Oscillating when not tracking

[nakbrooks]

Is it normal for a DDM85 to oscillate visibly (about 1-2 arc min on a 2-3 second cycle) when the motors are running but tracking is off? No problems at all when tracking is on. Both axes are well balanced and have been tuned.

[RamaSpaceShip]

Hi Nigel,

 

When tracking is off, my DDM85 remains steady. As I don't do this very often (mainly at startup when setting it up: balance and tuning), this is not a complete proof.

 

I will do a longer try during the day.

 

 

Best regards.

 

Bernard

Edited July 22, 2013 by RamaSpaceShip

[nakbrooks]

Bernard

 

What is the weight of your payload and what are your servo settings?

[RamaSpaceShip]

Nigel,

 

My payload until now was small (less than 20kg) because I used a temporary pier which was unable to carry the full load of my ODK16. I used another tube (250mm) to learn how to manage the DDM85. This is not at all equivalent, but it is better than waiting, and it helps a lot understanding how this mount works and how to set it up properly..

 

My new pier is ready soon (I have to wait the 6th of August to get the concrete dry enough) and the payload will become ~40kg.

 

I will then tell you more about the servo settings at that time.

[nakbrooks]

Mine is about 80kg, excluding weights, although this is well within the 100kg+ capacity of the DDM85 Premium. Maybe it is normal behaviour. There is no oscillation at all when tracking.

[GeorgeCarey]

Which axis oscillates? I have never seen this on my ddm60, and would be surprised if this was 'normal' on a ddm85.

 

I assume you used the auto tuning in Autoslew. Perhaps it needs fine tuning because an oscillation usually implies that the I value is not perfect.

[nakbrooks]

Yes I used auto tuning.  I've now done a fine tuning and the problem seems to have gone away.  I'll keep an eye on it.  I'm doing the initial setup of the whole system (pier/mount/OTA/rotator/focuser/camera/filter wheel/dome) so there are a lot of variables I'm juggling (although I'm trying to do things in a logical sequence).  It may or may not resurface.

[P.Keller]

the automatic tuning does not always work well if the mount is heavy loaded and you might try to tune it manually.

A low frequency shaking is always a to high P+I or to low D.

A noisy high frequency is a to high D.

 

What you observe is low frequency, so you have to reduce P and I (if you move the I slider a little down it will work).

 

Philipp

 

 

 

Is it normal for a DDM85 to oscillate visibly (about 1-2 arc min on a 2-3 second cycle) when the motors are running but tracking is off? No problems at all when tracking is on. Both axes are well balanced and have been tuned.

[nakbrooks]

Thanks Phillip.

 

Now that I have better balance and have retuned the oscillations have gone away.

 

Nigel

[DaveK]

I had an oscillation problem on the RA axis and I discovered that turning off the noise filter solved my problem immediately.

 

I started with an auto tune and then tuned manually for optimal settings. Each time I turned on the RA noise filter the oscillations returned.

 

I never use the noise filter on the RA axis.

[nakbrooks]

Interesting Dave. If my oscillations return I'll try that.