Closed Loop PC Based CNC

I have a Bench Mill currently running with stepper motors in an open
loop configuration. We are running production parts in fixtures and
after running several parts to several dozen parts, sizes and
locations start to drift due to missed steps. Slowing traverse rates
down and extending longer acc/dec ramps helps but the problem is still
there depending upon cutting forces and the shear number of axis
reversal moves. Additionally, slowing the machine to fit the
speed/torque curve of the stepper motors increases the part cycle time
and the part cost directly.

As a solution I want to take this machine to a true closed-loop system
using AC Servo Motors. While I have retro-fitted several large
machines (25'+ travel) with Fagor and AB 9-Series controls,
maintaining a PC based control presents some problems.

A likely scenario would be to use a matched servo drive and motor
combination such as the Yaskawa DR-1/SGM set for each axis. The
encoder feedback from the motor for commutation can be redirected to a
Controller board in the PC for position error control. The controller
would output a +/-10VDC analog signal to the Servo Amps to control
speed and direction. The calculated following error can then be
intergrated into a PID Loop to control speed, torque and location.

There are several interesting controllers on the market such as the
Galil, Delta Tau and Servo to Go. The Galil especially looks very
powerful and is sometimes found on eBay for a reasonable price. The
big question however is the Front-End software to drive the
controller, does for example EMC support these controllers?

Advice would be appreciated, practical experience perferred -
theoretical suggestion catalogued. If we can find no other solution we
will refit the machine with a Fagor 8025M control.

nitewatchman