The most significant difference between servo and stepper motion control systems is the use of feedback in servo controlled systems - that is there is a position encoder attached to the drive motor that reports the actual position of the motor shaft back to the controller. If there are any errors present, the servo controller may take corrective action to insure the motor reaches the proper position. Stepper controllers can only issue a move command and hope that the motor is capable of following it. Servo motors do not have this shortcoming and may generate full torque at high RPMs. Servo systems are inherently smooth due to their high encoder resolutions, typically at least ten times finer than the number of stepper motor positions per revolution.
Acoustic noise is virtually nonexistent in servo systems since there is very little motor resonance.
Servo systems meter out power only as required - proportional to the load torque applied to the motor. Repeatability and torque production are a problem for steppers in this mode of operation. Again, servo systems overcome this problem by supplying power as indicated by the on-shaft encoder. The most common way to keep stepper motor systems from losing steps is to over design the system such that there is a very high probability that no steps will be lost due to high loads or friction. When current is fed into the windings of the stator it generates and electromagnetic field.