By Don Talend
High-speed, precision industrial applications are benefiting from recent advancements in stepper motors. These units provide reasonable positioning accuracy whether or not they provide feedback to the controller, and because they represent mature technology, prices are trending downward.
Suitable uses include label application and cylindrical plastic joint welding. Advancements in stepper designs beyond variable reluctance (VR) and permanent magnet (PM) designs are finding a niche in these types of high-volume applications without sacrificing positioning accuracy. Hybrid motors, which have a toothed stator and magnet in a salient-pole rotor for a flux boost, are the first step beyond VRs and PMs less-suited to these types of applications.
Despite the increasing popularity of servos and brushless DC (BLDC) motors for many industrial uses, steppers maintain a strong presence where speed and precision are at a premium. Although they typically don’t achieve sub-micron precision to the extent that servos can, steppers are well-suited to tasks such as barcode scanning where reasonable precision is necessary, and at a lower cost than servos. Also, the stepper generally provides greater reliability and precision than the BLDC. A major reason for this is the increasing use of rare earth magnets in steppers.
Resolution and Torque Are Keys
One high-speed industrial application that uses a stepper for considerable precision is the SL1500 Modular Label Applicator from Universal Labeling Systems, St. Petersburg, Fla. “The use of a stepper gives the machine the capability to dispense labels at speeds to 1,500 in./min,” says Frank Gruetzner, senior project engineer at Universal Labeling Systems. “The high starting torque reduces downtime for the machine operator while increasing machine utilization and productivity.”
Oksana Petrova, motion product specialist with Schneider Electric, says that high starting torque makes the stepper motor a better fit than a BLDC for the high-speed labeling machine. “Stepper motors have a higher starting torque than that of a brushless DC motor of a comparable frame size,” says Petrova. “A high starting torque is essential in labeling applications to overcome loads encountered with the label-application process. Stepper motors also provide a higher resolution than brushless DC motors. This higher resolution translates into better positioning accuracy, which is essential for labeling applications. Also, the stepper used in the SL1500 has low resonance. There’s no tuning involved with stepper motor systems, and they historically have a long service life and good speed and torque ratings.”
The permanent magnet design of the stepper used in the labeler is intended to provide high magnetic-field intensity at a reasonable price point, says Petrova, adding that the strong magnetization is caused by the use of rare-earth AlNiCo magnets (Figure 1).
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THE KEY TO PRECISION
An advancement in steppers that has been used for very-high-speed applications such as wristwatches has been transferred to an industrial application: barcode scanning. Among other technologies, Sick AG (sick.de), Dusseldorf, Germany, manufactures barcode scanners used in tasks such as classifying and sorting products and calculating volumes. This application requires rapid oscillation of a mirror to deflect laser light. A photodetector measures the amount of laser light reflected back by each element of the barcode and the images are decoded into numerical data.
Acceleration and precise positioning are critical performance criteria for barcode scanners, says Christoph Anselment, mechanical design group leader for Sick AG. “We want to switch the focus position in the shortest time,” he notes. In its scanners, Sick AG uses a 16-mm Turbo Disc motor from Portescap, a variation on the stepper motor that uses disc-shaped magnets instead of more conventional cylindrical magnets.
Dave Beckstoffer, product manager for Portescap, explains that the Sick AG scanners can provide greater resolution and thus accuracy because the Turbo Disc motor can achieve 40 steps per revolution—about twice that of conventional steppers. The motor’s disc-shaped magnets provide axial, as opposed to a conventional stepper’s radial, magnetization to yield greater acceleration (Figure 2).
MORE PULSES PER REVOLUTION
Both of those applications illustrate a greater suitability for steppers versus a likely alternative: the BLDC. A stepper in general offers advantages over the brushless DC motor alternative for such high-speed precision applications, argues Beckstoffer. “The migration toward the use of steppers for these applications,” he notes, “is directly correlated to the growth of the semiconductor industry. Steppers rely on the number of stages and pole pairs for the number of steps per revolution, whereas the BLDC uses a feedback transducer such as an encoder or a resolver. The stepper offers greater reliability than the mechanical commutation system of DC motors.”