Direct-Drive Linear Motion Lives!
Technologically, they've made a straight line to success. Direct-drive linear motion systems eliminate gearboxes, ballscrews, belts, couplings, or other rotary-to-linear motion converters between motor and load—offering superior speed, acceleration, load-positioning accuracy, and rapid stroke cycling, compared to systems based on rotary motors.
Technologically, they’ve made a straight line to success. Direct-drive linear motion systems eliminate gearboxes, ballscrews, belts, couplings, or other rotary-to-linear motion converters between motor and load—offering superior speed, acceleration, load-positioning accuracy, and rapid stroke cycling, compared to systems based on rotary motors.
Yet all this has come at higher cost and complexity. Manufacturers are hard at work to cut into these limitations, generating new life into direct-drive linear (DDL) motion systems. Their task is made more difficult by the ongoing economic malaise, coupled with the reluctance of customers to move from the short-range comfort of an established design to newer technology with potential for longer-range gains.
One way that suppliers inject added value into DDL motion products is by integrating motor, bearings, and feedback device into a complete assembly and providing matching controls.
Direct-drive motion systems presently account for a relatively small portion (6-10%) of all linear motion applications—but the DDL segment is growing. One indication of market size comes from Motion Tech Trends (Inglewood, CA) in its report, “Direct Drive Linear Motors in North America, 2002 and 2007 .” MTT estimates that the North American DDL motor market stood at $108 million in 2002, with nearly 78% of usage going to factory automation (including semiconductor equipment and machine tools), almost 20% to instrument systems (including medical), and 2.6% to all others.
MTT president George Gulalo expects growth of DDL devices to spurt 15-20% per year over the next few years, reaching about $242 million in 2007. Breakdown by applications is forecast to be nearly the same as in 2002. The U.S. represents about 35% of the worldwide market for DDL motion devices.
Figures from Europe along the same lines come from a November 2002 Frost & Sullivan (London, U.K.) report, “The European Market for Linear Motors,” which predicts “robust, sustained growth” for this sector in the near future. The F&S report expects revenue to double from $57.0 to $125.0 million and sales to nearly quadruple from 33,300 to 125,500 units between 2001 and 2008. It cites “a larger, expanding customer base” as fueling growth of applications, volumes, and revenues for linear motor solutions.
“Direct-drive linear motion technology is in a maturing phase,” says Scott Hibbard, vp of technology, Electric Drives and Controls, Bosch Rexroth Corp. (Hoffman Estates, IL). DDL technology is no longer limited to a few leading-edge applications, such as high-speed machining and semiconductor production. Today, there is a wider knowledge base on applications, along with improved economics or “cost-per-unit-force,” as Mr. Hibbard puts it, stemming from larger production quantities and motor design improvements.
Better sealing, thermal management
Rapid point-to-point motion with high acceleration, made possible by DDL technology, creates added heating in the motor. Applications that require rapid cycling exacerbate the condition.
Among current initiatives to advance DDL motion, Bosch Rexroth is improving the sealing and thermal management of linear motors, which many machine developers expect to match those of rotary motors. This is a substantial task, given the open characteristic of linear motors exposed to harsh machining environments. “We have instituted a design principle that encapsulates both primary and secondary sides of the motors and achieves an IP65 rating as well,” states Mr. Hibbard.
Thermal management is a special problem for linear motors because all generated heat has to be dissipated through the surrounding machine parts, in contrast to rotary motors, which transfer a sizable part of heat directly through the case into ambient air. “Thermal growth can negatively affect the machine process, such as in metal cutting,” he adds. Conventional fluid cooling helps, but can still leave “hot spots.” Bosch Rexroth reports its thermal encapsulation technique limits temperature differential to 2 °C between fluid entry and the case, across the motor’s whole mounting surface.
Thermal management of high-performance linear motors is likewise a focus at Siemens Energy & Automation, Motion Control Systems (Elk Grove, IL). “Heat loss occurs almost exclusively in the primary [stator] section and is dissipated via an integrated, precision cooling system,” says Meredith Johnson, Siemens E&A project manager. A dual-circuit cooling concept is used on Siemens’ two main lines of permanent magnet sealed synchronous linear motors, 1FN1 and 1FN3. Trademarked Thermo-Sandwich, the low-cost cooling design (using air or water) limits heat transfer to the surrounding structure by thermally decoupling the motor from the machine. Avoiding thermal distortion is crucial to precise machining.
Design of 1FN1 and 1FN3 motors features larger air gaps between the stator and permanent magnets in the secondary section to accommodate machine construction variations. “Greater thrust with minimum force ripple is achieved for smaller frame sizes due to winding slots in the primary section with slanted magnets in the secondary section,” remarks Mr. Johnson.
Intended for machining applications, 1FN1 motor features accuracy and low force ripple. The 1FN3 suits high-dynamic applications and offers numerous motor sizes and options. Multiple motors of either type can be applied on one axis (or track) to increase continuous and peak force output, explains Mr. Johnson.
These linear motors work with Simodrive digital 611D and analog 611U drives. The CNC portion of the system, which enables machine tool as well as other motion-control applications, can be integrated within the drive, in a separate unit (e.g., Sinumerik 840D), or in third-party controls.
Material improvements, cost issues
Improved materials and thermal performance of motor coils, plus cost reduction due to higher volume production are common themes that run through developments of several manufacturers. Anorad Corp. (Shirley, NY), a Rockwell Automation business, sees the above factors helping DDL motors gain on traditional rotary motor/ball-screw combinations in new applications.
|“Balanced iron-core” construction of Yaskawa Electric’s TW linear motor (right) negates effects of the magnetic attraction force between iron-core coil and permanent magnet track members. This contrasts with detrimental,high-attraction forces developed in a normal iron-core design (left).|
Availability of higher flux magnet material provides advances in two directions. Higher output forces are possible from motors with existing dimensions; also “high-force, mini-linear motors” can be developed, explains Anorad product manager Michael J. Pitka. New motors resulting from each approach have expanded the company’s application of DDL technology versus alternate designs.
Advanced materials include improved molding compounds for encapsulating the motor coils, which lead to better thermal performance as measured in terms of a cooler running motor. Less heat raises motor efficiency, resulting in higher output force per frame size. Better molding compounds are part of Anorad’s new LC steel-core brushless (synchronous) linear motor family that has recently received UL recognition (see sidebar).
From Anorad’s perspective, DDL motors are experiencing increased usage as the technology moves beyond semiconductor production into the industrial sector, for example, in packaging machines. Therefore, Mr. Pitka notes, advantages of high-volume manufacturing processes can lower the cost of these motors. “With linear motors meeting industry standards in a greater variety of size and force ranges, coupled with greater affordability, linear motors are well positioned to significantly displace rotary motors,” he adds.
Other initiatives at Bosch Rexroth tackle cost and size efficiency of DDL motors. Designs under development promise double-digit percentage increase in continuous and peak force output for a given frame size, according to Mr. Hibbard. This has major implication on pushing performance benefits and improving the cost issue of DDL motion systems.
Bosch Rexroth refutes the claim that DDL technology is more costly than a comparable rotary-motion system, citing substantial system cost savings in some applications. The company’s direct-drive linear products include motors with 200-22,000 N (45-4,950 lb f ) output and Ecodrive controls with up to 20 kW power rating and various standard communication interfaces.
Yaskawa Electric America (YEA, Waukegan, IL) focuses on removing concerns of potential users unsure of DDL technology as a way to widen applications. Recent developments enhance performance of all three types of YEA’s linear motors. For example, high-density patent-pending coil winding methods (a carryover from Sigma rotary motors) result in a 70% space-fill factor, which is substantially higher than in non-optimized motor designs, explains Fred G. Zahradnik, engineering manager for Linear Mechatronics at YEA. Use of high flux density neodymium-iron-boron (Nd-Fe-B) magnets also pushes the performance. Yaskawa stresses “seamless integration” of its Linear Sigma motors into the common Sigma-II product line of rotary ac servo motors and controls.
Coreless construction of GW-Type linear motors eliminates cogging. Force ripple is reduced to 1/4 of the magnitude of current competition, according to Mr. Zahradnik, via a double-sided, phase-shifted winding design (patent-pending) placed on either side of the moving coil. The coil rides inside the “U-channel” magnet track fitted with Nd-Fe-B magnets on each side.
FW-Type linear motors consist of a moving coil and a single-sided stationary magnet track of skewed rows of rare-earth magnets on a carrier plate. A patent-pending feature, “sub-teeth” formed in the ends of the iron core, help to dramatically reduce edge-cogging forces compared to designs without sub-teeth, Yaskawa says.
Balancing magnet forces
A third linear motor, TW-Type, significantly reduces high attraction forces inherent to iron-core motors, thus easing requirements on structural design and bearing load capacity. This “balanced iron-core design” locates magnets on either side of the moving coil assembly (see diagram), so that attraction forces between stationary and moving parts are negated. Without balancing, magnetic attraction forces can be as high as 5-6 times the peak thrust forces developed by the motor.
With the balanced design, a single rail can guide a TW linear motor on each side of a gantry-type application, instead of the typical set of dual guide rails. Alignment tasks and running parallelism can more easily be obtained.
Mr. Zahradnik mentions the issue of feedback information reliability for servo control and commutation in linear motors. He notes that noise induction and voltage drops are the two most prevalent linear feedback signal malfunctions, against which Sigma-II system encoder methods provide a “good margin of immunity.”
Another balanced design development comes from Parker Hannifin Corp., Compumotor Div. (Rohnert Park, CA) with the introduction of its BLMA (Balanced Linear Motor Actuator) line. BLMA locates the coil windings on either side of the vertically oriented magnet bar. The “balanced design” cancels magnetic forces perpendicular to the travel direction, thus “providing all the forces of an iron-core actuator with only half the magnet rail costs,” says Marc Feyh, product coordinator for Custom Servo Motors at Parker Compumotor. Heat generation problems associated with high magnet forces also are addressed in the actuator design. BLMA reportedly generates only 50% the heat of a comparable size standard iron-core linear motor.
Meanwhile, Parker Compumotor is widening the market range of its slotless SL Series linear motor line by enhancing performance and reducing costs. SL Series is available as a line of linear motor components, for direct OEM integration into machines for substantial cost and space savings. To ease installation and on-site engineering for smaller OEMs and end-users, SL Series components also make up a packaged line of linear motor tables supplied by Parker’s Daedal division. “Lower price and outstanding performance make it a competitive choice with a wide variety of traditional servo-based positioning systems, such as belt and lead-screw tables,” adds Mr. Feyh.
Don’t forget LIMs
Most publicity centers on synchronous-motor-based systems, but direct-drive linear induction motors (LIMs) also are part of the big picture. John Mazurkiewicz, product manager at Baldor Electric Co. (Fort Smith, AR), mentions recent developments around power factor correction (PFC) involving induction based linear motors. PFC seeks to minimize the phase angle between voltages and currents that control the motor. “The advantage of this is energy savings. And you can get up to 40% more force,” he states.
The obvious benefit of correcting the power factor is lower operating cost of the motor. However, Mr. Mazurkiewicz notes other advantages of PFC obtained through appropriate control of LIMs, as well. A higher power factor means higher power capacity (or force delivered) per size of motor, lower voltage drops and their related problems, and more efficiency because of a cooler running motor.
As one way to attract more applications and users, Baldor Electric has developed what it calls a “Hybrid Core” brushless linear motor. Only the motor’s primary (forcer) carries magnets; and the slotted secondary (platen) is cut to the desired travel length in this lower cost approach. Hybrid Core motor features two kinds of primaries, explains Mr. Mazurkiewicz. A two-phase, stepper-type primary provides open-loop performance using standard stepper controls. For more demanding applications, a three-phase servo-type primary delivers closed-loop control using a servo drive and customer-supplied linear encoder. Forced-air cooling extends the continuous force rating of the motor by about 20%, according to Baldor.
Danaher Motion/Kollmorgen (Radford, VA) is pursuing three development goals to position direct-drive linear products to be more competitive with traditional technologies: Increased performance, improved ease-of-use, and lower costs. Dave Riebe, DDL product manager, foresees added performance from “investigating new potential electromagnetic designs” at Danaher Motion. Other elements of the program are use of new sales tools and complete solutions—controller, drive, motor, and mechanical assembly—that make it easier for customers to use DDL motion systems. As for product economics, “Increased volume, supply-chain improvements, and more efficient manufacturing techniques have resulted in reduced costs,” Mr. Riebe adds.
The latest linear motor line from Danaher/Kollmorgen is Platinum DDL, available in either ironless construction (no attractive force between primary and secondary parts) or iron-core construction, offering high force output per frame size (see photo).
Direct-drive linear motors exhibit a rich variety of forms as illustrated by the cover graphic. A source for still other DDL motor types and a products table can be found in the “further reading” box.
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First linear motors gain UL recognition
Certified insulation system integrity and product safety may be taken for granted in rotary servo motors, but it’s big news for linear motors.
Anorad Corp. (Shirley, NY), part of Rockwell Automation, claims a linear motor “first” with the February 2003 recognition by Underwriters Laboratories (UL) of its new LC family of steel-core, brushless linear motors. Meeting UL’s extra insulation requirements posed performance and packaging challenges for the frameless construction of these motors, explains Anorad’s director of marketing Michael Backman. A new insulation system and motor-coil design provided the solution, achieving Class H 180 °C protection and 650 V dc bus-voltage rating. Moreover, use of the latest generation rare-earth magnets in the LC motor family development resulted in force density increase over previous models.
Finite element analysis was used to optimize LC linear motors’ patent-pending laminated steel-core design, allowing an unusually high 85% copper fill in the coil assembly, according to Anorad. (It’s 7% higher than the predecessor steel-core motor.) The relationship between the steel core and larger quantity of copper leads to improved thermal efficiency.
Initially, two coil sizes are available, delivering 62 lbf and 94 lbf output in two motor models. LC motors are compatible with standard 3-phase brushless servo drives. For servo drives requiring commutation feedback, a Hall-effect module can be mounted to the motor’s moving coil. Motor commutation by software is possible with controls that offer this feature.
Get Linear Motion without
Converting from Rotary Motion
Linear actuators and electric-motor-driven devices that produce linear motion are common in industrial and commercial products. Experts estimate that about 75% of all motion applications in industrial markets require moving loads along a linear axis.
Direct-drive linear (DDL) motion systems are much fewer in number, but offer superior speed, acceleration, load-positioning accuracy, and rapid stroke cycling than their rotary-motor-based counterparts. They accomplish these results by eliminating gearboxes, ballscrews, belts, couplings—in short, any rotary-to-linear motion converter between motor and load.
At the same time, DDL motion is more complex to apply and costs more. Manufacturers are making strides to simplify usage and reduce costs. One example of adding value to a direct-drive linear motion system is to integrate bearings and feedback device into the same assembly as the DDL motor, which lowers system cost and makes it easier to apply. Examples of this approach include linear motion products from
Direct-drive linear motion is experiencing growth into diverse new applications as it becomes known to more potential users. Siemens Energy & Automation, Motion Control Systems, mentions high-dynamic machine tools, laser machining, pick and place, and other niche applications as suited for DDL motion technology.
Automation and material-handling applications, such as flying cut-offs, unloaders for plastic injection molding machines, and electronic assembly systems can benefit from productivity advancements of direct-drive linear motion, according to Bosch Rexroth Corp., Electric Drives and Controls.
Etel Inc. cites and electronics/photonics assembly, semiconductor fabrication, and pharmaceutical production as likely applications.
Lower cost iron-core motor design
Iron- core (or steel) and coreless motors make up two basic design varieties of DDL motion devices, providing a tradeoff between cost and smooth motion; see more in the main article, linked above.
Motion Tech Trends (MTT) notes a recent development in lower force applications where coreless as well as iron-core linear motors are gaining acceptance, with the latter “experiencing the majority of the gains.” Linear motors with an iron core motors supply a given force at lower cost. “Linear motors using iron cores cost about half as much as ironless core [or coreless] linear motors. The iron-core variety is less expensive for a number of reasons, including smaller size, fewer magnets and lower assembly cost,” according to MTT.
Ironless-core, direct-drive linear motors trade-off lower force capability and higher cost against practically eliminating cogging, thereby providing smooth performance and high acceleration.
Direct-drive linear motion products are manufactured by numerous companies. For example, see the products table in “ Control Engineering , April 1999, pp. 62-70.
Here is a sampling of new DDL motion products:
IndraDyn L is a new generation of permanent-magnet synchronous linear motors from
TT Micro linear motors from
Performance features of TT Micro linear motors include peak force of 45 N, acceleration up to 250 m/s
Some other developers and suppliers of direct-drive linear motion technology include:
|Adept Technology SmartAxis;
(Livermore, CA) SmartModules
|Aerotech (Pittsburgh, PA)||ATS 1000 linear Stage||www.aerotech.com|
|CLD (Carlsbad, CA)||4000, 5000 Series tubular
|TT Micro linear motors;
M3810 Thrust Rod linear motors
|Etel (Schaumburg, IL;
|TLC Series positioning
stages; DSCD-P controller
|LinMot (Delavan, WI;
|LinMot P Family||www.linmot.com|
|THK Inc. (Schaumburg, IL)||LSA Series linear
|1-, 2-, 3-, or 4-axis linear
To request information, visit www.controleng.com/freeinfo and search under the company name.
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