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Drive jerk on LX85 mount

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#1 barnold84

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Posted 04 September 2020 - 07:17 AM

Hi,

 

I think it's better to post this in a new thread. I noticed some significant friction on my RA axis that I could resolve by opening the nut that fixates the cylindrical thrust roller bearing of the RA axis on my LX85 mount. Now the axes smoothly with open clutch.

However, what I noticed (don't know if it's been observable before) is that with intermediate slew speeds (16xGuide to 1.5deg/sec) the mount show a significant jerk when accelerating. No matter if it's directional change or accelerating in the same direction as it moved before. On acceleration the scope vibrates a very short time and then slews with continuous speed. With lower speeds and max speed (4deg/sec) it doesn't happen.

I think it's the servo that causes a jerk when accelerating and the now missing friction isn't damping the oscillation caused by it. 

 

Does somebody know if I can control this somehow? I simply don't know if this jerk leads to premature wear of the gear. 

 

Björn



#2 atsidor

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Posted 22 June 2021 - 12:04 PM

I'm adding onto this topic as I think I recently discovered the same issue for the LX85, almost exactly. On "max" slew speed, the motor slews smoothly. However, as soon as you switch to one of the intermediate speeds, there is significant jerking when starting up, as Bjorn mentioned above. 

 

Here is a video with the RA motor cover removed to see the internals. I switched from max --> 1.5 deg/sec --> 1 deg/sec to show the effect. (Note that this is with no OTA/no load, but the same thing happens with my OTA mounted.)

 

https://photos.app.g...jccs9ChXq2KR9X6

 

In my mind, it does not seem to be a mechanical issue (i.e. gear slipping or a blockage) because the motor turns fine at max speed. All the gears I can see look like they are moving fine. It's almost like the motor starts out turning too "fast" and the motor control tries to compensate and slow down but it's just locking up completely. I'm no electrical person though.

 

So far I have tried:

- Adjusting/reducing backlash between the RA motor/gear assembly

- Resetting the Audiostar controller

- Calibrate the motors, then perform drive training

 

But the issue still persists. Interestingly enough, my RA axis has a fair amount of friction unlike what Bjorn mentioned above.

 

Hoping someone out there can help shed some light on this!


Edited by atsidor, 22 June 2021 - 12:06 PM.


#3 barnold84

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Posted 22 June 2021 - 12:30 PM

Hi Adam,

 

Actually, I never checked what happens if one removes the mount head. It also appears to me that in your case it's much more intense than what I had observed.

 

My impression is that the drive is not very well tuned. These motors are equipped with encoders and I could imagine that the jerk from the drives, once the drives start to accelerate leads to a resonance and the drive tries to compensate.

 

If it's a resonance problem, one should be able to tune the resonance through the weight (the momentum of inertia to be exact) and the friction of the axis. Another approach could be a slight imbalance to the RA axis (which is always a good idea). Since the mount is at my father's place, I don't have direct access to test it immediately.

 

CS!

Björn


Edited by barnold84, 22 June 2021 - 12:30 PM.


#4 atsidor

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Posted 22 June 2021 - 12:52 PM

Thank you for you thoughts Bjorn! Good idea about RA imbalance. Also, I think I will try adjusting the axis friction via the nut and bearing you mentioned above. If it is a resonance issue, then that does seem like it could help with damping. Perhaps there is a comparable electrical way to do this as well (can you slow down the motor speed?...again not an electrical person).

 

I also ordered a new power supply to rule out the possibility of a dying/bad one. I will be able to try that later today.

 

Thanks again! If others have thoughts, please chime in.



#5 atsidor

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Posted 23 June 2021 - 04:04 PM

Well, so can rule out the power supply as an issue. I swapped to a new 12V/5A power supply and still seeing the same issue.

 

I also adjusted the nut on the back of the mount, both tightening and loosening the friction with no improvement. In fact, I completely removed the motor assembly so it was de-attached from the RA axis and could run freely... but still jerking. So it seems like the issue is isolated to the motor assembly itself. Anyway, not sure where to go from here... ideas are welcome.



#6 atsidor

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Posted 23 June 2021 - 04:45 PM

I'm attaching a photo of the RA motor assembly (which looks pretty identical to the DEC motor assembly as far as I can tell) for reference. I'm guessing the notched wheel in upper left is for the optical encoder? Or would the encoder be somewhere else? Wondering if I should break into the assembly further.

Attached Thumbnails

  • IMG_1307_mod.jpg


#7 OzAndrewJ

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Posted 23 June 2021 - 05:31 PM

Gday Atsidor

What you circled is the PEC sensor, which is only used to reset the PEC datum on rebooting.

The encoder is the black plastic box on the end of the motor

ie on the left side of the motor in your piccy.

 

Andrew Johansen Melbourne Australia



#8 barnold84

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Posted 24 June 2021 - 02:14 AM

It surprises me to hear that the drive still behaves strange even after you completely removed it from the mount. 

 

The LX85 mount is very unfortunate. While the overall mechanics isn't bad, they are using these toy motors and honestly, I don't get idea why they use servos in the way they do. If you have an encoder, you'd place it at the position WHAT you want to measure (i.e., the DEC axis) but not the drive. They should have used some good steppers and the mount would be a notch better.

 

Björn



#9 atsidor

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Posted 24 June 2021 - 11:19 AM

Thank you Andrew. The PEC sensor makes more sense now. I am hoping I don’t have to crack open the encoder enclosure but may be necessary. It’s just weird that there is no issue at max slew speed, but there is at slower rates.

My next step is to try swapping the RA and DEC motor assemblies and see if the issue is the motor itself. As far as I can tell, the two motors are identical (aside from PEC sensor on RA). If the DEC motor works in the RA plug, then it must be an issue with the RA motor assembly. If not… I guess a bigger electronics issue with the main control board or hand controller maybe.

#10 OzAndrewJ

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Posted 24 June 2021 - 05:06 PM

Gday atsidor

If you know someone with a logic analyser ( you can get cheap ones these days for $10 or so )

you could probe the encoder pulsetrain at different speeds to see whats happening.

No need to open it up ( and to be honest, you probably wont "see" anything wrong if it is only working intermittently )

From the logic traces, you can easily see lost pulses or rapid/uncommanded speed spikes

 

Andrew Johansen Melbourne Australia



#11 atsidor

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Posted 25 June 2021 - 06:25 AM

Thank you Andrew. You are very knowledgeable. Can you help me understand the pin out that I am looking at on the motor assembly? What should I be probing here?

 

Also, would it be beneficial to check voltage supplied to the motor to see if there is anything weird about that? I did replace the power supply but maybe there’s still some issue there.

Attached Thumbnails

  • F822F111-C33D-4B97-8D61-890044EC1471.jpeg


#12 atsidor

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Posted 25 June 2021 - 06:30 AM

Also, are these motors / encoders fairly common parts that could be replaced easily? I hesitate to try to go through Meade. And I am not the original mount owner so not sure what help I would receive.



#13 OzAndrewJ

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Posted 25 June 2021 - 06:47 AM

Gday Atsidor

I cant tell from that photo, but it looks like 6 wires go to the PCB.

I assume 2 of these come from the HBridge motor driver and are to supply power to the motor

one will be +5V to power the encoder

one will be ground for the encoder

two will be the encoder A/B channels.

I have attached a piccy from another user who annotated the pins, but i cannot guarantee it is correct

You can easily find the +5V and Ground using a multimeter

The others might require a bit of digging

but you need to watch the encoder A/B lines to see if the encoder is working or not

 

Andrew Johansen Melbourne Australia

 

LX85 PCB Labeled.jpg



#14 atsidor

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Posted 25 June 2021 - 09:33 AM

Many thanks again. I will check the pin out relative to the pic and report back. Probably later today. 



#15 rmollise

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Posted 25 June 2021 - 03:34 PM

If your guiding is normal, I certainly wouldn't begin tearing the mount apart and replacing stuff for something that's a minor irritation. 



#16 atsidor

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Posted 26 June 2021 - 08:49 AM

If your guiding is normal, I certainly wouldn't begin tearing the mount apart and replacing stuff for something that's a minor irritation. 

Well, that’s a fair point. It has been a bit more than a minor irritation as it’s impeded go-tos. Either way, probably should indeed have just left alone…

 

Feel like an idiot here… but I accidentally shorted out the motor +/- pins when I was checking them bawling.gifBurned out some component as there is now no voltage going to the motors. Ahhhh so dumb.

 

Is there anything that can be done short of trying to get a new control board from Meade at this point?



#17 barnold84

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Posted 26 June 2021 - 09:48 AM

I think it's fair to care about the issue as such motion can lead to increased wear of the drive train. However, it's indeed a good question how far to go with self-diagnostics and testing. Unfortunately, you obviously went a bit too far.

 

What could you do now? You could try to check if you can get a spare part for some good price (maybe approach Meade directly, hoping that they're running on 100% again after the acquisition through Orion). 

I don't know the background of your mount, your goals (did you mention Astrophotography somewhere?), gear or your possible budget but I would honestly consider a new mount. The mechanics of the LX85 isn't bad, except for the toy motors they're using and I still don't get the purpose of the encoders if you connect them directly to the motor axis (of course I know that they need to measure the motor movement but what matters is the RA/Dec motion)!? They should have used good steppers or connect the encoders to the actual axes (RA and Dec).



#18 atsidor

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Posted 26 June 2021 - 10:58 AM

The mount is new to me from CN classifieds (about 2 weeks). Unfortunately it arrived with the motor issue (unknown to the previous owner). Since it is a new purchase, I have unfortunately kind of exhausted my budget :) I think I have no choice but to at least try Meade for now & see if anything may be done about it. 
 

I would indeed like to do astrophotography. Was hoping the go-to would help me out as I’m in fairly light polluted area and have a hard time finding faint objects otherwise. It may not be the best mount but it is a huge upgrade for me from my previous, non-go to mount that I started out with as a kid. Thus, pretty disappointed that it ended up like this for my first big astro purchase in 20 years…and it’s my fault. Definitely bummed. 



#19 barnold84

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Posted 26 June 2021 - 11:45 AM

I'm absolutely no expert (experience wise) regarding repairs, spare parts for astro gear. I just tried to extrapolate and would assume that a replacement electronic board wouldn't be a cheap thing and usually these kinds of things can rather end up close to the price of a good used mount. Therefore, go for a second opinion on how to proceed best.

 

Assuming that you really short-circuited the motor power supply, I assume that you simply grilled the driver chip. One option to consider: if you know someone with good electronic skills, you/he might try to replace the driver IC. But better no more far-distance diagnostics from here.



#20 OzAndrewJ

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Posted 26 June 2021 - 05:45 PM

Gday Atsidor

What did you short?????

Can you see a damaged chip.???

As noted, if it was just the driver chip, they can be replaced pretty easily by a competent solderer

but looking at the PCB pictures i have, these boards dont use the normal chips the earlier boards use.

 

Andrew Johansen Melbourne Australia



#21 atsidor

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Posted 27 June 2021 - 10:32 AM

Gday Atsidor

What did you short?????

Can you see a damaged chip.???

As noted, if it was just the driver chip, they can be replaced pretty easily by a competent solderer

but looking at the PCB pictures i have, these boards dont use the normal chips the earlier boards use.

 

Andrew Johansen Melbourne Australia

Andrew, I touched the motor+ and motor- lines together when I was checking voltage. I know it’s dumb and in retrospect probably shouldn’t be messing with it but here I am. I smelled a burning smell as soon as it happened and the voltage went to zero after it happened. I’ve checked the board and there is nothing obviously burned/charred that I can see. Not sure how to even tell which component got fried. Rest of functionality is working ok (I.e. hand controller powers up and works). Just RA motor won’t move. 



#22 barnold84

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Posted 27 June 2021 - 10:51 AM

Well, you might have fried not just the motor driver but also some power supply parts. You need someone with electronics skills to diagnose what’s blown and to replace some parts.

Don’t know if Meade could provide you a spare mainboard and how much that would be.

#23 atsidor

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Posted 27 June 2021 - 06:09 PM

Well, you might have fried not just the motor driver but also some power supply parts. You need someone with electronics skills to diagnose what’s blown and to replace some parts.

Don’t know if Meade could provide you a spare mainboard and how much that would be.

Yeah seems like a pain to figure out what needs replacing. Will probably reach out to Meade to start with and see if it’s possible to replace the whole board. 



#24 OzAndrewJ

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Posted 27 June 2021 - 06:33 PM

Gday Atsidor

It sounds like one of the HBridge Mosfets may have died then,

Someone with  CRO could trace the signals from the CPU to the motor bridge

but you need the proper probes and skill set etc to do that.

That said, if all the signals look good out to the driver chip,

then replacing it should be simple, but paying time to a tech

might end up being more than a new board.

 

Andrew Johansen Melbourne Australia



#25 rmollise

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Posted 28 June 2021 - 03:30 PM

Well, that’s a fair point. It has been a bit more than a minor irritation as it’s impeded go-tos. Either way, probably should indeed have just left alone…

 

Feel like an idiot here… but I accidentally shorted out the motor +/- pins when I was checking them bawling.gifBurned out some component as there is now no voltage going to the motors. Ahhhh so dumb.

 

Is there anything that can be done short of trying to get a new control board from Meade at this point?

No, there is nothing more to be done unless you are experienced in electronics repair and troubleshooting.




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