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

[KingJL]

I just upgraded the schematic and gerbers to make it easier to accommodate Half-Sine operation. My original board was mainly focused on CC operation. Added the half sine coil capacitor and damping network to the board along with 2 jumpers to allow switching between HS or CC mode. For HS operation shunt J104 and remove shunt from J!05... For CC operation, remove shunt from J104 and shunt J105.

I just ordered a set of 5 boards from JLCPCB for $3.33 (would have been $2.00, but I opted for lead-free HASL) plus a $3.93 shipping charge.


TX.pdf
Moodz_TX(HS)_Gerbers(1-13-2023).zip

[KingJL]

Bom for the Moodz_TX(HS):
TX_BOM.pdf

[Tinkerer]

I'm getting confused over which system you're describing. Here are the TX current & target EMF for CCPI:



The target EMF comprises short impulses that will depend on how the kick-start is done but will usually be close to half-sine voltages. Because they are rapid the target eddies have little time to build up so the eddy responses are likely close to a t*e^-t/τ. A low TC target will get a higher initial eddy current but decays faster, while a high TC target will decay slower but with a much lower peak.



The half sine TX and target EMF look like this:



To imagine the target responses start with a continuous sinusoidal response and simply truncate it. Here is a set of plots for some targets at 10kHz. The gray dashed lines exactly frame a half-sine of the current, and the plots to the right are all the responses cropped to these lines.



But this does not account for the exponential turn-on (or turn-off) inertia caused by the tau of the target. Suppose we take the US nickel; its tau is ~10us which means that its 5*tau settling is about the same as the half-sine pulse width of 50us, assuming a half-sine "frequency" of 10kHz. The total response looks like this (ideal truncated in gray):



It is still very much a VLF-ish response during the TX time (from which you can literally extract a target phase), and you also get a separate PI response during the off time.

Both approaches give very interesting but very different results.

Sorry for the delay, I was travelling.
My explanations are probably not very clear, but I think the subject is important enough to be worthy of detailed description and full understanding for anybody wanting to be involved in metal detector design. In my case, I am mostly interested in bipolar, square wave, continuous current PI. Therefore I will make another try, including simulations, to explain how I think this type of TX works and how it affects the targets and the receive circuit.
Probably the best way is a discussion. This will help to clarify and if needed to point out any possible mistakes in my thinking.

[Tinkerer]

Here is a simulation of Moodz's excellent Square current wave TX.
Next we will look at the details and the induced currents in the targets.

If anybody has questions about the simulation, please ask.

[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..?

[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]

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..?

[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.

[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.

[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..?

[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..?

[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]

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

[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,

[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.