Re: Do Stepper Motors need "Drag?"

--- In CAD_CAM_EDM_DRO@y..., tom.kay@n... wrote:

> Hi Hugh
>
> 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?
>

Tom,

When you originally asked if drag would help with overshoot
on stepper motors, I assumed you were talking about resonance
problems, and I still think that is what the person who told
you this was commenting about.

Steppers can also lose or gain steps due to the effects of
inertia on the mass they are driving. This is a different
problem from the resonance one and can be controlled with the
acceleration paramaters in stepper controller. As noted before,
steppers have less torque at higher speeds. An ideal controller
would take this into account when generating pulse streams, but
as best I know this is not done and only a limit to maximum
acceleration and maximum speed is utilized rather than trying
to figure out how much torque is available at the current
speed and how much of it is consumed by acceleration versus
other loads (drag, cutting forces) on the system.

A related problem is the lost steps that occur with uneven pulse
streams from some controllers is due to the need for "instant"
acceleration to keep in sync with the uneven pulse stream.

Resonance is a different problem that can exist at critical
speeds even if acceleration is zero. It is this problem that
can be reduced by drag in the mechanical system or by micro-
stepping or other techinques in the driver electronics.

Hopefully this all makes sense and is reasonably accurate.

Regards,
Steve Stallings