Step-motor-based systems stay competitive

Motion control: Traditional stepper-motor systems represent the only motion-control technology able to operate in open loop—although the addition of position feedback to enhance performance is on the rise. Simpler controls, lower cost components, and other innovations keep stepper systems competitive with servo motion systems in numerous applications.

04/22/2013


Stepper motion systems shine in many applications that require less than critical speed and position accuracy. Among the technology’s drawing points are no need for system tuning (versus servo-based motion), motors less costly to produce, and simpler controls and cabling. System price advantage remains even when adding a low-cost feedback device. Other benefits of stepper technology include minimal system setup time, less need of user expertise, and better motor inertia matching for driven loads.

Thermal imaging shows dramatically less heating for Oriental Motor’s AR Series “constant-duty” step motor that allows continuous operation. Motor case temperature differential compared to a conventional motor is greater than 40 C. Courtesy: Oriental Motor

Cost savings is a common theme voiced by stepper product suppliers. “Step motors have always had a cost advantage over servos, but traditionally this has come at the price of performance,” said Clark Hummel, applications engineering manager at Schneider Electric Motion USA. Hummel listed three attributes that allow today’s stepper systems to take on many applications requiring servos in the past—while still maintaining a considerable cost advantage:

  • Elimination of stalling (desynchronization)
  • Ability to automatically adjust running current to load requirements
  • Ability to provide torque against an overriding load.

B&R Industrial Automation Corp. connects lower cost with wide application of step motors and drives. Feedback, if used, is implemented in a lower cost device, according to Corey Morton, B&R’s director of technology solutions. “In contrast, high-resolution absolute encoders used with servos can cost as much or more than the motor itself for small power ratings,” he said.

Morton attributes further cost savings to the stepper drive, especially for smaller sizes. Various drive form factors now available—machine mounted or in a control cabinet—add to stepper systems’ versatility. “Coupled with many motor variations available, machine builders have the flexibility to select the stepper motor/drive combination that best fits their design and application needs,” Morton added.

Kollmorgen likewise regards stepper motor/drive systems as less costly to produce and easy to implement. “The lack of a feedback device [in many cases] in particular reduces the cost of the motor, cables, and drive,” said Gene Matthews, a product manager at Kollmorgen. “Integrating a stepper into a machine can be as simple as pulsing an output on a PLC.” With microstepping technology available in the drive, stepper system performance can now come closer—though still not equivalent—to that of a servo system. “This provides a value solution for applications that can make the performance/price trade-off,” Matthews noted.

Beckhoff Automation considers the choice of stepper or servo as balancing performance and price. “It’s a balance of available technology against an application’s performance requirements—especially in terms of load—and the available budget,” said Bob Swalley, motors and drives specialist at Beckhoff.

Step motors, while sensitive in terms of load, are simpler to size than servos. Typically you need to find a motor that can handle twice the peak load torque at the application speed, according to Swalley. Good performance is obtained where required speeds are not substantially above 800 rpm. “It’s possible to get functionality suitable by today’s standards using steppers and still save money over a more expensive servo alternative,” Swalley said.

Constant duty, motor+drive

Standard step motors run at relatively high currents, resulting in excess heat generation that limits their duty cycle. Recently, Oriental Motor USA (and others) have developed a class of “constant-duty” step motors with higher energy efficiency than standard motors, noted Todd Walker, OM’s national marketing manager. It’s the result of several factors: control of running current to reduce heating losses, using higher grade steel laminations, magnetic flux pattern enhancement in the motor teeth, and a more efficient electronic drive. This combination enables torque production with lower running currents.

Another significant stepper system development has been the integration of the motor, drive, and ancillary components into one package. Benefits include simpler (less) wiring, matched drive and motor, plus options for closed-loop mode using motors with a pre-assembled encoder or with self-correction options similar to servo systems. A number of manufacturers offer such motor-drive packages. “Stepper packages also build communication options into the driver, which will become more common in stepper driver technology,” Walker stated. (See products table and more on motor-drive developments at Ref. 1 online.)

Hybrid Motion Technology from Schneider Electric Motion, now available with MDrive products, adds various enhanced control capabilities and features to stepper-based motion systems. Courtesy: Schneider Electric Motion USA.

Schneider Electric Motion also values integrated motion. It has long been an advocate of motor-drives, for example, with its MDrive product line, which comprises NEMA 14, 17, 23, and 34 step motor sizes (Ref. 2). Combining the motor, drive, encoder, and motion controller into one package expands the overall cost advantage by eliminating long runs of costly feedback and power cables, removing most of the control panel space requirements, and reducing potential failure points in a motion axis, explained Hummel. “This provides a true value proposition when evaluating a stepper system against a servo,” he stated.

Open loop or more

Properly sized for the application, a stepper system running in open loop offers repeatable positioning—typically within a half-step or better accuracy (0.9° or better for a 200 step/rev motor), according to B&R Automation. “Overall positioning accuracy depends on a number of factors associated with the driver and motor construction,” Morton noted. “Microstepping (for example, 1/10th of a full step or 0.18°) is a commonly used method to reduce vibrations associated with stepper motors as well as to improve positioning accuracy.”

In open-loop control, a generous torque margin (up to 50%) is used to select the step motor. This serves as a safety factor against unexpected load changes. Feedback control minimizes or eliminates the need for this provision.

For applications requiring better positioning accuracy or overall performance improvement, stepper motors can be fitted with different position sensors such as incremental or absolute encoders and resolvers. “A simple implementation may use the sensor just to verify reaching a commanded position and initiate correction moves. A more advanced implementation may use full closed-loop positioning, similar to servo system operation,” Morton added.

However, a simpler sensor can suffice in a typical stepper application. For example, Oriental Motor’s Walker mentioned a 200 pulse/rev encoder priced at under $70, while a servo system might need a costly encoder with, say, 16k ppr capability.

AR Series motor and driver packages from Oriental feature a self-correction option that simulates servo-like operation and reportedly maintains positioning even under load changes and accelerations. A sensor monitors motor shaft rotation and, in case of overload, AR Series regains control quickly by going to closed-loop mode. If the overload condition continues, the drive outputs an alarm signal.  

Advances implemented in newer stepper drives include control of synchronism between the motor and driven load. For example, Encoderless Stall Detect used in Kollmorgen’s P7000 Stepper drive gives users feedback on when the step motor has stalled. An actual feedback device isn’t needed since the method is implemented in software. Instead, the system’s commanded current is monitored and if demand exceeds an expected current limit for a positioning move, the drive shuts down, Matthews explained. It could indicate an abnormal condition such as a jam, obstruction, or other machine issue. “This allows for a more advanced control scheme at a low cost,” he stated. 

Applications galore

Oriental Motor sees stepper applications in CNC machines and short move x, y, z positioning for assembly or automation machines. “Applications that require short, quick moves (1 or 2 shaft revs)—or fine positioning and repeatability—are ideal for steppers,” Walker said. He cited examples in sampling, metering, or pumping applications where controlled volume is required. “Expensive gearboxes can possibly be eliminated in some applications since stepper motors tend to have four times the inertia of servos,” Walker added.

Stepper systems are well suited for indexing, dispensing, and positioning tasks. Kollmorgen mentioned examples in labeling, 3D printing, and driving linear positioners. “Step motor-based systems allow manufacturers to create lower performance and less expensive versions of machines than those outfitted with servo motors,” Matthews remarked. “Stepper motor technology performs well at speeds below 15 revolutions per second [900 rpm]. If you need significant torque at higher speeds, servo is the right technology.”

Beckhoff Automation’s Swalley concurred that steppers are not appropriate in applications with changing forces and/or masses or requiring higher operating speeds. “That arena remains strictly the domain of servo motion because of potential loss of synchronism,” Swalley concluded. (More Beckhoff coverage appears at Ref. 1 and in a packaging machine application using stepper motion control, Ref. 3, online.)

Hybridization

Motion control is moving toward what is variously termed “actuator independent” or “hybrid” technology—driven particularly by manufacturers that supply both servo and stepper systems. Among benefits of this development is that machine builders can provide lower cost step motor solutions with the same motion control software previously used with servos, explained B&R Automation’s Morton.

Schneider Electric Motion commented similarly that the advent of modern control designs, such as the company’s trademarked Hybrid Motion Technology (HMT), has dramatically narrowed the performance gap between steppers and servos. HMT features include ability to operate in torque mode and at 100% of motor torque, which is said to be unique for stepper motors (read more at Refs. 1 & 2, online).

Continuing innovations and adoption of new hybrid technologies help keep stepper motion systems in proper step.

- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart(at)sbcglobal.net

Consider this

Applying stepper technologies can require less system setup time and user expertise: Do you have an application that a stepper motor can simplify?

Key concepts

Stepper motion systems:

  • Excel in applications that require less than critical speed and position accuracy
  • Need no system tuning (versus servo-based motion)
  • Motors are less costly to produce, and have simpler controls and cabling. 

See additional links at bottom for:

Ref. 1- Stepper-based motion systems addendum

Ref. 2- “Integrated motor communicates…”

Ref. 3- Beckhoff-Pattyn stepper application

www.br-automation.com 

www.beckhoff.com/drivetechnology 

www.kollmorgen.com 

www.motion.schneider-electric.com 

www.orientalmotor.com   



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