Several types, makes, and models of motors are available. Usually, you can group them. AC servomotors are appropriate for some applications. So too, are dc servomotors. Step motors are dc motors of a sort. To avoid confusion and to simplify the point of this article, I will divide motor types into:
• Step motors, associated with openloop motion control.
• Servomotors, including ac and dc closed-loop servos.

Some step motors serve in closed-loop systems, though not so frequently. In general, though it is possible to build a closed-loop step motor system, the cost of feedback devices and other ancillary components makes the step motor-servomotor system cost comparison approximately a draw. “Step motors” as used in this article refers to step motors in openloop mode unless stated otherwise.

Choosing a motor type

In deciding whether to use an openloop step motor or closed-loop servomotor, five simple questions about the application will initially reveal the most appropriate motor. Once you decide that, you can select from the torque-vs.-speed graphs included in suppliers’ technical data. Some additional selection criteria are presented here. They are useful in optimizing the selection.

Ask these five questions:

Is the load variable or constant? For example, office automation products such as copiers, facsimile machines, and printers, have constant-torque loads. The load is usually paper. It is predictable, and it does not vary through a move cycle. Here, you can use step motors. Conversely, factory automation applications typically have unpredictable, variabletorque loads, and they may require feedback operation. Servomotors, because of their closed-loop configuration and overload capacity, can intermittently absorb variable-torque loads during a move or process and recover to the desired position. Step motors have no tolerance for overload and must be sized for the absolute worst case. Recommendation: If you don’t want to lose position, pick a step motor with 30% more holding torque capability than the maximum torque requirement at the motor shaft.

Will the motor operate above 1,000 rpm, or only below? Step motors have higher magnetic-pole count than servomotors, and the constant-torque range is limited. Above about 1,000 rpm, step motors produce less than 30% of the torque available at low speed. The lower pole count of the servo produces constant torque over a wider speed range — up to about 2,500 rpm.

Size for size, the servomotor produces less torque in lower speed ranges, but at higher speeds the servomotor achieves higher torque output. Figure 1 shows the difference in torque-vs.-speed characteristics. The motors are the same size and use similar amounts of copper, steel, and magnetic materials.

Can the application tolerate position loss? Most step motors operate open loop; there is no feedback on the motor’s true position. If the step motor loses position, you must manually reset it. Servomotors usually operate closed loop; the controller always knows the motor’s position and calls for the right current to maintain the commanded position.

Do you need positioning resolution higher than 1.8 deg? Position accuracy and position resolution are frequently confused. Although higher accuracy arises from higher resolution, higher resolution is not the single determinant of high accuracy. Parameters such as load, friction, and servo controller gain are also important. Typically, step-motor systems have motors designed for full torque production every 1.8 mechanical degrees; that is, for 200 steps/rev. The 200 step/rev is the full-torque resolution of the motor. A servomotor using a typical resolver or encoder for closedloop feedback, usually has at least 1,000 steps/rev as positioning resolution.

Electronic manipulation of step motors, called microstepping, improves resolution significantly, but cannot offer full torque production at resolutions higher than that of a 1.8-deg step motor. Accuracy is different. For example, a microstepping system with resolution of 25,000 steps/rev will lose about 1 arcminute of positioning accuracy for every ounce-inch of torque applied to the motor shaft. Here, a motor that drives a load having 7 oz-in. of friction torque will be only as accurate as ± 7 arc-min. This is about the same as a servomotor with an encoder.

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