Hi,
one of the principles on which field oriented control rely is accurate information of the angular position of the rotor,
ergo the use of an encoder or resolver. Any flexure or backlash between the rotor and the encoder will seriously
degrade the maintenance of quadrature flux angle thereby degrading motor performance.
Field oriented control on which AC servos rely is an interesting topic, I suggest a Google search 'field oriented control by
Texas Instruments'. It will reveal a couple of excellent introductory videos on the subject. The upshot is that ALL AC servos
capable of accurate position control must have an integrally linked encoder/resolver. That does not preclude a secondary means
of detecting position but the servo drive MUST have direct access to rotor angle with zero lash/lost motion. The encoder
in the B2 series doubles as position sensor (rotational position is converted into linear position through a low lash/zero lash
ballscrew) and rotor angle for the field control loop to operate.
The A2 series is dual reading, thus it uses the encoder for field control but can 'load sense' using the auxillary encoder/linear
scale channel. Very crafty indeed. Had I realized that for as little as $50 extra each, I would have bought them instead of the B2
series examples I did get. Having said that I have no need of the 'extra fruity bits', I doubt I'll exhaust the B2 series capabilities
let alone the A2 series!!
Craig