Re: [CAD_CAM_EDM_DRO] Preventing Servos from Jumping??

mmurray701 wrote:

>My servos sometimes jump slighly in one direction or the other on
>power up. This is probably considered normal but I was just
>wondering if there is anything I can do to prevent it.
>
>Sometimes I get halfway through a batch of parts and need to leave
>the shop or something, and when i power the controls up when i get
>back it often requires re-zeroing. Not a really big deal, but if
>there is a way to prevent it, id love to hear it. Thanks.
>
>

What servo drive are you using? It would help greatly if you were more
specific. I'm guessing that you are talking about something like Gecko
320 or Rutex drives, here. Since these are really open-loop controls
as far as the computer is concerned, there really is no way to prevent this.
You might be able to put brakes on the motors that keep them from moving
until the servo drive is powered up, but it will probably still shift the
position up to 3 encoder counts each time, as the logic in the driver
probably starts out every time with the A and B inputs from the encoder
set to zero, no matter what they were at the instant you turned them off.
The problem is the computer is unaware of the movement.

I have a solution that right now will only work on EMC. (You don't
say what CNC control program you are using, either.) It closes the
servo loop all the way back to the computer. It is designed so that
you can power the Gecko drives up and down, and go back and forth between
manual and CNC control without any loss of position. It also allows the
computer to constantly monitor the real machine position and report
excessive following errors or crashes (and stop all axes, spindle, etc.)

This is done using my Universal Stepper Controller board, and a
new board that I don't even have a web page for, yet, but I will call it
the Gecko interface. It handles switching power for the Gecko drives,
has a braking resistor to bring runaway motors to a stop, and optoisolators
to safely repeat encoder signals to the Gecko drives. (This unit should
also work with Rutex drives, but has not been tested with them.)

See http://jelinux.pico-systems.com/univstep.html for more info on the
USC. I will try to get some info on the web site in the next couple of
days. I will be demonstrating this system with the Gecko interface at
the NAMES show April 24-25 in the Detroit area, along with another
system using my own servo amplifiers.

Jon