Author Topic: Stepper Motor Selection  (Read 772 times)

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Offline jismal

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Stepper Motor Selection
« on: December 16, 2018, 01:37:47 am »
Hello Guys

Could you please suggest a stepper motor to rotate a load of 2kg.
 

Offline Brumby

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Re: Stepper Motor Selection
« Reply #1 on: December 16, 2018, 02:32:01 am »
What is the size and shape of the load?  Is the mass evenly distributed?  How many steps do you want per revolution and what is the fastest step movement you want to achieve?

I'm not up into steppers of that capability - but that looks like a big challenge.  Because of the (suggested) shape, the inertia would be significant and the acceleration and deceleration you would normally associate with a stepper would translate into rather large forces.

Edit:
You will need a good controller and properly tweaked settings - but that is true for most any stepper application.
« Last Edit: December 16, 2018, 02:37:44 am by Brumby »
 

Offline jismal

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Re: Stepper Motor Selection
« Reply #2 on: December 16, 2018, 02:43:40 am »
Mass is an evenly distributed.like a flywheel.1.8degree would be fine. Fastest movent will be around 10 revolution per minute.
 

Online beanflying

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Re: Stepper Motor Selection
« Reply #3 on: December 16, 2018, 02:56:34 am »
Providing it is in a fairly steady state (not accelerating or decelerating) a garden variety nema 17 should be fine but maybe consider an outboard bearing or supported shaft if your load isn't horizontal. So if as shown vertically run an extra bearing  :)
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Offline jismal

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Re: Stepper Motor Selection
« Reply #4 on: December 16, 2018, 03:07:02 am »
Thanks you
 

Offline Brumby

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Re: Stepper Motor Selection
« Reply #5 on: December 16, 2018, 03:08:20 am »
Mass is an evenly distributed.like a flywheel.1.8degree would be fine. Fastest movent will be around 10 revolution per minute.

You didn't provide the most important detail - SIZE.  What diameter is this disc?

Next question is on motion:  Are there start and stop movements or is it a continuous rotation?  If continuous, why do you want a stepper?
 

Offline jismal

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Re: Stepper Motor Selection
« Reply #6 on: December 16, 2018, 03:10:16 am »
Not a continuous rotation. Also dia would be around 15cm
 

Offline Nusa

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Re: Stepper Motor Selection
« Reply #7 on: December 16, 2018, 03:11:59 am »
If you want the physics and math, this is a decent writeup:

https://www.orientalmotor.com/technology/motor-sizing-calculations.html
 
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Offline Brumby

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Re: Stepper Motor Selection
« Reply #8 on: December 16, 2018, 03:13:44 am »
That is going to have some serious inertia issues.

How critical will skipped steps be for this system?
 

Offline jismal

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Re: Stepper Motor Selection
« Reply #9 on: December 16, 2018, 03:21:27 am »
I was considering a stepper with optical feedback to reduce the step skipping.
 

Offline Brumby

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Re: Stepper Motor Selection
« Reply #10 on: December 16, 2018, 03:22:44 am »
This is intriguing.  If I may ask - what is the application?
 

Offline Nusa

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Re: Stepper Motor Selection
« Reply #11 on: December 16, 2018, 03:24:44 am »
For prototyping at least, I'd suggest a belt-driven setup rather than direct drive. Then you are in a position to change the mechanical advantage of the motor simply by changing pulley sizes.
 

Offline jismal

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Re: Stepper Motor Selection
« Reply #12 on: December 16, 2018, 03:27:12 am »
Its a vertical scanner. I think i need to game up my mechanics
 

Online beanflying

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Re: Stepper Motor Selection
« Reply #13 on: December 16, 2018, 03:27:52 am »
With that diameter wheel and speed changing at a minimum you need to use gear or belt reduction to get more torque at the shaft. So your wheel now need to be on its own shaft supported at both ends with bearings.

Without checking a specific model Nema 17's will run at a few hundred RPM but 6 up to maybe 10 to 1 is possible without to much fuss in a single reduction so you multiply (rough but simple) the torque by the reduction. This gets you in the order of 2-4Nm (Nema 23 is about double this) depending on motor choice, reduction and layout.

You will then need to run at least some rough numbers on how fast you need to get to or from your 10RPM (or from Xrpm to Yrpm) to set the driver properly to avoid missed steps or overloading your system. If you can't meet your acceleration needs then you need to look at a larger motor and run the numbers again.

« Last Edit: December 16, 2018, 03:31:14 am by beanflying »
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Offline Brumby

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Re: Stepper Motor Selection
« Reply #14 on: December 16, 2018, 03:38:36 am »
Its a vertical scanner. I think i need to game up my mechanics

Yeah - so do I.

My biggest concern was the direct drive approach.  As suggested above, changing to a belt drive setup on the rim or with a large pulley will give you much better mechanical advantage - and you will be able to drive the stepper in the operating area that is better matched to its strengths.
 

Offline saike

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Re: Stepper Motor Selection
« Reply #15 on: December 16, 2018, 11:25:32 am »
I have just been trying some of the Ebay special, Far East nema23 steppers with encoder feedback and within their limits they work very well, but, as they come, it doesn't take much inertial load for them to start oscillating around a set position. There is software available to retune them but if speed permits it is easier to vary the gearing.
 

Offline TK0130

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Re: Stepper Motor Selection
« Reply #16 on: December 16, 2018, 08:52:31 pm »
Best price/performance cheap Chinese stepper I have found is this:
https://www.ebay.com/itm/Nema-17-Stepper-Motor-Bipolar-84oz-in-59Ncm-CNC-3D-Printer-Reprap-Robot/131904469344?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
More than enough for a 3D printer. However, for your application it sounds like you want a geared stepper.

3.7:1 seems to be a common ratio among cheap planetary gearboxes. (as low as $20 if you are willing to wait a while)
 

Offline Brumby

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Re: Stepper Motor Selection
« Reply #17 on: December 16, 2018, 11:52:15 pm »
I'm not sure a geared stepper would be a better idea.  Certainly the gear train would reduce the load on the stepper motor itself - but with the inertia of the flywheel, the gear train will have every ounce of play and backlash exercised every time the stepper starts and stops.

In fact, I have to wonder what is the purpose of the disc.  If it is to smooth out the motion, then there is a fundamental conflict between that sort of motion and the inherent nature of a stepper motor - specifically, a stepper motor needs to function in a stop/start manner on each step in order to function correctly.  You could add positional feedback to help with this, but that is an extra step in complexity and will need to be "tuned in", especially for a system with high inertia.


Without meaning to offend, what you have done here is select a solution and ask for assistance in finding the component that will do the job, rather than define the requirements and then ask for input on possible solutions.  This tends to lead into (potentially long) discussions that wander back and forth trying to fit a round plug into a square hole.  Don't feel embarrassed by this, we see it quite frequently here where the solution chosen is not optimal or has its own set of problems.

If you feel comfortable with sharing a little more detail about the requirements of the motion system, you might find some alternative ideas.  Details such as how often there will be start and stop actions; how much travel will occur in each movement; acceleration/deceleration; the total mass being moved and how (Is this disc the only mass that is being moved - or is there something else?) ... and so on.

You can sometimes find ideas offered by people who may have had similar requirements for a past project - and not only would you then have access to some possibly more suitable solutions, but to the problem solving that went with them.


Just a suggestion.
 


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