Ground Balance Theory

[PiTec]

I do not understand your problem … I can set the duration of the flyback pulse to any value by simply adjusting the flyback voltage while measuring the flyback pulse width with the scope. I described this in posts #545 and #549:

TX timing: 50 µs TXon (10 volts), 2 µs TXoff (flyback, determined by 250 volts avalanche breakdown of the MOSFET).
10µs TXoff are achieved by clamping the flyback voltage to 50 volts. 500µs TXon consist of 50µs current ramp followed by 450µs constant current to keep dI/dt at zero.

With the scope timebase set to 1 µs / div, I measure the time between trigger (2) and the end of the flyback pulse (3) directly at the TX coil. The end of the flyback pulse is t=0 (t0). The error should be quite small, maybe +/- 0.25 µs. Any delay in the driver and MOSFET would not be added.


Before we continue we should find an agreement concerning the role of the flyback pulse …

[Aziz]

Hi Aziz,

no need to repeat the measurements. If you want t0 to be at the end of the driving pulse, simply add the durations of the flyback pulses to the first time codes:

Dacay_curves_03: Flyback pulse = 2 µs -> first time code = 7+2 = 9µs
Dacay_curves_04: Flyback pulse = 2 µs -> first time code = 8+2 = 10µs
Dacay_curves_03: Flyback pulse = 10 µs -> first time code = 9+10= 19µs

Of course the curves are no longer straight then …

Thomas, do you recognize now, why I got totally confused?

How do you exactly know, when the flyback has been really finished?
Please, please, please, make your measurements once again at the right trigger point t0 this time.
We will see much clearer, accurate and consistent results this time.
Cheers,
Aziz

[PiTec]

Hi Aziz,

no need to repeat the measurements. If you want t0 to be at the end of the driving pulse, simply add the durations of the flyback pulses to the first time codes:

Dacay_curves_03: Flyback pulse = 2 µs -> first time code = 7+2 = 9 µs
Dacay_curves_04: Flyback pulse = 2 µs -> first time code = 8+2 = 10 µs
Dacay_curves_05: Flyback pulse = 10 µs -> first time code = 9+10= 19 µs

Of course the curves are no longer straight then …

[Aziz]

Ok guys,

Thomas measurement data is indicating a severe time base error (not consistent). We won't get reliable VRM model parameters. No chance.

I suggest, he makes his measurements once again, where t0 = t=0 = switch-off time = trigger time position for data acquisition.
The f$in switch-off time t0 is (time code = 0 µs), when the f$in mosfets has been commanded to switch-off.
Not when the flyback period ends. That is very very important.

Don't make any changes to the original time code. Never ever!!! If you sample later, the time code will be greater of course. It should be 0 at trigger point = switch-off time = t0 = t=0. You can even sample earlier where the earlier samples will have a negative time code. At at switch-off time the time code should be 0 µs.

Now I'm looking forward to more accurate measurements and results.
Cheers,
Aziz

[Aziz]

Hi all,

I have continued my numerical best fit simulations and found interesting facts, we can apply to our latest measurements. I have even added some noise to the numerical simulations.

The formula 17 in the mentioned scientific paper can be extended to detect different exponents b. But not phase shifts or time base corrections p. So the time code in the measurement data must be correct.

The formula 17 gets into the form, when adding exponent b:
G(t) = a*( t^b - 1/(t+w) ), (p set 0 and simplified the equation)

The Excel Solver works quite stable to find the parameters a, b and w. It seems, that in this case only one optimal solution is possible (no local minima). That's a good news.

But if your time base isn't correct (if you set your time code t0 at 0 µs but in reality it is time shifted like t0 = +1 µs) then we get totally inconsistent & not reliable model parameters. (see first pic).


If we set exponent at b=-1 (1/t term gets valid in formula 17), then we can detect phase shifts and time base correction offsets (see second pic).


But this wouldn't make sense. Better we eliminate p with correct time base and look at the exponent b variation. And we can test, whether the VRM formula fits well into the measurments.


Cheers,
Aziz


PS: The exponent b calculation is quite more stable and reliable than other parameters. We can compare the b=-1 best fit against the calculated b in the measurement data. And this is going to be very interesting, which one wins the run (with less fitting errors).

[Alexismex]

Hello Deemon,always good and clear informacion thank you very much,
Now
I want to tell you another factor that i think exist :
I have a good test field (5 years now) where i test my PI detector the target go from 60 cm to 3 meter , i have test more that 50 detectors always of my design, i have noted a strange phenomena
( in the test to detect a 200litters tank at 3meters) THAT sometimes i will detect perfectly moving the coil East / West above the target and more difficult moving the coil North / south
in one day but another day is the contrary , i detect the target very well North/ South and with more difficulty East /west, this i have repeated many times from years to years , and i Know very well my detector for make confirmation that the phenomena exist ...i understand it is in relation with the electromagnetic field of the earth but why sometimes it reversed ???

[deemon]

In reality , we need two things if we want to get a proper PI target response :

1. Magnetize the target properly . I mean that we need to "turn on" a magnetic field and wait enough time for the field to permeate the target completely . This time depends on the target size , its conductivity , shape , etc .

2. Change the field rapidly . When we turn the field off , we'll start the eddy currents in the target . It looks like the target tries to "hold" the magnetic field level has been established in the target before . Those currents will decay by the exponential law , and this exponential decay is what we call "target response" . Of course we may reverse the field instead of removing it , just quickly change its polarity ... This would double the target response , but the principle is just the same .

So we can easily understand that only the fast change of the field is the real factor that produces the target response , but the voltage spike ( flyback pulse ) is the only a "side effect" of the transmitter coil , and nothing more .

[Tepco]

This is a bit misleading. The flyback pulse is certainly a consequence of coil charging, but it can be produced by other means that are no less effective. Step pulse for example. Works the same.

So it is not a tail wagging the dog, but the other way around. There are also fully functional dogs without tails.

Extremely simplified view: consider a PI machine to be a simple transformer - what phenomenon transformers convey - voltage or current?

Lovely, so let's consider 1x1m loop detector running at few hundred uS pulse width, detecting some pile of rusted junk underneath. Now, if I fire some 2uS pulse in that same coil (by means of suitable hardware), containing exactly same energy, number of miliJoules, as original pulse, will I get same target response? Never going to happen.

[Tepco]

There is nothing about “conviction” or something, just experience gained in some tests. Somewhere in this thread I described simple CC (but this time, constant peak current control) test circuit, capable to produce variable pulses, ranging from few hundred uS, down to less than 500nS, and keeping total energy per pulse (1\2xLxIxI) about constant. As a result, flyback amplitude and shape is fairly constant over very wide range of TX pulse width. Observed on scope, any difference is indistinguishable, until DC coil voltage + flyback voltage combined is below avalanche voltage. In some conditions, using 1200V transistor and bit smaller peak current, up to 800V coil voltage and less than 500nS TX pulses are produced. Only deviation from “constant flyback” shape and energy is actually at very wide TX, circuit operating with only few volts col voltage, then nonlinear transistor output capacitance may have bit more influence. Otherwise, flyback time and shape can be considered quite constant and independent of actual TX time. Now, only varying TX time, I recorded response of different metallic objects, and find that it is variable, depending on object TC and applied pulse width. As theory predict, pulses shorter than TC produce less and less response, longer pulse, slightly longer than TC will not increase response above some point. Considering flyback conditions are identical to any practical meaning in this test, result can be attributed to TX drive pulse only. Note that soil, or ferrite behave bit differently.


Circuit I used is based on constant peak coil current control (not constant current source!!!) so linear current ramp-up time is controlled by applying variable coil voltage. I will comment more on your CC version tomorrow (sorry, have to go offline now) and results, your approach is quite different. My intention is not to advocate some “theory” or to “wage some war” here, ( I have to admit his is interesting part of forum life too, actually sometimes I missing it) but rather to learn something and exchange experience. Will be interesting, for example, to see how we, attempting about same thing, using different means, ended up with completely opposite conclusions. Also, soon I will be back to my hardware again, so building setup for any proposed “acid proof” test or something similar will be no problem anymore.

[PiTec]

Tepco, just build the CC circuit linked on Eric’s PI Technology Forum 11 years ago (or a similar circuit), and separate the driving pulse from the flyback pulse. You can see live on the scope how the target response increases when the flyback pulse is not weakened by the driving pulse. Or just simulate it with LTspice.

http://www.findmall.com/read.php?34,129617
www.tb-electronic.de/pi_tech/pulse_cc_circuit.pdf

Anyway, I will no longer spend time to convince anyone. Find out yourself or keep believing in the power of the driving pulse

[Davor]

TX pulse is ONLY thing that energize metallic target, flyback is just side effect, needed to quickly remove energy stored in circuit.

This is a bit misleading. The flyback pulse is certainly a consequence of coil charging, but it can be produced by other means that are no less effective. Step pulse for example. Works the same.

So it is not a tail wagging the dog, but the other way around. There are also fully functional dogs without tails.

Extremely simplified view: consider a PI machine to be a simple transformer - what phenomenon transformers convey - voltage or current?

[Davor]

I can’t believe that it is still not evident to some of you that the flyback pulse is the one that kicks the target.

That's nicely put

[Tepco]

Hi,

I can’t believe that it is still not evident to some of you that the flyback pulse is the one that kicks the target. From the target’s point of view it does not make any difference if the H-field change comes from energizing the TX coil, or from de-energizing it during the flyback pulse. The coil’s energy can either be dissipated as heat in the MOSFET during avalanche breakdown, or in a resistor or electronic load in parallel to a tank capacitor, or can even be recovered from a tank capacitor. The flyback voltage determines the pulse length, and this usually does not need to be shorter than 2 µs. The only excess energy that needs to be dissipated as fast as possible in a damping resistor in parallel to the coil comes from the parasitic capacitances of coil, cable and MOSFET – but this damping takes place after the flyback pulse has finished. No need to have the damping resistor in parallel to the coil all the time, it is sufficient to turn it on just when the flyback pulse stops. In case of energy recovery, this saves quite a lot of energy that is otherwise dissipated in the damping resistor.

See here: http://www.geotech1.com/forums/showthread.php?20038-Triangular-Wave-Technology&p=164732#post164732


Thomas

Sorry, but most of this is just plain wrong.. TX pulse is ONLY thing that energize metallic target, flyback is just side effect, needed to quickly remove energy stored in circuit. You can see this, by generating variable pulse width (but keeping same energy release and flyback shape and duration), or generating flyback like pulse alone in different circuit, without previous TX period, and observe response. I measured and demonstrated this number of times. And no one make detectors to generate high peak power 1-2uS pulses. Fact is that reversal during avalanche period tend to “undo” what TX pulse is achieved before and must be kept short or avoided completely if possible. Only ratio between TX pulse and metallic target TC counts. Also, it is absolutely necessary to have dumping resistor in circuit all the time, all stored energy,1\2 LxIxI must be dissipated this way, not just what is stored in residual capacitance, otherwise it will just ring down exponentially, that will take some time. Soil, or ferrite, (non-conductive material) is influenced differently, much less sensitive to TX width and much more influenced by flyback, this must be kept in mind in soil response test to avoid misleading results. All this can be easily demonstrated, make circuit and measure, not some sort of my “theory” or something.

[PiTec]

Hi,

I can’t believe that it is still not evident to some of you that the flyback pulse is the one that kicks the target. From the target’s point of view it does not make any difference if the H-field change comes from energizing the TX coil, or from de-energizing it during the flyback pulse. The coil’s energy can either be dissipated as heat in the MOSFET during avalanche breakdown, or in a resistor or electronic load in parallel to a tank capacitor, or can even be recovered from a tank capacitor. The flyback voltage determines the pulse length, and this usually does not need to be shorter than 2 µs. The only excess energy that needs to be dissipated as fast as possible in a damping resistor in parallel to the coil comes from the parasitic capacitances of coil, cable and MOSFET – but this damping takes place after the flyback pulse has finished. No need to have the damping resistor in parallel to the coil all the time, it is sufficient to turn it on just when the flyback pulse stops. In case of energy recovery, this saves quite a lot of energy that is otherwise dissipated in the damping resistor.

See here: http://www.geotech1.com/forums/showthread.php?20038-Triangular-Wave-Technology&p=164732#post164732

Aziz, my t0 is exactly at 'switch off' of the flyback pulse, i.e. exactly at the point where the H-field of the flyback-pulse has completely collapsed, and where the eddy currents in targets resp. the magnetization in MV soils reached their maximum.

Did you ever read my comments on the TX waveform of the MV document in post #545? It’s about the same problem – they do not take the flyback pulse into account, and instead they use pulse widths of the driving pulse in their equations.

With a standard PI timing, both the driving pulse and the flyback pulse affect the shape of the response curve of targets with multiple TCs and MV soils. Only when the driving pulse is separated in time it is solely the flyback pulse width that affects the response, so this could be used as a starting point to make things less complicate.

Thomas

[Tepco]

Be careful with hardware too. Fastest possible coil discharge with NO avalanche period, use 1200V IGBT and\or shorter pulse if needed, so most of energy can be dissipated in first few hundred nanoseconds. Absolutely NO dumper diode (bidirectional zener) or any other form of voltage clamping, this can mess up things slightly.