Controlling existing motors with resolvers

Nope, Variable Reluctance resolvers require quite a bit more excitation drive, and not all resolver circuits can handle these. Tamagawa is a leading manufacturer of these, but they also make "brushless resolvers" which require MUCH less drive. In the data sheets, they sometimes state the excitation impedance as a complex number, something like 95 + 135j Ohms at some frequency. The higher these numbers, the less drive current the resolver requires. Common brands of brushless resolvers seen on older equipment are Singer-Kearfott and Harowe Controls.
Sin/Cos encoders are not magnetic devices, but often optical linear scales (can also be rotary) where the raw optical signal is brought out, either as a voltage or a current. The reason for this is interpolator boxes can be attached to increase the encoder resolution. Back in the "old days" linear scales often had native resolution of 100 lines per inch or so, so interpolation was required to get proper machine accuracy. Nowadays, high-resolution optical gratings can be made easily, so this stuff is rarely seen today.
Jon

andypugh wrote:

What is the feedback system used for axis position? Are there separate scales, or does that rely on the resolvers on the motors too?
 

There aren't any other position sensors, so the motor resolvers must serve both commutation and position feedback.