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DEEPER PI DETECTION DEPTH

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  • 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

    Comment


    • 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.

      Comment


      • Originally posted by Davor View Post
        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

        Comment


        • 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)

          Comment


          • Originally posted by Davor View Post
            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.


            Originally posted by Davor View Post
            (it would be difficult to beat this, but I have some ideas)
            Easy and trivial.

            Aziz

            Comment


            • 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
              Attached Files

              Comment


              • 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
                Attached Files

                Comment


                • Originally posted by Aziz View Post
                  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

                  Comment


                  • Originally posted by simonbaker View Post
                    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

                    Comment


                    • 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
                      Attached Files

                      Comment


                      • Originally posted by simonbaker View Post
                        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.
                        For illustrative purposes, let's make a gross simplification:

                        Response "conductivity" is entirely due to the target metal type
                        Response "inductivity" is entirely due to the target thickness
                        Response "strength" is entirely due to target surface area

                        Ergo, a silver dollar and a silver half-dime have identical conductivity, but the silver dollar has a much higher inductivity (and therefore higher tau) and a much higher signal strength.

                        A silver half-dollar and an English penny have about the same thickness and same surface area, therefore will have the same inductivity and signal strength, but the half-dollar has a little higher conductivity and therefore a higher tau.

                        Again, this is a gross simplification and things are more complicated than this. But folks need to get away from thinking that conductivity is the major factor in tau variations. It is not. Target thickness is a far bigger factor.

                        How does thickness affect tau? Consider a small thin ring of metal; the incident TX magnetic field produces a primary eddy current in the ring, which creates a counter-magnetic field. OK, now consider a second identical thin ring that is right behind the first. The TX magnetic field also produces a (smaller) primary eddy current in the second ring, but the counter-magnetic field of the first ring also induces a counter-eddy current in the second ring. Ergo, the first ring creates a "drag" effect in the second ring. Likewise, the second ring induces a (smaller) drag on the first ring. Consider yet a third ring; the process continues, with the third ring having even more drag effect (and a smaller overall eddy current) than the second ring.

                        You can think of thick targets in the same way. The surface eddies are the strongest, but as you go deeper into the metal the eddies are reduced due to ever-increasing drag of the distributed counter-magnetic effect. The point where the eddies are reduced to 37% of the surface value is called the "skin depth."

                        At some depth, the eddies will be reduced to near-zero, and at the back side of the target there will be no more magnetic field. In this case, the target looks like a mirror and the resulting counter-magnetic field is about 180-degrees out-of-phase with the TX field. If you reach the back side of the target before the eddies die out, some of the TX magnetic field will appear to have "shown through" the target, as if it is (optically) translucent. In this case, you will have less than 180 degrees of phase shift, depending on the relative depth.

                        Did I mention all of this is dependent on frequency/slew rate? And it all works, whether you use a sine wave, a square wave, or a pulse. It doesn't matter. However, if you wait to look at the target right after you click off the flashlight then the target is harder to see. But when the flashlight is on, and it is a xenon bulb shining in your eyes, the target is also hard to see.

                        - Carl

                        Comment


                        • Originally posted by Davor View Post
                          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)
                          I'm glad you like it, so do I! We call this "truncated half-sine." Yes, it really works.

                          Comment


                          • Originally posted by Tinkerer View Post
                            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
                            Hi Tinkerer:

                            I would specify your mutual inductances just two at a time, because specifying more at once causes every combination of two to be specified, and some don't really apply.

                            You don't really need coupling from target to target, for example, although it's so small probably won't hurt.

                            Also, you don't want to specify .0001 between your two RX coils -- I think your two Rx coils representing a center-tapped coil probably need to be coupled with a coefficient close to 1 (or -1), in which case you should end up with an inductance approx equal to 1000u combined (is that what you want?).

                            Specifying the separate coupling between the Tx and Rx coils -- oooh, don't make my brain hurt. The phasing may require a negative coupling for one of them.

                            I'm not sure about the bucking coil -- seems you need to couple that to the RX coils also, although I'm not sure what physical configuration you are modeling -- concentric coils??? Maybe you should just assume the bucking coil works perfectly and omit all the couplings between the TX coils and the RX coils?

                            But I think specifying only pair at a time is the way to go.

                            Regards,

                            -SB

                            Comment


                            • Originally posted by Carl-NC View Post
                              For illustrative purposes, let's make a gross simplification:

                              Response "conductivity" is entirely due to the target metal type
                              Response "inductivity" is entirely due to the target thickness
                              Response "strength" is entirely due to target surface area

                              Ergo, a silver dollar and a silver half-dime have identical conductivity, but the silver dollar has a much higher inductivity (and therefore higher tau) and a much higher signal strength.

                              A silver half-dollar and an English penny have about the same thickness and same surface area, therefore will have the same inductivity and signal strength, but the half-dollar has a little higher conductivity and therefore a higher tau.

                              Again, this is a gross simplification and things are more complicated than this. But folks need to get away from thinking that conductivity is the major factor in tau variations. It is not. Target thickness is a far bigger factor.

                              How does thickness affect tau? Consider a small thin ring of metal; the incident TX magnetic field produces a primary eddy current in the ring, which creates a counter-magnetic field. OK, now consider a second identical thin ring that is right behind the first. The TX magnetic field also produces a (smaller) primary eddy current in the second ring, but the counter-magnetic field of the first ring also induces a counter-eddy current in the second ring. Ergo, the first ring creates a "drag" effect in the second ring. Likewise, the second ring induces a (smaller) drag on the first ring. Consider yet a third ring; the process continues, with the third ring having even more drag effect (and a smaller overall eddy current) than the second ring.

                              You can think of thick targets in the same way. The surface eddies are the strongest, but as you go deeper into the metal the eddies are reduced due to ever-increasing drag of the distributed counter-magnetic effect. The point where the eddies are reduced to 37% of the surface value is called the "skin depth."

                              At some depth, the eddies will be reduced to near-zero, and at the back side of the target there will be no more magnetic field. In this case, the target looks like a mirror and the resulting counter-magnetic field is about 180-degrees out-of-phase with the TX field. If you reach the back side of the target before the eddies die out, some of the TX magnetic field will appear to have "shown through" the target, as if it is (optically) translucent. In this case, you will have less than 180 degrees of phase shift, depending on the relative depth.

                              Did I mention all of this is dependent on frequency/slew rate? And it all works, whether you use a sine wave, a square wave, or a pulse. It doesn't matter. However, if you wait to look at the target right after you click off the flashlight then the target is harder to see. But when the flashlight is on, and it is a xenon bulb shining in your eyes, the target is also hard to see.

                              - Carl
                              Thanks, that is interesting.

                              For comparison, I looked at some formulas for a single loop coil -- they are confusing but it seems that most of the formulas suggest that inductance of a single loop scales linearly with diameter. Since the circumference is also proportional to diameter, and resistance should be proportional to circumference, resistance should also scale linearly with diameter.

                              This implies that L/R is constant for all different sized loops of a given conductivity -- which also means if you're looking for rings, maybe tau will actually help discriminate rings of different metals.

                              However, the thickness of the wire is also a factor (maybe a log function???), so there is still room for confusion.

                              This perhaps corroborates Carl's statement that differences in inductance are due to thickness, since simple eddy rings of the same thickness would seem to have the same tau regardless of diameter (or target size), but stacking such rings would change the relationship.

                              BTW: does conductivity affect skin depth?

                              -SB

                              Comment


                              • Originally posted by simonbaker View Post
                                Hi Tinkerer:

                                I would specify your mutual inductances just two at a time, because specifying more at once causes every combination of two to be specified, and some don't really apply.

                                You don't really need coupling from target to target, for example, although it's so small probably won't hurt.

                                Also, you don't want to specify .0001 between your two RX coils -- I think your two Rx coils representing a center-tapped coil probably need to be coupled with a coefficient close to 1 (or -1), in which case you should end up with an inductance approx equal to 1000u combined (is that what you want?).

                                Specifying the separate coupling between the Tx and Rx coils -- oooh, don't make my brain hurt. The phasing may require a negative coupling for one of them.

                                I'm not sure about the bucking coil -- seems you need to couple that to the RX coils also, although I'm not sure what physical configuration you are modeling -- concentric coils??? Maybe you should just assume the bucking coil works perfectly and omit all the couplings between the TX coils and the RX coils?

                                But I think specifying only pair at a time is the way to go.

                                Regards,

                                -SB
                                Thanks SB,

                                This is for a concentric coaxial IB coil. The coil works great, but I would like to be able to simulate it, because it is easier to simulate many variations than building many coils.

                                The RX coils are center tapped, but I just realize that the phase dots are wrong on the circuit drawing. The coils are of the same phase or just one coil, center tapped. The RX coils are wound on the BU coil. Coupling near 1 between the 3 coils.
                                The BU coil is counter phase of the TX coil, but the coupling is about 0.25.

                                The RX signal is received by the RX, BU and TX coils. Due to the coupling and precise balancing, the R signal and the X signal are of opposite phase at the output of the RX coil.

                                Easy FE discrimination.

                                Tinkerer

                                Comment

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