Stepper motors tread on servosystem turf

Stepper motors are frequently used in motion-control systems because they can position loads relatively accurately with or without feedback. Moreover, stepper motor costs continue to drop, making them even more attractive than servomotors for many new designs.

Hybrid stepper motors, for instance, use a toothed stator and a magnet embedded in a salient-pole rotor to provide a boost in flux. The active rotor lets the motor run at relatively high slew speeds and efficiency. The hybrids are 25 to 60% efficient, filling the void between permanent-magnet steppers and brushless dc motors. The most widely used motors produce 200 step/rev, although they come in other step angles as well.

Hybrid motors are usually wound with bifilar windings, connected either in series or parallel to use all the copper. An axially magnetized concentric magnet surrounds the rotor shaft, and ferromagnetic iron cups or laminations mount on each end of the rotor and surround the magnet. The rotor teeth on each end of the magnet are offset half a tooth pitch from each other. This prevents the rotor teeth on both ends of the magnet from lining up with the same stator teeth.

Because stepper motors do not share the same qualities with conventional motors, different terminology describes their performance in terms of speed, torque, accuracy, and resolution.

Position accuracy, for example, is critical in this system, and the major component that influences position is resolution. For rotary stepmotors, it is expressed in steps/rev or °/step. For linear stepmotors, resolution is expressed in fractions of inches or millimeters/step. In both cases, it's a fixed, built-in parameter.

Step accuracy is the degree to which a stepmotor exactly repeats an increment of movement. The magnetic detent position affects this accuracy which is largely determined by numerous part and subassembly dimensions. Tolerances in these parts and internal friction produce variations in step-to-step positions. Accuracy is expressed as a plus or minus angular error in any step position. It may also be expressed as a percentage of the step angle or as an angular dimension. For example, error of ±0.25° in a 24-step/rev motor (15°/step) could be given as a step accuracy within ±1.67%.

These built-in position errors are systematic and repeatable for a particular motor, and they exist under all load conditions. Moreover, loads having sufficient friction add nonrepeatable random position errors which are unrelated to the accuracy of the step motor. The total error is also noncumulative, except as noted, and its value can be obtained from the partial plot of the Holding torque curve. The curve depicts a deadband where the frictional load stops the rotor.

The holding torque curve represents a stepper motor's most fundamental torque. The origin of the curve corresponds to an energized motor, but it's at rest in any step (detent) position. The curve also shows the holding torque developed when the rotor is displaced from its step position. The torque tends to force the rotor back to the zero-torque step position.

The holding torque segment of the total torque function curve corresponds to each phase of the motor. All other segments of the phase-torque function curves are formed from this one holding torque curve. In other words, its images are formed by rotation about the curve's vertical and horizontal axes. Thus, the holding-torque curve alone completely describes the instantaneous torque of the motor under all static conditions of excitation and rotor positions. All other torque characteristics, static or dynamic, have their origins in this holding torque curve.

Along with the static holding torque, the dynamic characteristics must be considered. One important issue is the combined resonance and instability which appear at certain stepping rates, manifested as oscillations which disturb normal operation. In some cases the magnitude of the oscillation increases with time and eventually the motor loses synchronism. This is shown as a torque dip in the Pull-out curve.

Resonance and instability are usually classified in three categories: low frequency, midrange, and high frequency. In addition, because permanent-magnet motors are inertia sensitive, a stepping motor cannot start at certain speeds under some loads. These speeds correspond to resonance regions in the pull-out torque curve, and are determined by the rotor's natural frequency.

Electronic stepping controllers usually reduce instability. For example, they can produce signals for half stepping, or one-phase-on, twophase-on sequencing to reduce instability over a wide speed range. Moreover, many controllers produce microstepping signals that drive motors hundreds of steps/rev. Microstepping does three things: It lets a stepping motor stop and hold a position between full and half-step positions, it largely eliminates the jerky motion of low-speed operation and noise at medium speeds, and it reduces resonance problems.

Although microstepping offers hundreds of positions in one revolution, nonlinearity and static friction in the motor prevent it from substantially improving precision. Another accuracy limitation involves the nonsinusoidal torque versus shaft-angle curves, attributed to the inherent detent torque on permanent magnet and hybrid motors.

Information for this article was contributed by John Anderson, Thomson Airpax Mechatronics LLC, Box 590, Cheshire, CT 06410, (203) 271-6406, Fax: (203) 271-6400, www.thomsonindustries.com