The very early servo systems were DC with drives that used voltage loops instead of the later DC type that used torque mode (current) loop control.
These motors used a DC tachometer that resembles a small DC motor. Servo-Tek was one popular manuf.
These voltage loop drives have to be tuned, first by the inner loop, tach feedback to drive, and then the outer loop, encoder to controller, which were a pain to tune.
The later torque mode drives to not require any feedback to the drive, all is done by the encoder to control.
This is also why you do not see tachometers on motors anymore. So if converting to later drives, the tach can be left on and the tach brushes removed.