, that can be switched for a large coil of 1 meter diameter and a smaller coil of 0.5m diameter. It is the best I can come up with.

If you find something better, I would like to try it.

I would also appreciate suggestions for improvement.

I attach a screen shot and the LTSpice file.

TX parameters
Large coil 100cm diameter, 20 turns, 1 Ohm, 1000uH
Peak coil current 4500mA at 24V
Resonant frequency 50KHz = 10us
Di/dt= 450mA/us
Axial magnetic field at center of the coil= 1.131 Gauss
Field at 100cm depth= 0.101 Gauss
Power consumption 3W, including conversion losses.

Small coil, 50cm diameter, 30 turns, 1 Ohm, 1000uH
Peak coil current 2600mA at 12V
Resonant frequency 100KHz = 5us
Di/dt= 460mA/us
Field at center of the coil= 1.960 Gauss
Axial magnetic field at 50cm depth= 0.175 Gauss
Power consumption 800mW, including conversion losses.
Reference: Earth’s magnetic field= 0.5 Gauss
Question: what influence does the Earth’s field have on the target?
When we pulse the target with a field that is 4 times stronger than the Earth’s static field, at an angle of about 45 to 90 degrees of the Earths field, how does that affect the magnetic moment?

Tinkerer

[Aziz]

They also completely given up monocoil. So basically this design uses IB only for GB, and in a most logical way: at zero crossing.

The current storage part (TX coil) can't interrupted in another way. It must be at zero current crossing time.

(it would be difficult to beat this, but I have some ideas)

Easy and trivial.

Aziz

[Davor]

Gee, they made it with complementary transformer gated mosfets and a complementary power supply, EXCELLENT WORK!!!
To make it a power saving PI they use complementary pulses too - brilliant!

OK, they in essence use Current maximum/voltage zero to discriminate ground in IB style, and target detection in PI style, and all of it fully symmetricaly due to the complementary pulses. The current pulse is shortened to gain a voltage spike - something that my exciter lacks. (I'll make it better though...)

They also completely given up monocoil. So basically this design uses IB only for GB, and in a most logical way: at zero crossing.

Complementary pulses are a bonus in case of demining because of the potential triggering of magnetic fuses with pulses of single polarity.

Shortly, I'm impressed. A large smile is flying around my head

(it would be difficult to beat this, but I have some ideas)

[Aziz]

I'll much sooner give up PI

Terrains I intend to search with MDs are far too hard and rocky to go for every pull tab.

Davor,

have a look at this patent application:
http://www.google.com/patents?id=XJw...Detector&hl=en
(I think, it's a White's engineer)

As it is already patented, we can discuss it and find a workaround.
Aziz

[Davor]

I'll much sooner give up PI

Terrains I intend to search with MDs are far too hard and rocky to go for every pull tab.

[Aziz]

Hi SB,

remember, that the target inductivity is dependent on:
- shape of target
- orientation of target
- target position to the TX coil

TX magnetic fields aren't uniform and their direction changes on position of the target to the TX coil. The induced eddy current path changes due to this fact. (Note, eddy current path = coil winding geometry). Well, the coil coupling coefficient between TX and target changes as well.

The corresponding target time constant tau = L/R changes.

When will you give up the discrimination?
Aziz

[simonbaker]

Davor, target inductivity is mostly controlled by skin effect. Target R is largely determined by metal conductivity. Target size & shape affect both. However, it is true that, for modeling purposes, you only need to get the tau right.

- Carl

I'm very interested in how the "skin" effect comes into play and how it varies for different targets. Are we also talking about a "shielding" effect where it might be difficult to stimulate much current in a thick target due to surface currents?

What can we say about targets that are identical size and shape but different metal conductivities, especially their taus?

-SB

[simonbaker]

Can't do it. At least, not with induction phase/tau.

To follow up:

Aziz showed a PI simulation above with targets whose taus had 4 orders of magnitude difference.

Typical metals we are interested in are within 1 order of magnitude in conductivity.

To do discrimination, it appears we need to map 4 orders of magnitude of tau onto 1 order of magnitude of conductivity.

That would not be surprising if tau had some exponential relationship to conductivity that spread it over a greater range.

However, if I understand correctly, it might be inferred that targets with the same shape and size mainly differ in tau due to conductivity alone, and that the "R" of L/R is based on the metal conductivity (since the target shape and size is identical).

In other words, to me it looks like there is a disconnect between the statements that a) targets have 3 to 4 orders of magnitude tau range and b) that tau roughly corresponds to metal conductivity and can be used for discrimination.

Perhaps (b) holds if targets are typically the same size and shape; however, Aziz's simulation would not be realistic representation of that assumption.

Forgive me for beating this, but it seems important to clarify how targets behave to better design our MDs.

-SB

[Carl-NC]

Ok, but how do you plan to use it to discriminate a US dollar from a Mardi Gras token?

-SB

Can't do it. At least, not with induction phase/tau.

[Aziz]

Those discontinuities look wide-band to me... like part square wave, which is broadband.

Can you show the last one you described that is supposedly narrow band?

-SB

Ok,

I admit, I was wrong. (Good mistake, which keeps learning.)
The full wave sine and pause pulse has a wide band spectrum as the sine wave abruptly discontinues in the time-domain. So the spectral energy isn't totally focused to the fundamental frequency.

See below.
Cheers,
Aziz

[simonbaker]

Target tau is to time-domain what target phase is to frequency domain. They are interchangeable. Mathematically,



In the time domain you really need to be looking at a driven response to determine tau. In a normal PI, we look at the decay way too late for this; we need to look at the first 1 or 2 us, right in the peak of the flyback, or look during the TX period. Either of these require an IB coil.

- Carl

Ok, but how do you plan to use it to discriminate a US dollar from a Mardi Gras token?

-SB