Re: Do Stepper Motors need "Drag?"

On 5 Nov, tom.kay@... wrote:
>
> Thanks for the info. I am pretty sure I understand what you mean by
> "watch the acceleration or ramping" but just to be sure, do you mean
> that the problem of overshoot or losing steps is biggest as you
> start, or reach the end of a move? And this can be solved by
> deccelerating smoothly instead of crashing to a stop, thereby giving
> the motors time to slow down and dissipate their energy?
>
Yes.

The force applied by the motor is to overcome friction and to accelerate
the table mass. When stopping, the friction helps but there must also be
a force to deccelerate the table mass. If the controller tries to
accelerate or deccelerate the table too fast it asks the motor for more
torque than it can deliver. With a stepper this means lost steps.

I think we want to stay far away from this. Lost steps in a stepper
system is a very bad thing.

This should apply at all table speeds. However, as I understand it,
steppers can give less torque at higher speeds, so the problems will
appear first in rapid moves.

I would think you'd be better off with the least friction possible. This
should give the best performance for a give motor.

> And can this be typically all pre-programmed into the software so
> that the whole system knows to slow down at the end of cuts?
>
At least in EMC you can specify the maximum acceleration. I suspect
other control software has the same option.

Hugh Currin
Klamath Falls, OR