PT scanner question

[dnar]

I have 2 sats with PT-20 scanners (fine for beam shows) and I note the scanners run quite warm when idle.

I suspected they are being driven to the center home position, so I unplugged the cables between amps and scanners to check, the mirrors are now off home and the projected beams at extreme top-left of my usual window.

Question: do these scanners have a spring or torsion bar to home them when unpowered? If so, I suspect it might be worth mechanically centering both galvos in the block while unpowered, and then finally adjust the electrical center with the position trim pots.

If my theory is correct I will be correcting two issues, 1) the scanners will operate in a more natural +/- range and 2) the idle power/heat will be reduced as the coils wont have a constant DC bias to hold them centered.

Thanks in advance.

[absolom7691]

There is no zeroing spring. There is a compression spring on the lower bearing to keep the entire rotor assembly pressed against the top of the unit. That is about it.

[dnar]

Ok, so I guess my next question is this. Is there any benefit in removing any DC offset at the scanner coil when idle and then re-positioning mechanically to suit? I am assuming there will be a benefit simply to reduce idle heat and balance the deflection drive.

[The_Doctor]

I delayed trying to answer because there's too much I don't know, but one thing suggests itself... Is there any sign of very high frequency oscillation? As far as I know in a scanner with no torsion bar, any rest position should be possible within positional feedback range. Like CMOS (bad analogy alert), only when switching states is significant power used. Even if two voltages are 'fighting' each other for positional control, the current gain stage should only be seeing the resulting voltage. So if it's getting hot when idle, perhaps the energy is alternating too fast to easily detect.

[dnar]

Good point, more information required, let me check.

/whips out the scope to investigate further.

So from the replies above, I would assume there should be no drive at idle, as drive is only required to make a movement, as determined by the positional feedback loop.

[The_Doctor]

As far as I know, yes.. Which makes the top-left parking thing interesting. Maybe not related to the heat thing, but it may be worth looking for pull-up or pull-down on the amp inputs. And if the inputs are balanced, that doesn't make the picture any clearer.

On my WideMove scanners, they'll park where told with no heat lost, but sometimes throw the mirror angles wide when power is lost (one of many reasons for scanfail boards...). And sometimes it leaves them parked in a centred position. There is a slight magnetic detent at that position even without power. Actually a sort of inverse detent, metastable. A slight manual movement either side will cause the angle to 'fall' wide, but there is also a very slight positive detent at peak centre that is stable enough to keep them that way against vibration. In other words, there may be some mechanical complexity involved in the 'decisions' torsion-bar-less scanners make when parking mirrors. I don't know if any of this is useful info but it may be that playing with power switching, and grounding inputs, might reveal useful info.

Steve Roberts (mixedgas) will likely have much more precise, less vague ideas about this.

[mixedgas]

Dnar,

Enjoy the pdf here. Unless its capacitive sensing, nearly all position sensors use a variation on this circuit:

http://www.photonlexicon.com/forums/...3&d=1383206690

You can find out what is going on there, and and if there is a series resistor (usually 0.1 or 1 ohm) sensing the current and back EMF from the galvo.


Steve