Fast Galvo or Slow X-Y Stage?
X/Y Stage for Fixed Beam Delivery, Galvo for Moving Beam
High accuracy is a tradeoff for speed in applications such as patterning displays that involve hundreds of millimeters per second. Here galvos play a role. But higher power machining typically requires only tens of millimeters per second so an X-Y stage is a good match.
The X/Y Stage in an industrial laser tool is characterized by the following features:
- standard size: 12″ x 12″ (actually sized in metric at 300 x 300mm)
- dc servo motor drive is standard with optional linear motor drive
- linear optical encoders—one half micron resolution.
- typ max servo motor speed: 200mm/sec, cruising speed 150mm/sec
- includes motor driver/amplifiers interfaceable to motion control board and then to computer
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 half 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 = certainty of positioning
No encoder = open loop = no feedback = “hope for the best” in positioning
All moving elements are sealed against environmental contamination such as vaporized and redeposited abrasive ceramic or silicon. (Whether you need this or not, it’s good to have!)