Re: [CAD_CAM_EDM_DRO] encoder installation on servo drives

embedded_tom wrote:

>I asked this on cnczone, but no replies. So...
>
>I've noticed that almost all the servo-based setups I've come across:
>1. Utilize a belt drive to connect the servo to the leadscrew.
>2. Place the encoder on the servo.
>
>I understand the reasoning for the belt drive, but wouldn't it be
>better to put the encoder on the leadscrew shaft?
>
>
>

I did mine this way. Many high-end machines with precision-ground leadscrew
do this so as not to add any cyclical error due to belt or pulley
imperfections.

>In other words, doesn't the belt drive add some backlash effects with
>the encoder on the servo?
>
>Or does the slop help prevent servo hunting when there's no motion?
>
>

A tight belt doesn't add any backlash. There shouldn't be any slop in
the belt,
but there could, possibly be some elastic resonances in a long belt.
The belt reduction increases the torque available at the leadscrew, and
smaller
servo motors have RPMs to burn, so it is a desirable tradeoff. The
encoder's
resolution is multiplied by the belt ratio, too, so that is an advantage
as long as
the errors induced by the belt don't sacrifice accuracy.

One other thing is high-torque flex couplings are tricky to make, and
can snap
without warning if subjected to a lot of shock loads. If you want to
direct-drive
a leadscrew with a servo motor, you need a very stiff and stout coupler, yet
not overload the motor bearings from misalignment. The belt avoids all that
hassle.

Jon