BIPOLAR ALTERNATIVE TO H BRIDGE - GENERATING SINE WAVEFORMS - HALF / SINGLE & FULL

[Carl-NC]

The eddies in the target are proportional to the derivative of the Tx current, therefore a linear ramp causes a constant offset eddy current in tge target. The signal at the Rx (assuming a balanced coil) would be the derivative of the target eddy currents, therefore the constant eddy causes zero offset in the Rx coil.

With a properly balanced Rx coil the Tx ramp should have little to no effect. I believe extreme efforts to minimize the ramp won't pay off, better to spend the energy in balancing the coil.

Ummm, no. A linear ramp in the TX current causes a constant induced EMF in the target, which creates an exponentially rising eddy current. The signal at the RX coil is a derivative of the rising exponential, which is a spike followed by a falling exponential. In a standard PI the TX turn-on creates a negative RX exponential which, if it has not decayed to zero, will subtract from the flyback's positive RX exponential and reduce sensitivity.

Dean, your question specifically refers to the TX pulse width of a monopolar PI in a thread that's all about bipolar PI, mainly constant current & half-sine. Mixing the different technologies can be confusing as the results can often look radically different. Here is a thread that discusses the target responses to a whole bunch of TX waveforms:

Optimizing Target Responses

I think post #12 might answer your question. Feel free to post further questions about monopolar pulsing to that thread to minimize confusion here.

[KingJL]

I have been playing with the Moodz Alt2H-Bridge for a while now. The PCB that I am using uses SMD components (except for the coil connector and a 100K KEMET potentiometer). If anyone is interested, I am posting the schematic and gerbers...

If anyone is interested, I have 2 extra bare PCB's of the original Moodz alt2h-bridge that I have been using for about 4 months. This version does not have the half-sign capacitor or damping network on board... have to add externally for experimenting with half-sign.


After the PCB is populated:


Ref post #11 of this thread for schematic.

[Tinkerer]

Targets
L2 is the RX coil is its simplest form, concentric about half the diameter of the TX coil
R3 is the damping of the RX coil
R12 limits the current to the input of the preamp
D1,D2, protect the input of the preamp

L3,L4,L5,L6,L7, represent targets with TC's from Ius to 100us.
A nickel, (US$ 0.05) is a common target with a TC of 10us
A very small gold nugget could have a TC of 1us
The (transformer) coupling of the TX coil L1 with the targets is exaggerated to make it easier to see.
The picture shows the current wave form of L1 in red
The eddy currents in the targets L3,L4 in green and blue

[Tinkerer]

Hi Tinkerer,
Thank you for sharing your circuit. Just wondering how you are connecting in the different coil + snubber options which you have labelled with gold and nickel for example..? I gather these are different RX coil options which you are simulating is that correct..?

[Teleno]

Yes continuous bi-polar circuit...could you clarify please " you won't be measuring any target voltage on the Tx coil."..cheers

To put it differently, the Tx coil is always transmitting. In the high current state is equivalent to a conductor shorted to ground. Think of a cable shield, any induced signal find a a small resistance path to ground and therefore the induced voltage is minimal.

In ioder to acquire the target you need an Rx coil connected to a damping resistor where the only current woud be the tiny one induced by the target and the voltage would be the current multiplied by the resistance.

[Teleno]

Yes continuous bi-polar circuit...could you clarify please " you won't be measuring any target voltage on the Tx coil."..cheers

The Tx coil is always connected to a low impedance source, conducting high current. You can only sense the target induced voltage when the Tx coil is connected to high impedance, but this never happens in a continuous bipolar circuit,

[Dean Sarelius]

Yes continuous bi-polar circuit...could you clarify please " you won't be measuring any target voltage on the Tx coil."..cheers

[Teleno]

As per the usual PI design samples will be taken after the flyback pulse but what I am trying to understand is if the initial charge to the coil before the flyback pulse if this will have any de-sensing effect on the target..?

Are we talking about a continuous bipolar circuit? In that case, each flyback pulse is immediately followed by a new low impedance high current condition, you won't be measuring any target voltage on the Tx coil.

[Dean Sarelius]

Hi Tinkerer,
Thank you for sharing your circuit. Just wondering how you are connecting in the different coil + snubber options which you have labelled with gold and nickel for example..? I gather these are different RX coil options which you are simulating is that correct..?

[Dean Sarelius]

As per the usual PI design samples will be taken after the flyback pulse but what I am trying to understand is if the initial charge to the coil before the flyback pulse if this will have any de-sensing effect on the target..?

[Teleno]

OK I should have mentioned that I was referring to a mono coil. Would the linear ramp cause a target offset if I were using a mono coil..?

You can't use a mono coil because the Tx coil is always carrying high current, in other words, it's connected to a very low impedance at all times.The target signal can't change the voltage across the Tx coil in these conditions, there's nothing to be measured. You forcibly need an Rx coil.

[Tinkerer]

Here we see the switching.
The red trace is the current in L1
The blue trace is the voltage in C1
The current starts with about +500mA. When the Mosfet is switched off, the coil L1, discharges into the capacitor C1.
When the current in L1 reaches 0A, the Voltage in C1 is at its peak.
The capacitor C1 immediately discharges into the coil in the opposite direction.
The capacitor C1, (blue trace) is discharged when the coil current reaches about -500mA

In the next session we will look at the eddy currents in the different targets.

[Dean Sarelius]

OK I should have mentioned that I was referring to a mono coil. Would the linear ramp cause a target offset if I were using a mono coil..?

[Teleno]

From what I have been reading in this thread my understanding is that the EMF caused by the coil charge ramp current actually influences the target response sensitivity. Referring to the SD2000 timing below of 240us it appears that they are attempting to chrage up the coil as slowly as possible to minimise this effect would you agree..? So the approach should be to apply a low noise current source to control the rate of coil charge current in order to minimise the negative target offset influence. Or am I completely wrong in that a shorter ramp up would be less likely to create a negative influence on the target response..?

The eddies in the target are proportional to the derivative of the Tx current, therefore a linear ramp causes a constant offset eddy current in tge target. The signal at the Rx (assuming a balanced coil) would be the derivative of the target eddy currents, therefore the constant eddy causes zero offset in the Rx coil.

With a properly balanced Rx coil the Tx ramp should have little to no effect. I believe extreme efforts to minimize the ramp won't pay off, better to spend the energy in balancing the coil.

[Dean Sarelius]

From what I have been reading in this thread my understanding is that the EMF caused by the coil charge ramp current actually influences the target response sensitivity. Referring to the SD2000 timing below of 240us it appears that they are attempting to chrage up the coil as slowly as possible to minimise this effect would you agree..? So the approach should be to apply a low noise current source to control the rate of coil charge current in order to minimise the negative target offset influence. Or am I completely wrong in that a shorter ramp up would be less likely to create a negative influence on the target response..?