To combine what Glen and Al said about encoders:
Use linear scales only if you have true zero backlash components throughout the system. Scales will give you better accuracy, as Al says.
However if there's even a small amount of backlash present, use motor mounted encoders. Otherwise the motors will hunt back and forth, trying to "compensate" for the backlash and quickly wearing out themselves and the screws. On the whole, I think you will find motor mounted encoders more forgiving.
Arvid |