Model M1 X/Y Stage
The Model M1 X/Y Stage in an industrial laser tool is characterized by the
following features:
- standard size: 10" x 10" (sized in
metric at 250 x 250mm)
- dc servo motor drive is standard with optional linear
motor drive
- linear optical encoders---one micron resolution.
- max servo motor speed: 200mm/sec, cruising speed
150mm/sec
- includes motor driver/amplifiers interfaceable to
motion control board
We prefer closed loop stages in our laser machines. Following is a
discussion of closed versus open loop:
An open loop X/Y stage has no feedback. Closed loop
has feedback. Feedback from the stage to the computer is stage position which the
computer translates into position coordinates. Encoders are rotary or linear, we prefer
both in a dc motor driven stage.
Stepper motors never have rotary encoders, always
linear. But encoders on a stepper motor are optional. With no encoders a stepper motor
driven X/Y stage is open loop.
Rotary encoders are directly on the motor shaft,
encoding rpm which is velocity to the computer. In contrast, linear encoders provide
X/Y position coordinates.
Linear optical encoders are as long as twice stage
travel. So for a 1m x 1m X/Y stage, you need two 2m long linear optical encoders,
one for each axis. Physically, the linear encoder is a long skinny piece of glass
having typically chrome "fingers".
Finger spacing is, say, 50 microns. Then an
electronic interpolator takes you down to selected resolutions, usually one micron. When the
stage moves, the computer counts the fingers going by through a light beam. The
counting mechanism, the computer's eyes, is a light beam through which the fingers pass.
There is no friction, thus "infinite" life.
Encoder = closed loop = feedback = "hope for
the best" = inexpensive in old days
No encoder = open loop = no feedback = certainty
of positioning = low cost now
All moving elements are sealed against environmental
contamination such as vaporized and redeposited
ceramic or silicon. (Whether you need this or not, it's
good to have!)
|
