PI: Effects of pulse shape and transmit time.

[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]

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?

[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.

[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) 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.

[Tinkerer]

Superposition applies to magnetics, and when analyzed this way the Earth field has no effect.

Looking at it from examples, if the coil generates +1 Gauss which is positively aligned with the 0.5g Earth field, then the net field is 1.5g. At turn-off the field returns to 0.5g, which is a transient of -1g.

If the coil generates +1 Gauss which is negatively aligned with the 0.5g Earth field, then the net field is -0.5g. At turn-off the field returns to 0.5g, which is a transient of +1g.

No matter how the TX field aligns with the Earth field, the transient field is always 1g.

In most places on earth, the EF is about parallel to the surface, while the coil field is perpendicular.
One way to check would be with a free floating compass needle that could align in the coil field.

[green]

There are several significant to consider:

1) For the ON time, the TX current pulse shape is very important.
To get similar results, the pulse shape needs to be the same.

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

3) The induction balance of a DD coil is not perfect.
This caused the switching noise and coil transients to be relatively more visible than the target response.

4) The ferrite core.
There is a very vast choice of ferrite cores available. A core might be optimized for a 50Hz frequency or 5MHz frequency.

Running out of time, will continue...........

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.

[Carl-NC]

Superposition applies to magnetics, and when analyzed this way the Earth field has no effect.

Looking at it from examples, if the coil generates +1 Gauss which is positively aligned with the 0.5g Earth field, then the net field is 1.5g. At turn-off the field returns to 0.5g, which is a transient of -1g.

If the coil generates +1 Gauss which is negatively aligned with the 0.5g Earth field, then the net field is -0.5g. At turn-off the field returns to 0.5g, which is a transient of +1g.

No matter how the TX field aligns with the Earth field, the transient field is always 1g.

[Tinkerer]

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

Moving a coil through the Earth's magnetic field generates a current in the coil. This is something very different from what I am talking about.

Let's see If I can explain what happens when the coil and the target are static:

We all including the coil and the target are surrounded and penetrated by the constant (nearly) magnetic field of the Earth. The field intensity is about 0.5 Gauss. No movement, no change, no eddy currents.

Now, let's assume we switch this field OFF. At the moment of switching OFF, eddy currents will be generated within everything that is permeated by the Earth's field.

OK, we can not switch the Earth's field OFF, but, within the sphere of the magnetic field that we generate with our coil, we change the field vectors of the Earth's field. Depending on the strength of our dipole coil field, which can be up to several Gauss, the Earth's field vectors will be totally displaced, deflected or enhanced.

When we switch the coil OFF, the Earth's field lines will return to their original alignment. In doing so, the Earth's field lines cut or move across the coil. We know that a coil moving across magnetic field lines generates current in the coil. It is the same when magnetic field lines move across a coil. The speed at which this happens, determines the amplitude of the current generated.

[green]

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

Don't know if it matters. All of my tests are static. Both the coil and target are stationary. The circuit is DC coupled, no capacitor coupling.

[Qiaozhi]

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

[Tinkerer]

If I plot the target during coil turn on with LTspice I get a similar response as you did in reply #44. If I plot the on response with a DD coil on the bench I get something different. Low tau targets have a higher response at turn off, high tau targets have a higher response at turn on. The turn on response is highest at the start, then decays instead of increasing. Any idea why? Been thinking about looking at the target during coil on and off times.

There are several significant to consider:

1) For the ON time, the TX current pulse shape is very important.
To get similar results, the pulse shape needs to be the same.

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

3) The induction balance of a DD coil is not perfect.
This caused the switching noise and coil transients to be relatively more visible than the target response.

4) The ferrite core.
There is a very vast choice of ferrite cores available. A core might be optimized for a 50Hz frequency or 5MHz frequency.

Running out of time, will continue...........

[green]

While we have the sim at hand, we could look at the target response.

We see the TX eddy currents in the targets, to the time of TX switch Off and then the new eddy currents generated by the switch OFF di/dt.

v(n011) is the coil voltage decay

I(L2), ..L3, L4, L5, L6 are the responses of targets with TC's of 5us, 10us, 100us, 500us

We see how the short TC target eddy currents raise to a certain level during TX, but then start decaying again.
The long TC eddy currents keep increasing right to the time of switch OFF, because their TC is longer that the TX time.

We see that the switch Off must first destroy the negative eddy currents from the TX before it can build the positive eddy currents from the switch OFF.

Now, just one more hint:

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.

Of course, if we really want some more target information, there are many many more ways to extract this information from these target response curves.

If I plot the target during coil turn on with LTspice I get a similar response as you did in reply #44. If I plot the on response with a DD coil on the bench I get something different. Low tau targets have a higher response at turn off, high tau targets have a higher response at turn on. The turn on response is highest at the start, then decays instead of increasing. Any idea why? Been thinking about looking at the target during coil on and off times.

[Teleno]

Here is one idea:

A good way to reduce capacitance is to put the capacitances in series.

A Tx D-coil connected in series would have half the capacitance of each half D coil, but the total inductance will be larger than the sum.

It can also work during flyback for the components. If instead of one series diode you use two, their joint capacitance during flyback (reverse polarized) is halved.

The diode's capacitance gets higher as reverse voltage gets smaller. This technique has more effect as the coil voltage approaches zero, improving stability at the time of sampling.

[Tinkerer]

Tinkerer: Have you taken into account both the parasitic capacitance and resistance of the coil? I believe green had left them out.

Yes, the parasitic capacitance is very important. For this sim I used 200pF (rather on the low side) to include the coil inter-wire capacitance and the twisted pair cable and mosfet/diode capacitance. You can see the effect of this capacitance clearly in the red current trace, which goes down, then up and down again. Since the di/dt of the current is what kicks the target, this explains well how important the parasitic capacitance is.

The green voltage trace does not show anything of that.

While we are looking at the parasitic capacitance, we might try and find ways to reduce it, specially the coil to shield capacitance.

Green has been doing good work on that. Maybe, if we all put some effort into it, we find some good solutions?

Here is one idea:

A good way to reduce capacitance is to put the capacitances in series.

More ideas?