Strange anomaly

[Bernd_Eppinger]

Hi, George,

 

Were you able to watch the power consumption of your mount during the 10min exposure in your post from 12.05.2013 12:08AM?

 

If so, did you perhaps see an abnormal increase in power consumption, which "suddenly" (i.e. within far less than 5 seconds) decreased back to normal? That might be an indication of a mechanical problem, in your case probably in the DE axis.

 

There is a comfortable method of recording motor currents, encoder positions and position errors: Drive -> Servo-Setup, then Settings -> Log File Settings. Switch everything on and use an interval of e.g. 1 second. The log file is in Autoslew's profile-directory, subfolder ClServoLogs\. Don't forget to switch it off when you don't need it anymore.

 

Best regards,

 

Bernd

[GeorgeCarey]

Thanks, Bernd.

I will set up the log recording next time.

[GeorgeCarey]

I should never have said that I had cured the over current error - I got several tonight.

Problems started as soon as I slewed to ngc4725. Over current errors and position errors were frequent.

I had to seset and start again.

The error log is here:

http://geoastro.co.uk/may2013/autoslew/

 

I don't really understand 'magnetic angle'. Once it has been established (manual homefind) why does it vary?

 

George

[GeorgeCarey]

I then switched to counterweight West and started again. Apart from a 'Velocity exceeded' error the mount is behaving and currents are stable.

Over 2 hours of good tracking.

Edited May 14, 2013 by GeorgeCarey

[GeorgeCarey]

This is a graph of current for the good tracking period.

 

Edited May 14, 2013 by GeorgeCarey

[Bernd_Eppinger]

Hi, George,

 

Thank you for creating the graph, that facilitates the evaluation.

 

I see a handful "jumps" from high to low current. If they are fast (in the log, I think at least 2 of them might be fast), and if they are caused by friction and not by wind etc, then I think such jumps may cause an excursion of the DE axis that might be visible in the image.

 

I don't know if the excursion is large enough to cause an intermittent slew to 30 and 31 comae berenices. At least the position error in the log is small. But it is possible that the excursion happens between the 6 sec. sampling interval, so that the log doesn't show it. It is also possible that when you made the "bad" photographs, there was more friction and a larger current drop when the friction was (suddenly?) released.

 

In the first 2 logs, I find the following lines interesting (something like this occurs several times):

 

22:46:27.725: Axis 2 trying to correct OverCurrent Error
22:46:27.741: Axis 2 slewing to -4.78
22:46:32.171: Axis 2 slewing to 1.22
22:46:36.960: Axis 2 slewing to -4.78

 

 

That seems to show an intentional slew which Autoslew makes in DE in an attempt to get out of an over-current condition. Could something like this have resulted in the slew to 30 and 31 comae berenices?

 

 

> "I don't really understand 'magnetic angle'. Once it has been established (manual homefind) why does it vary?"

 

I am not sure about the precise definition which Dr. Philipp Keller uses for the "magnetic angle". My interpretation is, that in the Manual and in the User Interface, he actually means something like an offset between the magnetic field angle and the angular distance from the reference mark. (Note: probably multiplied by the number of the poles.) However, I have the impression that in the log-file, "magnetic angle" really means the absolute angle of the magnetic field vector. This would have to change when the geometric angle (e.g. of the RA axis) changes.

 

Well, only my interpretation. I think Dr. Keller will know it better.

 

But I have the impression that the magnetic angle might partly play a role in your problem. Did I understand it right that "I had to seset and start again" means you repeated the manual homefind? I get this impression from the log.

 

If so, it is possible that you got a better magnetic angle the second time, resulting in a higher torque for the motor or less current consumption for the same torque. When I played with the Autoslew version which first had the manual homefind, I found that my magnetic angle was not very reproducible. I then reduced the magnetic angle find speed. This also seemed to influence the speed at which the reference marks are approached after the motor switches on during manual homefind. In my case, it improved the manual homefind precision drastically at the expense of a slower automatic homefind. However, I must admit that as a mobile user, I have a very special set-up, and I do the manual homefind without telescope and hence with bad balance.

 

Best regards,

 

Bernd

[GeorgeCarey]

The graph was made with the data when the mount behaved perfectly. There were no spoiled images and I don't think there were any excursions. I would need to aim at a very bright star to check this.

 

The deliberate slews after the over current errors are almost certainly what caused my 30,31 Com B stars. The odd thing is that this happened on 8th May with no errors reported by Autoslew. 

 

When I do a 'Reset', I switch off Autoslew, turn off the power supply to the mount for 10 seconds and then start again. I do a manual homefind. Sometimes when I move the mount to the reference points by hand the homefind fails and the mount jumps around a bit. Because of this, after a manual homefind,  I always offset the mount a little with the controls and then do another homefind. The motor speed is low and the mount moves more smoothly than I can push it by hand. 

 

I suspect that you are correct in thinking that the magnetic angle in the log file may be a different representation than the magnetic angle found after a homefind.

 

I put in a support request yesterday - we will see what transpires.

Edited May 15, 2013 by GeorgeCarey

[Bernd_Eppinger]

Hi, George,

 

thank you for the update.

 

So let's wait for ASA's reply. If ASA agrees, I would be interested in what they say.

 

 

Best regards,

 

Bernd

[GeorgeCarey]

No reply from ASA. I have sent another email today.

[GeorgeCarey]

Reply from ASA today. They suspect that it is indeed due to a bearing problem. The mount will go back to Austria next week.

Thank you to ASA for good service!