Altitude going haywire #tickmanagement #oscillations


Rob Brown
 

I'm getting very strange behavior from my alt-az drag-n-track all of a sudden. It has been working very well for over a year. Now this:
1) GoTo is tens of degrees off and isn't improving as I build PXP points. In particular, scope wants to move too far in Altitude.
2) I checked Tick Management in Servo Config. Sure enough, the encoder ticks were 10X higher than the motor ticks, instead of the other way around, roughly speaking. So I ran a few cycles of Alt tick management and all the encoder readings come back as "Bad". Motor readings are good though.
3) I saved the new ticks anyway, because they were at least in the expected ballpark.
4) Now in GoTo, the scope oscillates in Alt over the target object and never finishes. The oscillations are at least 2 to 4 degrees in amplitude. 

I am totally lost. Is my encoder bad? Controller1? Or just bad connections? HELP!

Thanks,

Rob Brown


Don W
 

Hi Rob,

Do you have a record of the tics per rev for you mount and the servos?  Check using ServoConfig.exe to see that the tics per rev for all the encoders are correct.  If your mount uses friction drives, slippage might be your problem.  You can check the tics per rev by carefully moving the mount axis through a measured angle (Azimuth can go 360°, Altitude maybe 90°).  Check ALL the encoders.

What version of SiTech are you using?

Don W


Rob Brown
 

Don,

 

Thanks for responding! Here’s what I have:

 

From Controller in current state:

Alt Motor Ticks 6,891,738

Alt Scope Encoder Ticks 765,361

Az Motor Ticks 3,640,499

Az Scope Encoder Ticks 1,214,385

 

From working calibration file (May 2018)

Alt Motor Ticks  6,813,919

Alt Scope Encoder Ticks  2,239,383

Az Motor Ticks 3,640,499

Az Scope Encoder Ticks 1,214,385

 

As you can see, the big change is in the Alt scope encoder. But it’s definitely not slipping. In fact, it can’t, because it is on a separate shaft from the drive and slip clutch and that shaft is free-running (idling).

 

I was going to try swapping the encoders to see if the problem followed them, when I noticed that the Alt encoder was actually wobbling on the shaft! It turns out that the brass shaft of the encoder was ever so slightly bent. I gently pushed it back, checking the wobble until it looked good. I then ran the Ticks Determination routine again, and Voila! Fixed! The average of three runs for the Alt Encoder is now 2,268,068. That’s very close to my last calibration, so I will now try re-loading the old cal and see if GoTo works.

Done! Problem solved!

 

Lesson learned: Protect your encoders! Both of them are highly exposed, and I simply must have banged it loading the scope (40 pounds) into the cradle. I’ll build enclosures.

 

-Rob Brown

 

From: Don W
Sent: Monday, November 4, 2019 11:42 PM
To: Sitechservo@groups.io
Subject: Re: [Sitechservo] Altitude going haywire #tickmanagement #oscillations

 

Hi Rob,

Do you have a record of the tics per rev for you mount and the servos?  Check using ServoConfig.exe to see that the tics per rev for all the encoders are correct.  If your mount uses friction drives, slippage might be your problem.  You can check the tics per rev by carefully moving the mount axis through a measured angle (Azimuth can go 360°, Altitude maybe 90°).  Check ALL the encoders.

What version of SiTech are you using?

Don W

 


Don W
 

Hi Rob,

Great that you found and fixed a problem.  However your tics/rev, particularly for the Alt Mount encoder may still need some refinement.  Use what you have and see if it is accurate enough.  Measuring the tics/rev is not real easy, so it may take many iterations to zero in on the best setting.

Don W