field test unit no 001 "model T"

[moodz]

It may even be worth revisiting the damping across both coils using this circuit (If it works at all of course, no guarantees there, its untested, and I'm not an engineer...). I seem to recall you saying you were getting good sensitivity with a 1k resistor in this arrangement. So with the body diodes neutralized.. who knows..

It could work .. I will probably play more with the damping once the rest of the detector is put together. At the moment the existing damping circuit can balance the detector even at extremely high gains ( 1000 - 2000 x ) on the diff amp.... however it drifts but that is for other reasons ... not the damping.

moodz.

[moodz]

i slowly understand what you did , i tested with a small circuit.

Very good .... post your results so others can see also.

thanks

moodz.

[Midas]

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showpost.php?p=121759&postcount=8

moodz

No.. that's really not what I was trying to describe. In fact that circuit would suffer even worse since its main positive pulse on driven side has a low resistance path through the body diode (and the two 330ohm resistors) to the un-driven side of the coil. The single sided damping scheme without the extra blocking diode wouldn't have suffered as badly since it was only reflected negative pulse being over-damped.

Now you've got the blocking diode in place its a different story. The difficulty may be with this circuit is that you've only got one oscillation to get all your damping done because if it goes negative you don't have any damping at all. You may be able to do just that of course but is that level of damping required to do that really optimum?

I suspect that only being able to control the resistance in one direction may put you at a disadvantage. If that does prove to be the case then I was thinking something along these sort of lines may work better. You can juice up the gate capacitor during quiet time when you not transmitting and the coil has settled so you can rely upon the mosfet sources being at the supply voltage.

It may even be worth revisiting the damping across both coils using this circuit (If it works at all of course, no guarantees there, its untested, and I'm not an engineer...). I seem to recall you saying you were getting good sensitivity with a 1k resistor in this arrangement. So with the body diodes neutralized.. who knows..

[Midas]

Bi-directional Resistance Control

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showpost.php?p=121759&postcount=8

moodz

No.. that's really not what I was trying to describe. In fact that circuit would suffer even worse since its main positive pulse on driven side has a low resistance path through the body diode (and the two 330ohm resistors) to the un-driven side of the coil. The single sided damping scheme without the extra blocking diode wouldn't have suffered as badly since it was only reflected negative pulse being over-damped.

Now you've got the blocking diode in place its a different story. The difficulty may be with this circuit is that you've only got one oscillation to get all your damping done because if it goes negative you don't have any damping at all. You may be able to do just that of course but is that level of damping required to do that really optimum?

I suspect that only being able to control the resistance in one direction may put you at a disadvantage. If that does prove to be the case then I was thinking something along these sort of lines may work better. You can juice up the gate capacitor during quiet time when you not transmitting and the coil has settled so you can rely upon the mosfet sources being at the supply voltage.

[ashkelon]

i slowly understand what you did , i tested with a small circuit.

[moodz]

Thanks Midas ... the diode in the descrim balance control disconnects the damping resistance during receive .. otherwise the coils would be unbalanced during receive and you are correct about the mosfet body diode and output capacitance effects. I tried the floating drive technique but could not stabilise the balance point ... it kept drifting off.

There is another problem ... the method of producing the +VE supply rail via D5 and C4 is too clever by half.... the problem is that a large target or ground effect will substantially effect the magnetic coupling between L1 and L2 this causes a relatively large change in the voltage stored on C4. Because C4 stores a charge that is somewhat proportional to the coupling between L1 and L2 there is a "memory" effect a large target induces a voltage which remains for some time after the target has been removed from the coil....
The effect of this is to unbalance the balance point ... especially at high gain.
.. I will have to use a more conventional synchronised bias generator to generate the +VE.


However all is not lost ... the charge on C4 via D5 is actually a representation of the ground effect ... and could be used for ground balance .... C4 will have to be alot smaller .... so stay tuned .... I have a plan for this piece of information.

The 4066 sampler starts sampling at about 6 - 7 microseconds after TX switch off ... the sampler turns off at about 22 microseconds ... this gives a good descrim point.

I am commencing another rebuild of the board with the alternative +VE power supply.

moodz.

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showpost.php?p=121759&postcount=8

moodz

[moodz]

Hi Paul, Thats some interesting changes to the design. One question have you changed the code and is it now sending pulses starting +5 and pulsing low ?




Mick

Mick ... this post answers your question also ....

There are two versions of the firmware "H" and "L" the H is non inverted TX and the L is inverted ... you could substitute TC4422 non inverting or some similiar type ... I only put the 4421 because I have it in stock ( but I am going to order some 4422 ).
To invert a sample pulse you set S1 off before S1 on time instead of S1 on before S1 off.

http://www.geotech1.com/forums/showpost.php?p=134961&postcount=35

PS the ground balance sample needs more work and is only experimental at this stage

moodz

[moodz]

Where is the pulses timing diagram ? i understand nothing about this circuit

Ashkelon ... these are development threads ... not a recipe for making a detector ... if you read all you will see the timing here

http://www.geotech1.com/forums/showpost.php?p=134961&postcount=35

moodz

[ashkelon]

Where is the pulses timing diagram ? i understand nothing about this circuit

[mickstv]

Hi Paul, Thats some interesting changes to the design. One question have you changed the code and is it now sending pulses starting +5 and pulsing low ?




Mick

[moodz]

Now with added ground sampling

During the TX pulse valuable information about the ground is thrown away in the previous design version ... also it caused balance point instability.

Below is the revised schematic with supply rails referenced to the signal ground ( +12 ) and the addition of a sample channel for the ground signal. This value will be sampled during the TX pulse and fed to a ground balance GB channel circuit ( not shown yet ). Happily the TX pulse drives the ground sample channel.

The supply rail has been lifted from 7.4 to 12 volts


moodz.

[moodz]

Thanks for your continuing contributions Moodz! I see you have the gain cranked to 2000 now, brilliant! Hows the stability ?

I think the diode in series with the descrim balance mosfet is a positive step as my own tests seemed to indicate the over-damping in the one direction through the body diode of the mosfet in the previous circuit was reducing the sensitivity compared to just a pot (equal resistance in both directions). This new version is completely un-damped in that direction, which I guess is better from a sensitivity standpoint, however I would assume (though happy to be proved wrong) is also going to extend settling time to some extent (since its only damping the positive oscillations). I'm wondering if it might be worth implementing a full AC switch, ie. two mosfets source to source with a floating drive. That way you get proper bi-directional control of the resistance. Not sure if its worth it or not, what do you think?

Thanks Midas ... the diode in the descrim balance control disconnects the damping resistance during receive .. otherwise the coils would be unbalanced during receive and you are correct about the mosfet body diode and output capacitance effects. I tried the floating drive technique but could not stabilise the balance point ... it kept drifting off.

There is another problem ... the method of producing the +VE supply rail via D5 and C4 is too clever by half.... the problem is that a large target or ground effect will substantially effect the magnetic coupling between L1 and L2 this causes a relatively large change in the voltage stored on C4. Because C4 stores a charge that is somewhat proportional to the coupling between L1 and L2 there is a "memory" effect a large target induces a voltage which remains for some time after the target has been removed from the coil....
The effect of this is to unbalance the balance point ... especially at high gain.
.. I will have to use a more conventional synchronised bias generator to generate the +VE.


However all is not lost ... the charge on C4 via D5 is actually a representation of the ground effect ... and could be used for ground balance .... C4 will have to be alot smaller .... so stay tuned .... I have a plan for this piece of information.

The 4066 sampler starts sampling at about 6 - 7 microseconds after TX switch off ... the sampler turns off at about 22 microseconds ... this gives a good descrim point.

I am commencing another rebuild of the board with the alternative +VE power supply.

moodz.

[Midas]

Thanks for your continuing contributions Moodz! I see you have the gain cranked to 2000 now, brilliant! Hows the stability ?

I think the diode in series with the descrim balance mosfet is a positive step as my own tests seemed to indicate the over-damping in the one direction through the body diode of the mosfet in the previous circuit was reducing the sensitivity compared to just a pot (equal resistance in both directions). This new version is completely un-damped in that direction, which I guess is better from a sensitivity standpoint, however I would assume (though happy to be proved wrong) is also going to extend settling time to some extent (since its only damping the positive oscillations). I'm wondering if it might be worth implementing a full AC switch, ie. two mosfets source to source with a floating drive. That way you get proper bi-directional control of the resistance. Not sure if its worth it or not, what do you think?

[moodz]

updated front end.

FETs now balanced NP on each input.
NP Mosfet pack used ( 300 volts .... but good for 350 )
4066 works better than other more expensive solutions but use mosfet switch to drive ( like a tc4421 ) Make sure you use the high voltage 4066 ( eg 15 volt rating ).
The THAT amplifier is not the best here ... use opamp DC diff amp.

moodz.

[Midas]

Have you tested the frontend with any other targets like say some PCB board/copper, lead and steel pipe. Just wondering what sort of results you get.

I just tested it with a big piece of roofing lead I think about 1.5mm thick, it responds in the same way as the aluminium chunk. Then I tested a piece pcb 150 x 150, and an aluminium can, both respond in same direction as iron. It all seems pretty consistent with the discriminating capabilities of multi-coil metal detectors.