Re: stepper vs servo

Jon Elson <elson@p...> wrote:

> Servos don't stall, unless the physical limits of the machine are

reached.

Trying to get exact meaning of terms:
stall = stop?
stall = stop temporily?
Servos don't stall = there are no conditions, however severe, that
they will stop/stop temporily when they are supposed to be moving?
When torque is insufficient, only the feedrate fall below the
commanded feedrate?

For steppers:
stall = stop?
stall = lose step?

> They can slow down, however. Some systems (the Allen-bradley 7320
> I worked with before, for instance) had a feature called feed-rate
> compensation.
> Whenever one axis lagged more than some limit behind commanded

position,

> all axes cut their feedrate in half, hoping that would allow the

lagging

> axis
> to catch up.

Meaning servos alway act in unison? When Y slow down, X will slow
down simultaneously, and there is no time-lag?

My feeble mind: The only way that X will know Y has slowed down is
when Y has already slowed down and a signal is issued that this has
happened, by which time X may already have proceeded some steps
beyond Y? By which time there will already be some deviation, even if
minute, from the planned path? Is the answer below:

> And, it is most likely that the lagging axis will leave extra

material in

> place,
> rather than remove extra material, and a later pass can clean it up.

Thanks in advance

Peter