Binding or backlash in DEC axis?

[Patrizio Boschi]

Hi,

i'm experiencing a strange behavior on DEC axis (Bresser EXOS-2 mount relubed and with new worm shaft bearings, with DIY OnStep goto/tracking system).

It seems that a certain point (especially near meridian) the DEC compensations desn't work well; the same command is sent over and over again without any relevant DEC movement; then, sometimes (ref. recording 01:01:33), an almost periodic peak movement is detected and that is compensated.

Also, a lot of DEC backlash is detected by PHD2. However, it's strange because I don't "feel" so much backlash by hand, when replacing the DEC belt/motors with the spin handle. In fact, I managed to get what I feel as almost zero backlash, but the axis is now a bit tight. Maybe it's too tight, and the slow response of DEC is due to binding and not due to backlash?

The motors are NEMA17 400 steps / 32 microsteps, tracking at 0.830mA. The mount is well balanced, with weights kept very "high" on the shaft.

https://openphdguiding.org/logs/dl/PHD2_logs_ySuU.zip

[Patrizio Boschi]

Sorry, I was obviously meaning 0.830A, not mA. For comparison, the slews go with no problem / no noise at 2.5°/s with 0.950A, that is why I'm doubtful about binding at tracking rates.

[wave...@talktalk.net]

Hello patrizi...,,
As a PHD2 learner myself, I reckon your guiding results are basicaly pretty good, but compromised by the odd events seen in the Dec trace, which takes a dive every 10 minutes or so. I think your suspicions are correct.
The 13 minute GA run holds some useful data on periodic error, but its recommendation to set the Dec guiding to
unidirectional (to the South in the curent setup) because of the almost 4 seconds of backlash, should be followed. Remember this slop is measured at guide speed so you'll not be able to 'feel' it by hand.

The 59 minute guiding session shows several of the Dec excursions and each time the guide pulses slowly bring it back, when it overshoots and is slowly returned again. This is the first one:

Reading the PHD2 documentation on Analysing Guide Logs, I'd say these events point to stiction and the approx. 10min glitch in the Dec system. Is the worm period around the same? Maybe it's related to the new bearings and the tension you mentioned?
Cheers,
- Jack

[Patrizio Boschi]

Hi, thank you for the answer.

I'll try to reduce stiction by loosening the worm shaft a bit, even if this will reintroduce some of the initial backlash in some DEC positions.

However, the unidirectional DEC guiding hint is not applicable in this case, because after the DEC "glitch" it would never recover. In fact, I continue to think that in reality there is no backlash at all, but only stiction which is eventually forced out abruptly during the guiding glitches and more softly during the guiding assistant, resemblinflg the effect of backlash.

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[Bruce Waddington]

The Guiding Assistant can't make any distinctions about the underlying mechanical problems - it is really measuring "reversal delay".  We labeled it 'backlash" because that's a more familiar term and it is usually the source of the problem - but not always.  Here's what we have to work with from the GA backlash test:

This is the result from issuing 14 "north" guide pulses of 500ms followed immediately by 14 "south" guide pulses of the same size.  You can see there is a long delay before the axis gets moving south at anything close to the expected rate.  Based on the later guiding results, this is probably static resistance of some kind.  The gear mesh on the Dec axis may be too tight, the shaft may not rotate smoothly and easily, etc.  You can't "feel" any of this because of the very low speed (and corresponding low energy) of guide pulses.  The implication is that any reversal in Dec guiding direction is going to incur these unwanted, long delays.  The next question is why the reversals are happening.  Here's a typical sequence:

The important thing to remember here is that the Dec motor is normally idle, starting and stopping only for very short periods (e.g. < 100ms) in response to guide commands.  It is therefore extremely unlikely that any of that is going to cause a 20 arc-sec Dec excursion of the guide star.  Those things nearly always originate from forces external to the mount drive system.  i notice that these events became more frequent after you did the meridian flip.  This suggests to me that you've got a mechanical problem with the guiding assembly and the cables that are routed to it.  Cables that dangle, drag, or snag can easily create these problems as can any looseness in any of the mechanical fittings that hold the guide scope and camera.  In this case, you were working around Dec = 38 degrees, so you could re-position the scope at that Dec location pointing east, force the meridian flip, and then carefully look for cabling and rigidity problems in the new pointing position.  These Dec excursions correspond to mechanical shifts of of 2.5 to 7.5 pixels  (7.5 - 22.5 microns), so much smaller than the thickness of a human hair (50 microns).

Good luck,

Bruce