Re: Retrofitting a Fadal with a new controller. Where do you interface new to old ?
If they are brushed DC motors then Gecko makes a drive that will probably work. If they are AC servo motors then just replace them with modern AC servo motors and drives that will accept step & direction control if that is the controller type you want to use. I'm guessing that the Fadal AC servos are probably analog control, step & direction is not common on industrial class machines because it does not afford the fine control of an analog control. DMM Technology servos are my favorite retrofit servos. https://store.dmm-tech.com/ Trying to make non-matched motors & drives work together is not worth the effort.
If they are DC motors then by definition they are brushed. Most AC servo motors are 3 phase brushless DC motors. A resolver and an encoder perform the same function, they output position data. The resolver outputs a 1V P-P sinewave, the encoder outputs a pulse train, the controller or the drive receives that data and does something useful with it. Either device could be on either type of motor, although I don't recall ever seeing a resolver on a brushed DC motor. Older brushed DC servo systems with velocity drives used tach feedback from the motor for speed control. Newer ones operate in torque mode so do not require tach feedback.
Jim Dawson
Sandy, Oregon, USA