Triangular Wave Technology

[Ferric Toes]

This paper is worth reading. It should be downloadable off internet. I have run out of time to scan it today, but will do it later if needed.

Eric.

[Aziz]

Ok guys,

I have been told, that you all love math.
Check this zipped Excel sheet out. Just enter the PW field and see the spectrum distribution for up to n=8.

[ATTACH]22829[/ATTACH]
(I hope I didn't make a mistake.)

Cheers,
Aziz

Who is going to worry about the spectrum distribution? (Not equal?)
Everyone? Me not.

The first Excel sheet wasn't induction law compensated. The RX coil will pick up the response direct proportional to the frequency.
If you multiply n (frequency) and B[n] together you get the induction compensated response.
Ok, a new Excel sheet for you:
FFT-TriangleWave2.zip

Try PW=0.1 and see what comes out.
How to overcome the less power through the coil for PW = 0.1? Just increase the source voltage or decrease the TX inductance and resistance (becoming more exponential rise/decay character). Max output power is given for PW=0.5 of course.

Cheers,
Aziz

[Tinkerer]

That is exactly what I would expect to see. The total decay of the target trace post switchoff extends for about 50 uS, so as you say, it has a TC of 10uS. Looking at the negative decay it would likely extend for the same time, except that it is curtailed by the 40uS to switch off. However as the starting amplitude is lower, for the reason you give, it appears flatter. At 5 TC's though, there is only 1% of this smaller signal that subtracts from the switchoff decay, which is insignificant.

Eric.

I think we perfectly agree so far. Below is another simulation, this time it is an attempt to simulate a FE target.
I changed the TX to 50us, so that the blue target trace with a TC of 10us gets 5xTC, TX ON time.

The red target trace should represent a FE target. I would appreciate your opinion if this is an acceptable approximation of what it should look like.

Next I will do the short, high voltage pulse. I will try to calculate the same amount of joules into the coil, to keep some constant.

[Tinkerer]

I would still like to hear an answer to this question - "Are you saying that if you pulsed a coil with, say, just a 2uS 500V pulse, then that is a good as a long low voltage (100uS or more) conventional pulse that generates a 2uS 500V flyback anyway?" Unless I am mistaken, that was what was implied a while back in this thread. We do need to get the basic fundamental facts right to be able to progress PI. The only use I can see for a high voltage pulse is to "kick start" the otherwise slow growth of current in the coil inductance. After that, the normal power supply voltage would take over until the switch off. Instead of wasting the stored energy in a damping resistor, it could be recycled to do this kick start on the succeeding pulse: which is easier to do on a bipolar design.

I still firmly believe that an appropriate length TX pulse is necessary prior to switchoff and flyback in order to set up the necessary field and energise the target. However, I am always ready to listen and learn.

Eric.

Below is the closest quick simulation of the 2us pulse. I could only use 200V, because I have no Mosfet over 800V. Even like that, the Joules in the coil are many times more.

Using the same coil, 300uH, 200pF stray capacitance, 0.5R, the problem is with the stray capacitance. the resulting target response from the same 10us TC target is much lower that by conventional pulsing.

Using the TEM TX method, we use a short, high voltage TX and a long slow charge ramp. Maybe this is a better approximation to a very short TX pulse.

[Aziz]

The Missing Harmony Link to TEM TX.

Hi all,

where is the missing harmony link of the "Triangular Wave Technology" to the TEM TX transmitter?
(Harmony? Harmonics? PJ's harmonics? Where is PJ? )

If you carefully look at the current wave form of the TEM transmitter, it looks like a PW modulated triangular current wave. Put for PW = 0.95 (long pulse width mosfet switch-on gate = shallow current rise ramp, short off-time = high flyback voltage = steep current decay cos(x) ). We now have the PW modulated triangular current wave approximation at its best.

You can use the latest Excel sheet to calculate the TEM transmitter frequency response. And look, how the spectrum response gets more equal and wide band (with 0.95).

BTW, a wonderful harmony link.

Cheers,
Aziz

[Davor]

PRO FLYBACK. Are you saying that if you pulsed a coil with, say, just a 2uS 500V pulse, then that is a good as a long low voltage (100uS or more) conventional pulse that generates a 2uS 500V flyback anyway?

Yes.
There are caveats though. There is no way that current will just stop flowing with no apparent reason, so the end result, observing from a coil current point of view, is a triangular wave. The advantage is that a Tx coil is at a very low impedance all the time, thus not influencing the target response at all, and energy losses can be negligible.
It just can't work as a monocoil ...

A first example is a simplified classic flyback PI:



And the other one is an equivalent voltage spikes generated one:



To be fair and square to both cases I used a kind of IB topology, but also a parallel resistor in spikes case (which basically does nothing).

[Tinkerer]

Here is the short and long simulation.

Same coil, same target of 500us TC

Same TX amps

The blue trace is the long TX
The green trace is the short TX

The red target response is from the long TX
The pink target response is from the short TX

Quite clear I would say

[Midas]

Here is the short and long simulation.

Same coil, same target of 500us TC

Same TX amps

The blue trace is the long TX
The green trace is the short TX

The red target response is from the long TX
The pink target response is from the short TX

Quite clear I would say

OK I'm not exactly sure whats clear but I'll have a stab. What I see in your sim is that the difference between the TX on sample and peak fly-back response sample is the same either way. However you have a lot less time to sample the peak with the shorter on pulse so I guess that means a crapper SNR. But then of course you can pulse more frequently and probably make it back. Hmm yes.. not super clear... not to me anyway.


Midas

[mikebg]

Note what is written in patent US 4,506,225 by A. Loveless and A. Barringer:

"The primary field is defined by a primary waveform of known shape and it contains a plurality of frequency components appropriate for measurement of the desired electrical parameter or paramaters of a conductive body in the target zone. The frequency components are of known frequency, phase and relative amplitude."

Remarkable is, that designing a device which takes samples in time domain, the authors use terminology attributable to frequency domain.

Freuency analysis illustrated in post #31 showed that triangular wave technology is not efficient for WB (wide band or PI) metal detectors because harmonic components have low amplitudes and are placed in large frequency distance.


However as a bad educator, in post #31, I use dB scale for amplitude comparison where visual differences reative
to amplitude of fundamental frequency seem small, and log scaled frequency axis where visual distances between
frequencies of harmonics also seem small.
Here below is given for comparisn the drawing which is more suitable for educational purpose. Both axes are linear.
What next?
Can you tell which form of TX current is most inappropriate for the WB metal detector?

HINT: It looks very much like a triangle wave and in radio ingeneering is called CW (continous wave).

[Davor]

I prefer the dB scale. Lots of crappy antennas, filters, etc. look fabulous in lin scale. So in this case it is more marketing than educational ... unfortunately.

This accentuates the need for giving the higher harmonics a helping hand and using more complex waveforms. In my pursuit of a perfect signal source for a chirp air sonar I did the opposite - combining two binary sources in a bridge configuration to suppress higher harmonics. It works fine. It can also work fine in the opposite direction. Two independent binary sources produce a zero as well.

[Ferric Toes]

How about a trapezoidal waveform with leading edge modulation?

http://www.ias.ac.in/sadhana/Pdf2008Oct/537.pdf

Eek

[dougAEGPF]

Also here
Posted August 26 ,2012
http://australianelectronicgoldprosp...-applications/
dougAEGPF

[Aziz]

How about a trapezoidal waveform with leading edge modulation?

http://www.ias.ac.in/sadhana/Pdf2008Oct/537.pdf

Eek

Hi Eric,

this isn't much power efficient for a hand-held detector however.

I for one would use the TEM TX as it has both low frequency and high frequency current components. Both parts can be adjusted to the frequency range of interest. The power efficiency and its simplicity is really superb. Compared to the classical PI, you even get more response from the targets.

Cheers,
Aziz

[Aziz]

Another frequency equalising trick:
(It has been already mentioned).

The chirp wave of the source power frequency. You can use either rectangular step voltage source or even sinusoids.

So "chirping" the triangular current gives you a "wide band" response. This is quite expensive to realise it in a circuit (requires a VCO and saw-thooth/triangle generator). But very simple in the digital software domain.

Aziz

[Davor]

Unfortunately most of the references listed in the paper are inaccessible

The paper as it is simply describes a method of voltage conversion to meet the design goals, and the mechanisms to maintain it at a desired value, by applying a flyback principle in TEM coil. Guess nowadays PI machines could benefit a little from something like this to maintain a PI pulse at desired voltage by means of manipulating the charging period.

Otherwise this whole ... thing ... can be easily replaced by a simple PSU at a desired several kV value. In a sense this approach is more of a flyback PI approach, but "encouraged" to produce a TEM excitation signal.

A trapezoid current shape seem as a perfect approach to achieve the most bang per high voltage limit, as it maintains the sharp edge, hence a possibility of immediate or very early sampling.

Now that here is Dave J. with thorough knowledge of this technology applied to metal detecting, I'll shift some more focus to it.

So far I was quite unhappy with PI as we know it in a sense that there is something wrong with the excitation signal. There is something that Tinkerer calls a pivot that I clearly observed in simulations, which is a direct consequence of a coil speed (R/L). It becomes apparent in log scale. An infinitely fast coil would have a pivot at the switch-off, yet all realistic coils will have it at over 4µs.

Ferric Toes' experiments with special lightning fast coils just confirm the obvious - there is an odd limit in using a critically damped coil as an excitation source.