PI active damping

[Teleno]

What signal do you use for feedback?

Integrate (output level - reference level). Adjust reference level to get the desired response in the absence of a target. Time constant must be long enough, in the order of seconds.

[green]

An alternate to constant current damping. Think might be the fastest damping possible. Coil decays at avalanche until near zero current, then switch shorts capacitor charge. Would need to be in a control loop.

[moodz]

Also known as Constant Current damping as disclosed in this patent by P. G. Moody, whose nick in this forum was "moodz" (he no longer participates).

Normally a "critical damping" resistor is used, calculated as:

At the end of the flyback the voltage induded by the target is measurable across resistor R.

Active damping replaces R by a constant current sink. This way the resistance seen by the coil higher than R at the beginning of the flyback and then much lower at the end. As a result the flyback period is shortened allowing for earlier sampling. The dI/dt is also faster inducing a higher signal in the target. Sounds like it's all benefits, doesn't it?

But... at the end of the flyback (Icoil near 0) the R of the current sink is very small. The coil is practically shorted and we cannot measure the voltage induced by the target.

Possible approaches to measure the target signal with this arrangement:

1. placing a diode between current source and the reference voltage (GND or VDD depending on the coil driver being P-MOS or N-MOS)

This classical solution increases the equivalent R as the voltage at the coil goes below 0.7 volts, causing ringing on top of the target's signal.

2. Using a separate receiving coil.

3. Measuring the end current in the coil instead of the voltage.

Let's see if we can materialize a circuit to do this.

huh ?.. the internal impedance of an ideal current sink / source is infinite ... the R does not become small and short the coil if you implement correctly.

[Teleno]

huh ?.. the internal impedance of an ideal current sink / source is infinite ... the R does not become small and short the coil if you implement correctly.

The dynamic impedance is infinite (current does not change with a voltage change), but the impedance is finite otherwise no current would flow int he "current" sink.

[moodz]

The dynamic impedance is infinite (current does not change with a voltage change), but the impedance is finite otherwise no current would flow int he "current" sink.

You are right ..... I put a diode in series with the current sink so it goes open circuit when the voltage across the coil falls below 0.6 volts ... this would normally cause unacceptable ringing ....

HOWEVER .... if the front end preamp has a response as plot below then the ringing is eliminated :-) the clue is in getting the coil to cancel its own ringing.

This preamp has a gain exceeding 50db but attenuates ringing and 0db DC gain. Its verrry niiice.

[moodz]

.... we can bust the hum also :-)

[Teleno]

You are right ..... I put a diode in series with the current sink so it goes open circuit when the voltage across the coil falls below 0.6 volts ... this would normally cause unacceptable ringing ....

HOWEVER .... if the front end preamp has a response as plot below then the ringing is eliminated :-) the clue is in getting the coil to cancel its own ringing.

This preamp has a gain exceeding 50db but attenuates ringing and 0db DC gain. Its verrry niiice.

[ATTACH]44114[/ATTACH]

I don't understand what you mean, is the ringing being cancelled by the notch response of the preamplifier, by the coil itself or both?

With the active damping method, the current sink adjusts its static resistance to the coil's voltage so that the current remains constant. As the coil voltage drops the static resistance becomes lower (overdamping), which causes the decay time constant to become longer and look like a target. Now if the current sink is set up to have minimum resistance equal to the critical resistance then you can avoid ringing at low voltages and avoid underdamping. This can be accomplished by placing a resistor in series with the current sink.

[Goaty]

Teleno
PM sent - could you please check your inbox
Thanks

[moodz]

I don't understand what you mean, is the ringing being cancelled by the notch response of the preamplifier, by the coil itself or both?

ANS: The coil itself is used to cancel the ringing ...too fiddly to match a notch response of preamp to coil ringing frequency ( would need to track phase also )

With the active damping method, the current sink adjusts its static resistance to the coil's voltage so that the current remains constant. As the coil voltage drops the static resistance becomes lower (overdamping), which causes the decay time constant to become longer and look like a target. Now if the current sink is set up to have minimum resistance equal to the critical resistance then you can avoid ringing at low voltages and avoid underdamping. This can be accomplished by placing a resistor in series with the current sinkt

ANS: The energy stored in the coil is proximate to 0.5 x L x I x I where L is inductance of coil and I is current (max) ... Energy is stored in the magnetic field during TX ON and then converted to other forms beginning at TX off. So to damp the coil what constant current can you draw from the coil for what duration of time to EXACTLY dissipate the energy stored in the coil ? This is the question that if solved perfectly damps the coil without using resistors and in minimum time.

PS ... cant see your PM in inbox .. I think its working OK.

[Teleno]

PS ... cant see your PM in inbox .. I think its working OK.

I didn't get your PM.

I understand the principle of active damping, but the devil is in the implementation details with non-perfect components. Even if you apply the correct current for the correct theoretical time, as the coil voltage approaches zero the MOSFET of the current sink enters the linear zone and it no longer behaves as sink but as a low-value resistor that causes a slow decay of the final millivolts. I don't believe the series resistor is a problem because even with constant current damping the static resistance at any goven moment has to be higher than the critical damping resistance, otherwise you'd be slowing down the decay, not accelerating it.

An advantageous alternative that's used in the Vallon is to decay the coil current as a 1/4-cosine (discharge on a capacitor). This has the advantage that the slope at the last part of the decay - the closest to sampling - is the highest, and since the signal induced in the target is proportional to this slope, the sensitivity for fast targets is better that that using a constant slope. In other words: decay is slowest at the peak voltage and fastest zero crossing, which maximizes the target signal just before sampling takes place.

[Goaty]

@ Teleno

I didn't get your PM.

Sorry, I didn't want to hijack this thread.
I have resent message - re my post in MPP topic.

[green]

I assume active damping is used to allow earlier sampling. I keep trying to get the delay as short as I can for target testing. Is there a delay time that wouldn't make sense to go below for detecting?

[moodz]

I didn't get your PM.

I understand the principle of active damping, but the devil is in the implementation details with non-perfect components. Even if you apply the correct current for the correct theoretical time, as the coil voltage approaches zero the MOSFET of the current sink enters the linear zone and it no longer behaves as sink but as a low-value resistor that causes a slow decay of the final millivolts. I don't believe the series resistor is a problem because even with constant current damping the static resistance at any goven moment has to be higher than the critical damping resistance, otherwise you'd be slowing down the decay, not accelerating it.

An advantageous alternative that's used in the Vallon is to decay the coil current as a 1/4-cosine (discharge on a capacitor). This has the advantage that the slope at the last part of the decay - the closest to sampling - is the highest, and since the signal induced in the target is proportional to this slope, the sensitivity for fast targets is better that that using a constant slope. In other words: decay is slowest at the peak voltage and fastest zero crossing, which maximizes the target signal just before sampling takes place.

...yes correct ... optimal damping requires very fast switching of the current sink ... the current sink goes open circuit when vcoil = approx 0 volts ... however there is always a small residual. However the results are spectacular .... the PIC below shows a saving of over 6 microseconds in the coil damping using an active damping circuit and ring cancellation. The peak coil current was 6.5 amps. The green trace is active damping .. the red trace is 500 ohm damping resistor for comparison.

[moodz]

I assume active damping is used to allow earlier sampling. I keep trying to get the delay as short as I can for target testing. Is there a delay time that wouldn't make sense to go below for detecting?

... there are two questions ( and many answers here ) ... First it may or may not be desirable to sample earlier ... however the second question is that for a given delay ... the more the coil has "settled" after the flyback the higher the gain that can be applied without overloading the front end preamps. ...and also keep noise out etc. A major noise source is the hot damping resistor in high gain systems for example. The bandpass response in the active damping scheme reduces hum and other magnetic noises below the TX frequency which would otherwise be sampled and produce unwanted noise in the detection band.

So there are some improvements provided by active damping in earlier sampling but I have found you can use much higher gains in the frontend prior to sampling which greatly improves the sensitivity of the PI detector for a given coil size.

[green]

...yes correct ... optimal damping requires very fast switching of the current sink ... the current sink goes open circuit when vcoil = approx 0 volts ... however there is always a small residual. However the results are spectacular .... the PIC below shows a saving of over 6 microseconds in the coil damping using an active damping circuit and ring cancellation. The peak coil current was 6.5 amps. The green trace is active damping .. the red trace is 500 ohm damping resistor for comparison.

[ATTACH]44133[/ATTACH]

I've been using I/T=E/L to calculate current decay. Your graph shows infinite change, wondering how that can be.