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Topic: bipolar stepper losing torque (Read 704 times) previous topic - next topic

treebykooba

Oct 03, 2013, 05:12 pm Last Edit: Oct 03, 2013, 05:34 pm by treebykooba Reason: 1
hello,

I made a 3d printed zoetrope using an arduino to control the startup and RPM of a bipolar stepper (this one: https://www.sparkfun.com/products/9238)

I am using this code: https://github.com/xtremd/Zoetrope/tree/master/zoetrope

When I originally made it, I had a CNC'd piece of plexi around the shaft of the motor and everything was working fine. This is obviously temporary as the plexi began to strip a bit so I had a piece of aluminum made for the motor shaft with a set screw (the plexi just fit around perfect on the shaft)
Now, for a reason I do not understand, the motor is unable to spin the 3d print around.

Why would a stepper work okay with a crummy plexi shaft coupling and fail with a nice aluminum coupling? Both couplings weigh around the same but the 3d print weighs quite a bit so a small weight difference doesn't seem like it would matter.

I am pretty new to steppers and they are always really confounding me.

Powering it through an ATX psu...5amps. Would more amps help you think? Does that matter?

Any ideas much appreciated!!

MarkT

Resonance.   Resonance is the seldom-acknowledged bugbear of stepper motors and
is in practice what limits their performance.  The two basic steps to solving resonance
issues are using microstepping and adding viscous friction or other oscillation-damping
mechanical tricks.

The perspex coupling was clearly absorbing resonant oscillations more and stopping them
reaching the level where the motor mis-steps (which can easily cause stalls, double-speed
or random motion).

The other change is the MoI of the system, if the new coupling has significant MoI that
would affect the frequency of resonance.  Since different couplings change both friction
and MoI you may need to experiment a bit to see what helps (adding mass, trying a rubber
coupling, just increasing static friction)

But if you are using full or half-stepping you should consider moving to microstepping,
which removes a lot of the driving force behind these torsional oscillations in the first place.
[ I won't respond to messages, use the forum please ]

o_lampe

Remove the zoetrope from the shaft and see, if it is still not running. Maybe you've stripped a cable, while replacing the plexi-shaft?
It's not my fault when my Posts are full of Errors. This stupid autocorrection from my browser is set up for german grammar.

treebykooba

#3
Oct 03, 2013, 06:16 pm Last Edit: Oct 03, 2013, 06:40 pm by treebykooba Reason: 1
thanks so much for the feedback! when i remove the zoetrope from the shaft everything works fine. I am half stepping the motor, is that the same as microstepping? it's not whole stepping. i tried doing quarter steps and even 1/8 steps but that made it worse. i put it on whole stepping and the motor started up better but now the timing is off for the zoetrope effect. also...what do you mean by Mol? and i was also curious about amperage. would giving the circuit more amps help? right now it's getting 12amps which already seems like a lot...thanks again!

dc42

What stepper motor driver board or chip are you using? If you want to maintain torque at high RPM, you need a proper chopper-type driver.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

MarkT

microstepping is usually 1/16 or 1/32 or so to give good suppression of torsion harmonics,
its worth trying all the options.  You must use smooth acceleration profiles too, otherwise
the step-change in angular velocity is a big kick to the resonant system.

MoI is Moment-of-Inertia, torque =  moment-of-inertia * angular-acceleration,
analogous to force = mass * acceleration
[ I won't respond to messages, use the forum please ]

icio


Keep in mind that microstepping decrease the motor torque

polymorph

Perhaps things are not aligned correctly. The plexi maybe had more give than the aluminum, so now it binds too much to move.
Steve Greenfield AE7HD
Nick Gammon on multitasking Arduinos:
http://gammon.com.au/blink
http://gammon.com.au/serial
http://gammon.com.au/interrupts

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