How do the Tradeoffs Work? - Steppers for Horizontal Rails

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Doppler9000
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How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by Doppler9000 »

I hope its okay to post these related topics as separate items.

IN sizing a motor, it seems like the end goal is to have a stepper with enough power for the task but little extra.

If this is true, the optimally-sized motor for a dedicated horizontal rail should be significantly less powerful than one for vertical operation.

First, in terms of the practical realities is it a worthwhile exercise to try to semi-optimize for the lower power requirements of the horizontal orientation?

Assuming it is worthwhile…

It seems that 0.9* motors offer some net advantages over 1.8* alternatives at a somewhat higher price.

In terms of the other specifications, as always, there are tradeoffs, and I am hoping to better understand them.

The motors that got me interested in learning more are some Lin (Moons-owned), 0.9*, ‘zero detent’ units.

Low inertia, zero detent torque, and up to 0.2 N-m holding torque.

https://www.moonsindustries.com/products-launch/z417

To approximately what magnification is this motor powerful enough to drive a horizontal KR2601B (2mm pitch) with single outrigger idler rail reliably (StackShot 3 controller)? Max load would be 5kg or less.

Is low inertia a net positive or negative for a macro rail?

More generally, I wondered how one would go about establishing a system’s minimum power requirements, then analyzing the tradeoffs of adding more power due to concomitant changes in inductance, detent torque, resistance, etc., as well as deciding when to go up in frame size.

Also, I wondered if anyone had tried the Lin above or their 0.45* high torque, high inertia motors.

Chris S.
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by Chris S. »

Doppler9000 wrote:
Sun Sep 19, 2021 7:26 pm
I hope it’s okay to post these related topics as separate items.
Not only okay, but quite a good way to do it. Separate threads on different topics are likely more valuable for people doing research later on.
In sizing a motor, it seems like the end goal is to have a stepper with enough power for the task but little extra.
I don’t think I’d agree with that appraisal. I’d get a stepper with a moderate amount of extra power—nothing fancy or expensive—and not overthink it. A decently designed macro rig isn’t all that picky about which stepper motor you use. No one could accuse me of not being persnickety about the quality of my macro rig, but the stepper motor I use for focus stacking is not an especially good one. You’ll recall that I found it online, and the folks at Cognisys did me the favor of terminating and testing it for me. They were not particularly impressed with this motor, and noted that it has more runout than the motors they use. But nothing about this motor has any negative effect on my work, which is often at 100x. If the motor were causing any hardship, I’d have upgraded it in a heartbeat, as doing so is not difficult.

I’ve since done a few other projects with stepper motors, and I get better ones than that first one, but nothing fancy. There are now two stepper motors in my macro rig. The second stepper is on a large Velmex linear stage that can rapidly move the camera rig (including the focus-stacking assembly) a good distance away from the subject stage and (later) back again. This allows me to change lenses without bumping the subject or lighting. Although there is nothing special about this stepper, it invariably returns the camera to the exact point at which it was previously focused. Since this is true even with microscope objectives having sub-micron depth of field, I think this motor is doing fine.

My most recent project involving steppers is building a pair of atomic absorption spectrophotometers for a university research lab. Although their measurement resolution is two nanometers, the mechanics aren’t all that picky. For these, I’m using 0.9 degree steppers that cost less than $22 each.

All of these steppers are somewhat more powerful than they need to be, but not enormously so. I haven’t attempted to calculate how much power is actually needed, as I don’t think it matters much.

Gut feel, the stepper motor you linked to is smaller front to back than I'd go with. Equally using gut feel, I do like the NEMA 17 frame size, which I think is a good fit for much of what photomacrographers do. I suspect, but am not certain, that 0.9 degree steppers offer more real-world precision in my builds than 1.8 stepper motors. I've wondered if they really do, but haven't cared enough to test.

There are of course other use cases for which stepper choice is more critical. Or if carefully specifying your stepper is an element of the build that is interesting to you, there are members of this forum who have gone much further into the weeds than I have, and will likely offer their thoughts.

--Chris S.

chris_ma
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by chris_ma »

Chris S. wrote:
Sun Sep 19, 2021 10:28 pm
I suspect, but am not certain, that 0.9 degree steppers offer more real-world precision in my builds than 1.8 stepper motors. I've wondered if they really do, but haven't cared enough to test.
I've read somewhere that the 0.9 deg motors are more difficult to microstep, but that source was not really scientific, and in practice it doesn't seem to make too much of a difference.
what seems more important is to use a high quality motor driver.
chris

JohnDownie
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by JohnDownie »

Chris S. wrote:
Sun Sep 19, 2021 10:28 pm
I’d get a stepper with a moderate amount of extra power—nothing fancy or expensive—and not overthink it.

--Chris S.
Good advice for car buying, as well.

Thanks, Chris.

John

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by ray_parkhurst »

Those Zero Detent Torque motors are very iteresting. I can see them being useful for horizontal systems, and the zero torque would make accurate microstepping much easier. For vertical systems you probably want at least a little detent torque to ensure the system does not slip. Some folks here seem to have problems with vertical slip, so would need more detent torque to keep the system from slipping down.

For most horizontal systems, very little torque is needed. But for vertical systems, obviously more is required given all else equal. But "more" is relative, and if you actually measure the torques needed they are quite small relative to typical rated motor torques. In fact, if you are building a system from scratch, I'd recommend measuring the torques required for movement and holding of the system. Overcoming static friction is the limit, so the measurement should be done slowly, and the torque required to "break free" the system recorded as the absolute minimum required. I suspect you'll be hard pressed to find a motor that has that low a torque, so a broad range of motors should suffice.

Note also that "holding torque", ie the torque provided by the motor when driven by DC (non-stepping), is irrelevant for our applications. This is because most controllers will turn off the current drive after some time to keep from overheating the motor, and at some point you'll probably turn the power off, so the motor will be unpowered and thus revert to the "detent torque". No issue for horizontal systems, but may have impact on vertical systems if the detent torque is insufficient to keep the rail from turning.

mjkzz
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

0.9 degree motor helps a lot for smooth motion, this might not be significant for low magnification, but for higher magnification smooth motion, ie, less vibration can be very helpful.

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

ray_parkhurst wrote:
Tue Sep 21, 2021 5:15 pm
Note also that "holding torque", ie the torque provided by the motor when driven by DC (non-stepping), is irrelevant for our applications. This is because most controllers will turn off the current drive after some time to keep from overheating the motor, and at some point you'll probably turn the power off, so the motor will be unpowered and thus revert to the "detent torque". No issue for horizontal systems, but may have impact on vertical systems if the detent torque is insufficient to keep the rail from turning.
I would not recommend turning power off when idling, but lower the current so that there is some kind of force to make the whole system "rigid", ie, staying at same state as before (at higher running power).

One example is a rail with a lot of wobble, one method people use is to apply a directional force, like a rubber band, so to keep your system steady, to overcome this directional force, regardless horizontal or vertical, the "holding torque" is needed.

Another good practical experiment is the first time you turn on the whole system, regardless how good your rail system is, you will observe some kind of change, maybe under higher magnification to observe it. This can be observed when you turn off your system, too. The bottom line is, whenever you turn off the power, the state of your system changes.

Lowering the current and still maintain the state of your system, as well as keeping your motor cool is more desirable.

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by ray_parkhurst »

mjkzz wrote:
Tue Sep 21, 2021 6:55 pm
I would not recommend turning power off when idling, but lower the current so that there is some kind of force to make the whole system "rigid", ie, staying at same state as before (at higher running power).
...
Lowering the current and still maintain the state of your system, as well as keeping your motor cool is more desirable.
In your older mjkzz controllers, the running and idling current can be chosen independently, which is very convenient. As an example I typically run 0,5A running current, and 0.125A idling. However, after a fairly short time, the idling current turns off, leaving the motor in "detent torque" mode. Even if it did not turn off, and I am not sure if your current controllers turn off, or if controllers from WeMacro or StackShot turn off, at some point the system will be powered-down, and no current will be present. I turn my system off at night, so each night the motors go to detent torque. I think it is not prudent to count on any level of holding torque to ensure a system does not slip (crash!) down.

mjkzz
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

ray_parkhurst wrote:
Tue Sep 21, 2021 7:19 pm
mjkzz wrote:
Tue Sep 21, 2021 6:55 pm
I would not recommend turning power off when idling, but lower the current so that there is some kind of force to make the whole system "rigid", ie, staying at same state as before (at higher running power).
...
Lowering the current and still maintain the state of your system, as well as keeping your motor cool is more desirable.
In your older mjkzz controllers, the running and idling current can be chosen independently, which is very convenient. As an example I typically run 0,5A running current, and 0.125A idling. However, after a fairly short time, the idling current turns off, leaving the motor in "detent torque" mode.
No, it should not turn motor off, the idle current should stay at set level, ie, in your case 0.125A, if not, let me know, I have not heard any complains about this. It might be that 0.125A is not enough to "feel" it is still on. One way to test it is to set it to 0.375A and see if the motor is off. The idle time is 10s, if no movement, the controller will go to idle state and set phase current to idle level.

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by ray_parkhurst »

mjkzz wrote:
Tue Sep 21, 2021 7:48 pm
No, it should not turn motor off, the idle current should stay at set level, ie, in your case 0.125A, if not, let me know, I have not heard any complains about this. It might be that 0.125A is not enough to "feel" it is still on. One way to test it is to set it to 0.375A and see if the motor is off. The idle time is 10s, if no movement, the controller will go to idle state and set phase current to idle level.
Perhaps it does not turn off, but my point is that it is impossible to always keep the idle current on. You would need to guarantee it is on 24/7.

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

Yes, it is 24/7 as long as you have power plugged in with my controllers, all of them as matter of fact.
I think it is not prudent to count on any level of holding torque to ensure a system does not slip (crash!) down.
It is prudent to prevent over heating for step motors with permanent magnet inside (most of them do, except one type, I forogt), as we all know what heat does to magnets

mjkzz
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

I think we are seeing things from different angles. My initial response to the "powering down" issue is from a system designer point of view, powering a motor is a significant contributing factor in the system, removing or changing that factor will have impact on your system.

I now see your point of view -- from user point of view, the system is powered down by operator, so it is prudent NOT to crash down when powering down by operator. Sure, 100% agree, but the right solution is NOT to have a system set up like that (I deleted the video, people start to subscribe to it) -- crashing down.

But this does not mean having idle power is not necessary. I have played with TMC drivers and love them, and one key feature is idle power, TMC implementation is much advanced, it does not drops current level suddenly, there is a gradual drop, but it means this feature is rather important.

Also think about this, when step motor is idling, little mechanical work performed, yet it is being powered at same current as running power, where does energy go? Heat is one way . . .

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

Here is one concrete example of "holding torque". I had a vertical setup using THK2001A which is known for its efficiency, so under heavy load (it was about 3.5kg), it definitely slides down, albeit slowly, if I do not power it up. For such heavy load, idle power need to be set to about 0.375A to hold it without slipping. Also, to move it, I had to set running power to 1.5A.

Well, we all know how long it will take to position the subject, to get the right composition and then finally start stacking. During this time, if the motor is running at 1.5A, it will become very hot, and it is not good for the motor. Running at 0.375A idle power, the motor was barely warm yet the rail is held securely. But if the power is removed, the rail will slide down.

Of course, in case of power outage situation, I installed limit switch so that it can stop the rail from sliding (not electrically, but mechanically/physically) too far to damage the lens.

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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by Pau »

Maybe off topic, but, at least for high magnification and not high suspended weight coupling the motor to a microscope focus block will be superior both for precision and to prevent sliding down
Pau

mjkzz
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Re: How do the Tradeoffs Work? - Steppers for Horizontal Rails

Post by mjkzz »

OK, just for the purpose of education, here is an experiment one can perform:

Get a step motor, say a bi-polar one, not powered up, try to turn the shaft with fingers, one would feel certain effort to turn it.

Now, connect the two wires for the same phase together for both phases, forming closed circuit, try to turn the shaft again, what do you expect? I am sure many here know this and I will keep it interesting for people to experiment.

Now, connect that motor to a step motor driver, before powering it up, feel the torque, for SOME driver, you would feel the same as if the wires were connected, for some OTHER drivers, it would feel like open looped one, meaning as if the motor is not connected to anything. To power down a step motor driver, mostly is to inactivate the EN pin (either low or high), some driver might disconnect power from the motor, forming an OPEN loop, leaving the motor in detent state.

However, keeping the driver in a low current state means keeping the motor in a closed circuit loop, for either type of driver, hence avoiding detent state.

Anyways, I am just trying to provide some fun info here because it was fun for me when I played around with step motors.

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