Hello. I completed building the cheap microscope V7 and it works, but the Z axis motor becomes too hot to touch (so in excess of 50C) within 10 minutes. Since the 'scope is PLA with a glass temperature of 50C this is not safe.
I have seem a couple of old threads on this here, from 2020, but nothing recent, and no solutions. There is some suggestion that the motors remain energised after moves. In this instrument, this makes no sense as the mechanism is stiff and will not move without the motors, so there is no need to holding torque.
I’m driving with a Sangaboard v.05. The mechanism is easy to move with a finger when the motor is disconnected, so it’s not an excessive fricion issue. I’ve swapped the motor for another and the problem remains so its not the motor either.
Does anyone else see this?
More importantly, does anyone know how to solve it?
If not, can I obtain the source and maybe edit and recompile it?
Thanks
Well, I had an idea and it substantially works. I removed the sticker over the base (uppemost in this instrument) of the Z motor. This exposes some punches holes where the core of the motor is formed. There is some plastic behind those holes, so I have not exposed anything dangerous to the motor, but air can now circulate much better, and it now never gets too hot to touch. It comes close, but not on the sides touching the plastic - only on the top. So I think this is a reasonably effective workaround.
I’d still like to solve the problem though. It’s still running too hot for a motor doing nothing almost all of the time.
I’ll leave the post in place because of that, and becaise it might help others.
Something else. I found a problem with the self-tapping screws which are inserted to hold the small gear to the motor shaft. Their heads were rubbing on the main body housing, causing intermittent impedance ot the Z motor turning. This is why focus was unrelaible, and that is why I couldn’t get the autocalibration to work.
I considered adding spacer washers to pull the motor back off the housing, but decided instead to just remove the self-tappers; the gears are a very tight fit anyway. The problem is gone.
The most recent discussion on motor temperature is in the middle of the thread Powering the stepper motors - #17 by mikromidas - posts 17 to 29 . It seems to be an issue that comes and goes. I have never found a problem with it, but my tests in that thread do suggest that there is not much headroom and so small variations can easily tip over between fine and too hot.
I have seen some pictures of builds with little heatsinks stuck on the motors (the kind that they sell as cooling kits for a Pi or something like that).
As a stepper motor the current is basically the same whether it is moving or not, unless it is actively released. Releasing the motors has not been seen to be necessary so far, and one would need to test whether it would alter stability. There were indications from testing positioning of the Block Stage that positional dither was worse on un-controlled axes.
Instructions for mounting the motor gears is an issue that we are addressing. The gear needs to be pushed fully onto the motor shaft, and then the screws need to be done up so that they sit fully within the gear. With standard screw head sizes that will fit. With the screws buried in this way there is plenty of clearance.
(from Assembly Instructions)
Also issue: https://gitlab.com/openflexure/openflexure-microscope/-/issues/302
and Forum thread: freshly-assembled-upright-microscope-v7-0-0-beta1 post 3
Thanks for youer reply William. Your diagram and comments are interesting - my self-tappers do not fit below the surface of the gear face as shown; they are stuck above it. I got them from Filip of Sangaboard fame in his “Bag of Bits”. The 3D print is very good - from a Bambu Lab P1S so I’m pretty sure thats in spec. So now I thnk these screws are too wide - both heads and shafts. Anyway, as I mentioned - I have left them off the Z motor and all is now well. Thanks again.
I’m having fun with the 'scope and compiling a list of issue and ideas which I hope will help.
Chris
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If the screws are too wide that is a concern because these are the exact Westfield Fasteners screws listed in the instructions. I don’t have a recent small gear print here but I’ll check on Tuesday to make sure the head fits.
During assembly, I tightened those screws very tight and thought they had stopped because the shoulder of the scew had met the gear face and there was a hairline split in the gear.
But just now, I put one into the recess upside down to see if the diameter was indeed too large, and it is not. It fits.
So - I was wrong. Sorry about my mistake.
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Something else to consider is that these motors are very cheaply made. I built two microscopes over the last two years and needed to buy twelve motors (three separate lots of 4) to find six that worked correctly. I had the same overheating problem and replacing the motor solved the problem. It’s possible that reinstalling the parts on the new motor was part of the solution but there’s definitely more going on.
I’d still love to see a different kind of gear motor used, but I’ve personally looked for an alternative and really didn’t find a suitable one. Good luck.
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That’s an excellent observation - thanks. I have a few spares so I can keep swapping for a while.
I’ll try it.
Thanks again
Chris
Yes, quality control on those motors is an issue. Unfortunately, I’ve never been buying enough of them to really get a handle on that - though I have not had many duds (a 50% hit rate does sound nasty, I don’t think we’ve had that many problems). I suppose current control (rather than just voltage control as we do) might help, but that would be a big change to the Sangaboard.
From what I’ve understood, those motors are specced to be driven with 5v Darlington pairs, so if they’re overheating it does suggest the motor is not right. However, I’m very much open to suggestions if there’s a nice way to fix it, other than swapping motors.
I do confess myself curious whether swapping your motors around results in a change (e.g. is it something about the motor, or about the Z axis being more insulated)? My money’s on the motor…
Because of the gearing and the nature of the transport, I suspect there is no need to maintain a holding current to maintain position, and so each stepper motor could be powered off between moves. This would completely solve the problem.
I suppose it would be possible to write a test movement doing this and then see if any motion is lost. Set up a dial test indicator on each axis under test, do a series of descrete movements in one direction, then back again and see if you end up where you should have - accounting for backlash at the direction change step.
This should completely remove the issue - probably even for the dodgy motors.
You’re right that it is more or less self-locking, but I am not sure that avoids missed steps if you release the gearing. My guess is that if we did that, there will be some residual stress in the (plastic) gear train that results in the motor pinging back a bit, even if the stage doesn’t move.
The experiment you suggest would, for sure, be interesting. It’s worth noting that I think you can release the motors by sending a release command on Serial. This isn’t currently exposed in the HTTP API, but could be added simply enough. Also, you would probably get a more accurate result by using image analysis (because then it’s a non-contact measurement with easily <1um resolution) - and that could be done with the code that already exists in the camera_stage_mapping module. So this would be something that could be done without too much pain using an unmodified microscope.
I don’t currently have time to do this, but it’s the sort of thing that would be useful to keep in mind for student projects in the future, unless someone pops up here and does it 
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Thanks. yes, I suppose it comes down to time, and the number of people who are prepared to turn ideas into relity )i.e. do work). I’m increasingly dumb these days, so 'd be more hindrance than use, but I’d be happy to try the DTI experiment if you can tell me what I send on serial to do the release - it is literally the word “release”? But then I’d need to speciufy which stepper, and how to I “grab it” again?
I’ve done from DTI tests already with stage movement in the X axis. Overall it’s pretty good, but sometimes it sticks for a move, then catches up on the next one. I’ve found my backlash is about 70 steps when reversing direction, but the backlash adjustment seems to do nothing to change that. Any idea why?
BTW is there a way to make the step size value permanent? I don’t see the benefit of a step value of 20, which just jogs the image a small percentage of the screen width. I’d rather move to the next visual field with some small overlap so I can maintain context. I can’t see a way to do that.