Re: [CAD_CAM_EDM_DRO] Using linear encoders.

Peter Reilley wrote:

>Can you use linear encoders in a servo system? This would be on a
>Bridgeport type machine with ball screws. I can see where any backlash,
>no mater how small, would result in a continual hunting for position.
>However, I know that there is at least one commercial CNC conversion system
>that uses the linear scales for feedback.
>
>

Sure, you can use a linear scale. It is better to have the scale feed
back to the
computer so that a deadband roughly equal to the backlash can be set in the
software to stop the hunting.

>Is this a reasonable thing to do or should I go with the rotary encodes
>mounted on the servo motors.
>
>

You will get a lot more resolution a lot cheaper with rotary encoders.
A 1000
cycle/rev encoder gives 4000 counts per rev, if you count all transitions.
On a 5 TPI leadscrew, that is 20,000 counts/inch, or a resolution of .00005"
You won't get that resolution on a linear scale without a very expensive
interpolator. Most DRO scales are roughly .0002" (actually 5 uM), so
4 times greater resolution for roughly $100 - 150, while a standard 5 uM
30" scale will run $350 and up. Unless your ballscrew is of the
high-precision
type, however, the linear scale will be a good deal more accurate,
depending on
the accuracy of the screw.

Jon