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FrozenGate by Avery

Scanned images are waving






Just because you can connect them doesn't mean you should. The only reason you should tie grounds together is if you're using the ground as a signal reference. In the case of those galvos, the ground input on the signal port of the galvo drivers serves no signal purpose as only the differential pairs are used for position input.

If you connect the grounds, all you'll end up doing is creating ground loops as the ground hops from the galvo PSU, to the galvo driver, to the signal ground, to the other galvo driver, and back to the PSU. You don't want that, as it'll soon be picking up a 50hz/60hz signal from the power mains, causing the wavy distortion that OP is seeing. Sometimes you can't avoid ground loops, but this kind of ground loop is completely avoidable and unnecessary.
 
Bionic-Badger said:
Just because you can connect them doesn't mean you should. The only reason you should tie grounds together is if you're using the ground as a signal reference. In the case of those galvos, the ground input on the signal port of the galvo drivers serves no signal purpose as only the differential pairs are used for position input.

If you connect the grounds, all you'll end up doing is creating ground loops as the ground hops from the galvo PSU, to the galvo driver, to the signal ground, to the other galvo driver, and back to the PSU. You don't want that, as it'll soon be picking up a 50hz/60hz signal from the power mains, causing the wavy distortion that OP is seeing. Sometimes you can't avoid ground loops, but this kind of ground loop is completely avoidable and unnecessary.
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ElektroFreak said:
^Listen to the man..
Click to expand...


Yea, that pretty much covered me. I had the impression that the grounds are needed. Thanks Badger, left you a +rep for that very well written comment. Not completely understandable regarding the 50/60Hz ground from the mains though, but sounds quite logical. Thanks.
 
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Well it is understandable that you'd want to tie the grounds together. Normally it's a good practice, as most of our signals are with respect to ground, and if we don't tie them together nothing works. Also, even with differential pairs of signals, it normally wouldn't hurt to tie all grounds together. The problem in this case is that the grounds are going all over the place, and we can create ground-loops. The threat of these is probably why the galvo manufacturers suggest not tying grounds together, and a reason we use differential signals.

Ground loops allow stray charge to induce differing potentials throughout the loop; they also act like loop antennas, picking sources of interference. Usually this interference manifests itself as a "hum" around 50hz or 60hz--very noticeable in audio-visual systems--caused by the 50hz/60hz AC power lines that are everywhere. Motors and sources--such as the galvos themselves--can also affect the ground loop, as they cause EM interference around them. The EM waves moving around may not seem like much, but depending on currents involved, the voltage potentials induced by the interference may have a dramatic effect--even dangerous. When the noise is also on the signal ground--such as connecting the galvo grounds to the DAC grounds--that noise can be signal-amplified as well.

Hopefully that explains some of it. It's one of those electrical non-ideal phenomena that may not be encountered much in circuits you build, but can have a big effect when it is.
 
So oddly enough, connecting the galvo modulation grounds to the same place as the laser modulation grounds got rid of the "waving". Upon simply disconnecting the galvo modulation ground wires, the "waves" were still present. It was only when I plugged them in to pin 18 with the red laser "-" modulation wire that the waves ceased (I'm pretty sure all the "-" laser modulation wires are connected to the same place). So it was a grounding issue, and you guys helped me solve it, so you're all awesome, but I'm not sure how my set up differs that makes it so that my galvos need to be grounded.

Also, I tried connecting the "-" power line for the laser drivers to the ground on the power supply (instead of the "V-" on the power supply) like the diagram DJZ posted suggested, but that simply didn't power the lasers. Hmm, I guess my power supply grounds and DB25 pin grounds are still separate. That might have something to do with it. Thanks again :beer:
 
Glad to hear you got it sorted! Those pesky ground issues can be funny things.. without actually getting my hands in your setup I can't say how yours differs from most others.
 
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I wish I had access to my small 473nm scanner setup. I would simply disconnect the galvo modulation ground wires to see if my images start waving like Scotoma's.
Glad you sorted this out.
 
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