Thanks Nick and Gary!

I need to find out if they have resolvers or er encoders. Will I make any damage if I dismantle them to see what's inside? If they are fitted with resolvers, I think I will be able to replace them with encoders.

Regarding the planetary gears, I intend to build a rotary axis and/or a two axis (B/C) head for light wood working and foam. But I thing in either situation there is no backlash allowed. Or am I wrong? At the moment I do not have the money for Harmonic Drives...