wrote in message ...

On Sat, 10 Sep 2011 14:46:27 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Ignoramus13208 fired this volley in
m:

Lloyd, this will work just fine. It is a known approach. If your DRO
is absolute, you can also avoid homing.


The scales are not absolute, but do have a built-in home index, so switches
(or choppers) would not be required except to provide an over-limit panic
stop. The fact that the index is built into the scale would, I think,
further improve repeatablilty of homing. I've seen home switches go
"wonky" over time, and choppers can get dirty, and not repeat the same home
position twice in a row.

LLoyd
Greetings LLoyd,
First of all, where are you getting the cheap magnetic scales?
Secondly, the biggest problem with using scales instead of a motor
shaft mounted encoder is backlash and resonance. As the control moves
the motor to correct the positional error if it has to turn the motor
shaft a lot before the scale moves then the motor can overshoot and
then this can lead to resonance problems. Inaccurate ballscrews aren't
a problem, just loose ballscrews.
Eric

I have a lathe and mill that works the way Lloyd is talking about. My
system works fine with mechanical slop in the system because it closes the
loop using tachometer feedback directly from the motor and closes the
position loop from the position encoder. My mill uses linear scales but the
lathe uses rotary encoders coupled to the ball screws. I would prefer the
lathe to have linear scales but at least with the encoders coupled to the
ball screws it takes out the motor/pulley/belt related errors, plus I get a
following error if a belt breaks hopefully instead of a destroyed part.

RogerN