What is following error?
Motion controllers contain an on-board PID control loop that allows them to use position and velocity feedback to adjust their command signal. This feedback, read from either analog voltage or pulse-train encoder signals, indicates the actual position/velocity of the motor. The controller compares its commanded signal to the feedback signal to verify that the motor is following the command. The difference between these values is called the following error.
Cause 1: Encoder input resolution is set incorrectly
Cause 2: Command or feedback wires are reversed
Cause 3: Improperly tuned PID loop in a servomotor system
Cause 4: Commanding a move that is too fast
http://digital.ni.com/public.nsf/allkb/899AE29CB4E3578B8625709D0070A725