PI: Effects of pulse shape and transmit time.

[Teleno]

Right.. I see your point. But, with PI detectors only a short portion of the decay at the very end is sampled. TX on time or ramp doesn't matter..

The targets are roughly equivalent to LR circuits. Diferent targets hace different time constants. As the magnetic field turns-on eddy currents are induced in the target. If the field is switched off before the turn-on eddy has had time to decay, both eddy's cancel each other and you dn't get a target response. So the TX time can be used as target filter, shorter TX leaving larger targtes non-responsive. My aim is to detect small gold nuggets, which have shorter decays while avoiding magnetic ground that has longer decay. So I thought making the TX time as short as possible can help... but the turn-on ramp was in the way and I have fixed the problem.

[dfbowers]

Right.. I see your point. But, with PI detectors only a short portion of the decay at the very end is sampled. TX on time or ramp doesn't matter.. so unless you do some sort of VFL/PI hybrid with an IB coil it should not make any difference. Also, peak current will be limited by wire resistance anyway. Or are you suggesting multi period sensing?

[Teleno]

Instantaneous current across the coil would be more like this:
Don

That applies to the turn-off. The tun-on is more linear and is ramp-shaped because it occurs at a constant voltage: V/L * t; if you neglect the coil's resistance. The damping resistor in parallel has no effect on turn-on.

I can go around this and do a sharp turn-on. This allows shorter pulses witout decreasing the peak current, but I don't know if it's a desirable feature or not.

I would suggest building the Hammerhead. Perfect learning platform for PIs! You can learn first hand what adjusting pulse width can achieve.
Instantaneous current across the coil would be more like this:
Don

I apperciate the suggestion, but Hammerhead and all PI's I know use a trapezoidal/triangular current waveform (turn-on is a ramp) with the problem of limiting the peak current available for very short pulses.

My approach allows the peak current to be the same regardless of pulse width. My question: is a short, square pulse at peakcurrent a desirable feature or not? would it improve PI performance for certain applications?

[dfbowers]

I would suggest building the Hammerhead. Perfect learning platform for PIs! You can learn first hand what adjusting pulse width can achieve.
Instantaneous current across the coil would be more like this:
Don

[Teleno]

The transmit current in current PI design have either a triangular or a trapezoidal shape due to the slow rise time of the inductance (V/L * t).

For a given coil, as the TX pulse gets shorter (T1 ... T5), the maximum achievable value of the TX current decreases like this:

Now I've designed a circuit capable of generating current pulses of a rectangular shape down to microsecond scale, like this:



Are there any advantages in a short, rectangular current pulse? for example: ground rejection, small object detection (large object rejection), or anything else?

Would it be worth the effort to go for such a design or is it just a waste of components and time?