Mount "shaking"
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Maxime Spano
Hello all,
I noticed that the mount we have at the observatory (a Mathis MI-1000/1250 fork mount) sometimes shake. It can be noticed especially when doing planetary imaging. If I recall correctly the shake happen in RA direction. On this mount we have a RCOS 24"+a FSQ106 which make the load a bit over the limit of the mount but the balance is ok (it probably could be improve a little bit more but it is not the easiest work to do with such a telescope). The mount has Renishaw encoders. We have SiTech 0.95A and we put the encoders in precise mode, I did some test changin to polite or cascade but didn't notice any change when the mount has decided to shake. When it shake, autoguiding is useless and deepsky images are awful... Thing is it doesn't seems to happen in any particular direction, but when it start to shake it will do it from time to time the whole evening. We can also have evening with no shake at all. I was wondering if there is any way SiTech could record what's happening on encoders, RA, DEC or any "correction" asked. I recorded a video on a very turbulent night on the moon where we can see the effect I'm talking about (229Mo): https://drive.google.com/file/d/1pY7Cu3dU9nVBrt59hvhpq7HysCSawd9z/view?usp=sharing Just to mention, the telescope is inside a coupola and there was a very mild wind this day (as it might looks like I'm imaging on a windy night outside) Any help or idea would be appreciated. |
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Don W
Hi Maxime, |
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Maxime Spano
Hi Don,
Thank you very much for your post. Before I answer your questions I would like to mention two points: - I took over the activity of the observatory while the telescope plus mount was already installed for several years. Many informations are missing (whether they have been lost or not transferred) so I am still discovering new things every day. - Last night I was at the observatory and while imaging Venus, quite low, I noticed the shaking was very strong (I confirmed it was RA axis). When I wanted to move to the next target, the mount stopped and the RA motor went blinky. It was hell to solve the problem as the protection plate screws for access to RA worm and drive was blocked by the mount fork. Had to stop and start SERVO multiple time to make the telescope move little by little (each time going blinky) until I can remove the RA plate. It seems that there is grease accumulated on the worm, I remove some, and release the pressure of the worm on the gear. It allowed me to move the telescope again. However there is still a shaking when tracking, as well as when pointing, if I put my hand on the fork while the telescope point RA movement is not smooth at all, It feels like the speed is not constant (like a bumpy movement). To answer your questions: 1- I am not sure. I found these informations encoders: RESR20, USA 200, LINE COUNT 31,488, SERIAL No 59L347, Head:RENISHAW, patented 06F461, RGH20H30D30A. I doubt we have only 31 488 tics… 2- To me, Installing V0.95A is not the cause of the shaking 3- I confirm it is RA 4- I will have to test, but one time as the autoguiding was very bad (I found later that it was probably because the telescope was shaking at that time) I tried all the mode for the encoder but didn’t noticed any improvement. 5- The noise is definitively coming from the RA motor/belt/worm. I wonder if an excess of old grease (9 years) could block the motor in some way (I would be very surprised if it was the case, it is sticky grease but there is not that much on the gear itself). As mentioned our encoder were always in precise mode. Sorry for the very long post, Maxime |
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Don W
Hi Maxime,
OK, I see that you posted questions last December and didn't seem to have the shaking, so the shaking is recent. I am going to guess that the shaking in RA is too tight a fit of the worm against the main gear. That might also be due to old grease. Grease does tend to dry out over long time (years). You mention that there is not a lot of grease on the gear. It also depends on what kind or brand of grease is there. Whether the old grease is the problem or not, it is probably time to re-grease your mount. I use CMD #3 extreme pressure grease on my mount http://www.cmdlubeoil.com/ My guess is that the tightness of the worm against the gear for a moment, then the error builds up and the servo jumps ahead. This would occur regardless of the mount encoder setting, especially if it occurs with no mount encoder. So this is a very important test - with the mount encoder set to IGNORE. I could not tell from the numbers for the Renishaw what the tics/rev are. You can read their setting using ServoConfig.exe, or read the SiTech.cfg file in a text reader like NotePad. Don W |
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Dan Gray
Go to the controller stuff, and take a look at the position error. You have to move your mouse to the textbox. If you have a lot of position error, you could try lowering the integral, and raising the derivative. Also, Don brought up a good point about the worm maintenance. Dan On Sun, May 17, 2020 at 10:45 AM Don W <westergren@...> wrote: Hi Maxime, |
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jan@...
That setup works out to 0.1 arcseconds per tick. RGH20H is 50nm per tick. The RESR20 31488 ring is 200mm in diameter. 50nm / 100mm = 0.5 microradians = 0.1 arcseconds.
Jan |
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Maxime Spano
Hi,
Thanks for your answers. Don, the shaking was difficult to identify as it was not present all the time and not every night (it just shake for 4 or 5 sec then it's ok for 30s). It's just that some nights autoguiding was crazy and some other nights ok. I thought it was something to do with the balance, the encoders, PXP model, Scitech param... Remove the old grease and put new one is on the planning now. I will test probably tomorrow night, first to see if the shaking is still there and with different encoder mode. I will see what tics/rev we have in our Scitech param. I am still surprised that grease can have such a high impact on the mount and blocking the motors (tightness of the worm on the gear was not touched for years) Dan, ok I will look and let you know about the position error we have Jan, thank you for these info. So it means we have a circonference of 628.32mm and at 50nm per ticks we obtain 12.5 millions ticks per rev, right? |
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Don W
Hi Maxime,
My suggestion about the worm/gear fit and grease is based on your description of having to release the fit a bit to be able to move the mount away from viewing Venus at low altitude. You should not have to keep getting out of Blinky mode - which was probably triggered by too much load on the servo. Note you can look at the current to the RA servo by going to the "Controller Stuff" as Dan suggested. Also note that the RA drive does not have to be extremely tight to eliminate backlash, since the RA always drives tracking in one direction. Any backlash from too loose a fit only affects slewing accuracy and is quickly eliminated as soon as it starts tracking. Monitoring the position error as Dan mentioned, and the servo current will tell you if the fit is too tight. Old lubricant that is "sticky" can act more like glue than lube, or also old lube might have acquired dust/debris/insects over time depending on how well covered your main gear and worms are protected. Good luck next time out. Don W |
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Maxime Spano
Hi all,
So I went to the observatory yesterday. The shaking was discrete but the mount was still not behaving well. It was blocked again while pointing low toward West. The shaking was present in precise, ignore and cascade mode. However in cascade mode the star was almost constantly going up and down (i.e along the RA axis) like if there was backlash to compensate even if there is not (I didn’t find where was the backlash compensation in SiTech but I guess it’s there somewhere). For the encoder, the number of ticks per rev, on ServoConfig I read 15097436 for RA and 15038461 for declination. I found these numbers too on SiTech.cfg plus two other ones: AlScpTicksPerRev=12505200 and AzScpTicksPerRev=12595200. The 12.5 million(ish) is coherent with the number calculated according to what Jan indicated in his previous message (and I guess means Scope ticks per rev). For the 15 million(ish) I don’t know where they could come from and if the difference with 12.5 millions affect anything.
Toward West (Leo const.): PRiERROR was around -90 while pointing toward the horizon but only around 5 when I was pointing away from it, when tracking it oscilated around +-5. SecError was around +-1 While tracking the RACur was around 3.3amp with spike at 4.5amp (lowest to 2.9amp) while DecCur was at 0.4amp Pointing toward the horizon keep RACur around 3amp (it was between 2.8 and 6amp very variable), pointing away from horizon RACur was more around 4.5amp (between 2.8 and 6.8amp). Another test with the telescope pointing toward South at 45° altitude (which is a position when telescope usually working fine, i.e no blinky ) showed roughly the same numbers for RACur.
https://drive.google.com/open?id=1_HOy3zgWh12k9cWed1X9Y0zcLWRlp0X3 |
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Don W
Hi Maxime, Don W |
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jan@...
I can't help with the oscillation, but I wanted to mention that 12595200 is the correct number for ticks/rev (31488 * 400x interpolation = 12595200). I'm a little bit surprised that your Al ticks per rev is different. Perhaps it was a typo in your email, or maybe they really use a different setup for the Altitude encoders.
Jan |
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Dan Gray
Did you try my suggestion of lowering the integral and raising the derivative? Dan On Thu, May 21, 2020 at 6:15 AM <jan@...> wrote: I can't help with the oscillation, but I wanted to mention that 12595200 is the correct number for ticks/rev (31488 * 400x interpolation = 12595200). I'm a little bit surprised that your Al ticks per rev is different. Perhaps it was a typo in your email, or maybe they really use a different setup for the Altitude encoders. |
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Don W
Hi Maxime,
Please try the PIDs settings that Dan suggested. They might be able to smooth out the RA motion. Use the ServoConfig.exe utility to adjust the PIDs and check for RA motion. Do this first, before going on to mechanical evaluation. However I still think that there is a mechanical reason for the oscillation. Since you observe very high current to the RA servo, that means the servo is under high load. I don't know how the Mathis mounts set the clutch, nor the clearances to the covers over the RA section. But please inspect the mount to see if there is any thing rubbing or cable that could be pulling on the RA motion. it may be possible that something inside the RA housing, or the RA shaft bearing is going bad. A way to check all of that is to release the worm drive and check the movement of the RA manually from looking East to looking West. The motion should be smooth, no tightness, and this will allow you to verify RA balance. Obviously be careful when releasing the worm, the mount and scope are big and heavy. Don W |
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jmgoldba
We can also have evening with no shake at all.I would apply Don's second paragraph of checks before the changing PID parameters. I would not be playing with the loop parameters unless there was a consistent issue and I was sure the physical mechanics of everything is as good as it's gonna get. Physical mechanics includes balance, clutch, and gear meshing such that the current you're applying to the motors is within expected range and I'm thinking, independent of position. The "shaking" in the video is when you are slewing, not tracking. It can be noticed especially when doing planetary imaging Is it me, or are there a few things going on here that don't make sense? Jesse |
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Dan Gray
It's a really easy thing to try, and also, I've seen too much integral on may systems. Can you tell me what your P I and D parameters are? I'm not 100% sure, but I'd be willing to bet that changing them will solve the problem. Not too much, maybe I'd bet a good strong I.P.A. though. If you tell me your P.I.D. settings, I'll recommend new ones. Dan We can also have evening with no shake at all.I would apply Don's second paragraph of checks before the changing PID parameters. I would not be playing with the loop parameters unless there was a consistent issue and I was sure the physical mechanics of everything is as good as it's gonna get. Physical mechanics includes balance, clutch, and gear meshing such that the current you're applying to the motors is within expected range and I'm thinking, independent of position. |
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Bob Harwell
We have a 25” mirror on a German Equatorial Mount that uses SiTech and Renishaw absolute encoders. We have had issues with our mount, as it was built back in the 1950s. But something I found, that was exceedingly useful, was connecting a shaft extension on each of our axes where the motor would normally connect. That allowed me to turn the axes by hand. It became instantly obvious that our DEC axis was WAY too tight. Our worms are spring loaded and the spring on the DEC axis had been adjusted too tight. We were experiencing positive feedback (caused oscillation) on the DEC axis. Reducing the stress solved the problem. A misaligned worm into the gear can cause very high frictional forces. That in turn makes turning the worm very difficult. I suspect that you will find that the RA is difficult to turn by hand.
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Bob On May 21, 2020, at 11:02 AM, jmgoldba via groups.io <jmgoldba@...> wrote:
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Maxime Spano
First, thanks to all of you who try to help solving this problem. It is very helpful and instructive for me.
With the current situation I don't go everyday to the observatory, next time will be monday. The worm is already quite loose (the spring holding the plate where the worm is fixed is almost completely out), but I don't see changes, I mean not like on amateur mount where it is clear when the worm is tight or loose...but there are 11 screws for holding/tuning the worm and what each of them is doing is not obvious (I should get an explanation from Mathis). The grease is very sticky, not what I would expect from grease, but visually quite clean, there is a big cover on the whole RA motor system (that I removed to make the video) and I didn't find a lot of dust when I removed it. There is one thing strange thought is that on the inside of the cover there is a shape that give more space for the worm but above these shape there were stripes of grease. I assumed it was because of previous manipulation of the cover, I cleaned it before putting it back, I would be very surprised that the worm is touching the cover there... We checked the balance like 6 month ago and it was pretty good, but I agree it looks like all the weight load the motors a lot when it's toward West (I didn't notice this when it's East but I will test). Jan: the numbers I put are copy/paste and it's not a typo. So our Al and Az Scp ticks per rev are different in our SciTech.cfg (but I don't know as well if it's because the setup are different in Al and Az). Dan: I didn't try to change the PID as I was not sure what was "a lot of error" and if it was important during pointing or tracking. As I got 100 errors on pointing I assume it was 100 ticks "missed" and on 12million ticks I thought it was not that much so I prefered not to change it. As well I didn't know how much I could change the PID to see any change on the behavior of the telescope. But I saw your second post and as soon as I go back to the observatory I wil let you know the PID settings we have. Jesse; I plan to remove the two covers (one at the bottom for the worm, motor, one around the axis for the gear) on the RA axis and check carefully for any mecanical problem. I assume the cause of the blinky motor and bumpy slew (shown on my second video) is the same as the shaking i see when I do planetary imaging (visible on my first video). It is just more obvious when I slew. Bob: thanks for the tips. Maxime |
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Maxime Spano
Hi all,
Finally I just went today at the observatory and spend the afternoon making test. I have to say with the version 2018 of the manual for the mount it is easier to understand what each screw is doing to the worm plate and helped me improve the situation.
The results are encouraging compare to the last values I had: Encoder on ignore mode: -Low West PriError: pointing: -60 to -70, tracking +-1 Pointing away from horizon : RACur around 1.7amp, varies 0 to 3.8amp Tracking RACur: 1.7amp, varies 1.5 à 2amp
Tracking: Prierror +-1; RACur: 2.2amp, Pointing away from horizon Prierror: around -70; RACur 3.2amp varies 1 to 6amp And most important, motors don’t go blinky and the telescope moves more smoothly, it’s not perfect though.
P: AD 13056, dec: 12000 I: ad 2500, dec: 2500 D: ad 5088, dec: 5000
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Dan Gray
I would recommend changing the I to 1200 and the D to 9000. Dan
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