Right Al. Heidenhain glass scales are just like a rotary encoder straightened out. There are others that don't use glass scales, but still output quadrature pulses in realtime.
Snagglet may be referring to the capacitance type of DRO scales that typically have a readout on the slider with a possibility of connecting a remote readout. They have a serial transfer of position at a much too slow update rate for a feedback loop. It's the same as the digital calipers that came as a blessing to us which grew too short arms to see the traditional calipers without hanging glass on our noses.
There is one point though. Using a rotary encoder, it is (usually) right on the motor = very stiff connection. While the linear scale is on the axis. Then you (machine buider/retrofitter) need to make sure the connection motor-axis is stiff. Otherwise the drive will hunt continously and be impossible to tune. So beefy preloaded ballscrews and solid construction is a requirement. With encoders the motor can be tuned to follow closely to where it should be, and the flexibility will just result in the axis not being exactly where it should be.  |