Stability and Resolution of Closed-Loop Steppers Compared to Servo Motors
Position-controlled stepper motor axes are known for excellent in position stability. Traditional classical (DC) servo systems exhibit typical controller oscillation up to a few encoder counts. For low-friction, low-damping situations, this can lead to instability if the servo parameters are not set carefully.
A typical servo system feeds off errors, i.e., it needs to see an error before it reacts. These effects do not occur with a stepper motor, because it does not need the servo to maintain a stable position. Actually, when a current is applied to the stepper motor, it creates a moment that keeps it in place, while a servo motor wants to move the moment a current is applied. Stepper motor controllers provide an intrinsic velocity feedforward, which allows them to run at extremely constant velocity, even at very low speeds, without the help of an additional tachometer.
However, if the microstep resolution of the controller is too low, a disparity between the motor and the encoder resolution can lead to a quantization of the achievable positions. In case of the SMC controller with 3000 microsteps, the angular motor positions are virtually continuous – for a 1mm drive screw, the calculated resolution is 1.6 nanometers- typically below the achievable resolution of a positioning stage with mechanical bearings.