### Author Topic: Just got new MINI steppers. What is the smallest way to do PWM?  (Read 3560 times)

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#### waltr

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##### Re: Just got new MINI steppers. What is the smallest way to do PWM?
« Reply #30 on: July 09, 2013, 06:46:32 PM »
Ahh...a four wheel drive buggy with a stepper on each wheel.
How fast do you want it to go?
Geared or direct drive?
How large are the wheels (diameter)?
How steep of an incline? surface roughness? or a smooth, flat, level surface?

With my PIC test code I was running 4 steps in 100ms. I counted 10 steps in the motor so this works out to 240 RPM. The torque seemed to still be good so the motor could go faster with some PIC code changes.
If the wheels are 2cm diameter and are directly driven for the motor and the H-bridge delivered 3.0V to the coils then each wheel would push along the surface with a force of 0.012 Newtons.

References:
http://en.wikipedia.org/wiki/Torque
http://en.wikipedia.org/wiki/Force

#### ROBOT420

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• The Geeks shall inherit the earth!!
##### Re: Just got new MINI steppers. What is the smallest way to do PWM?
« Reply #31 on: July 10, 2013, 09:19:09 AM »
Sir Newton would be proud! LOL I have a new TX RX coming in the mail and I guess I need to weigh the RX and battery before we can figure the rest. I will have a look around for some lightweight rims/wheels. Thanks, again!
Don't know jack, but want to know it ALL....let the journey begin!

#### waltr

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##### Re: Just got new MINI steppers. What is the smallest way to do PWM?
« Reply #32 on: July 10, 2013, 07:51:37 PM »
Forgot to point you to the motor calculator in the Robot Tutorials on this web site. That can give you an idea as to how much weight in a 4 wheeled buggy these steppers could move.

The calculator says 20gms total weight (mass) with four direct drive wheels (no gear reduction) on a smooth, level surface at 3V.
If you do a 2:1 reduction then the torque doubles (not considering gear losses) and the buggy weight increases to 37gms. A greater gear reduction increases the torque and load capacity but decreases to top speed.
How fast have you gotten these stepper to run?

Still to measure on these steppers is the coil inductance and try for the maximum speed.

Do you have some H-bridge circuits built?
Wheels could be made from thin plywood or balsa wood (local hobby shop) and glued to the gear on the stepper.

#### ROBOT420

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##### Re: Just got new MINI steppers. What is the smallest way to do PWM?
« Reply #33 on: July 11, 2013, 06:15:15 PM »
Good thinking! I have a few parts in mind, I will add them soon. Let me know when/if you need some more of them. Have a good one, and enjoy those steppers, Steve.
Don't know jack, but want to know it ALL....let the journey begin!

#### waltr

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##### Re: Just got new MINI steppers. What is the smallest way to do PWM?
« Reply #34 on: July 14, 2013, 04:35:37 PM »
Here are the results of more measurements.

These have 20 Steps per revolution which is 18° per step.

The maximum speed I was able to achieve is 2860 RPM. Just over this speed the stepper motor stopped running which probable hit its resonance frequency (torque goes to near zero).

Measurement setup:
To measure to RPM I used an opti-interrupter salvaged from an old printer. The interrupters output pulse time was measured with an O'scope. I left the wood arm that I glued onto the motor's gear for the torque measurements and aligned the opti-interrupter to have the wood arm pass through the interrupter. Therefore, for each revolution of the motor I see two pulses on the scope.
First picture is the motor running and the opti-interrupter. Note the blurred wood arm.
Second picture is from the scope. Top trace is interrupter output, bottom trace is the Voltage at one motor coil. This only shows a half a revolution.
« Last Edit: July 14, 2013, 04:37:16 PM by waltr »

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