PI Power Output

[Tinkerer]

You guys have been busy with some good replies.

Gday Aziz, i agree with your explanation with increasing power to the coil.

Tinkerer, we talk the same language--plain ol english!

Yes higher voltages do work & it is easier & faster for higher voltages to travel through things rather than trying to force Amps through something.
I have been working on a system that has 90v & 600mA hence the original question to get a conversation going about it.
I have had to refine it & detune it a bit as it is capable of much higher output, it's all but finished & i will test again in the next few days hopefully.
I have been working in terms of total power to the coil where a more std setup has around 15v & 1.2A=18W, mine has 90v & 600mA= 54W.

I have had near 360v & 1.6A from the same setup but of course battery life suffers & personal life expectancy lowers dramatially especially on wet days. Also i didn't want to have a big rock on the coil to stop it bouncing hahaha!


Battery size is not an issue at this stage, the minelab batteries last from 8 to 10hrs from full charge & there not heavy at all. If these style of batteries only lasted a couple of hrs before recharge it would suit me if the detectors were stronger.

Tx on time?
The thing to remember is that the TX on time makes NO difference as the Target Decay signal is the same regardless, something to do with the time constant maybe?.
In Fact Long TX on times is an absolute no no as it induces the ground more & then things become troublesome.
Short sharp pulses are in order here as the ground is not induced as much & we get longer stronger target signals to read.

TX signal shape?
Does it really matter what the TX signal shape is?
Ok in the past with low power we have seen the TX switch off very fast for flyback reasons i guess.
My question is that with a higher output TX signal do we need to be as fast with the switch off?
For a given TX power output what time frame is sufficient, you would think for low output figures that very fast would be correct, but what about with higher output, how slow is to slow in microseconds?

Having a strong magnetic field from a coil is only part of the equation, the thing we really need is a strong magnetic field that has a lot of Push associated with it. So i guess a fast switch off is still necessary, but how fast?

Hi B^C,

higher voltage will get the same amps faster into the coil. However, for the ground and target penetration the important part is the magnetic field strength.
What counts is the Ampere Turns.
Now, I have heard that it is an advantage in the aggressive Australian soil to use very short TX pulses. I believe ML uses short and long pulses.
So there may be a real advantage in using high voltage to reduce the TX time.
In a coil with 325uH inductance and 2.8 Ohm DC resistance, it takes 100uS to bring the coil current to 2.5A, at 12 V
With 120 V it takes only 7uS to reach 2.5A coil current.
It also looks like a saving of about 30% power.
Tinkerer

[Guest]

Hi Tinker,

I have been following this thread and find it very interesting. I believe some of the questions you are asking are due to inductive coupling vs true pulse induction. Many of the great answers from the others are explaining what happens when the current is switched off and there after. Your observations are during switch on while the current is building. From what I have read you are using an induction balanced coil. It can be looked at as an air core transformer that has very low coupling. When a resistive target is placed into the changing magnetic flux, it creates an impeadance to the flow. This in turn upsets the balance in your transfomer and causes coupling. Ractive targets, in simplistic terms inhance the flow of flux and cause a disbalance in the transformer and coupling. A true PI is measures the decay of eddy currents induced into the target after the coil current collaspes. If you see how standard TR detectors operate it is the same as what you are doing during switch on. You are using a step function vs a sine wave. Time domain vs Frequency domain remember T=1/f and F=1/t. Keep up the interesting experiments

Regards
Mark

[B^C]

Tinkerer

I understand that ampere turns is important but only if we can take full advantage of it, it's hard to explain. Let me test something out & i'll try to answer this differently. I tested some things & found that sometimes a little less is more so to speak. It's like have a 500hp engine but you can only get half throttle, i found something interesting & putting it words is not easy.


Yes short TX is the only way to go.

I guess the figures above are calculated from your operating frequency which is important.

What calculation are you using for your current rise vs time vs voltage?

[Aziz]

Hi B^C,

I think, there are benefits from other side using shorter TX pulses. The CMRR, PSRR will be better, if your power supply won't be stressed much. There will be less ripples and regulatation in the supply part. Also less EMI during TX switching. This will improve the analog part of the front-end and SNR of course.

But from the point of target stimulation, there is definitely no other way. The more, the better. At the end: The higher the flyback voltage, the more the target stimulation and target eddy currents, as this gives the highest dB/dt damping. It doesn't matter, how you get the high flyback voltage: either with strong short pulses or weaker long pulses.

A common misinterpretation is the ground: Ground should also be seen as a weak target: It has conductivity, inductivity, capacitance, permeability, permittivity, coupling to the transmitter coil, parasitic capacitances to the transmitter coil, ...
It is obvious, that the ground will react to the transmitter stimulation. And it is also obvious, that all parameters will interact with each other, which makes the discrimination and ground balancing difficult. So varying the transmitter pulse width gives some help to get some of the parameters (time constant of the target for instance). It is also obvious, that high excited pulse energy will give more ground signals. And it is only a question of SNR (signal compared to noise level) and dynamic range of the measuring system.

I think, there is still a common misinterpretation of target eddy current stimulation in a PI device. We had here an interesting contraversal discussion last year (or was it this year?) . As we all know, the transmit on pulse can be neglected due to less stimulation on MONO coil systems. I think, not all people have really understood the importance of the damping process.

Regards,
Aziz

[ZED]

But from the point of target stimulation, there is definitely no other way. The more, the better. At the end: The higher the flyback voltage, the more the target stimulation and target eddy currents, as this gives the highest dB/dt damping. It doesn't matter, how you get the high flyback voltage: either with strong short pulses or weaker long pulses.



Regards,
Aziz

Aziz i beg to differ,your not considering the inductance of targets,targets need time to build up eddy currents,your suggesting to reduce this time in favour of increasing the speed of the coil field,it is true that the faster a field moves the stronger the eddy currents but what about the inductance of targets that resist the growth of eddy currents,increasing the speed and reducing the window for which targets are exposed to a moving field in my opinion produces a negative improvement or zero improvement for targets with high inductance.
Speed of the collapsing field and the exposure time to this field are both important criteria.

Zed

[Aziz]

Hi ZED,

it is only a matter of exposed energy and how you control this at damping process. The target has some capacity to store and consume energy (due to inductivity and resistance and has some tiny capacitance too). It cannot produce more energy than it is available. Then you have two options:
- slow damping, longer sampling with lower target signal response or
- fast damping, shorter and earlier sampling at higher target signal response

So there is really no free lunch. This is a fact.

Sure, making the system faster has some disadvantages: higher noise susceptibility due to shorter detection window and reduced bandwidth of the front-end (slow response). On the other side, slow response needs really more time to sample at low signal levels. If I consider, then you are right, that the slow system has more obvious advantages compared to a fast response system.

So using an integrator for sampling is very convenient, as the target response is a time-domain response (distributed over time).

Now the question is: What is better? Using higher target signals for short sampling periods or using lower target signals for longer sampling periods. I think, the latter one is obvious as the ground itself has not much energy storing capability and will (should) decay faster than the targets.

From the point of processing (circuit), whether which option is better, should be analysed further.

Aziz

[Tinkerer]

Aziz i beg to differ,your not considering the inductance of targets,targets need time to build up eddy currents,your suggesting to reduce this time in favour of increasing the speed of the coil field,it is true that the faster a field moves the stronger the eddy currents but what about the inductance of targets that resist the growth of eddy currents,increasing the speed and reducing the window for which targets are exposed to a moving field in my opinion produces a negative improvement or zero improvement for targets with high inductance.
Speed of the collapsing field and the exposure time to this field are both important criteria.

Zed

Zed,

I agree with you.
Lets look at the sampling DURING TX. There we can read the signal at the time it is generated, not some undefined time later like the OFF transient signal.
Carl states above. "The maximum di/dt is right at turn ON" (for the ON transient)
There is no doubt that this is right.
However, the maximum response from the target is not at that moment.
Also, the moment of maximum response from the target varies in time with the TC of the target and the material of the target.

THIS IS THE KEY TO DISCRIMINATION AND TARGET IDENTIFICATION

Once we fully understand why the signal amplitude is higher than expected and why there is a difference with the TC and target material at the ON transient, we will be able to fully discriminate and identify the target.

Simulations give the wrong results if the target is not realistically represented with a TC.

Tinkerer

[Tinkerer]

Tinkerer

I understand that ampere turns is important but only if we can take full advantage of it, it's hard to explain. Let me test something out & i'll try to answer this differently. I tested some things & found that sometimes a little less is more so to speak. It's like have a 500hp engine but you can only get half throttle, i found something interesting & putting it words is not easy.


Yes short TX is the only way to go.

I guess the figures above are calculated from your operating frequency which is important.

What calculation are you using for your current rise vs time vs voltage?

I think I know what you mean. I have found something similar, but am not ready to go public with it yet. I need to find the right terms to present it first.

I find Misc.EL a great tool to calculate all sorts of things related to PI.

Tinkerer

[Aziz]

Now, everybody can get crazy!

Hi all,

I made some circuit simulations. A four equivalent systems were analyzed:
- Fast coil damping, with target (left/top)
- Slow coil damping, with target (right/top)
- Fast coil damping, no target (left/bottom)
- Slow coil damping, no target (right/bottom)

The slow damping is achieved with higher coil capacitance. The flyback voltage is quite lower than on fast response configuration. Same transmit pulse energy on every system. You can see, that only few mA are missing on the slow damping. Where the hell is the missing energy gone? -> it is stored in the higher capacitance.

The signals are named systematically:
F for fast, S for slow, 0 means no target, xxclip.. means the coil voltage at the clipping diode. The Rin resistor is set to 10k to avoid damping resistor inbalance (for critical coil damping).
Please play with the circuit simulation (LTSpice model) and make different sweep analysis for:
- different pulse width
- different target inductivity (target size)
- different target resistance (target conductivity)

You can scope the signals, make the comparison between the simulation models. You will find interesting facts.

Now see the following response comparison at the diode clipping:
The fast response system gives tremendous big target signal response for bigger targets (higher inductivity). That is quite interesting.

The simulation models are very basic but quite accurate. The coil is splitted into three parts: series coil resistance, coil inductor and coil capacitor.

Have fun with circuit analysis.

Aziz

[Aziz]

here is the basic circuit of the models...

[Tinkerer]

Hi Tinker,

I have been following this thread and find it very interesting. I believe some of the questions you are asking are due to inductive coupling vs true pulse induction. Many of the great answers from the others are explaining what happens when the current is switched off and there after. Your observations are during switch on while the current is building. From what I have read you are using an induction balanced coil. It can be looked at as an air core transformer that has very low coupling. When a resistive target is placed into the changing magnetic flux, it creates an impeadance to the flow. This in turn upsets the balance in your transfomer and causes coupling. Ractive targets, in simplistic terms inhance the flow of flux and cause a disbalance in the transformer and coupling. A true PI is measures the decay of eddy currents induced into the target after the coil current collaspes. If you see how standard TR detectors operate it is the same as what you are doing during switch on. You are using a step function vs a sine wave. Time domain vs Frequency domain remember T=1/f and F=1/t. Keep up the interesting experiments

Regards
Mark

Hi Mark,

I think you come closest to what is actually happening.
OK it is not traditional PI. It is probably more of a hybrid.
I attach 2 pictures of the current wave form. If we look at the current wave form instead of the voltage wave form, we see that it is not all that different from a VLF.
Then maybe it is easier to understand where to look for the information that we can read in it.
This is TIME DOMAIN, the information is locked in a specific time frame.

Now, I have found where to look, that is I know at what spot in time I can read some information, but I do not yet fully understand what I am reading.

Much less I can explain exactly what is happening.

All I know, is that the information is there.
I find it every time.
It tells every time the same story, but I don't understand all of it.

The one picture shows 10k PPS of coil current pulse. I don't remember the TC , but it looks like somewhere around 3 TC.

The other picture shows just two coil current wave forms. The TC is around 1 TC.
You can see that the switch OFF is fast and it produces a spike in the opposite polarity.
I think it is this spike that kills a good part of the OFF time signal.

You can also see on the ON slope 2 small switching noise spikes close together. This is one of the sample pulses.

Several samples along this slope give different information. At a certain time the FE signal is higher, at another time the FE signal all but disappears.
It is like an invisible sine wave is riding on the slope, the line of the slope being the 0V line.
Sampling where the sine wave crosses the 0V line gives no signal.
Non magnetic metals give a peak response at a different spot.
The TC of the target moves the peak spot to a different time.

Tinkerer

[Tinkerer]

here is the basic circuit of the models...

Hi Aziz,

nice simulation.

Could you show it with a target with a TC of 10uS and again with a target with a TC of 100uS?

Could there be a correlation between a fast switch OFF and a target with a short TC?

Or, could there be a correlation between a slow switch OFF and a target with a long TC?

There are targets with low conductivity and others with high conductivity. We know that the low conductivity targets give a short TC relative to their size, while high conductivity give long TC's.
I don't think we know much about the inductivity and the capacity of the targets, maybe this is where the secrets are hidden?

Tinkerer

[Aziz]

Hi Tinkerer,

Could you show it with a target with a TC of 10uS and again with a target with a TC of 100uS?

See below: Ground is taken as TC=1µs and target with 10µs and 100 µs.
The TC=100µs signal will last very long at low signal level, whereas the others are almost zero.

Could there be a correlation between a fast switch OFF and a target with a short TC?

I don't know. The switch off time is same for all systems. However, the higher coil capacitance will slow down the damping and decrease the flyback voltage. The critical coil damping resistor will be lower with increased coil capacitance.

Or, could there be a correlation between a slow switch OFF and a target with a long TC?

I don't know.


Interesting fact on the fast damping system is, that the clamping diodes are recovering faster from the conductive state, which allows earlier sampling (4-5 µs). This gives a higher sampling integration possibility, which contains effectively more target response signals.

Aziz

[Tinkerer]

Hi Tinkerer,



See below: Ground is taken as TC=1µs and target with 10µs and 100 µs.
The TC=100µs signal will last very long at low signal level, whereas the others are almost zero.



I don't know. The switch off time is same for all systems. However, the higher coil capacitance will slow down the damping and decrease the flyback voltage. The critical coil damping resistor will be lower with increased coil capacitance.




I don't know.


Interesting fact on the fast damping system is, that the clamping diodes are recovering faster from the conductive state, which allows earlier sampling (4-5 µs). This gives a higher sampling integration possibility, which contains effectively more target response signals.

Aziz

Thanks Aziz, great job.

Now lets see if I understand right.

There is a time delay until the response of the targets reach their peak. The peak response is not at the same time for targets with a short TC and a long TC.
This is exactly what I am talking about. It makes it possible to read the TC from the response.

Interesting about the slow damping. So if we were to sample at 1.020mS, the 10uS target would actually have a higher amplitude?

Tinkerer

[Aziz]

Hi Tinkerer,

Interesting about the slow damping. So if we were to sample at 1.020mS, the 10uS target would actually have a higher amplitude?

Tinkerer

it is a misleading display. Note, that the shown display is the coil voltage difference of two systems at the clipping diodes (target/no target). If you scope the target voltage only (see below, represents target eddy current and it is direct proportional to target induction voltage), then you will see that fast decaying system has a higher target induction voltage, but which decays faster than the slow decaying system. The integral of the target induction voltage over time (after switch-off) should be equal for slow and fast decaying system (due to same transmit pulse energy and TC). So there is really no free lunch.

Somehow, the clamping diodes in the simulation give some advantage on the fast decaying system here, which allows a bit earlier sampling. That is quite interesting, which I didn't expect.

Aziz