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Topic: Step motors with integer steps per degree (Read 2277 times) previous topic - next topic

AngeloGomes

Guys, I am working on a project for a experimental setup in the Labs where I need to put a sample in a fixed angle from the magnetic field direction. For this , I am using a step motor and a dial. The problem I have facing is that I donĀ“t have integer steps to rotate the motor to some angles.

example:

2048 steps for 360 degres gives 5.688 steps/degree.

I would like to have this precision of 1 degree, at least.

Do you guys have some tips (electronics or motors) for this ?


Regards.

Angelo

TomGeorge

Hi,

You would probably do better with a DC motor and gearbox, with an encoder connected, to give you position feedback, with a resolution more suited to your 1deg requirement.

Or put a gearbox on the stepper and use the encoder to give you positive position feedback.

Is 1deg steps fine enough, or do you need fractions of degrees?
What is the overall application with the bit of lab gear.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

jremington

Quote
2048 steps for 360 degres gives 5.688 steps/degree.

I would like to have this precision of 1 degree, at least.
With this setup, you have a precision of better than 0.2 degrees per step. What is the problem?

Johan_Ha

To reach exact degrees like 1, 2 or 3 steps per degree, you need a gearbox with some exact ratio. Perhaps 3D printed cog wheels for standard dent belt, but with your own cog ratios to accieve 360 steps per revolution.
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AngeloGomes

Thank you guys.

Well, I would like to rotate the sample in 1 degree steps. In each degree step, I stop the motor and then perform a resistivity measurement in the sample.

Someone said something about a dc motor, I did look for some small ones, and I found one with an encoder. The max speed is 100 RPM. In my case, to obtain 1 degree resolution, I think the motor should have low rpm, but even 100 rpm would give 600 degrees/sec ou 1 dg every 1.67 msec, in other words, I had start and stop the motor within 1.67 ms !!

Back to step motors: I've been trying the 28byj-48, but the main problem is the motor displacement is not reproducible when it rotates back and forth sometimes.

jremington

#5
Apr 12, 2018, 05:38 pm Last Edit: Apr 12, 2018, 05:38 pm by jremington
Quote
the main problem is the motor displacement is not reproducible when it rotates back and forth sometimes.
That is called backlash, and the problem is easily solved.

Just make sure you approach the final position from the same direction of rotation, every time. That may require you to overshoot the new position by some fixed amount, and then back up.

Paul_KD7HB

That is called backlash, and the problem is easily solved.

Just make sure you approach the final position from the same direction of rotation, every time. That may require you to overshoot the new position by some fixed amount, and then back up.
If you are lucky, you can find spring-loaded split gears which will eliminate the back lash problem.

Paul

Southpark

#7
Apr 12, 2018, 10:35 pm Last Edit: Apr 13, 2018, 02:49 am by Southpark
2048 steps for 360 degrees gives 5.688 steps/degree.

I would like to have this precision of 1 degree, at least.
Somebody already asked you what accuracy you need. You haven't told them yet.

Somebody else mentioned mentioned 0.2 degree per step, or 0.176 deg per step for 2048 steps over 360 degrees.

Could possibly - look into half-stepping .... maybe.

'Precision' is something like a variance around some kind of averaged angle value (from repeatability measurements - like..... if you measure a something, and you measure and again, and again, and again and again...... you'll get an average...and a spread around that average.... giving some idea about repeatibility). So, at this point, you'll probably need to say what you need... like +/- 0.2 degree maximum uncertainty (relative to the value).

TomGeorge

#8
Apr 13, 2018, 12:29 am Last Edit: Apr 13, 2018, 12:32 am by TomGeorge
the main problem is the motor displacement is not reproducible when it rotates back and forth sometimes.
If you put the encoder on the output shaft that you want to position, then you will be avoiding the backlash.
Place a constant load on the output shaft for the motor to work against, such as a rubbing device in contact with the output shaft.

Look for encoders with many thousands of steps per revolution, not 360 or 720.
5000 counts per revolution will give   5000/360 = 13.88 counts per degree.
0.072 Deg resolution.

Also you do not need to have the encoder connected directly to the output shaft.
It can be on another shaft driven by the output shaft, that has the required gear ratio to give your precise encoder outputs per degree.

You need to talk with encoder distributors of  SICK, IFM, Turck, industrial encoders, they will be expensive, but that is what you pay for precision/consistency.
If you chat with the right sales rep or technician that is is turned on to this sort of thing, you may have some quick success.

ANOTHER solution, you have the output shaft/disk lock into mechanical detents that are 1 deg apart, no need for precision encoder just something to get you into the ball park and the detent or locking pin will pull the shaft to exactly integer degree positions.

There is a servo resolver solution, but that is very expensive and has some specific hardware requirements.

Just some thoughts.

Tom..  :)
(PS read my side bar  to this post.)
Everything runs on smoke, let the smoke out, it stops running....

outsider

#9
Apr 13, 2018, 07:07 am Last Edit: Apr 13, 2018, 07:22 am by outsider
You could do it with a 200 step per revolution stepper and a gear reducer with a 5:9 ratio (if you could find it) or a cog belt / sprocket contrivance with 10 teeth on the motor sprocket and 18 teeth (or 36 or 72, etc.) on the driven sprocket. A gear reducer of 45:1 would give 1 degree on the output to 25 steps on the input.

Johan_Ha

#10
Apr 13, 2018, 07:10 am Last Edit: Apr 13, 2018, 07:12 am by Johan_Ha
If your stepper motor makes 2048 steps per revolution and you want a shaft that can perform exact degrees (360 steps per revolution), have a toothed belt wheel at your stepper motor with 128 teeth, a wheel at the shaft with 225 teeth. Ten steps will rotate the stepper motor 10/2048 revolutions. It will rotate the shaft 10/3600 revolutions.

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10 / (128 * 8) and 10 / (225 * 8)

 The 8 is there to make the cog wheels as small as possible. So, to turn one degree, your stepper motor should make 10 steps.
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If you ask for help and write 'u' instead of 'you' because you think it's convenient, I will write 'no' instead of 'yes'. For same reasons.

Southpark

#11
Apr 13, 2018, 06:06 pm Last Edit: Apr 13, 2018, 06:06 pm by Southpark
If your stepper motor makes 2048 steps per revolution and you want a shaft that can perform exact degrees (360 steps per revolution), have a toothed belt wheel at your stepper motor with 128 teeth, a wheel at the shaft with 225 teeth. Ten steps will rotate the stepper motor 10/2048 revolutions. It will rotate the shaft 10/3600 revolutions.
Nice. Are those 128 teeth and 225 teeth components readily available online?

Johan_Ha

#12
Apr 13, 2018, 09:26 pm Last Edit: Apr 13, 2018, 09:38 pm by Johan_Ha
I have no idea. But they could be 3D printed.

With 32 and 45 teeth you could get a working setup. But according to this page,  there are no wheels with 45 teeth.
____________________

If you ask for help and write 'u' instead of 'you' because you think it's convenient, I will write 'no' instead of 'yes'. For same reasons.

AngeloGomes

Somebody already asked you what accuracy you need. You haven't told them yet.


Oh sorry.

at most 0.5 deg error would be fine

Southpark

#14
Apr 15, 2018, 02:57 am Last Edit: Apr 15, 2018, 02:59 am by Southpark
Oh sorry.

at most 0.5 deg error would be fine
Angelo..... the 2048 steps per revolution stepper will nicely handle your requirement.

Each step for your stepper covers approx 0.176 degree. If you use half-stepping, then that could get down to approx 0.09 degree per half-step.

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