How servos and steppers stack up

Engineers who work in motion control often start out using a relatively simple approach involving stepper motors to synthesize moves. Steppers are appealing in that their controllers can be simple and inexpensive, at least for undemanding tasks. And most steppers run open loop, needing no position or velocity feedback to hit their targets.

The problem comes when the application demands appreciable torque and relatively fast speeds. Stepper motors can handle a range of torques and speeds that overlap those of more-expensive servomotors, but the economics for the stepper approach often don’t work out.

It is useful to examine some of the key differences between stepper and servomotors as a means of understanding which applications each type of motor fits. Steppers and servos are both synchronous motors. In both cases, the rotation period exactly equals an integral number of ac cycles. Both motor technologies employ a rotor with permanent magnets and a stator with coiled windings. Both systems operate by applying a dc voltage to the stator windings in a specific pattern that results in movement of the rotor, and both technologies are capable of position and speed control.

Two key differentiators between stepper and servo systems involve the use of a feedback device and the complexity of the amplifier electronics. Servos, by definition, are closed-loop systems utilizing a feedback device. Steppers are open-loop systems with no feedback.