Cutaways of the Powerpac hybrid step motor. Servo motor and step motor constructions still differ, but their capabilities grow more similar.

There was a day when the design decision of whether to specify a step motor system over a servo system was simple. If you needed repeated motions with good accuracy at low speed, stepper systems were the right choice. All that was necessary was some form of sensor to mark the boundaries of where movement took place.

However, if you needed reliable precise positioning, then you needed the feedback from a servo motor's position sensors. Servos were also the choice for any application requiring high speed. And if you needed a higher level of resolution, then you needed to go to a servo system.

Step motors were inexpensive and servos weren't. Steppers were simple to apply. Servo systems usually needed a systems integrator to get them up and running.

The lines separating these technologies are less distinct now. The costs of servo technology have dropped dramatically, making the price differential less of an issue. Plus, both technologies have made advances. The result: More flexibility to tailor your motion control solution.

Less is more

Step motor construction follows the same basic design today as it did a dozen years ago, but there have been many refinements. Continual advances in magnetic circuit design have reduced flux losses that rob a motor of useful torque. New magnetic materials with a stronger field help boost torque output, enabling higher torque densities. This not only lets engineers squeeze more power into a smaller space, it improves positioning precision as well.

PMA Series brushless servo motor. Servo motor and step motor constructions still differ, but their capabilities grow more similar.

Accurate positioning, or control of the step angle, is important for some stepper applications. Accuracy is influenced to a lesser degree by the motor because of inexact motor and stator tooth profiles plus mechanical tolerance buildup of parts. It's influenced to a greater degree by the displacement angle created by load torque. In other words, the initial step angle of a motor and the ability of motor torque to consistently overcome load torque dynamics dictate stepper accuracy. When there's little variation in friction and load torque, a stepper can precisely recreate a motion repeatedly.

In general, the higher the torque generated by the motor, the lower the displacement angle produced by the load torque and the higher the position accuracy of a step motor. Steppers, with their high pole count, have greater torque for a given volume than servos. Ultimately, they are more precise. Because of their open-loop operation, though, they don't have the repeatability of constantly monitored servos.

The lower pole count of the servo motor allows it to operate at higher speeds than step motors. Although the stepper has a greater inherent angular accuracy, the servo uses a high-resolution position feedback device for repeatable accuracy with high precision, even under changing loads.

Step drive design has also influenced stepper system capability. Microstepping, for example, lets motors run more smoothly and with less audible noise than full-step drives. These "subdivided" steps boost accuracy and repeatability, assuming a consistent load. Idle current reduction circuits reduce the heating that occurs in steppers at rest. And drives with digital control offer such features as indexed moves and electronic gearing.

Although low-cost step modules will continue to drive step motors in an open-loop control scheme for the near term, closed-loop step drives and indexers are gaining acceptance. In these systems, position feedback information, typically from an absolute encoder, feeds back to the step drive. The result of closing the position loop is low-cost repeatable position accuracy.

There are slight but important differences in position feedback between servos and closed-loop steppers. Position feedback enables a servo loop to react quickly and constantly in near-real time to operate the motor. Steppers, on the other hand, use feedback in a delayed mode to confirm that the move arrives at the expected end location. It's important to note that the feedback here is not real time. Adding a feedback loop will slightly increase stepper system cost.

Servos gain ground

Ten years ago, installing servo motors required motion control "wizards," system integrators who had the experience necessary to set up and tune them for reliable operation. These engineers' servo tuning "black magic" relied heavily on oscilloscopes and calculus to tweak high-strung feedback loops into useful submission.

Software plays a large role in reducing the differences between steppers and servos. Several drives include system troubleshooting programs that can monitor system operation. In servo drives, software takes over much of the tuning process that was done manually.

Today's advanced digital electronics have changed all that. Servo tuning can now be done by system software, making it possible for anyone to optimize a servo system.

Digital electronics and recent servo motor construction developments have all but eliminated low-speed jerkiness, or cogging, that often plagues servo systems. A digital approach also eliminates the drift that was a hallmark of analog circuits.

Developments in servo motor design incorporate new magnetic materials and new manufacturing methods to reduce cost. In addition, servo motor design has migrated from super high performance down to a level appropriate for most machine designs, and thus benefitting from the resulting economies of scale. Today it is possible to find a servo system priced in the range where simple stepper systems were just a few years ago.

Software programming of digital servo drives has also evolved. The new drives are easier to use. In the same way that servo motors have changed to suit a larger cross section of motion control needs, servo drives have found a middle ground between "dumb" drives that function as torque or speed controls and fully programmable drives. The new drives can execute simple preset move profiles or move at the command of a controller or outside input.

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