RE: [CAD_CAM_EDM_DRO] servos, steppers, limit switches

One way to zero (reset, home ) without limit switches is to run the axis at
a slow speed to a mechanical stop. The controller would determine the
mechanical stop by monitoring the encoder as you guessed. Once the encoder
stops counting, and if the encoder has an additional index (Z or reference
pulse), the controller would move back until it saw the index pulse at which
point it would stop the axis and set this as a zero or home position. I am
sure you can see the inherent disadvantage in this; one you have to run the
machine slowly so as to not damage the axis when you run against the
mechanical end of motion. Second, if you do not have a index pulse on the
encoder, the repeatability of the zero position would not be very good as
the controller might either zero on the leading edge or the falling edge of
the A channel or the B channel. A index pulse on the other hand gives one
better repeatability.
Madhu

>-----Original Message-----
>From: Brian Sherwood [mailto:lurch@...]
>i'm in the process of converting my taig to servos...and someone told
>me i don't need limit switches on a servo setup although the stepper
>setup has one at the max-value of each axis. the wiring diagram for
>the servo setup shows the limit switches wired in series with the e-
>stop switch.
>
>My first stupid newbie question is, how does a servo system ever do
>a 'home' command to find out where it is without limit switches? Or
>does it just run to the end of travel and drop out when the encoder
>stops encoding, then you set your known value for that axis to that
>point, knowing the mechanical limits? Like, for example, if I know
>my x-axis travel is 12 inches, do I just run it til it is against the
>stop and when it stops, I set x to 12 in the program and call it done?
>
>Thanks in advance.
>
>Brian
>