Re: [CAD_CAM_EDM_DRO] close loop and stalling

george_barr wrote:

>I have a question regarding "close loop" systems using stepper motors
>with encoders. The software is EMC. What happens when the motors
>accelerate to fast and stalls using the above setup? Does it stop
>all the motors when it stalls?
>

Yes. You set the following error limits such that under normal
operation, the limit is just above
the normal following error. If the following error exceeds that value,
an emergency stop
is called, and all axes are stopped.

> Or, does it try to correct the motors
>until it arrives at the determined position? I read somewhere that
>with a close loop system you can run the motors as fast as you can
>and let the encoders compensate for slow motors and it will do the
>routing correctly. Can someone explain some of these things?
>
>

This will work with servo motors, which do not have the abrupt stalling
characteristic of
steppers. If a servo motor is driven to the speed or torque limits set
by the servo amplifier, it
will continue producing torque at that limit, just no more. If a
stepper motor is driven
so fast that it loses a step, it usually falls severely out of sync, and
just buzzes and vibrates,
with little or no movement. Encoders cannot fix this, but they can
detect it. If a stepper
misses an occasional step at a discontinuity in speed or acceleration,
the encoder feedback
can correct it. But, if the motor is driven so hard it stalls, then a
large following error will
develop within milliseconds, and the only logical thing to do is stop
all axes.

With an open-loop stepper system, steps are lost when an abrupt stop is
commanded, and
therefor, the CNC computer has no idea where the machine actually is.
it has to be
rehomed to regain the position alignment. The closed-loop system, with
the encoders
feeding back to the computer, allows the computer to monitor the
position even though
an e-stop happened.

Jon