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Novoflex Castel Micro- stepper controlled
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19318
Location: Richland, Washington State, USA

PostPosted: Thu Oct 04, 2018 5:01 pm    Post subject: Reply with quote

ray_parkhurst wrote:
I'm not sure why you downplay the missing of these microsteps.

I'm hoping the comments below will answer this question. Your "edited to add" is definitely on the right track -- my example implicitly assumed that the smallest microsteps were smaller than I really cared about.

ray_parkhurst wrote:
we seem to have a fundamentally different understanding of what a microstep is.

I expect we agree on the physics of how microstepping works, so the difference is in how we think about it at a higher level.

I think about microsteps in two different ways, depending on context. One way is to focus on what happens when the motor stops, trying to make that position as accurate as possible. The other way is to focus on what happens while the motor is moving, trying to make that movement as smooth as possible.

In the first case, position when stopped, I care about how close the device is to the position that I intended, or at least how close it is to the intended distance between two stopped positions whose absolute locations I may not care much about.

In the example that I gave, the context would be something like intending an interval of 20 microsteps, in a 200-frame focus stack. In that situation, being off by a few microsteps in an individual focus step is tolerable, but accumulating a high fraction of missed steps would be a disaster because then the start/end positions of the stack would be wrong.

In the second case, smoothness while the motor is moving, I would care mostly about "jerk" -- changes in acceleration. Even if the instantaneous motor position lags by a lot of microsteps, say 1/3 full step behind the commanded microstep position, I'm probably happy with that as long as the lag doesn't trigger some other issue like a nasty resonance. In particular I would have no concern about the low nominal torque of a 1/256 microstep, as long as in the end, when the motor is stopped, there's enough torque to reach and hold whatever final accuracy I care about. In those videos of silent motor motion, I would place a large bet that at the bottom of the hardware they're doing something equivalent to running with tiny microsteps and not worrying about whatever lag is introduced by the correspondingly tiny nominal torque.

ray_parkhurst wrote:
I also don't understand your comment about this being more like closed loop than open loop, as I look at it exactly opposite. In fact it is more like a very bad open loop!

Adding emphasis, exactly what I wrote was "From a standpoint of overall system behavior, this is a lot more like closed loop then open loop."

My comment was intended to underscore the point that microstep errors don't accumulate, while skipped full steps do.

The full steps are clearly open loop, since without adding feedback you can get arbitrarily large errors and nothing will correct for them.

But for the microsteps, if we stand back and squint it sure looks like something is monitoring the motor shaft position and somehow making adjustments so the shaft never gets very far away from the nominal microstep position. Described like that, the behavior sounds like "closed loop" or at least "proportional control". But I agree that using either of those standard phrases was probably a bad thing to do, because they have so many other connotations attached. Typically those phrases imply more hardware added specifically to provide the feedback, where in this case it's really just the "springiness" of the magnetic field doing the job.

--Rik
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Adalbert



Joined: 30 Nov 2015
Posts: 465

PostPosted: Fri Oct 05, 2018 1:30 am    Post subject: Reply with quote

Hi Ray,
Quote:
but repeatability suffers due to backlash and such

Yes, the biggest problem with the backlash occurs during the changing of the direction.
But if you stack by moving in the same direction you can get a repeatable step 140nm (it has been measured by Zerene).
http://www.photomacrography.net/forum/viewtopic.php?t=30220&postdays=0&postorder=asc&start=30
BR, ADi
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mawyatt



Joined: 22 Aug 2013
Posts: 1618
Location: Clearwater

PostPosted: Fri Oct 05, 2018 6:13 am    Post subject: Reply with quote

Another consideration for rail quality in automated stack and stitch efforts is the ability to precisely return to a given position from various other positions and directions.

Consider a vertical stack & stitch effort with camera/lens on Z axis and subject on X & Y axis. Where one starts say in the lower right corner, does a vertical Z stack, then moves an incremental X subject movement to the left, another Z stack, another X movement and so on. Then at the end of the X range to the left a incremental subject backward Y movement followed by sequential incremental right X movements and carried on for maybe 4 Y movements and similar X movements. This will produce 16 individual Z axis stacks. Rails with considerable backlash will cause this sequence to be more complicated because of the approach from only one direction requirement to eliminate/reduce backlash. Now add what I call "rail wobble", where the rail pointing angle changes because of bearing slop (or other causes), this causes rail stage to cock and release (loading & direction dependent, sometimes periodically, sometimes random) and things become even more difficult.

Backlash and rail wobble can be handled somewhat easily with directional control and off axis loading for single axis stacking, but under the more difficult conditions of stack & stitch, lack of good rail performance in both the Z axis and in X & Y can be very frustrating, especially of one is attempting to completely automate the process.

Anyway, just some random thoughts on this topic.

Best,
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1683
Location: Santa Clara, CA, USA

PostPosted: Fri Oct 05, 2018 6:45 am    Post subject: Reply with quote

mawyatt wrote:
Another consideration for rail quality in automated stack and stitch efforts is the ability to precisely return to a given position from various other positions and directions.

Consider a vertical stack & stitch effort with camera/lens on Z axis and subject on X & Y axis. Where one starts say in the lower right corner, does a vertical Z stack, then moves an incremental X subject movement to the left, another Z stack, another X movement and so on. Then at the end of the X range to the left a incremental subject backward Y movement followed by sequential incremental right X movements and carried on for maybe 4 Y movements and similar X movements. This will produce 16 individual Z axis stacks. Rails with considerable backlash will cause this sequence to be more complicated because of the approach from only one direction requirement to eliminate/reduce backlash. Now add what I call "rail wobble", where the rail pointing angle changes because of bearing slop (or other causes), this causes rail stage to cock and release (loading & direction dependent, sometimes periodically, sometimes random) and things become even more difficult.

Backlash and rail wobble can be handled somewhat easily with directional control and off axis loading for single axis stacking, but under the more difficult conditions of stack & stitch, lack of good rail performance in both the Z axis and in X & Y can be very frustrating, especially of one is attempting to completely automate the process.

Anyway, just some random thoughts on this topic.

Best,


This indeed is a frustrating situation! It is the main reason I went away from doing the SnS manually with micrometer XY stage. Even though the stage was preloaded, there was enough variability in position repeatability that I had to set many manual alignment points in the stack. I tried ensuring that I always moved in one direction, but it's quite easy to mess this up, or not move far enough back before forward to get proper alignment, etc. The KR15 rails essentially eliminated this issue.
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1683
Location: Santa Clara, CA, USA

PostPosted: Fri Oct 05, 2018 6:56 am    Post subject: Reply with quote

Adalbert wrote:
Hi Ray,
Quote:
but repeatability suffers due to backlash and such

Yes, the biggest problem with the backlash occurs during the changing of the direction.
But if you stack by moving in the same direction you can get a repeatable step 140nm (it has been measured by Zerene).
http://www.photomacrography.net/forum/viewtopic.php?t=30220&postdays=0&postorder=asc&start=30
BR, ADi


Good ref, I remember reading that long ago when I first joined the forum.

It's reasonable IMO to constrain the system to only stack one direction, and with custom software it's not an issue.

I am still concerned about the mechanical limitation. How long would it take to move 100mm if the system is geared to a low ratio? And how reliable are these gearboxes?

Ultimately I'd wager that the Novoflex black box actuator is just a simple 400-step motor, and that the 0.2um claim is due to microstepping. I suppose we'll need to wait for the rail to be released, and someone with deep pockets to buy and test it, before we'll know. Or perhaps a query to Novoflex may help...
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Macro_Cosmos



Joined: 15 Jan 2018
Posts: 136
Location: Sydney

PostPosted: Sun Oct 07, 2018 9:38 pm    Post subject: Reply with quote

The official page boasts a .2um repeated resolution... which I am highly sceptical about.

No indication of the step resolution however, which is calculated as about .15um, which I am sceptical about as well. Ultimately, the price means I won't be able to have one for testing. Novoflex products are high quality whilst retaining the beautiful looks, something I like. However, performance is the most important factor.

It's a shame that they don't offer the rail for purchase, has to be bundled with their controller.

~ MC
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bralex



Joined: 22 Jan 2018
Posts: 33
Location: Colorado, USA

PostPosted: Mon Oct 08, 2018 7:55 am    Post subject: Reply with quote

I can just about follow this conversation, so this may be way off base. Noting the comments about gearing - I have run into circumstances before where a harmonic drive was used as a minimal-backlash and compact gear set. I know some of them get down to the 20 mm size range. Given the system price, I can conceive of one being incorporated. The gearing ratios might even allow the rated resolution without microstepping. Puts a sharp limit on the maximum speed though. Anyway, just a thought.

http://www.harmonicdrive.net/
https://en.wikipedia.org/wiki/Harmonic_drive

PS once I learned about them a few years ago, I picked one up surplus. They are super fun to play with, fascinating.
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1683
Location: Santa Clara, CA, USA

PostPosted: Mon Oct 08, 2018 8:28 am    Post subject: Reply with quote

bralex wrote:
I can just about follow this conversation, so this may be way off base. Noting the comments about gearing - I have run into circumstances before where a harmonic drive was used as a minimal-backlash and compact gear set. I know some of them get down to the 20 mm size range. Given the system price, I can conceive of one being incorporated. The gearing ratios might even allow the rated resolution without microstepping. Puts a sharp limit on the maximum speed though. Anyway, just a thought.

http://www.harmonicdrive.net/
https://en.wikipedia.org/wiki/Harmonic_drive

PS once I learned about them a few years ago, I picked one up surplus. They are super fun to play with, fascinating.


Wow, that's really neat! Never saw one of those before. Fascinating invention and looks like it could work well for small movements. Another thing to study...
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bralex



Joined: 22 Jan 2018
Posts: 33
Location: Colorado, USA

PostPosted: Mon Oct 08, 2018 8:44 am    Post subject: Reply with quote

ray_parkhurst wrote:


Wow, that's really neat! Never saw one of those before. Fascinating invention and looks like it could work well for small movements. Another thing to study...


I'd be happy to point to some surplus ones I've seen for sale, can't speak to their size/suitability, just something to play with...
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1683
Location: Santa Clara, CA, USA

PostPosted: Mon Oct 08, 2018 8:46 am    Post subject: Reply with quote

bralex wrote:
ray_parkhurst wrote:


Wow, that's really neat! Never saw one of those before. Fascinating invention and looks like it could work well for small movements. Another thing to study...


I'd be happy to point to some surplus ones I've seen for sale, can't speak to their size/suitability, just something to play with...


Sure, that would be good to start with...
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bralex



Joined: 22 Jan 2018
Posts: 33
Location: Colorado, USA

PostPosted: Mon Oct 08, 2018 9:03 am    Post subject: Reply with quote

ray_parkhurst wrote:


Sure, that would be good to start with...



One quick one - not sure if these disassemble. This place also sometimes has some built into motors that aren't otherwise labeled as harmonic drives, but I don't spot any in a quick look today.

https://hgrinc.com/?all=1&view&aisle&from&to&markdowns&newarrivals&sort&kw=harmonic&per_page=24&min_price&max_price&pn=1

I like the component set, I think I paid $40-50 for mine, eBay is crazy!
https://www.ebay.com/sch/i.html?_odkw=harmonic+drive&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=harmonic+drive+component&_sacat=0

I don't see any in my other usual nerd surplus places today. Should have looked before I offered!
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