Re: Digital scales for servo positioning.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "roboticscnc" <gsm42@o...>
wrote:

>
>
> Hi, I'm new to the group. I have a mill and a lathe with "Easson"
> digital scales and I would like to use the scales for the

positioning

> feedback function as this would eliminate backlash issues and missed
> steps. Has anyone used this method ?
> I would also like to use an encoder on the lathe spindle for feed
> rates per revolution and threading. Can anyone point me to a block
> diagram or schematic, preferably with specifications ?
> Glen

Because of the nature of the lower cost step to servo systems (Gecko,
Rutex) the feedback loop is closed in the hardware and although
tunnable it assumes a 'tight' feedback loop. Any amount of backlash
in the leadscrew will cause a hunting effect since the servo will
constantly oscillate in the range of the backlash. Mounted on a
motor this is virtually zero (one encoder count). Mounted on a
system with .010 of backlash it will oscillate violently and cause
your machine to shake itself like a wet cat.

If you have ballscrews and backlash in the range of .001 or less it
is probably doable.

You also need a stardard dual (AB) 'quardrature' (actually two phased
square waves) output from the encoder. It may be difficult to get
those signals from your linear scale.

I am getting ready to hook up my big mill to CNC control. I just
replaced the standard leadscrews with dual nut (antibacklash)
ballscrews. I have a Shooting Star DRO that uses standard rotary
encoders for positional feedback. Even with all of that I have a
backup plan to mount encoders on the motor shafts if the feedback
loop proves to be unstable. My other servo tables all have encoders
mounted on the motors.

So, the scales do not eliminate the backlash issues in a closed loop
servo system but instead serve to be a source of instability.