MechMate CNC Router Forum

Go Back   MechMate CNC Router Forum > Common Assemblies & Parts > Motors & their mountings
Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Reply
 
Thread Tools
  #1  
Old Wed 15 July 2009, 02:05
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Position accuracy & torque of stepper before slip & slip amount

Interesting posts on the Geckodrive Yahoo group last night:
(from http://finance.groups.yahoo.com/group/geckodrive/message/19229)

*************************************
Hi,

Rather than doing the research myself , does anyone know if a stepper will
hold its position at rated torque, or will there be some error? If so, is it
up to a full step?

Also my guess is that at a full step position the accuracy is greater than at
a microstep position.

Cheers,

Peter.

************************************************** *
Yes, standing still, at rated holding torque, the error is almost
exactly two full steps.
If the error ever exceeds 2 full steps, the motor will jump to the next
detent, which is every 4 full steps.
Note that when running, the motor cannot deliver the holding torque, but
something quite a bit less, and any load that approaches the 2 full
steps lag is dangerously close to stalling the motor.

Jon

************************************************** *********
Hi,

not exactly, I think the correct formula is

torque = sin (pi/2 * dp) * rated torque

with dp = position deviation in full steps. So the maximum torque is
delivered at one full step from the unloaded position. It decreases to zero
between one and two full steps and becomes negative between 2 and 4 full
steps and that's why the motor jumps 4 full steps when overloaded. Most
manufacturers specify 20% tolerance for the max. torque and up to 5%
distortion from the ideal sine waveform. Microstep or not does not matter,
only the absolute value of the current vector counts. Actually,
microstepping can result in higher torque when running because less
resonances means less risk for stalling.

One tricky thing is that the max holding torque is reached only if both
windings are energized with the full rated current what results in a vector
of 1.41 times the rated current. You cannot run the motor continously with
that current level at any speed considerably higher than zero or the motor
will overheat. If you run the motor with only one winding energized (or a
current vector of 1.0 times rated current when microstepping) you will only
get 0.71 times the rated torque. So the rated max. holding torque is a
"marketing trick".


Best Regards

Nicolas Benezan
Reply With Quote
Reply

Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -6. The time now is 15:34.


Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.