Direct-Drive Linear Motion Matures

Rockwell Automation LZ Series direct-drive ironless linear servo motors (top left) optimize smooth motion, delivering 68-850 N (15-190 lbf) continuous thrust. LC-100 Series iron-core linear servo motors (lower right) are intended for high force applications.

Compared to conventional linear motion systems, direct-drive linear (DDL) motors deliver superior speed, acceleration, load-positioning accuracy, rapid cycling, and more. However, DDL motion systems have higher initial cost and require tighter design integration with a specific machine or overall system.

Core of a typical DDL system is the linear motor. In simplest terms a linear motor consists of a primary and a secondary element. Respectively, these correspond to the rotor and stator of a rotary motor, with the stator rolled flat. The primary (or slider), moves linearly relative to the secondary, separated by an air gap. Several linear motor design variants exist.

Mature, yet advancing

Bosch Rexroth, Electric Drives and Controls Division, regards DDL motor systems to be a mature technology, yet one that continues to make incremental advancements. The company cites examples, such as improved coolant jacket designs that minimize temperature differential to less than 2 K between stator and machine bed, optimized magnet shapes to reduce force ripple and material cost, and magnet tracks protected by stainless steel covers supplied pre-assembled in various lengths, to ease system installation and operation.

“Direct-drive linear motor systems combine high speed and acceleration with high accuracy in a quite simple machine construction,” says Karl Rapp, industry sector manager, at Bosch Rexroth’s Automation & Machine Tool unit. “This enables machine builders to develop new designs that were hard or impossible to perform before.”

For example, in an industrial glass-cutting application, DDL linear motion improved production time by around 20% compared to a conventional servo motor and rack system. Galactic plate-glass cutting machine from German company Hegla achieves up to 3.5 m/s (11.5 ft/s) x-y axis speed and 12 m/s2acceleration using Bosch Rexroth IndraDyn L linear motors, while holding tolerances of around2glass area. This compares to only

Today’s intelligent digital drives combine with DDL motors to achieve very fast control-loop closures in the drive, rather than in the controller. Functions in IndraDrive, such as force/torque ripple compensation, nanometer-level interpolation, and ultra-high resolution sinusoidal feedback, are said to result in superior dynamic/static stiffness and motion accuracy. “Additionally, automatic commutation functions can eliminate the need for absolute feedback or Hall-effect sensor boxes, although they’re supported as well,” states Rapp.

Yaskawa Electric emphasizes the importance of quick and simple DDL motion system setup. Advanced features in its new Sigma-5 Series servo amplifiers make that possible, explains Paul Zajac, product engineer at Yaskawa Electric America (YEA). These features include a tuning-less function, said to provide “instant performance,” and an advanced autotuning function for higher performance.

Another feature is enhanced vibration suppression, which reduces machine resonance and settling time. It also eliminates vibration due to machine resonance and load disturbances, according to Zajac. “This function automatically detects and suppresses oscillation frequencies under 1 kHz. A notch filter is also available to control frequencies of 1 kHz and above,” he says.

Parker Hannifin Corp., Electromechanical Automation NA, connects increased performance and reduced motion errors in DDL motor systems to the power of today’s controllers and drives. “Controllers increase performance because they can preemptively solve servo errors associated with the first and second derivatives of speed (acceleration and jerk, respectively), which are typically caused by friction, stiction, and other repeatable disturbances,” states Jay W. Schultz, product manager at Parker. “Error reduction and performance benefits are especially noticed in DDL systems because the load is coupled directly to the motor and thus to the feedback device.”

Siemens Energy & Automation agrees that continued innovation has marked succeeding generations of linear motor products. A 6th version linear motor introduced earlier in 2008 is evidence of the company’s commitment to the product line, notes Jeff Gerlach, Siemens E&A consulting business developer. “New 1FN6, a self-cooled, brushless synchronous linear motor series, features a magnet-free secondary (track) for cost efficiency and easier assembly,” says Gerlach. Track sections without magnets, which are ideal for applications requiring long traverse, and elimination of water cooling add up to simpler, less costly installation.

Dominant motor type

DDL motors fall into two main categories: brushless PM (permanent magnet) synchronous and LIM (linear induction motor). The first type dominates. Bosch Rexroth’s Rapp cites several reasons:

Higher force density and efficiency;

Faster response due to an existing magnetic field;

Less heat loss transferred into the machine structure; and

Lower stack height.

Siemens’ first linear motors were LIM type, but technology has migrated to brushless PM synchronous motors. Advantages include power/force density, efficiency, and less energy usage, explains Gerlach. “For example, our old asynchronous LIM had 30% of the force density of our new 1FN6 Series,” he states.

Rockwell Automation also considers brushless synchronous linear motors more cost-efficient than LIMs. For example, cost per Newton thrust force is much lower.

To illustrate, Steve Feketa, product manager, Linear Motors and Actuators at Rockwell, points out that a typical PM synchronous motor with 2,356 N (526 lbf) peak force has a coil volume of only 1,816 cm3(or 1.30 N/cm3), while a LIM producing 2,224 N max., would require 25,659 cm3(or 0.08 N/cm3)! Also, users have wider selection of drives to control brushless synchronous motors versus LIMs.

Yaskawa continues to offer several PM synchronous linear motor types:

Coreless U-channel GW-type—intended for minimal cogging and force ripple;

Iron-core FW-type—with moving coil and one-sided stationary magnet track for higher force output, plus improved system rigidity, deceleration, and settling time;

Iron-core TW-type—with magnets on either side of the moving coil to balance high magnetic attraction forces developed between the coil and magnets in the FW design. TW-type also eliminates special structural and support bearing design; and

Cylinder-shaped, ironless CW-type—which forms the basis of a non-contact, maintenance-free linear motor system.

LIMs have market appeal in transportation, warehousing, and some entertainment applications, but most general automation vendors offer only brushless PM synchronous linear motor systems.

Vital feedback devices

Rockwell Automation sees another significant change for DDL technology in the progression of feedback devices. Until recently, glass-scale optical encoders were the only feedback devices suitable for DDL motion systems. Moreover, encoder cost and sensitivity limited DDL motion systems to high-end electronics and semiconductor applications in clean environments, explains Feketa. (An exception is in machine tools where optical feedback, including metal scale, is successfully applied.)

“Advancements, such as lower-cost magnetic technology, now allow automation control suppliers to apply magnetic encoders,” Feketa says. Magnetic encoders bring two main benefits that justify DDL motion as a solution for more applications: They’re more rugged for industrial usage —less sensitive to dust, dirt, and debris than glass-scale encoders—and new feedback technology has helped lower DDL motion-system cost.

Glass-scale encoder feedback can cost as much as $2,000 for one meter length, while newer magnetic encoder devices can be integrated for under $200, according to Feketa. “While optical encoders are more precise, new industries adopting magnetic encoders find that the 1 and 5 micron resolutions typically offered are more than adequate,” he states. (See Online Extra for more on newer applications.)

A further big change noted by Rockwell is development of advanced algorithms and camming routines that help reduce machine jerking and shaking. “This allows customers to achieve high performance while preserving the life cycle of the motion system and its components,” Feketa adds.

Yaskawa DDL systems interface with various linear encoders, including optical and magnetic types. “Optical linear encoders are the most commonly used feedback devices because they’re capable of higher resolutions compared to magnetic encoders,” Zajac says. YEA’s linear motor systems are also compatible with absolute linear encoders—an electromagnetic induction type device highly resistant to oil and water contamination (rated IP65)—available in 0.1 or 0.5 micron resolution and lengths up to 6 m. “The non-contact design is optimal for high-speed, high-acceleration linear motor applications. Using an absolute encoder eliminates the need for homing as well as for magnetic pole detection,” he adds.

Parker Hannifin sees magnetic linear encoders applied in significant numbers to industrial markets. “They’re less expensive than optical encoders, easier to install, still give good resolution, and have very high degree of resistance to environmental contaminants,” Schultz says.

Total cost of ownership

First cost of direct-drive linear motion systems (mainly the motor) can be higher than the alternative. However, “many OEMs admit that once linear motor technology is embraced and properly designed into a machine, cost of the total machine is lower,” says Bosch Rexroth’s Rapp. It’s due to simpler construction, reduced assembly and startup time, and better machine performance for product quality.

“Costs decline with fewer moving parts and things to align and maintain,” he says, but cautions that, “just slapping linear motors onto an existing design has proven to be less successful than designing for direct drive from the start.”

Higher performance means benefits to OEMs, especially when multiple machines are used to meet production rates. “Direct savings from fewer total machines needed, plus indirect savings (less floor space, fewer machine operators) result in lower total system cost,” Rapp adds.

Siemens’ Gerlach also believes that long-term costs—operating, maintenance, and machine downtime—are less. He cites DDL motion systems’ low parts count in contrast to “all the parts that can fail in a servo motor/ball-screw system.”

Gerlach adds, “Using ball screws also involves more steps, like mapping torque and pitch variations into the control calculations. With a direct-drive system, potential component failures are limited to the linear motor’s primary section and feedback device.”

Rockwell Automation focuses on lower total cost of ownership (TCO) and faster return on investment (ROI) to make DDL motion systems competitive with lower-initial-cost alternatives. Productivity-raising advantages and less downtime for maintenance or equipment failures must be clearly tied to lower TCO for customer awareness, suggests John Good, director of marketing for Rockwell’s Linear Motion Solutions.

DDL motion systems’ superior accuracy and repeatability can translate into productivity gains. Good mentions a food industry example, where strict regulations require actual product weight to meet or exceed advertised weight. To avoid underweighting, one manufacturer opted for a DDL motion system that was able to cut food product to more precise weight, instead of the wasteful but common practice of packaging slightly more product than necessary per container. “The company experienced a return on its investment in approximately 18 months,” Good says.

Yaskawa likewise believes that total cost of ownership for a DDL motor system is far less than for other linear-actuator technologies. The company cites much the same benefits discussed above that “far outweigh” higher initial costs.

Parker Hannifin connects DDL motion systems’ cost-effectiveness to complementary benefits of precision, speed, and flexibility. Customers trading precision for speed when using belt-and-pulley transmissions, can now switch to DDL motors and increase precision by an order of magnitude without sacrificing speed, explains Schultz. Also, it’s possible to run multiple motor coils on the same secondary—the most costly part of a system with a magnet track—because each coil is physically decoupled from the force transmission. “This increases flexibility, reduces cost per axis, and can result in more throughput and space savings in many applications,” Schultz says. “Other cost-cutting potential comes from a relatively large number of DDL motion suppliers and low-cost feedback innovations.”

Market status

“Industry is seeing an annual growth rate of around 10-12% for DDL motion systems, whereas growth of general motion-control solutions is around 6%,” according to Rockwell Automation’s John Good. While a comparatively smaller market than general motion, DDL technology currently has an attractive market base of $725 million worldwide.

Market growth is due largely to more industries turning to DDL motion systems (see more online). Helping this increased adoption are developments like magnetic encoder feedback and less costly magnet materials that have significantly lowered overall cost of the technology. “In addition, machine builders and manufacturers are starting to reach performance limitations of rotary servo technology, and look for new solutions to help raise machine performance to the next level,” Good concludes.

Siemens notes more inquiries daily for DDL motion systems, particularly from custom-machinery builders. “We also see increased interest in converting linear (and rotary) axes to direct-drive motors on existing machine designs as OEMs update their product lines,” Gerlach says. He suggests, “the next logical step is replacing the servo motor, ball screw, and associated components with a linear motor primary and sufficient secondary sections to cover the traverse length. Fewer components, less assembly time, and more efficient operation would result.”

Yaskawa has seen a 10-fold increase in DDL motion sales, specifically to semiconductor and solar-panel industries,” says Zajac. “We expect this increasing trend to continue.”

According to Parker Hannifin’s Schultz, DDL linear motor market growth is highly volatile. “Some markets are expecting growth as high as 15% CAGR over the next several years, while others, such as semiconductor fabrication, are in decline,” he says. “Parker has steadily increased capacity to meet demand for linear motors. Even in this uncertain economy, we continue to see strong sales for our linear motors.”

Incremental advances for DDL motion systems include large expenditure of engineering resources to simplify motor design to ease customer installation. Parker claims its linear motors are the easiest to integrate into applications, largely due to their “generous air gap” design. Schultz emphasizes the point by reportedly having assembled a DDL motor system using everyday tools, even though he is “only a marketing guy.”

Direct-drive linear motion systems may have reached maturity, but have not lost the ability to innovate.

System attribute
DDL motor system
Rotary motor with

Ball screw
Rack/ pinion
Belt drive

Source: Bosch Rexroth Corp. and Control Engineering .

Velocity
++
o
+
+

Acceleration
++
+
o
—

Accuracy
++
+
—
—

Thrust force
o
++
+
o

Travel path
++
—
++
+

Wear out
++
o
o
o

Design effort
+
o
+
+

++ very advantageous

o neutral

+ advantageous

— less advantageous

ONLINE extra Also read:

Direct-drive linear motion expands its applications

and

Linear motion: New patent addresses maglev stability

.

Author Information

Frank J. Bartos, P.E. is a consulting editor with Control Engineering. Contact him at braunbart@sbcglobal.net .

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