PI Power Output

[Tinkerer]

Hi 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

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

[Unregistered]

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

It is generally accepted that to fully energize a conductive target the tx pulse length should be at least 2xtarget TC.

[ZED]

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

Very good question Tinkerer ! consider this,why do m/lab pi`s for the most part only find nuggets smaller than 120 oz [there are a couple of exeptions]

Zed

[B^C]

Gday Aziz, the simulations are good, thank you!

Couldn't help but ask & off the subject a little but maybe relevant.
Different diameter inductors-coils have a distinct magnetic field strength surrounding them, how can we add or subtract this into the spice sim to compensate for this?
Can we just drop the supply voltage to do this or change the coupling of the target?--calibrating is another thing i know.

The point i am getting at is that the transient spike voltage & characteristic would seem to be dependant on the strength of the magnetic field surrounding the inductor at the time of collapse. The diameter of the coil has an influence on this as well it would seem.
If i'm wrong just belt me over the head!
Eg:
If we have two coils with the same inductance etc & fed from the same power supply a 8" coil is going to have a more intense magnetic field surrounding it than say a 16" coil has so the transient spike must be affected?
It would seem as though this may be the reason why large coils won't detect small nuggets, the spike ain't big enough to induce them hence the use of smaller coils?

So one would think to look at things better we need a Test std especially if the transient spike voltage changes with different diameter coils. This way we can all look at & test the same things?

Or i may need a kick in the butt!

Or how much of a transient spike is measured from a 16" coil compared to an 8" with the same specs, inductance etc?

[ZED]

Aziz looking at your last graph i dont see anywhere where your faster flyback has any real advantage,sampling one to two u/s after switchoff is not realistic,five to six u/sec after switchoff is a more likely scenario in which case it would appear that the slower system has more to offer.

Zed

[Aziz]

Hi B^C,

Gday Aziz, the simulations are good, thank you!

Couldn't help but ask & off the subject a little but maybe relevant.
Different diameter inductors-coils have a distinct magnetic field strength surrounding them, how can we add or subtract this into the spice sim to compensate for this?
Can we just drop the supply voltage to do this or change the coupling of the target?--calibrating is another thing i know.

The point i am getting at is that the transient spike voltage & characteristic would seem to be dependant on the strength of the magnetic field surrounding the inductor at the time of collapse. The diameter of the coil has an influence on this as well it would seem.
If i'm wrong just belt me over the head!
Eg:
If we have two coils with the same inductance etc & fed from the same power supply a 8" coil is going to have a more intense magnetic field surrounding it than say a 16" coil has so the transient spike must be affected?
It would seem as though this may be the reason why large coils won't detect small nuggets, the spike ain't big enough to induce them hence the use of smaller coils?

So one would think to look at things better we need a Test std especially if the transient spike voltage changes with different diameter coils. This way we can all look at & test the same things?

Or i may need a kick in the butt!

Or how much of a transient spike is measured from a 16" coil compared to an 8" with the same specs, inductance etc?

Indeed, the spice simulation can be weight up with some gold! (psst, almost nobody realized that ).

To simulate different TX coil diameter, the parameter TgtCoupling=0.01 must be changed. For a given target (smaller than the coil radius), bigger coils have less coupling to the target. Smaller coils have more coupling. But the distance, shape, size and orientation of the target has also an influence to the coupling factor. So it is difficult to calibrate a system. Therefore only relation of the response is important. Not the absolute values. If you know all of these parameters, than a more realistic simulation could be done for a specific configuration.

Supply voltage Ubat and ton (transmit pulse width) defines the exposed magnetic field energy and is not convenient to simulate different TX coil sizes.

An 8" and 16" coil with same inductivity L would give for the same target different response. The coupling of TX coil and target differs in this case. Small targets on bigger coils would therefore have less response due to less coupling. Coupling means: How many and how strength magnetic fields are going through the target?

The basic spice simulation indeed allows deep analysis of very interesting questions. I have taken an ideal switch for the model, to make a comparison between fast and slow damping as the flyback voltage shouldn't clipped due to mosfets avalanche breakdown voltage.

---

I have found, that the conductivity of the target matters much in respect to the target response! If you take a comparison for a fixed time constant target TC and different conductivity of the target (target inductivity differs then), then the target response differs quite much. Particularly long TC targets with low conductivity have less response.

We should all play a bit more with spice simulations.

Aziz

[Aziz]

Hi Zed,

Aziz looking at your last graph i dont see anywhere where your faster flyback has any real advantage,sampling one to two u/s after switchoff is not realistic,five to six u/sec after switchoff is a more likely scenario in which case it would appear that the slower system has more to offer.

Zed

Thats true from the point of target response only. If you can not sample earlier, then you will loose signal (integration area of signal).

Darn, but the clamping diodes 1N4148 are responding better with fast system and give some real advantage over slow damping. That is, which surprizes me.

Aziz

[B^C]

Aziz, yes i thought simulating this wasn't an issue.

One other thing, the conductivity of copper vs gold for the simulation, if we add some resistance to the target inductor this should do the job --yes!
Obviously the copper inductor will show better results than we would expect?.

My question is how much resistance to add as a constant?

Or do we just adjust the coupling as talked about?

http://environmentalchemistry.com/yo...lectrical.html

[Aziz]

Hi B^C,

Aziz, yes i thought simulating this wasn't an issue.

One other thing, the conductivity of copper vs gold for the simulation, if we add some resistance to the target inductor this should do the job --yes!
Obviously the copper inductor will show better results than we would expect?.

My question is how much resistance to add as a constant?

Or do we just adjust the coupling as talked about?

http://environmentalchemistry.com/yo...lectrical.html

I never did a real calibration yet (still haven't the instruments to measure the parameters). The target model parameters are just estimated. You can see below the very basic target model. It is really difficult to model a real target. But the basic model will give quite good results to the interesting questions anyway.

A target consists of some inductivity (parameter LTgt), capacitance (parameter CTgt) and will dissipate the induced energy through its resistance path (RTgt). It is more or less coupled to the TX coil (parameter TgtCoupling).

Fortunatelly I bought today an oscilloscope (Tektronix 455), which will help to measure or give a better estimation of the target parameters. Then a more realistic simulation could be done. The target model itself could be modelled with a lumped model. But this is quite more complex to do it.

Conductivity of the target is indepent to the coupling factor. I will list the dependencies of the parameters now (might not be complete):

TgtCoupling (target coupling) is dependent on:
- Transmitter coil size, transmitter coil orientation (to the target)
- Target size, targe distance, target orientation, target shape
- other objects nearby the coil/target, that affects the magnetic fields

Target LTgt (target inductivity) is dependent on:
- Target size, target shape, target material (permeability)

Target RTgt (target resistance) is dependent on:
- Target size, target shape, target material (conductivity)

Target CTgt (target capacitance) is dependent on:
- Target size, target shape, target material, other materials nearby the target

So it is quite difficult to model a target as the parameters interact with each other.

Aziz

[Tinkerer]

Aziz, yes i thought simulating this wasn't an issue.

One other thing, the conductivity of copper vs gold for the simulation, if we add some resistance to the target inductor this should do the job --yes!
Obviously the copper inductor will show better results than we would expect?.

My question is how much resistance to add as a constant?

Or do we just adjust the coupling as talked about?

http://environmentalchemistry.com/yo...lectrical.html

The surface area of the target has a very big influence on the coupling factor.

Tinkerer

[Tinkerer]

Hi B^C,



I never did a real calibration yet (still haven't the instruments to measure the parameters). The target model parameters are just estimated. You can see below the very basic target model. It is really difficult to model a real target. But the basic model will give quite good results to the interesting questions anyway.

A target consists of some inductivity (parameter LTgt), capacitance (parameter CTgt) and will dissipate the induced energy through its resistance path (RTgt). It is more or less coupled to the TX coil (parameter TgtCoupling).

Fortunatelly I bought today an oscilloscope (Tektronix 455), which will help to measure or give a better estimation of the target parameters. Then a more realistic simulation could be done. The target model itself could be modelled with a lumped model. But this is quite more complex to do it.

Conductivity of the target is indepent to the coupling factor. I will list the dependencies of the parameters now (might not be complete):

TgtCoupling (target coupling) is dependent on:
- Transmitter coil size, transmitter coil orientation (to the target)
- Target size, targe distance, target orientation, target shape
- other objects nearby the coil/target, that affects the magnetic fields

Target LTgt (target inductivity) is dependent on:
- Target size, target shape, target material (permeability)

Target RTgt (target resistance) is dependent on:
- Target size, target shape, target material (conductivity)

Target CTgt (target capacitance) is dependent on:
- Target size, target shape, target material, other materials nearby the target

So it is quite difficult to model a target as the parameters interact with each other.

Aziz

Thanks AZIZ, you have characterized the target parameters very well.
Please ignore my post about target surface area. It was posted before I saw this post.
Tinkerer

[Unregistered]

Gday Aziz, the simulations are good, thank you!

Couldn't help but ask & off the subject a little but maybe relevant.
Different diameter inductors-coils have a distinct magnetic field strength surrounding them, how can we add or subtract this into the spice sim to compensate for this?
Can we just drop the supply voltage to do this or change the coupling of the target?--calibrating is another thing i know.

The point i am getting at is that the transient spike voltage & characteristic would seem to be dependant on the strength of the magnetic field surrounding the inductor at the time of collapse. The diameter of the coil has an influence on this as well it would seem.
If i'm wrong just belt me over the head!
Eg:
If we have two coils with the same inductance etc & fed from the same power supply a 8" coil is going to have a more intense magnetic field surrounding it than say a 16" coil has so the transient spike must be affected?
It would seem as though this may be the reason why large coils won't detect small nuggets, the spike ain't big enough to induce them hence the use of smaller coils?

So one would think to look at things better we need a Test std especially if the transient spike voltage changes with different diameter coils. This way we can all look at & test the same things?

Or i may need a kick in the butt!

Or how much of a transient spike is measured from a 16" coil compared to an 8" with the same specs, inductance etc?

A bigger coil has a bigger field which means the field lines extend further from the coil before the curl region compared to a smaller coil.This is why you need bigger coils to detect larger objects at depth.However the field strength of a bigger coil (all other things being equal) is smaller than for a small coil and hence has less sensitivity to small objects.The s/n of bigger coils is also often poorer because a bigger antenna is more effected with EMI noise than a smaller antenna (coil).The ground signal amplitude from a bigger coil may also be very much larger than with a small coil because the "ground volume" under a big coil is larger than with a small coil and bigger coils (particularly mono's) are more susceptible to induced geomagnetic field noise than smaller coils.

[nick_f]

Hi Tinkerer,
The CRT monitors have a circuit with MOS-FET and a 1mH ferrite coil that generates about 90V if I remember right, at a decent power (like 50W I think).
If you say your intention is to sample during the on time and looking to use a high voltage, you should charge a larger capacitor to whatever voltage you want (even 100V or more) and connect it to the coil for a limited period of time, until the maximum current through the coil is reached.
Based on your observations (I have yet to check if they are working for me), I have some other interesting idea. There are some voltage converters for cold cathode neon tubes that are very small in size and they can generate up to 600 or 1000V. I have one at home, I had to put a few fast diodes (1N4937) in series to withstand the voltage. I think it sort of doesn't matter if you monitor the target response during the fall of the flyback pulse or the raise of the flyback pulse (as long as there is a fast change in the magnetic field). When the coil is not energised, you can apply a high voltage (let's say 400V...1000V), that will create a fast raising magnetic field. If we can maintain the magnetic field constant in the coil for 10us, I think we can sample at any period of time that doesn't exceed that interval. Which is the opposite of the standard PI circuit and in this case, it wouldn't matter about the falling part pulse. I have to do some simulations. And a coil with a low inductance will raise the current (and magnetic field) much faster than a large inductance. A balanced receiver coil with many turns could check for the target.

Regards,
Nicolae

[Tinkerer]

LONG OR SHORT TX PULSE

Long or short TX pulse.
Does it make a difference?
Do the eddy currents caused by the ON transient diminish the response of the OFF transient?
I set up the following experiment:
TX coil = 325uH, 280mm diameter, 2.8 Ohm DC resistance for a TC of 116uS
RX coil = 300uH, 140mm diameter
Test # 1, coil current of 1.65A, using 8V for 100uS TX
Test # 2, coil current of 1.65A, using 15V for 43uS TX
The samples were taken at 2.5uS after switch OFF
The total gain is 69 by the preamp (OP37) single stage.
The output of the preamp was held in a Sample & Hold (NE5537) to facilitate the reading.
The targets were placed on top of a plastic box at a distance of 150mm from the center of the coil.
The targets were chosen to provide a wide variety of TC, from 5uS to about 200uS
# 1 -- ½” square alu foil with a TC of 5us Results: Test # 1 = 1mV Test # 2 = 1mV
# 2 – 1” square alu foil with a TC of 10uS = 3mV = 3mV
# 3 – US$ 5c coin, with a TC of 20uS = 7mV = 7mV
# 4 – US$ 1c coin, with a TC of 70uS = 6mV = 6mV
# 5 – US$ 10c coin, with an estimated TC of 100uS = 5mV = 5mV
# 6 _ US$ 25c coin with an estimated TC of 200uS = 14mV = 14mV
If the Eddy currents generated by the ON transient have any influence on the signal amplitude of the Eddy currents generated by the OFF transient, the difference would be most visible on the targets with long TC’s.
If the amplitude of the OFF transient Eddy currents is influenced by the TX pulse time relative to the TC of the target, this should be most visible for the US$ 1c, with a TC of around 70uS and being pulsed with 43uS or about 60% and 100uS or about 143% TC
Results:
There is no difference between the results. The only difference was in the noise level. With 15V – 42uS TX, the noise level is considerably higher. I don’t know the reason yet. Possibly it could be that I did not sync the timing with the 60Hz power line.

Tinkerer

[Tinkerer]

So I found the source of the noise. With the higher voltage, the power supply
makes more noise.
Here is the power supply.
How can I reduce the noise?

Tinkerer