Re: deskcnc servo driver

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Gregory Kamysz
<smsupply@a...> wrote:

> With servo controls how does the software know the machine has

reached

> it commanded position if it starts to lag behind? I understand

that the

> servo control knows that the motor is behind but if the pc sends

the

> next command and it hasn't reached it's destination from the

previous

> command what happens?
>
> I guess Geckos will only allow 125 step lag before faulting. Is

this

> really a problem?
>
> Greg
>
>
>

With the semi-closed loop controls like the Gecko and Rutex the
controller software does not "know" the position of the machine. As
far as it is concerned it is talking to a standard Step and Direction
control from the parallel port. The machine position is maintained
(sort of) by the Step to Servo control module (Gecko 320/340) and as
Fred described it keeps moving the motor until the encoder steps have
matched the commanded steps. The 128 step error limit is normally
quite adequate since properly tuned using the acceleration and
velocity parameters, the machine should be able to keep up with the
stream of move commands. Conditions that would cause a fault are
commanding too quick an acceleration, mechanical issues (making too
heavy a cut for the motors to handle) or other out of spec
conditions. Under most circumstances you WANT the system to fault
and stop if the error is too great since it indicates a possible loss
of position.

As for getting another command before the last one has finished:
This is the function of the Controller Software. It is calibrated to
know what the fastest the machine can move and how long it takes to
get there (and how many step pulses it has to send to do that). It
will only issue moves at a rate that it knows/assumes the machine can
handle. This works for steppers and step to servo systems. So when
you program a part and apply a feedrate like 60 IPM, the software
pumps out commands that in real time do not exceed that rate so if
your machine is working properly it will be where is supposed to be
when the next move command is received.

The dynamics of stepper motors are much different than servos and it
is not always a 1:1 swapout in the mechanics to get best
performance. There have been reams of information about the pros and
cons of steppers VS servo's.