×

INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

How can I enter a backlash value for my focuser?

  • Posts: 222
  • Thank you received: 20
That sounds great. Unfortunately my Ekos doesn't provide "Out step multiple". I've got one that I built from source awhile back so I could try the Bahtinov focus metric. (Hopefully that's obviated by the autofocuser!) Like many folks, I'm pretty much a late adopter when it comes to Ekos updates, not wanting to risk a working system for a given bell or whistle. Probably worth it here. Is that in the latest release, or do I have to build from source, do you happen to know? [EDITED TO ADD:] OK, overnight I got a ton of updates and I see that it's in the release. Thanks! I've got the analysis tab too, woo!

SEP star detection did not work at all well for me. I got much, much more reliable results with "Centroid".

> So for your case, that step should be larger than your backlash.

Just to be clear, "that step" means the initial step size * multiplier, right? So in your example I should ensure that my backlash is smaller than 5 * 20 = 100. Which I'm pretty sure it is, but only just barely.

I probably should have been taking better notes in "debug equipment" mode Friday night rather than "get it focused and acquire some data", but it was such a nice night... ;-}
Last edit: 3 years 5 months ago by Rick Wayne.
3 years 5 months ago #61443

Please Log in or Create an account to join the conversation.

  • Posts: 222
  • Thank you received: 20
So I tried a few combinations of initial step size and multiplier after updating to the latest KStars release. Mostly I went with 20 and 5. Sometimes Linear achieved good focus; other times it goofed. I neglected to run the logs (sorry) and didn't get a screenshot since I was using my iPad most of the night. But what often seemed to happen was that the software would go way outward to clear backlash, then progress inward for a while, leaving a nice half of a V until it had started climbing the other side. Back outward -- not quite as far -- then inward again. But the dots for the second round wouldn't overlay the original slope, rather they'd extend a sort of shelf out over the deeper part of the curve. I hypothesize that the software's idea of the X coordinate (focuser position) is just dramatically wrong, so its mechanics get messed up. The outward and initial inward moves are much larger than step size * multiplier, which is probably as intended.

And then sometimes it would nail it right at the bottom of the curve on the second or third try. Sometimes Iterative would work, or even Polynomial.
3 years 5 months ago #61865

Please Log in or Create an account to join the conversation.

  • Posts: 239
  • Thank you received: 38
What focuser do you have? On my moonlight CRL I see some backlash, but your focuser plots look dead on (no backlash at all). Looks excellent!
3 years 5 months ago #61897

Please Log in or Create an account to join the conversation.

  • Posts: 118
  • Thank you received: 16

What are your current settings for the focus module?

Have you tried adjusting the backlash setting? Just because the plot looks good does not mean they have ZERO backlash. You set the backlash accordingly and your plot should like theirs. A focuser with improper backlash settings will show readings all over like a scatter plot.

If anyone else has anything to add or has to correct me please do so, however, that is how I understand the focus module and the plot.
Last edit: 3 years 5 months ago by Daniel DeSclafani.
3 years 5 months ago #61898

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182

I presume that comment is directed at my post, because that is the only one that has posted a V-curve.
This is entirely self-built autofocuser on a WO WhiteCat (helical focuser). It is a geared solution driven by a NEMA17 stepper motor and MyFocuserPro using the MoonLite protocol.

The design of the focuser (3D-printed) allowed me to virtually eliminate backlash, so the focuser hits optimal position literally every time.
The following user(s) said Thank You: Daniel DeSclafani
3 years 5 months ago #61899
Attachments:

Please Log in or Create an account to join the conversation.

  • Posts: 118
  • Thank you received: 16
Thats awesome! On a side not, how are you liking the Ioptron? I want to go with a CEM70 in the future.
3 years 5 months ago #61901

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182
removed since it duplicated my post above and I don't seem to be able to delete the post in its entirety.
Last edit: 3 years 5 months ago by Jose Corazon.
3 years 5 months ago #61902

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182

It took me a little tinkering to get used to, but now it is working very well for me. The problem is that the stepper motor in DEC is underpowered or poorly matched to the board, which results in it missing steps, if there is too much resistance. It is essential to balance the mount perfectly and also not to tighten the tension screws too much. Just enough so that all discernible wiggle in DEC has been eliminated. That way, I can get good performance out of it.

I wish iOptron were offering the option to modify the firmware on the DEC board to allow me to fine tune the stepper to the board. If I could do that, this mount would be perfect in all respects.
I got it primarily because it is light enough so I can carry the setup full assembled from the living room into the backyard. 2 min setup and tear-down time. That was my primary criterion for going with the CEM25P instead of an HEQ5.
3 years 5 months ago #61903

Please Log in or Create an account to join the conversation.

  • Posts: 222
  • Thank you received: 20
So to the extent that the last couple are directed at the OP, please read the thread. It becomes clear that the driver offers no backlash setting (at least not yet), and the very first sentence of my initial post was:

Still, I appreciate the attempt to help!

If the skies ever clear again I'm going to try with a smaller initial step value -- the smallest that will cause a visible change in the focus -- and a bigger multiplier, so that the initial move definitely takes out the backlash.

I also may change the driver a bit, my motor combined with microstepping makes 10 or even 100 steps completely invisible with Kevin's driver out of the box, so I put in a big multiplier to actually get the shaft to move.
3 years 5 months ago #61905

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182

What you are describing is the result of excessive backlash. The linear algorithm assumes there is none and that is what the step calculations are based on. The shift in the curve after the V-curve has been established and then the focuser moved out again is a direct measure of the backlash in your system.

I would think that it would not be too difficult for Hy to implement a backlash compensation in his algorithm, once he is done with his current course work.

But from what you are describing, your system needs some serious tightening up.

Also, you may want to play around a little with the tolerance value. You need at least 5%, otherwise the focuser will fail too often. It never fails in my setup at 5% tolerance.
3 years 5 months ago #61906

Please Log in or Create an account to join the conversation.

  • Posts: 222
  • Thank you received: 20

Wait, what? Did you not previously write:

One of these things is not like the other :-) Hy's answer up the stream also seems to indicate that the algorithm is designed to minimize the effects of backlash.

I mean, "excessive" I might certainly buy, since a step size of 100 makes a visible focuser tube displacement but can be nearly eaten by backlash.

So, as nearly as I can tell, the backlash is actually in the motor; there's really nothing I can do about that without replacing it. Although I recognize that geartrain backlash in a commercial motor seems unlikely compared to my laughably crude setup displaying, shall we say, idiosyncrasies...still, when I reverse direction the thing gronks for a couple seconds and only then does the shaft start to turn. A subsequent move in the same axis makes the gronking and turning simultaneous.

When I ordered the timing pulley they sent me one with a bore about triple the size of what I'd asked for; coming up with a crude bushing for that left me with a setup that certainly might be susceptible to backlash, although as I noted that doesn't appear to be happening. I've ordered another, we'll see what arrives in a week or two on the slow boat from China.
3 years 5 months ago #61942

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182


Let me rephrase: The linear focuser inherently compensates for backlash, as it moves so far out, then back in, that it eats up all the backlash in the process.

HOWEVER: While establishing the V-curve, the Linear algorithm determines the calculated minimum on the assumption that there is no backlash. It moves out accordingly and then performs a calculated move towards that minimum. Any backlash will displace the second inward curve by that amount. In addition, if you excessive backlash, it is theoretically possible that the outward movement does not extend beyond the calculated minimum, which will send your focusing attempt off into Nirwana.

That's what I wanted to convey in fewer words and where I obviously failed miserably.

When you look at the curve I posted, it is obvious that there is essentially zero backlash, as the secondary inward curve nearly perfectly overlaps the primary V-curve.

If you have a geared stepper and a gearbox with a lot of play, that will be difficult to fix. I have opted against using a stepper with a gearbox and I am using direct drive microstepping instead. I was worried about steps being missed, but it seems to be working surprisingly well. No major problem with missed steps.

Jo
3 years 5 months ago #61946

Please Log in or Create an account to join the conversation.

Time to create page: 0.320 seconds