Re: Poor guiding with GM8
Alex,
You may benefit by using a run out gauge on your rotating parts. Basically this is a fixed needle point which rides right along the spinning surface and will tell you how far out of round you are. The base of the gauge clamps to some fixed surface which is not moving. Then you should be able to see any variations within 0.001 inches or more. These are common in auto shops and are used when testing brake rotors to see if they are warped out of plane, or are improperly mounted. Perhaps there are higher precision gauges which can better measure the variations in your mount machinery.
The finish of the worm has been discussed greatly here in the past. Each new worm is not perfectly identical and has a certain surface profile on the gear tooth faces. How to best break one in and get the best performance may have to do with wearing it it over time, though some have tried various polishing techniques and other methods. If you think your old ones are damaged you can order new, but don't expect super performance just because it is new. It may take a bit of use and adjustments over time to get it to perform to its maximum.?
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Hi
I have just acquired the OPW amd there exists a hole. Does anyone have purchased it recently? Is it for a spring mechanism??
Thanks
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Re: Poor guiding with GM8
Guilherme,
Could you please share your guide log with guiding disabled or graphs, similar to what I posted?
What kind of worm do you have - is it brass or steel?
Alex
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What result I can get with the current OPW in my old G11?
Hello all!
I live in Brazil and I have a G11 for about 10 years and I never could get really good tracking.?
I finally purchased the OPW (after a long wait) for the RA axis and the new "high precision" worm for the DEC axis.
I would like to hear from members here what result I can wait in terms of PE and guided RMS erros after installing the OPW?
Also does anyone can recommend some instructions on how to install it?
Thanks you so much!
Andre
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Re: Poor guiding with GM8
I use direct drive with my GM8, see in the showcase page. It works just fine with a 400 steps NEMA17 motor, with slews up to 3.5 degrees/s. I also use a spiral coupler instead of the oldham model that came with it.?
The problem with PE is still bad in my case though. I believe it has more to do with the old worm. I recently acquired an OPW with a new high precision worm for the GM8, and will compare the PE before/after.
Guilherme
On Wed, Jul 28, 2021 at 5:09 PM Alexander Varakin < avarakin@...> wrote: Pete,
I also have G11 and I am using Onstep with the direct drive there too.
Unfortunately, GM8 has only 180 teeth on the ring gear vs 360 for G11, so I believe that doing a direct drive for GM8 would be pushing it.
In any case, I did try direct drive and I still saw bad PE.
Let us know how your mod goes, I was wondering about using the Oldham too. I am currently using the spiral coupler, I believe that most folks do this as well.
Alan,
I will check it, but I think it is optical illusion.
John,
Good point, I will try to loosen up the belt a little.
Michael,
Good point too. Is there an article somewhere, describing how to align worm against the ring?
I am reading this forum on a regular basis and did not see a discussion on this subject.
All,
How consistent is the quality of the brass worms? Does it make sense to get a new one?
I guess I can try to pull one from my G11...
Alex
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Re: Poor guiding with GM8
Pete,
I also have G11 and I am using Onstep with the direct drive there too.
Unfortunately, GM8 has only 180 teeth on the ring gear vs 360 for G11, so I believe that doing a direct drive for GM8 would be pushing it.
In any case, I did try direct drive and I still saw bad PE.
Let us know how your mod goes, I was wondering about using the Oldham too. I am currently using the spiral coupler, I believe that most folks do this as well.
Alan,
I will check it, but I think it is optical illusion.
John,
Good point, I will try to loosen up the belt a little.
Michael,
Good point too. Is there an article somewhere, describing how to align worm against the ring?
I am reading this forum on a regular basis and did not see a discussion on this subject.
All,
How consistent is the quality of the brass worms? Does it make sense to get a new one?
I guess I can try to pull one from my G11...
Alex
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Re: Poor guiding with GM8
Alan,? By "main", are you referring to the pulley on the worm?? Given the 480 second PE this seems likely.??
If so, Alexander should check that this pulley is firmly located and tight on the shaft.? There is not much room to make the set screws in the pulley engage with the shaft.
I'm doing an OnStep install on my CG11 and I opted not to do what Alexander has done and use a belt drive.? Instead I am planning to use two new Oldham Couplers to fit the worm shaft in it original position and drill out the 3/16" end of the coupler by 0.235mm to 5mm and then directly mount a 0.9 degree step Nema 17 to that thus avoiding belt drives.? This will be my third OnStep conversion and I hope the best.
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Re: Poor guiding with GM8
Your main pulley is off-center, which gives a 480 s periodic error.
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
The full gory details of how the position counter value is determined are in Microchip document 70208C.pdf (¡°Quadrature Encoder Interface¡±): ?
? David ?
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From: [email protected] <[email protected]> On Behalf Of Paul Kanevsky
Sent: 27 July 2021 20:42
To: [email protected]
Subject: Re: [Losmandy_users_io] Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this? ? Peter,
Entirely possible and even likely. I'm not arguing with your results. All I was trying to do was to correct some information that was posted earlier on the encoder precision. Gemini PIC controller reads encoders in quadrature, and so the effective resolution is 4x smaller than the (Gemini-addressable) step size.
Regards,
? ? ? -Paul
On Tue, Jul 27, 2021 at 03:32 PM, <pcboreland@...> wrote: Paul,
This is where people are getting confused I think.? The positioning step accuracy is 0.5625". Therefore if you what to make a Dec correction it will be that large. You can not move the motor a 1/4 step.? ?If you install a 1024 encoder you can make a 0.1405" step move. The results are night and day as far as Dec guiding performance. In the chart just posted Dec rms was 0.15", about the same size as the step resolution. There is no point fighting the facts, a higher res encoder will greatly improve your Dec guiding.?
Peter
PS. Before this change by Dec guided at about 0.74" rms.
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Re: Poor guiding with GM8
Alex,
Quite an interesting pic showing your design. If there was ever an arrangement that should remove all gear slop from the stock mounts I would think it would look just like this. Since your deviations are multiples of the worm period, some kind of PEC correction curve should be of benefit. But how to program one into on OnStep I have no idea. You know the brass worms can be bent under the right amount of force, perhaps from your belt tension? If you place a worm in a cordless drill at low speed and stare down the end of the shaft, you should be able to see if the worm end spins in a tight circle, or starts to oscillate out of round. Or some other round piece is doing an ellipse, not a circle.?
You are the trendsetter here, and I certainly hope you can help with a future design model.
Good luck!
John
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Re: Poor guiding with GM8
Alex,
That's a very nice system.
But it is so different from anything "stock" that I'm at a complete loss to advise you.
Your system drive seems sensible, and as the oscillations are at the worm period, so they could be related to anything like the pulley on the worm or anything the worm is driving.? It cannot be anything related to the geared down (by the pulley ratio) in front of the worm.? Those would be at much higher frequency = shorter period.?
Definitely examine the worm center line height vs the ring gear center height.? ?
Best of luck,
Michael
On Tue, Jul 27, 2021, 6:52 PM Alexander Varakin < avarakin@...> wrote: Hi Michael,
Many thanks for the detailed reply.?
Here is my mechanical setup - I have the reverse worm setup with time gear attached directly to the shaft of the worm. The larger gear is 60 and the smaller is 16.
The aluminum parts form an arrangement, which works as OPW. I drilled and tapped the holes in the bearing blocks in order to make the whole thing as one piece.
So I don't have an Oldham coupler and there is no possibility of misaligning the motor and worm shafts.
I have ABEC 7 bearings and I also tried the Belleville spring, it did not help.
I also tried swapping the worms, looks like it does not help, based on few minutes of testing, before clouds rolled in.?
I have brass worms. I wonder if I should buy new worms.
Right now I am running out of ideas...
Alex
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Re: Poor guiding with GM8
Hi Michael,
Many thanks for the detailed reply.?
Here is my mechanical setup - I have the reverse worm setup with time gear attached directly to the shaft of the worm. The larger gear is 60 and the smaller is 16.
The aluminum parts form an arrangement, which works as OPW. I drilled and tapped the holes in the bearing blocks in order to make the whole thing as one piece.
So I don't have an Oldham coupler and there is no possibility of misaligning the motor and worm shafts.
I have ABEC 7 bearings and I also tried the Belleville spring, it did not help.
I also tried swapping the worms, looks like it does not help, based on few minutes of testing, before clouds rolled in.?
I have brass worms. I wonder if I should buy new worms.
Right now I am running out of ideas...
Alex
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Re: At some point, I may begin commenting on the G11 DEC axis...
Allan,
The Ra encoder upgrade was a bust I feel. I would not bother. While testing was limited, I did not see a significant improvement. I think it's because you are either slowing down or speeding up what is a continuously moving axis. Better to get? rid of the large period errors natively if possible. Funny thing is my Ra guiding seemed to improve as my Dec guiding improved. I thought that they were independent things?? Now my Dec is so well behaved, my Ra axis, seems to have gotten the hint to get better too?
The reason to upgrade the thrust bearing is not about smoothness parse. If you see a large low frequency spike which for me was around 2600s, I much reduced it with a German made bearing.? If you do not have this error? that's great. The other thing was worm mesh alignment, which proved to be an issue for me, and Michael Herman too.
Peter
Peter
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
Paul,
I appreciate that, but it's kind of irrelevant, and confuses most people into thinking you are controlling the motor at that resolution. Frankly, I can not recommend enough that Losmandy owners upgrade their Dec encoders and see what results they get. It's such a simple mod. Moreover, people should zero out their backlash. The Allen wrench method is balls on accurate. Zeroing backlash is what 10 micron mount do in their software.? If you change the slew setting to 1200 you can also move just as fast. Sticking with the 25:1 gearboxes make that possible.
?
Upgrading the Ra encoder was a bust. Folks should focus instead on elimination the large period errors. (80s (OPW), 240s and 2600s.? I've shone a light onto those problems and how I dealt with them. If you follow the steps I took you will have no use for PEC. The gearbox upgrade and thrust bearing upgrade are a must if you want a calm Ra guiding curve.? ?
All the best,
Peter
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
Peter,
Entirely possible and even likely. I'm not arguing with your results. All I was trying to do was to correct some information that was posted earlier on the encoder precision. Gemini PIC controller reads encoders in quadrature, and so the effective resolution is 4x smaller than the (Gemini-addressable) step size.
Regards,
? ? ? -Paul
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Show quoted text
On Tue, Jul 27, 2021 at 03:32 PM, <pcboreland@...> wrote:
Paul,
This is where people are getting confused I think.? The positioning step accuracy is 0.5625". Therefore if you what to make a Dec correction it will be that large. You can not move the motor a 1/4 step.? ?If you install a 1024 encoder you can make a 0.1405" step move. The results are night and day as far as Dec guiding performance. In the chart just posted Dec rms was 0.15", about the same size as the step resolution. There is no point fighting the facts, a higher res encoder will greatly improve your Dec guiding.?
Peter
PS. Before this change by Dec guided at about 0.74" rms.
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
Paul,
This is where people are getting confused I think.? The positioning step accuracy is 0.5625". Therefore if you what to make a Dec correction it will be that large. You can not move the motor a 1/4 step.? ?If you install a 1024 encoder you can make a 0.1405" step move. The results are night and day as far as Dec guiding performance. In the chart just posted Dec rms was 0.15", about the same size as the step resolution. There is no point fighting the facts, a higher res encoder will greatly improve your Dec guiding.?
Peter
PS. Before this change by Dec guided at about 0.74" rms.
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.This seems rather coarse.?
This is a single step addressable by the Gemini controller. The actual accuracy of the encoder reading is 4x the step or 0.1406" and even smaller for Titan. Regards, ? ? -Paul
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Re: Poor guiding with GM8
Alex,
Your frequency chart shows the 480 sec GM8 worm period is the biggest frequency, and the 240 sec period (occuring twice per worm period) is the second highest peak.? Your tracking chart shows a lot of high frequency noise too.
Your scope weight is not the big problem, but it is a long focal length so this magnifies any mount stability problems.
The tracking chart seems to show the DEC is pretty smooth, but the RA has all the oscillation and noise.
If the problem were purely the RA worm, I think you would likely see a smoother repeatable RA curve.? The fact that is is jumping around seems to me likely you have your RA gearbox drive shaft not well enough lined up with the worm drive shaft.? So I think your Oldham coupler white center part is jumping around.? Ideally when you are rotating the axis by the motor, you want to see the white center of the coupler just rotate, not shift up or down or in or out, no movement perpendicular to the rotation axis.
There are only a few parts in the drive train of the old mount design, which I prefer for that very reason.? It is easier to perfect the few drive parts.??
However, you have a belt drive, not the Gemini servo motor and gearbox, so you must be careful that the belt pulleys on both the drive (stepper motor, gearbox?) and the worm drive (a second axle like the tucked motors?) are in line so the belt won't snag, and the teeth are going into the pulleys smoothly.? (The Gm8 and G11 also could have come with the Hurst stepper motor drives too and a 492 unit electronic drive for that. )?
So there are other things to check not just the Oldham coupler.
Here are a few PDFs on related areas for mount improvement.? I did get my own GM8 down to about 1.5 arcsec RMS PE, and I was using it for deep sky imaging with an Orion EON 130mm triplet, also similar in weight (22.5 lbs)? to your scope, but a longer FL of 910mm (f/7). That PE report is attached.? I used a Losmandy GM8 OPW and better RA wirm bearings, and a Belleville R4 size spring in each axis...see the attached PDF ("Improving...")? for details on those.??
Have fun, Michael?
On Mon, Jul 26, 2021, 9:02 PM Alexander Varakin < avarakin@...> wrote: I tried swapping the worm gear without much luck. Should I try to swap worm?
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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?
Peter:? since you're doing a deep dive, might be interesting to check the orthogonality of the axis shafts to their top plate. I had an issue in mine leading to some stiff spots when rotating the axis by hand (similar to what one might feel with a wonky needle bearing).? See this thread if interested:?? /g/Losmandy_users/message/59944Keith
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Re: At some point, I may begin commenting on the G11 DEC axis...
Do you have plans to upgrade the optical encoder to 1024. I think you will see a very noticeable benefit. It's a $65 investment and you do not have to remove anything from the drive other than the motor end plate can do everything insitu.? It would be nice to have a second reference point to go by. With a minimum step size of 0.56 arc-sec and I would imagine longer settling times due to the weight, having the ability to make smaller 0.14 arc-sec steps would be beneficial. I've ended up with a min step size of 0.05 in PHD2, compared to 0.4 before I made the alteration. With good polar alignment I would expect on only having to make little tweaks.?
Here is my guide graph from last night showing just how small the corrections are and lack of reactionary overshoots. The scale is +/- 1s if the scale is not readable
I found it useful to remove the motor and use my fingers to judge the worm mesh to get both side touching. It's such a fine adjustment thing. Also, I would highly recommend changing out the thrust bearing. It made a large difference for me.
The encoder upgrade seems like a good thing to do at some point.? Have you tried it on the RA axis too? I don't plan to do anything about the thrust bearing because it is new and seems to spin smoothly.
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Pete
David C. Partridge
Michael Herman
Paul Kanevsky