There is one basic difference between servo and stepper control systems: the use of feedback. Servos have a position encoder attached to the drive motor that reports the actual position of the motor shaft back to the controller. If any positioning errors exist, the servo controller may take corrective action to ensure that the motor moves to the proper position.
In comparison, stepper controllers can only issue a move command, and you must simply hope that the motor is capable of following it. You have no way of being sure that the motor has indeed reached the desired position. The presence of feedback in a servo controller system results in several benefits, enumerated below.
1. No lost pulses. Servo systems know exactly where the motor is at all times, so all step commands are executed.
2. Full torque at high speeds. Stepper motor torque falls off as motor speed increases, due to electrical time constants and poor current utilization. Servo motors do not have this problem and may generate full torque at high rpm.
3. Quiet, smooth operation. Servo systems are inherently smooth due to their high encoder resolutions-typically at least ten times finer than a stepper motor’s number of positions per revolution.
4. Zero holding current. Stepper motors must have high currents applied at all times, even while they are stationary, with little or no load. Servo systems draw power only as required, and the power drain is proportional to the load torque applied to the motor.
5. True microstepping. Stepper motor resolution may be increased via a process, called microstepping, in which currents are applied to the motor windings in proportion to the desired position between normal steps.
