Stepper Motion Evolution
It’s the only motion-control method able to run in open loop, without the need for position feedback. This makes stepper-motor-based systems simpler than servo motion systems, with the lower cost of step motors adding to the attraction. Coupled with other evolving design enhancements—such as hardware miniaturization and higher torque density—stepper systems stay competitive for many motion applications that require relatively low speed and position accuracy.
Stepper based motion systems can range up to 0.75 kW (1 hp) equivalent power, but most applications run at much lower output. A large number of manufacturers serve this market.
Size, simplicity matter
Parker Hannifin Corp. sees size and simplicity as clear advantages for stepper systems competing with servo motion that’s also declining in price. “Stepper systems are now commonly available in miniature sizes at no additional price premium,” says product manager, Marc Feyh. “Some miniature servo systems exist, but they’re not as small and tend to be more expensive, as even more components need to be miniaturized.” Open-loop simplicity is a second benefit. However, competing on a price/performance basis grows tougher.
Innovation continues on size reduction and maintaining performance at “reasonable cost.” Parker distinguishes this trend from the case of costly high-end stepper systems that don’t compete as well with lower-cost servo systems. To meet size and cost demands, Parker’s Electromechanical Automation Div. is introducing Prostep drive/controller, a compact microstepping unit with a fully programmable controller and onboard I/O features.
The palm-sized package (1.1 x 1.5 x 4 in.) mounts alongside Parker’s Promech Series linear actuators to form a miniature stepper-based motion system. Parker rotary steppers range down to NEMA sizes 11, 14, and 17, which translate to 1.1, 1.4, and 1.7 in. per side dimension, respectively, on a square motor cross section.
According to Shinano Kenshi Corp. (SKC), smaller motor sizes, higher torque density, and lower noise/vibration are among enhancements keeping stepper motion control relevant. SKC claims improvements in all these areas, while holding (or reducing) the cost of its step motors. New, redesigned rotor and stator laminations have reportedly optimized internal space and construction of NEMA 17 and 23 steppers.
The redesigned steppers produce 20-30% more running torque than the prior SKC model, using the same power input and standard cost-effective magnet materials, explains Rex Bergsma, president of SKC (U.S.). Improvement comes from placing extra magnetic material into the rotor. With total space fixed, it required removing some stator space where the phase windings are located. Lamination redesign also improved the shape of individual rotor and stator tooth profiles for SKC’s new STP-43D step motors. The result is a stronger, more focused magnetic flux field that translates to less audible noise (see diagram) and “settling time (or damping) nearly twice as fast as previous models,” says Bergsma.
Not just rotary
A significant subset of linear stepper motor systems exists, besides traditional rotary steppers. Baldor Electric Co. notes increased interest in these linear motion products by application and design engineers. It cites advantages over rotary stepper (and servo) packages, such as reduction of parts, virtually no wear or maintenance, and ease of integrating into machines. “Linear steppers are well suited to applications with light loads and provide excellent open-loop performance in others, offering higher acceleration and faster speed compared to rotary,” says John Mazurkiewicz, Baldor product manager.
Baldor produces a wide variety of linear stepper motors, including single- and dual-axis (x-y) units, multi-axis gantry types, and custom designs. The company’s NextMove Series programmable controllers supply appropriate outputs to run linear and rotary steppers (as well as servo motors). A real-time motion controller is the newest NextMove product (see photo).
Oriental Motor Co. (OM) describes the development of new step motor and drive systems as a process of “refining performance attributes and integrating components.” Several trends comprise the process.
Higher torque density : Periodic redesign with higher energy product (strength) neodymium-iron-boron (Nd-Fe-B) rare-earth magnets, plus structural changes, such as larger rotor diameters, has improved motor torque performance. OM products benefiting include high-torque PV and PK-HT Series hybrid step motors (2-phase) and CRK Series, 5-phase motors.
Improved step-angle accuracy and drive smoothness : An advanced microstepping method is used to obtain higher step accuracy via closer current control. For example, Oriental’s RK Series, 5-phase microstep driver combines a special ASIC (application-specific integrated circuit), proprietary software, and current sensor to control current over a wide speed range. Current sensor and control software further provide a “smooth drive function” that limits vibration and noise during operation, independent of the driver’s input frequency, explains Nick Johantgen, engineering manager at Oriental Motor USA. Lower inherent vibration of 5-phase versus 2-phase step motors also aids the condition. Example products with “smooth drive” are RK and CRK Series 5-phase stepper controls.
More miniature sizes : “An overall trend for open-loop step motors and their associated drives is toward smaller sizes,” says Johantgen. “Many new applications in portable medical and industrial instruments require use of smaller motors.” To meet this challenge, OM has developed size 8 and size 11 CRK 5-phase motors as well as size 11 and 14 (1.4 in./35 mm) PK 2-phase step motors. New compact CRK microstepping driver measures just 0.98 x 1.77 x 2.56 in. and controls CRK motor sizes up to 1.4 A per phase.
System view, market view
Another trend at Oriental Motor is the bundling of various step motor/drive components into a coordinated system solution. This includes mounting plate, flexible coupling, damper, lead wire set, and controller. “Bundling individual components into a full system tends to reduce overall costs,” adds Johantgen. It further serves to aid competition with servo-based motion.
|Besides producing more running torque due to better magnetic flux focusing, redesigned laminations of Shinano Kenshi Corp.’s new stepper motors also offer lower magnetic-field vibrations and 3-6 db less audible noise-depending on running frequency.|
Three types of gear heads for CRK step motors are also part of the system offering. Lower cost hobbed-gear type has 10-35 arc-min backlash (gear-ratio dependent); planetary-gear type has wide gear ratios and 3 arc-min backlash; and harmonic-gear type essentially eliminates all backlash.
Overall size of the stepper motion market, including commercial and automotive sectors is quite large. The core industrial sector, factory automation (FA), represents a substantially smaller market, amounting to around $156 million for stepper motors and controls consumed in North America in 2006, according to the latest market study from Motion Tech Trends (MTT). FA markets include machine tools, robotic systems, OEM production machinery of all kinds (including semiconductor equipment, printing, textiles, plastics, etc.). Rough estimates by MTT size the stepper motor market for Europe and Japan 15% and 50% higher, respectively, than the U.S.
Authored by MTT analyst Muhammad Mubeen, the study indicates that two step motor designs dominate FA applications (see “Market” diagram). These are mass produced tin-can type (aka “can-stack”) and permanent magnet hybrid step motors. Another design—variable-reluctance (VR) stepper—has virtually disappeared in this arena.
“VR steppers are hardly ever seen in new applications, especially with new price levels of competing hybrid steppers from China,” says Mubeen. One exception for VR steppers is in very high-temperature applications, but not at the high-performance end of the market. MTT’s study, “North American Market for Step Motors and Controls (2001-2006),” can be found in the Information Product Center of MotionInfo.com.
Berger Lahr Motion Technology Inc. (a Schneider Electric company) regards compactness, simplicity, and networkability as key trends for stepper systems. Shrinking drive and motor dimensions help reduce control cabinet and machine space requirements, while system setup can be done in less than an hour via DIP- and rotary-switch settings, explains Sam Bandy, project manager-motion. The stepper drive needs no commissioning software. “However, some stepper systems with embedded functions need configuration software with screens for data input,” says Bandy.
Networkability of stepper drives via CAN-open, Profibus, Ethernet, etc., promotes multi-axis control, communication with higher-level controllers, and system status data acquisition.
Competing with servos
Bandy points out that, unlike servo motion, the drive-motor combination in a stepper system needs no tuning. “It eliminates the need of an expert to do the commissioning and also reduces set-up time,” says Bandy. Another stepper benefit is fast response time. With servo technology, corrective action is based on error content, which is the difference between intended action and an actual state—according to rules of closed-loop PID control. “This reactive, ‘lag-following method’ to obtain the position reference makes it slower in some applications like printing and labeling machines,” says Bandy. “There is no lag-following in a stepper system. The proactive action does not depend on an actual state and can be executed based on the control alone.”
|Beyond 2006, growth of North America stepper-based motion is expected in the 3-4% range, according to Motion Tech Trends.|
SKC’s Bergsma believes that competition between stepper-based and servo-based motion systems hinges on motor pricing. “Servo motors still can’t compete with stepper motors on price. Because stepper motor prices also continue to drop, they probably never will,” he states. Moreover, stepper motors lend themselves to high-volume manufacture, hence lower unit cost. “They offer a commercial value better than that of servo motors,” continues Bergsma.
Demand for what he calls “complete operating systems,” using either stepper or servo motors, is low. Such one- or two-motor applications typically use a “black box” driver/indexer controller, which sometimes comes with too many functions and may be overpriced. In contrast, “a majority of today’s motors are integrated into customized and standardized applications that require large quantities of motors,” says Bergsma.
Most high-volume motion applications are stepper driven. These price-sensitive systems take advantage of much lower production cost for stepper controls using available drive ICs. “Servo-driven systems still occur in OEM equipment but only up to low/medium volumes and only for applications requiring closed-loop feedback and acceleration/deceleration that servos can provide,” adds Bergsma.
Connie Chick, GE Fanuc director, Control Systems Business, voices a recurring theme that steppers suit low-end motion systems, where final position accuracy is less than critical. Steppers also work well for simple setup axes—examples include some indexing applications like feed-to-length, guide rails, and end stops. “Servo motors have much higher speed and torque ratings. So, high-speed machines requiring accurate positioning and/or high torque are not stepper opportunities,” says Chick.
One GE Fanuc development focuses on micro controls and “smart steppers.” It involves a stepper amplifier integrated into the motor housing with commands input over a simple device network. A system consists of GE Fanuc’s small form factor VersaMax Micro 64 PLC with built-in pulse/direction output control and MotorCube smart stepper that doesn’t take up panel space. Intended for simple indexing applications with very low cost targets, this approach seeks to hold machine control costs to under $2,000 installed (for one motion axis).
“Servo control continues to drop in cost, but there is still a place for a simple stepper application,” says Chick. GE Fanuc is a provider of stepper and high-performance servo systems.
|Portescap’s new h3 Series hybrid motors deliver up to 40% more torque in the same footprint as the standard h3 line (and competing motors). Stator-enhanced magnets contribute to higher performance by focusing the flux path for increased torque production.|
Portescap (a Danaher Motion company) likewise notes step motors’ inherent ability for positioning without requiring feedback—when “properly sized to the application.” It’s a competitive advantage versus servo-motor systems. “Open-loop control, plus motor design improvements make stepper-based systems cost-effective,” states product manager, Dave Beckstoffer.
For example, Portescap has substantially increased output torque in the newest member of its h3 Series hybrid step motor family. An aluminum housing design enhances heat dissipation, hence slows motor temperature rise that normally cuts torque performance over time, explains Beckstoffer. “It also reduces power consumption to benefit overall power requirements of the step motor system.”
Neodymium magnets used in the new step motor design reportedly optimize torque density without an increase in package size. Applied in can-stack type step motors for some time, Nd-Fe-B rare-earth magnets are relatively new for optimizing torque density in hybrid step motors, according to Beckstoffer. These innovations also help quiet the motor and reduce resonance. “Bearings of h3 steppers are secured via a retainer and o-ring, which prevents bearing spinout (axial movement during operation)—a typical noise source of step motors,” adds Beckstoffer. Larger bearings handle higher side and radial loads compared to standard hybrid steppers.
Where’s the action?
Stepper motion systems compete on an equal basis with servo systems in open-loop applications without need for encoder feedback—such as packaging, material handling, and assembly—and also where high speed or acceleration/deceleration isn’t critical, according to SKC. “Servo-based systems perform best in demanding closed-loop applications,” adds Bergsma.
Linear steppers compete in semiconductor and fiber-optics manufacture, wire bonding machines, laser trimmers, wafer-probe machines, and medical equipment, among other applications, says Baldor Electric. Linear stepper systems often garner praise for their low maintenance. compared to alternatives.
“Any point-to-point application in a controlled environment where tight coordination among axes isn’t important is an excellent candidate for a low-priced stepper solution,” adds Parker’s Feyh. He cites myriad such applications in the life sciences industry, medical instrument systems, and other desktop equipment.
MTT cites examples of tin-can steppers in FA applications such as pick-and-place machines, conveyors, and material handling/packaging machines of relatively small size.
Portescap considers steppers preferred technology for textile manufacture, electronic assembly, linear stages, and medical analyzers. Many step motors also are used in automotive applications, but they’re usually larger-sized, more powerful models.
Target applications for stepper systems mentioned by Berger Lahr/Schneider Electric include those with very low maintenance needs and OEM systems which require a low-cost motion solution. Because low-level skill can commission the stepper system, machine exporters also often prefer it over a servo system.
Overall, stepper-based motion systems are alive and well. Enhancements continue to evolve over the entire power range of offerings, but are most visible in the compact and lower power sizes of stepper motion systems.
|Frank J. Bartos, P.E., is consulting editor with Control Engineering. Reach him at firstname.lastname@example.org .|