Re: [CAD_CAM_EDM_DRO] stepper motor accuracy

It is a typo, they meant to say brake, not break. A mechanical brake will
hold until its mechanical limit is exceeded, but the shaft of the stepper
motor acts as a torsonal spring and deflects a certain rotational amount
which causes a movement before the torque rating is exceeded. I am not sure
any of this makes a damn bit of difference when it comes to CNC machining.

Dan.

----- Original Message -----
From: <palrajas@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, October 05, 2001 7:32 PM
Subject: [CAD_CAM_EDM_DRO] stepper motor accuracy


> Hi list,
>
> Trying to figure out what is involved in determining the accuracy of
> a stepper motor.
>
> Some extracts from Gecko's "ACCURACY AND RESOLUTION" for stepper
> motors.
>
> Finally, the static or frictional load applied to the motor affects
> accuracy. A stopped step motor, which has 100 oz/in of holding
> torque, is fundamentally different than a break that has the same
> holding torque.
>
> The break will not turn at all until its holding torque is exceed.
> However a step motor only generates restoring torque if it is
> displaced from its rest position Using the brake analogy, think of
> the output shaft being connected to the break with a torsional
> spring. Now when applying a load, the output shaft has to be radially
> displaced to apply torque to the break.
>
> When torque sufficient to overcome the holding torque is applied to a
> step motor, the shaft will jump to the next stable location, which is
> 4 full steps ahead or behind the original one, depending on which
> direction the load is applied. Peak restoring torque occurs a full
> step ahead or behind the original location, beyond which it weakens
> and reverses at the 2 full step ahead position to attract the shaft
> to a full step location ahead or behind the original one.
>
> QUESTION:
> What is "a break that has the same holding torque"?
> http://www.denford.com/
> What is the above trying to explain?
>
> What relevance has the above to, say, a step motor controlling the
> movement of the x-axis of a milling machine? The torque mentioned
> above originate from where in this milling machine - the frictional
> force that prevent the tool from moving in the x-direction or the
> turning force from the current applied to the coils of the stepper
> motor?
>
> Thanks
>
> Pal
>
>
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