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Posting for attachment. Note to you young-ins, this only works for galvos with Torsion Bars, Ie GSI open loop scanners such as G330 or G124.
This derives it's Back EMF feedback from the difference voltage between the purely resistive network and the inductive load from the Galvo. Aka a Bridge Circuit.
The value of "C3" is critical in the design of the bridge. Terminals CDJ go to the external DC offset potentiometer. A value of 1OK Ohms is just fine for that part.
Steve
Open Loop Sample Back EMF amplifier, fleshed out on a sample design handed out by GSI to "select" customers who got the large brochure, not the data sheet.
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Known informally as the " RoC = L" Back EMF amplifier , from the title line in the schematic. R and C are two components in the circuit, L is the inductance of the Galvo coil. Please note again, this only works with galvo scanners and actuators with Torsion Bars..
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Posted for another fellow who has a pile of G124s. As it is set up for G115 in this version, he'll have to alter R or C a bit.
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GSI would never suggest parts values for R and C, leaving that as "An Exercise for the Reader".
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A TL082 would drop right in if you do not have the Raytheon or Maxim part.
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I do believe this edition came from either Laserist or PhotonBeam.
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Steve
That is my drawing from the early '80s based on GS brochure info and I believe I've posted it before. Photonbeam could have as well as I probably gave him a copy. I still have this original.Quote:
Originally Posted by mixedgas
I can attest that this really peps up the step response for the G-series with decent damping characteristics. The key is to not drive the galvos beyond half their maximum angle of scan in order to get the full benefit from the Back EMF feedback.
I still have at least 4 DIY circuit boards I made back then for my G-115 scanner pairs. Here is a photo of one axis driver I used in one of my disco projectors. A TLO84 would be an excellent alternative to the 4136 in my drawing.
Attachment 57520
Thank you for owing up to your copyright!
It had been posted here before, so I snagged it. Right now that is helping a long time retired laserist have fun with eight 124s he just received.
I used to have the GSI brochure/ book but I cant find it in the piling system.
So I hope you don't mind if I reposted it.
Thanks,
Steve
I'm glad to hear that it is helping your friend and hopefully others will find benefit from it, too. I'll see if I can find my GSI source...and I did. Here it the 2 1/2 page Series G Scanner brochure with the general schematic mine is based upon:Quote:
Originally Posted by mixedgas
Attachment 57537
Attachment 57538
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Attachment 57540
I'm curious how these amps would work on some Stocker Yale R4-077's. I have an old XYP-1000 missing a scan head and was thinking of putting my R4-077's to use as a replacement.
I know nothing about your Stocker Yale's and can only suggest you compare the GS G-Series specs in the scanned brochure pages I attached. If their coil characteristics are similar then it might work for you. (today, I'd use a modified push-pull driver output section but the one shown in my schematic works like a champ).Quote:
Originally Posted by absolom7691
Not exactly but workable.
Attachment 57541
Thanks. So, your galvo coils are 15ohm each and wired in parallel would be close to 8 ohm. Your galvo coil's inductance is 5 times larger, however. You would have to play with the RC values to tweak it, but workable.Quote:
Originally Posted by absolom7691
Does anyone out there have a photo of the old GS A-102 amp card?
I need to see the parts layout- going to build a couple
Thanks,
Jay
Yes, I do, and will post shortly.Quote:
Originally Posted by glorocks
(Oops, sorry. I thought I did but only have their A-600 driver board.)
If interested, here is my version of an open-loop drive amp that I used on the GS-115 galvos back in 1977-1979 using the single uA791 op-amp driver. This could easily be modified to be a non-inverting version and/or differential balanced input version. I used 7812 abd 7912 1amp voltage regulators. The nice thing about the 791 op-amp driver was that its output was current limiting to 1 amp, bipolar and therefore protected the galvo load. I used 4 pairs of these drivers on my 1st RYGB projector. Never had a failure.
If you need to drive heavier current loads like for the GS-300 series or similar you can use the 791 as the pre-driver to an differential output using two 3055 transistor drivers in a push-pull arrangement and using + and - 24vdc supply rails on the 3055's.
Attachment 58157
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Edit: Dec 14, 2021
I found in one my old TV interviews open-loop scanner examples of the resonant frequency ringing in the General Scanning G-115 scanner I used. I clipped a few frames from the video to show as examples.
I'm fairly certain (after 41 years) the first two images show a 45Hz square wave (1st harmonic signal) with 90 degree XY phase shift, the second just slightly phase-shifted from the first.
Attachment 58558 Attachment 58559
The third image shows a 90Hz square wave (2nd harmonic) with a 90 degree XY phase shift.
Attachment 58560
I still intend to get these old scanners hooked up to a pair of u791 opamp drivers shown earlier in this post and take some videos to post here that shows the "magic" of the ringing at or near the scanner's open-loop resonant frequency.
Thanks for looking-
Can anyone else help?
I built a DIY A-102 drive amp based on GS's schematic in their GS-100 series open-loop galvos, shown here that uses back EMF feedback to improve step response:Quote:
Originally Posted by glorocks
Attachment 58159
Unless you are just wanting an exact duplicate of their original A-102 drive amp, layout isn't that critical. My version is shown here, front and back:
Attachment 58160
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My DIY A-102 schematic shown here:
Attachment 58162
The trade-off for improved, more critically damped step response is reduction in maximum rotor deflection angle (about half). Overall it is an impressive improvement given this downside.
(dang, should have looked at the first pages of this thread...sorry for the duplications...doah!)