Looking for a solution to improve slow coil electronically to let it decay fast and keep R-dump high

[Ism]

I am not only making shaft flat but hiding cables inside and making it sharp like two edges sword using sandpaper.

[ATTACH]28786[/ATTACH]

Sorry Waikiki, I now remember seeing your coil/shaft projects. I went Anderson website and saw round shafts so I thought I would suggest but you are way ahead of the curve.

[Silver Dollar]

Active damping sounds like a good idea but can you implement it without a microprocessor?
I just got all the parts for a Chance and have it all built. It has some sort of flyback clamping.
I wonder if this active damping can be added?

Here's a circuit Moodz was playing with a while back. It has semiactive damping and power
generation for the front end;



R18 sets the damping...

[Waikiki_Sweep]

Active damping sounds like a good idea but can you implement it without a microprocessor?
I just got all the parts for a Chance and have it all built. It has some sort of flyback clamping.
I wonder if this active damping can be added?

Here's a circuit Moodz was playing with a while back. It has semiactive damping and power
generation for the front end;

[ATTACH]28844[/ATTACH]

R18 sets the damping...

Chance is using active damping. You can see a second IRF9640 which dumps flayback when microcontroller sending a signal.
You can see what happens when Chance damper fails on charts here: http://md4u.ru/viewtopic.php?f=5&t=3033&start=2320

[baum7154]

I think TEPCO studied the timings of the signals to the gate of the Q9 MOSFET and has said that it's purpose is to short the coil between pulses which inhibits flyback if I understand correctly.

Dan

[Waikiki_Sweep]

Active damping sounds like a good idea but can you implement it without a microprocessor?

Sure it is easy to build timing circuit using micricontroller. TX impulse, samples, damping impulse, VCO sound. To make it analog there are too many parts. Also if everything will work from +5V single supply it will help to avoid all those transistors to convert signals for different voltages.

[Waikiki_Sweep]

I think TEPCO studied the timings of the signals to the gate of the Q9 MOSFET and has said that it's purpose is to short the coil between pulses which inhibits flyback if I understand correctly.

Dan

Probably yes. It is easy to check with two channels oscilloscope. I will try.

I am thinking about another use of this circuit: to accelerate damping for a moment before to take sample to allow early sampling.

[greylourie]

Active damping sounds like a good idea but can you implement it without a microprocessor?

I dont know enough electronics, but the sd2000 schematic damps/snubs and that doesn't have a micro.

[Silver Dollar]

Well that second FET shorts out the flyback but there is still a damping resistor so no active damping.

I would tend to call it snubbing as it limits the flyback via shorting...

[Waikiki_Sweep]

Well that second FET shorts out the flyback but there is still a damping resistor so no active damping.

Damping resistor will be there anyway. I do not think you can left coil without load at all.
But damping resistor can provide "slow" damping and FET for used "fast" damping for that periods you want.
Why not to damp 200 volts fast than slowly read that microvolts from targets?

[Silver Dollar]

Well you need to provide a path for the current to flow to but the patent I previously mentioned
used a FET dumping the current into a diode. No resistor! He claims less noise and perfect damping.

It is not clear if chopping off the flyback is a good thing. Some claim it is counteracting the field generated
by the TX pulse and others claim the TX is forgotten and the flyback is the real field producer! In either case
it doesn't speed up the decay but perfect damping could speed things up a bit...

[Waikiki_Sweep]

Well you need to provide a path for the current to flow to but the patent I previously mentioned
used a FET dumping the current into a diode. No resistor! He claims less noise and perfect damping.

It is not clear if chopping off the flyback is a good thing. Some claim it is counteracting the field generated
by the TX pulse and others claim the TX is forgotten and the flyback is the real field producer! In either case
it doesn't speed up the decay but perfect damping could speed things up a bit...

Yes. I see no damping resistor in patent AU2013101058A4. May be it works without resistor or may be high impedance input creates more noise if there is no any resistor because that diode is not conducting after decay drops lower 0.6v.

As I understand targets in PI detectors has excited by flyback because high current spike from high voltage flyback provides that energy. If you short flyback by opposite diode (like you see it in electric motors) you will have no signal from targets. It is easy to try on your PI detector.
Because diode left you 0.6 volts to measure lower decay part so if TX excites targets there will be a signal.
So short high voltage flyback is important.

Also you can see that in any PI detector coil decays with two different speeds already.
First it decays faster with damping resistor AND 1k resistor with diode from opamp protection.
Than when decay drops to less than 0.6 volts protection diode stops to conducting and only damping resistor is responsible for decay speed.

Anyway all it needs experimental proof.

And regular damping is not solving problem with high capacitance coil. But that one from patent gives more hope.

[baum7154]

As I understand it the flyback IS what creates the high voltage and hence high energy pulse to the targets. Too low a damping resistor approaches the effect of a short causing the coil hold energy longer and to flyback too slowly, and too high a damping value underdamps the coil.

[Waikiki_Sweep]

As I understand it the flyback IS what creates the high voltage and hence high energy pulse to the targets.

Yes. I believe so.

Too low a damping resistor approaches the effect of a short causing the coil hold energy longer and to flyback too slowly,

Low damping resistor value shorting coil and sinking flyback to ground faster. So flyback decays faster and coil loosing energy faster.
But damping resistor shorting targets signals too and sensitivity with low value of damping resistor drops down.
Also there is no influence on TX stage because 12V TX not driving any significant current into 600 Ohm damping resistor but 200v flyback does.

and too high a damping value underdamps the coil.

Yes. And coil ringing. You can see some curves and even oscillations with underdamped coil. But high value of dumping resistor increasing sensitivity of PI metal detector.

Interesting that in real life it looks a little different.
Properly damped coil shows on oscilloscope a long flat line for several microseconds where flayback is high and preamplifier is in saturation.
And in some cases underdamped coil shows some (ringing) curves instead with shorter flat line in the beginning so it may let to sample earlier and use high dumping resistor.
Such a "witchcraft" method of damping should not be recommended but I see better results for small gold using it.

[Silver Dollar]

Well I've never seen any pics of Chance's TX with a flyback pulse on it as it shorts that pulse out
with a second FET. The primary TX pulse fills the target with magnetism and the flyback reverses
that field so try's to erase that magnetism. So by shorting the flyback we get a better return.

QED does the same with a snubber. Minelab limits flyback to 200v. I've seen others trying to get
a bigger flyback so I'm still unsure which is the proper explanation.

A quicker field collapse makes for a stronger field but slower can help with IDing a target. But
for those tiny bits of gold you have to look quick as they decay very fast!

There's something going on with the damping resistance too. You may want to disconnect that
resistor during transmit and after we get below the diode drop and start sampling so a quicker
way to dump the voltage without a resistor is probably a good idea.

I like the FET with a pot as you can easily adjust it. The problem is if you don't protect the input
parts can fry if the flyback gets through. Here's a protected version of Moodz frontend;

[baum7154]

Ran across this early post by Joe R. (BB SAILOR)on this topic and it includes interesting reading on a patent website (link is included)about active Mosfet damping. I have not looked at the CHANCE PI Q9 Mosfet gate control voltage to correlate if the Mosfet is switched with a fixed voltage or a variable voltage to create more active damping. What is known is that on CHANCE PI the mosfet is in play during the period when the 16 short pulses are sent to the coil, or rather between the pulses. It does not appear to be in play after the last long 120us pulse to the coil and it's subsequent flyback. I will try to get a better look at this on my CHANCE PI tonight.

Dan

bbsailor
Guru


Join DateFeb 2005LocationBarnegat, NJPosts338

It's all about what happens between the pulses.

A pulse induction coil that operates at the coil's natural self resonance (between 500Khz and 1000 KHz) will only have the pulses spaced one or two microseconds (uS) apart. All the processing would need to occur in that space. This space is too short to stimulate a target with enough energy to cause eddy currents to be generated in the target and also have enough time to process the receive signal. Also, once a damping resistor is added across the coil it stops being a resonant circuit and will have a certain bandwidth based on the value of the damping resistor.

Even continuous wave metal detectors need to operate in the VLF range 1KHz to 20KHz) to penetrate the ground enough to obtain a response from a metal target.

Go to the following web link for an excellent patent explaination of pulse induction coils, damping resistors and sensitivity to small objects. http://www.freepatentsonline.com/7075304.pdf.

The two paragraphs below, taken from that patent, provide a good description of PI coil bandwidth and damping to make a sensitive coil for small objects.

"[0007] The induced eddy currents in a metal target are proportional to the change in magnetic field with time (.DELTA.B/.DELTA.t) at the metal target location. For high sensitivity, one would like to have dB as large as practical and .DELTA.t (the change in time) matched to the metal object's time response (bandwidth). For a small metal object with a fast time response (high bandwidth) the optimal detector sensitivity would be achieved with a small .DELTA.t matched to the small metal objects response (matching bandwidth of sensor and target). For a large metal object with a slower time response the optimal detector sensitivity would be achieved with a larger .DELTA.t matched to the metal object's time response. The magnetic field (B) is proportional to the current (I) in the transmitter coil and the number of coil turns (N), thus B.about.IN. More coil turns (N) increases the magnetic field at the target depth for a fixed current. However, increasing the number of coil turns also increases the kick-back voltage across the transmitter coil and switch due to the increased inductance. The voltage across the transmitter coil and the electronic switch turning off the coil current is V=Ldi/dt and L.about.N.sup.2. More coil turns also increases the capacitance C of the coil due to the potential (voltage) differences that exist between the individual turns of wire which makes up the coil.

[0008] Consider the transmitter coil. The same effects apply also to a receiver coil that is being excited by a transmitter coil. At the moment of current change in the transmitter coil, a high voltage appears across the coil. A fixed shunt resistor R is typically placed across the transmitter coil to dissipate the current in the coil. The resistor is called the damping resistor since it is used to dampen or suppress coil oscillation caused by the LCR circuit formed by the coil. The larger the shunt resistor, the greater the current dissipation and the faster the current decay. Fast current decay allows for small metal targets to be more easily detected since the coil has a higher bandwidth. If the damping resistor value is set too high, [my spelling and grammar correction added] current is forced into the coil where the capacitance and inductance combination causes voltage/current oscillations: the oscillations will mask small metal target signals. A small damping resistor slows down the coil decay and lower the sensitivity of the coil to small metal targets. Controlling the damping resistor effects the performance of the PIMD."

The value of the damping resistor is critical when attempting to operate at very low delays. The coil takes about 5uS after current shut off to spike and decay to zero plus a few more uS for the front-end amplifier to stabalize. Once it reaches zero at the output of the front-end amplifier, because of the amount of energy in the flyback spike, the signal wants to oscillate, like a ball being dropped from some height. The damping resistor works like putting a pillow under the ball to dampen it's bounce and attempt to quicken the front-end amplifier's recovery time. As you reduce the delay setting on your PI machine, you are sampling closer and closer to the bounce point. Once you get to the point where you are starting to sample the tail-end of the bounce, the PI machine will lock-up, indicating that you have reached the natural delay of that coil. When the MOSFET "flat-tops" it starts to conduct, generates internal heat, and tends to exted the time it takes for the signal to reach a stable sampling point. Anything that adds capacitance to the coil circuit such as: amount of coil turns, wire insulation dielectric, shield spacer dielectric, shield spacer thickness, the shield itself, MOSFET output capacitance and the type and length of the coax used, will tend to slow down the potential performance of the PI machine.

Coils with higher self resonant frequencies require less damping, (larger damping resistor values) and allow a little closer sampling to the the point where the signal stabalizes at zero (or the impact location of the ball analogy). Some desired targets, like gold, have very fast decays and require sampling at the earliest possible time to obtain the strongest response from a fast-decaying target signal.

The referenced patent reveals the inventors use of a MOSFET to act as a variable damping resistor to optimize the coil's bandwidth and ultimate performance in detecting specific targets. The inventor describes the use of a computer to control the voltage on this damping MOSFET which controls it's resistance in an attempt to optimize the signal form an unknown target. Once the signal is optimized, the computer then can identify the nature of the target by comparing the optimized target response from a library of target responses stored in the computer. This is all an attempt to recover additional information from the target to aid in it's identification. PI machine designs that integrate the samples over some period of time (a few ms) loose the ability to extract this additional target intelligence.

This long-winded explaination should help you better understand what is happening between the pulses.

bbsailor