PI: Effects of pulse shape and transmit time.

[Tinkerer]

1) Any idea what the transmit pulse would have to look like to cause the on target signal to be opposite what we both get with the spice simulations, Or is something else causing it?
2) If there is an effect, doesn't balancing the coil for a zero signal cancel it out?(Missed Carl's reply while I was replying)
3)The switching transients are less than 3 usec long. The rest of the no target trace is a lot less than the target traces.
4)I have a couple different ferrites. Both have a similar response as some clay from the yard, maybe the same.

1) Using an inverting opamp configuration gives the opposite polarity response than a non inverting configuration. I am not sure if it is that what you mean. Also, on the simulation I usually look at the current in the target inductor. I look at it as the TX coil current going one way and the eddy currents going the other way. The current in the target resistor is of opposite polarity.
2) I am not sure if there is always an effect and if there is, it may only be under certain circumstances.
3) I think I misunderstood what you meant about the differences in the traces. We should take a close look at the details. In general the traces look normal to me.
4) Eric Foster said that some ferrites can be used to simulate magnetically viscous ground. In my experiments I found that some ferrites give very little resistive response, only reactive response, which is of the opposite polarity. The response of iron is mixed, resistive and reactive. with an IB coil both responses can even cancel each other.

How is your TX pulse? What is the TC of it? With many PI's the TX voltage droops during the pulse. It still works like that, but the ON time target response curve is different.

[green]

1) Using an inverting opamp configuration gives the opposite polarity response than a non inverting configuration. I am not sure if it is that what you mean. Also, on the simulation I usually look at the current in the target inductor. I look at it as the TX coil current going one way and the eddy currents going the other way. The current in the target resistor is of opposite polarity.
2) I am not sure if there is always an effect and if there is, it may only be under certain circumstances.
3) I think I misunderstood what you meant about the differences in the traces. We should take a close look at the details. In general the traces look normal to me.
4) Eric Foster said that some ferrites can be used to simulate magnetically viscous ground. In my experiments I found that some ferrites give very little resistive response, only reactive response, which is of the opposite polarity. The response of iron is mixed, resistive and reactive. with an IB coil both responses can even cancel each other.

How is your TX pulse? What is the TC of it? With many PI's the TX voltage droops during the pulse. It still works like that, but the ON time target response curve is different.

1 and 3)[When we use an IB coil, we can sample the TX eddy currents just before the TX switch OFF, invert the result and add it to the sample after switch OFF, obtaining an enhanced peak target response.] From your reply #44. If you look at the target traces in your reply #44 the target signal is higher amplitude at the end of coil on time. I get the same with spice. The target signal is higher at the start in the scope traces. What controls what the on time target signal looks like with spice?
4) I included the ferrite because it gives a response the same as some clay from the yard. To cancel ground I would sample an inverted signal at the end, gain it to be equal amplitude to the sampled start signal and add them.
)The Tx pulse is about 85 usec, close to the coil TC of 93 usec.

[Tinkerer]

1 and 3)[When we use an IB coil, we can sample the TX eddy currents just before the TX switch OFF, invert the result and add it to the sample after switch OFF, obtaining an enhanced peak target response.] From your reply #44. If you look at the target traces in your reply #44 the target signal is higher amplitude at the end of coil on time. I get the same with spice. The target signal is higher at the start in the scope traces. What controls what the on time target signal looks like with spice?
4) I included the ferrite because it gives a response the same as some clay from the yard. To cancel ground I would sample an inverted signal at the end, gain it to be equal amplitude to the sampled start signal and add them.
)The Tx pulse is about 85 usec, close to the coil TC of 93 usec.

I start with the last/easy one: The TX current pulse is near linear up to about 1 TC. If we make the TX longer than that we see the eddy currents droop more. Do you include the Mosfet RDS ON, and the cable resistance when you calculate the TX coil TC?
With the TX pulse at 83us, your 200us target eddy current ramp remains in the near linear region, but the short TC targets are already decaying considerably.

So there are 3 ways to look at it:
1) make the TX pulse long enough so that the eddy currents for the most important target has decayed.
2) Make a flattop TX pulse. The eddy currents will start decaying as the TX pulse reaches the flattop.
3) or use an IB coil and enhance the peak target response by adding the TX ON sample to the TX OFF sample. This can be specially useful for long TC targets where it is not feasible to make the TX pulse long enough for the TX ON eddy currents to decay. For the Ground Balance we need to know the characteristics of the ground. This is really the big question:

What are the characteristics of the soil (Ground) I think we should open a new thread for that.

If you look closely, the ON response of the long TC target, L5, has more amplitude than the OFF response. But, if I remember right, the TX pulse length is about 200us.

I am preparing a set of IB coils at present so I will be soon capable to show real target curves to compare with the simulations. There are always some differences, but I started with real components and then tried to make simulations that give me similar results.

This is another separate thread we should start: "Why are the simulations and the real PI traces different?" Looking at each difference and analyzing the cause of it, should help a lot in understanding and fixing the underlying problems.

[CAS]

Teleno, using a constant current TX with different pulse widths is a very worthwhile approach. Due to my involvement with prior and current employers I cannot offer much more detail, other than to suggest you should pursue this. Minelab has filed a couple of patents on CCPI, White's has one and should have another in the works, so CCPI methods are being commercially developed.

I have been experimenting with CC coil drive circuits and seeing what difference they make on the MPP. I do notice a slight noise decrease as well as a very slight increase in detection distance with small objects. As I am using the MPP only as a test backend for the TX circuit, I am yet to try different pulse widths.

What would the advantage of a CC drive be over say the 'standard' TX drive circuit?

[Teleno]

I have been experimenting with CC coil drive circuits and seeing what difference they make on the MPP. I do notice a slight noise decrease as well as a very slight increase in detection distance with small objects. As I am using the MPP only as a test backend for the TX circuit, I am yet to try different pulse widths.

What would the advantage of a CC drive be over say the 'standard' TX drive circuit?

Care to post some of those CC drive circuits?

CC drive increases the rise time of the pulse. Standard rise is a ramp while CC rise is as fast as the turn-off. For a given current CC allows shorter pulses because turn-on is immediate. Shorter pulses mean that responses of longer tau's are damped or eliminated altogether. Ground response is also reduced because it is k*(1/t - 1(t+T)) where T is the pulse width. As T tends to 0, t+T gets closer to t, and the difference 1/t - 1(t+T) gets smaller.

All in all: better selectivity and signal/ground ratio for smaller targets.

[CAS]

Thanks for the answer. While I have a good understanding of electronics theory and practice, how it all relates to MD theory is new to me.

Now as I understand it, max coil current will always be relative to the combination of resistance in the TX circuit. The fastest rise time will always be limited by the capacitance/resistance/inductance combination of the coil and fet. Other than picking the best spec components and having a super fast coil, I dont see any way of over coming this.
By using a CC circuit, the max voltage will be reduced as compared to no CC. This means that there will not be as much energy put into the coil in a CC circuit.

Will this result in a reduction of detecting depth or because, of the benefits of CC, sensitivity of the receive circuit can be increased ?

[Teleno]

The fastest rise time will always be limited by the capacitance/resistance/inductance combination of the coil and fet. Other than picking the best spec components and having a super fast coil, I dont see any way of over coming this.

It is overcome by a very high voltage, as high as the transient's peak at cut-off.

By using a CC circuit, the max voltage will be reduced as compared to no CC. This means that there will not be as much energy put into the coil in a CC circuit.

If you turn the coil on using the high voltage you reach the top current instantly rather than havoing to eait for the ramp to crawl up as in standard. The energy stored in the coil is the same: E = (L x I^2)/2, but it is stored faster.

In the first post of this thread I posted a graph explaining the difference.

Now what happens in the target is this:

- The rapid chage at turn-on induces the typical exponential decaying voltage in the target.

- If the pulse-width is longer than the target's tau it will have decayed completely by the time of turn-off. The exponential decay induced by turn-off (of opposite polarity) starts fronm zero ásn is fully available after turn-off.

- If the pulse-width is longer than the target's tau, by the time of turn-off the target is still decaying. This residual voltage is subtracted form the decay amplitude generated at turn-off (remember, they're opposite polarity). The target's signal is weaker.

In short: targets having tau's longer than the pulse duration get damped, targets with shorter tau's pass through.

[CAS]

I had already been playing with injecting a high voltage pulse to speed rise time and have managed to get a constant 1.8A coil current at about 12uS after turn on. If I use a lower current (1A) that time can be reduced significantly (to about 2uS) but I dont want to start reducing the energy put into the coil. This means I can have a TX pulse down around the 12uS but surely there will always be some bit of rise time just due the the coil/component effect.

[Teleno]

I'm retaking this thread to share a quick thought on pulse width and ground elimination.

Let's say a PI has two different transmit periods and with . The target's tau is such that .

The target signal is for both and (because tau is small).

The ground signals are different: for and for .

because , therefore causes more ground response.

is calculated directly as:

;

where t is the moment of measurement counted from the end of flyback.
(formula from a previous post in this thread).

Then we can eliminate the ground signal by solving the linear system:

where and are the measurements taken after the short and long pulses respectively.

This gives:

It appears a sequence of two such pulses could be used to cancel ground for all taus up to the shorter pulse n a simple way.

[Qiaozhi]

Then we can eliminate the ground signal by solving the linear system:

where and are the measurements taken after the short and long pulses respectively.

This gives:

It appears a sequence of two such pulses could be used to cancel ground for all taus up to the shorter pulse n a simple way.

I think your solution to the simultaneous equations is slightly incorrect:




Solving by elimination gives:

and not

I also double-checked using determinants:







Resulting in:



and therefore:

which is the same result.

Hope this doesn't screw up what looks like a potentially elegant solution.

[Teleno]

...
and therefore:

which is the same result.

Hope this doesn't screw up what looks like a potentially elegant solution.

Correct. Thank you.

Actually is the ground signal rather than .

can be simplified as .

In practice the k for ground balance would be simply calculated as when s = 0 (no target) and g > 0 (hot ground).

[Qiaozhi]

Correct. Thank you.

Actually is the ground signal rather than .

can be simplified as .

In practice the k for ground balance would be simply calculated as when s = 0 (no target) and g > 0 (hot ground).

Actually, the ground signal is: due to the the negative sign on , but that's not important. It's the target signal (s) that you want to extract.

Can you explain the derivation of: ?

;

[Teleno]

Can you explain the derivation of: ?

;

Sure, it's here: http://www.geotech1.com/forums/showt...063#post197063



The formula comes from this paper.

above is the "on" period, that I noted as T. From the paper you'll see that the origin t = 0 is the end of the flyback transient.

Derivation: k represents the ratio between the ground responses to the "wide" and the "narrow" pulses, whose formulas are and respectively.

[Qiaozhi]

For those who are wondering what's being discussed here, you can easily understand it with the minimum of mathematics, as follows.

In a standard ground-balancing PI (like White's TDI) two samples are taken that are fairly close together. First you have the main sample, closely followed by a second ground sample (maybe 10us to 15us later). This technique is very similar to the method for Earth Field cancellation, in as much as the Earth Field exists in both samples, so can easily be removed by subtraction. However, with the TDI method, the target signal in the second sample is lower than the first sample, and needs additional gain when compared to the first sample. By adjusting the gain on the second sample, it is possible to cancel targets (specifically ground) with a particular decay constant (or ). One benefit of this approach is that it can be used as a simple form of discrimination based on conductivity. The result is that the audio tone will rise for low conductivity targets, and will lower for high conductivity. In this way, small iron targets (such as nails) can be identified. But, one unfortunate side effect is that it creates a hole in the target response. In other words, any targets that happen to match the selected will also be rejected.

What Teleno is proposing here is subtly different. In this case two pulses are transmitted of different widths. The first pulse is narrow, and the second is wider. The proviso is that both pulse widths are sufficiently wide that they are capable of saturating the target. Or (in other words) the target is much smaller than the time constant of the first TX pulse. The result is that the sample taken after the first TX pulse will return the same amplitude signal as the sample taken after the second [larger] pulse. This is because the target has a finite size, whereas the ground appears to the coil as effectively infinite. Hence the ground signal will be different for the two samples. i.e. larger for the sample following the second [larger] TX pulse.

Consider this formula:



where:
s is the target signal
is the signal from the sample following the first [narrow] TX pulse, which contains both target signal and ground.
is the signal from the sample following the second [wide] TX pulse, which also contains both target signal and ground.
k is a user-adjustable gain to eliminate the ground signal

Here's how it works in simple terms, using "easy" numbers:

If we take . This contains the target signal (let's say 5 units) + a contribution from the ground (let's say 1 unit). So the total for is 6 units.
Next we take . This also contains the target signal (5 units) + a larger contribution from the ground due to the wider TX pulse (let's say 3 units). Therefore the total for is 8 units.
As you can readily see, the ground signal in the second sample is 3x the ground signal in the second sample. So we need to set k (the gain) to 3 if we want to eliminate ground.

Now we're ready to remove the ground signal:



which is the target signal minus the ground signal, but without the hole in the target response that you get with the TDI.

[Qiaozhi]

Sure, it's here: http://www.geotech1.com/forums/showt...063#post197063



The formula comes from this paper.

above is the "on" period, that I noted as T. From the paper you'll see that the origin t = 0 is the end of the flyback transient.

Derivation: k represents the ratio between the ground responses to the "wide" and the "narrow" pulses, whose formulas are and respectively.

Ah, yes! ... I remember that paper now.
We discussed it just over one year ago. How time flies.