Re: which commercial linear scale for EMC & S2G card?

["Matt Shaver" <mshaver-at-erols.com>]

> From: Doug Fortune <pentam-at-home.com>
> **** Can someone save me the "experimentalism" and recommend a
> vendor for the linear scales (which has been tested successfully with
> the S2G cards and Linux/EMC) ?   *****

See:

http://www.sonypt.co.jp/en/products/p_4.html

Any of the scales that say "A/B phase output and zero point signal by
differential line driver" should work, the ones that say "Analog output"
won't. If you pick one of these and are unsure of the compatibility, I'll
download the data sheet and have a look at it for you. I like the look of the
SH50 for your application. It's possible to use the analog output scales by
purchasing Sony's MJ series of "Interpolators". These are an analog to
digital interface that can output quadrature or up/down counts and also
allows you to select the desired resolution via a dip switch. I installed
that setup on an Excello horizontal CNC mill for a shop near Baltimore. The
downside is that the MJ boxes are expensive (I think like $1000/each), and
you need one/axis plus the scales which aren't exactly cheap either.

Having said all that, I have to warn you that there's another problem with
using linear scales for servo feedback. The farther away (mechanically) the
motor shaft is from the position feedback sensor, the more effect things like
backlash, slack in the gibs, and lead screw rigidity have on the tuning of
the servo loop. These issues are null when a rotary encoder is coupled
directly to the servo motor's shaft. I'm not saying it won't work, just that
it's going to be more difficult to get the axis motion to occur smoothly,
without oscillation, during the accel and decel periods.

It would be interesting to see someone try this, just to see if it would work
;)

> Where can I find more information about the ISA encoder card?

They're probably referring to:

http://www.mcs.net/~kulaga/dro.html

This does work with the EMC software, but it's just the quadrature input
circuitry. You'll also need some DACs to output your control voltages to the
servo amps.

Matt