Linear Motors and Controls Seek the Mainstream

Hardly new as a technology, direct-drive linear motion systems show a growing variety of configurations. Motor and load are directly connected—as the name implies—doing away with gear trains, belts, ballscrews, and pulleys to achieve remarkable speed, precision, and reliability. Linear motion systems shine where performance is crucial.

04/01/1999


KEY WORDS

 

  • Motors & motion control

  • Direct-drive linear motors

  • Brushless motors

  • Machine control

Sidebars:
Linear motor terms in brief
Configurations galore


Hardly new as a technology, direct-drive linear motion systems show a growing variety of configurations. Motor and load are directly connected—as the name implies—doing away with gear trains, belts, ballscrews, and pulleys to achieve remarkable speed, precision, and reliability. Linear motion systems shine where performance is crucial.

At the same time, linear motors are more complex to apply and costly to produce than their rotary cousins. Now, newer developments, design refinements, and a downward trend in manufacturing costs seek to open direct-drive linear systems to wider audiences.

Much effort centers on making the technology simpler. At Anorad Corp. (Hauppauge, N.Y.), motor features to enhance machine tool applications are among current developments. For example, its new LXB-S Series moving coil (MC) motor (see products table) covers magnet plates with stainless steel—allowing wipers to keep the surfaces clean of ferrous chips—and metallic encapsulation improves protection against coolant contamination. Connectors are attached to the coil assembly for ease of installation. "Cooling manifolds, used for both the coil and magnet assemblies, trap the heat from penetrating sensitive structural material," explains Boaz Eidelberg, director of Linear Motor Business Development.

Lower costs and ease of use in general automation are behind Anorad's introduction of moving magnet (MM) linear motors (see diagram). This motor type (model MM-NS) eliminates the moving cable, increasing motor reliability. Each coil is energized only when the magnet is directly above it. Thus, in high duty cycle applications the MM motor runs much cooler than a moving coil motor, provided the travel is longer than magnet length. "The coil has plug-and-play connectors for ease of installation. The moving magnet is also safer to operate since all the magnetic flux is trapped by the coil," says Dr. Eidelberg.

Anorad, a linear motion technology pioneer, became a Business Unit of Rockwell Automation (Milwaukee, Wis.) in fall 1998.

Kollmorgen (Radford, Va.) notes growth in linear motor solutions for applications where high reliability , rather than high performance, is the main criterion. And the cost outlook is likewise brighter. "Today, linear motors' cost differentials can be as low as 10 to 20% compared to traditional approaches (ballscrews)," states Claude Chirignan, Kollmorgen's technical support manager for linear motors. "Motor suppliers have improved their designs for manufacturing ease and employ lower cost magnet materials."

An end-product with "distinctive market differentiation" is a further attribute of linear motors. Mr. Chirignan cites emerging examples in pharmaceutical processes, people transportation, and robotics applications. He considers high-end applications, like laser cutting, water-jet cutting, CNC machine tools, and press feeding as still strong. "General-purpose, precision X-Y tables also get recognition. Here, a pre-engineered solution can drastically cut development time, quickly bringing a new product to market," he adds.

Simplify, improve

Aerotech Inc. (Pittsburgh, Pa.) likewise mentions developments to simplify linear motors. Among examples cited are actuator packages combining an encoder plus support bearings; and the moving magnet linear motor that eliminates "cable-management problems" with the coil allowed to remain stationary.

For greater force output from a fixed coil size, higher density magnets represent one approach to magnet track design. "This allows use of a smaller frame motor where a larger motor with equivalent output force would not fit," says Ed Novak, product manager, Drive Components Div. at Aerotech.

Last year was prominent for acquisitions in linear motion technology. In March 1998, Baldor Electric Co. (Fort Smith, Ark.) acquired Northern Magnetics (Santa Clarita, Calif.).

Roger Bullock, linear products specialist at Baldor, notes emergence of applications requiring higher force, low-inertia motors. Non-cogging (ironless) linear brushless motors are under development with forces up to 1,500 lb (6,672 N) peak, slated for release in second-half 1999. "Noncogging, low-inertia motors have greater acceleration capability with excellent low-speed performance (no force or velocity ripple)," comments Mr. Bullock. Other developments seen at Baldor include prepackaged products that marry linear motors, encoders, bearings, etc., to reduce the user's intensive tasks of product integration, testing, and design.

Cost, risk, change

Bruce Beakley, president of Trilogy Systems (Webster, Tex.), sees a growing reliance on linear motors for "high-performance positioning applications" over the last decade. But demand has been limited to a few markets able to justify the cost and demanding the highest output, for example, semiconductor fab and electronic assembly.

Mr. Beakley cites barriers to wider user acceptance. These include high cost of motor and other system elements; risk , that is, reluctance to try new (and costlier) technologies; and unfamiliarity that forces engineers to "think differently" about machine design, for example, minimizing weight to reduce motor heating. And changes in design approach occur slowly. "Developments that eliminate the three barriers expand and drive linear motors' markets," he states.

Trilogy has just introduced the Linear Encoder Module (LEM), said to be the first linear-motor-based encoder that uses actual permanent magnets of the magnet track as the position feedback. A Trilogy invention, LEM costs less than standard linear encoders (no cost penalty for longer length), works in harsh environments, and supplies motor commutation signals as well as home and end-of-travel limit sensors. Resolution and repeatability is 5 microns (0.0002 in.).

Etel SA (Môtiers, Switzerland) stresses competence in precise positioning and speed stability—prime motion requirements for visual inspection and scanning applications that can only be met with linear motors and high quality electronic controls. Examples of capability are 1 nm position stability for wafer inspection and speed stability of 0.1% at 6 mm/sec for image scanning.

Denis Piaget, head of Etel's Direct Drives & Systems Div., cites electronic chip fabrication as another area where linear technology can improve productivity and reliability. Prime requirements are to reach a position very quickly, "but also with a minimum settling time and a smooth movement profile." Mr. Piaget lists typical performance in a pick-and-place machine using Etel equipment as: 50-mm movement in 40 msec—within a 1 m window—with a 2-kg payload.

Not all high accuracy

Not all applications require high accuracy. Mr. Piaget believes a market is opening for long-stroke conveyors (horizontal and vertical), where position accuracies of 1-2 mm suffice, "but smoothness of the movement is important." Electronic controls and a low-cost encoder complete this economic solution.

Baumüller (Nürnberg, Germany; U.S. office in Bloomfield, Conn.) also specializes in modular, long-stroke linear motion systems for heavy loads in marine container terminals and parking garage applications. The company manufactures PM brushless motors and amplifiers for these and higher precision industrial applications (see products table).

Improved cooling and encapsulation of PM linear motors are ongoing developments at Rexroth Indramat (Hoffman Estates, Ill.). Its standard PM linear motors, available with a liquid-cooled primary, have IP64 protection rating for general automation. However, new thermo-encapsulated motors with IP65 rating target machine tool and high-precision uses. An epoxy-filled primary enclosed in an aluminum jacket with a liquid-cooled surface surrounds the coils. "Maximum temperature rise—from coolant input to outside of motor—will not exceed 2 °C," says Karl Rapp, machine tool applications engineering manager.

For precise usage, Indramat PM linear motors rely on an absolute scale for feedback of commutation signals. Positioning accuracy of 0.1

New products from Indramat include an ultra-flat PM motor for space-critical applications. The cooling channel built into the primary minimizes motor height. Also introduced is a small linear motor for applications requiring feed forces up to 1,000 N.

Indramat sees no great motor improvements coming from current magnetic materials, but is looking at various primary coil winding methods to raise force density. Overall, its linear motors operate at speeds up to 600 m/min and forces up to 22,000 N/motor.

Simpler and more practical is likewise the theme at Sulzer Electronics AG (Zürich, Switzerland) and its Linmot Div. Linmot actuators feature a simple moving slider, fixed stator, position sensors complete with electronics, and bearings integrated within a compact, cylindrical metal body. The plug-and-play approach promotes simple connection to drive electronics and interfaces to PLC and PC control, via popular communication networks.

Linmot-P actuators target precision industrial linear motion control, typically in textile weaving, material handling, packaging machines, and robotic manufacturing.

PM brushless vs. induction

Of the two main linear motor technologies (see first sidebar), permanent magnet (PM) brushless dominates the market, most manufacturers say. Anorad offers these advantages of PM brushless versus linear induction motor designs:

  • Higher efficiency—PM type develops higher flux without heating; LIM generates magnetic flux by induction, thus increasing heat produced in the primary and induced into the secondary;

  • Higher continuous force per coil area—Typical values are 5 lb/in.2(3.5 N/cm2) with PM motor; 1.8 N/cm2with induction motor;

  • Smaller size—Magnets have low profile and no flux coils are needed for PM, compared to large flux coils and a secondary for cooling in LIMs;

  • Simple amplifier—PM uses standard 3-phase brushless commutation, while LIM needs a drive with amplitude and frequency control;

  • Simpler mechanical design—Air gap dimension is larger for PM motor (typically 0.5-1 mm vs. 0.2 mm for a LIM. Flux develops in higher concentration near the PM coils; and

  • Higher accuracy—Less heat distortion occurs with PM designs; LIMs pose greater heat distortion in long secondary, and heat removal is difficult at stand still.

For their part, induction motors eliminate the permanent magnets—a substantial benefit. Magnets need careful handling (also personal safety awareness) due to high attraction forces. They're also more difficult to install in a machine and attract ferrous chips even when the motor is shut off.

Kollmorgen's Mr. Chirignan adds lower audible noise and lower cost of ownership to the above benefits of a PM brushless solution. In his experience many users have found the perceived cost differential favoring ac induction "either not there or offset" by PM brushless solutions' advantages.

Baldor's Mr. Bullock concurs about higher efficiency and acceleration with PM brushless. He further notes more user acceptance and developments for this technology stem from most manufacturers' concentration today on only PM brushless motors. Still, Mr. Bullock sees vector-controlled LIMs less costly than PM linear motors, allowing closed-loop positioning to

Trilogy's Mr. Beakley adds, "With continual decreasing price of neodymium magnets, brushless motors will surpass induction motors and be the dominant technology."

Indramat likewise places PM brushless linear motors in the lead for user acceptance. "Permanent-magnet motors provide up to twice the force density compared to linear induction motors of the same physical size because of power losses to induce fields into the secondary," remarks Mr. Rapp.

At the same time, LIMs continue to be a viable solution, especially where a continuous magnetic field poses problems or long travel is necessary. "Permanent magnet costs outstrip induction motor costs as motor length increases significantly," he says.

Aerotech equates the market dominance of PM brushless linear motors to their performance—high output force from small packages. Ease of installation and numerous sizes available also contribute. It considers LIMs impractical in many high-speed OEM machine applications due to a high coil weight per output force ratio.

Controllers: the vital link

Drive electronics—including the motion controller, amplifier, feedback method, and control algorithm—play a major role in the overall performance of linear motion systems.

Etel agrees that electronics and the feedback method are essential for success. Also vital is a system capability to address mechanical design issues like structural stiffness, natural frequency, and oscillation, explains Mr. Piaget.

Advances in controls can help decrease what Baldor's Mr. Bullock calls a "fear factor" associated with using linear motors. This notion "keeps potential customers locked into 'safe' technologies while their forward-thinking competitors take advantage of this newer technology," he says. Examples of progress in this sector are higher usable encoder quadrature rates, all digital controls, and newer software interfaces and autotuning algorithms to ease system integration.

New directions

The union of Anorad and Rockwell Automation presents wider strategic possibilities for industrial linear motion applications. But, implementing a complex linear system, such as a CNC machine tool, is more complex than with traditional rotary motors. Rewards only come by way of a rigorous development process. Anorad's Dr. Eidelberg envisions a multi-step process "to bridge the gap between needs and know-how for potential linear motor users." For more detail, see an "Online Extra" to this article at www.controleng.com . (See also, CE , Mar. 1997, pp. 90-100 for other details.)

Looking ahead, Kollmorgen's Mr. Chirignan sees direct-drive solutions and design engineers helping to bring to market more reliable, quieter, and improved products—with either higher speed, acceleration, or reduced cycle time.

Baldor notes an expanding future for PM brushless motors. Lower costs due to advances in magnet technology and production economies will help. "Induction motor manufacturing costs are basically fixed with only modest savings for higher quantities," adds Mr. Bullock.

Aerotech sees increasing use ahead for direct-drive linear technology by machine builders. Simpler mechanical arrangement for motion, and improved speed and throughput are the goals. "Prices for brushless linear motors will continue to fall as more players enter the market and competition increases," says Mr. Novak.

As linear-motor systems become more robust and reliable, and lower in cost, Indramat envisions they will make significant inroads in general automation. "Higher voltage controls will be the norm because this provides the same power at lower current, while allowing use of smaller, less costly parts and cables, adds Mr. Rapp.

In the next five years, Trilogy Systems' Mr. Beakley sees more competitors and lower prices. Linear motors will narrow the price gap as motors, encoders, and drives all make advances. "The most successful companies will be those that help design engineers easily integrate linear motors with plug-and-play products."

For more information, visit www.controleng.com/freeinfo .

Representative Direct-Drive Linear Motors

Company

Product

Type

Travel mm (+)

Cont. Rated Force (N)*/ no cooling

Cont. Rated Force (N)* /cooling **

Pk Speed (m/sec)

Accel pk (G)

Features

Motor Types: PM - permanent magnet, brushless servo (synchronous); IND - ac induction (asynchronous); SR - switched reluctance
MC - moving coil; MM - moving magnet; IL - ironless IC - ironcore T - tubular (s) - single-sided magnet (d) - double-sided magnet
(+) - Unlimited travel, via modular magnet tracks, coil assemblies, or reaction plates linked end to end * 1 N = 0.225 lbf
** - Water cooling, except as indicated by (a) for forced air C - connectors built-in D - runs cool due to switched coils
G - Peak G value is for motor unloaded Packaged units: P - includes bearings PX - includes encoder & guides
PF - Peak force 3X continuous at low duty cycles (b) - bearing limited (x) - consult manufacturer
For linear control products, visit www.controleng.com

Aerotech

BLM Series

PM

Unlimited

280

427 (a)

5

90

Peak G for unloaded motor only

Anorad

LXB-S-12

PM-MC

"

4,000

8,000

5

10

MM-NS

PM-MM

"

210

5

8

No moving cable; C; D

Baldor/Normag

BLCM Series

PM-IL

"

to 265

to 530

>5

10

No magnetic attraction; PF

LIM Series

IND

"

to 1,800

to 3,600

>50

1

Cont. force at 10% duty cycle

Baumüller

LSE 1

PM

"

1,650 (s)

6,000 (s)

2.7

>10

IND motor avail; SS cover is std

California Linear Devices

Model 2024

PM-IC

to 305

to 3,110 (a)

1.3

50

Peak force approx. 2X cont; T; Self-contained; Accuracy 0.025 mm

Etel SA

LMA Series

PM-IC

Unlim.

4,000 (s)

9,000

5

10

High mag attraction preload

ILM Series

PM-IL

"

500 (d)

650 (a)

10

20

No magnetic attraction

GE Fanuc

Linear Motors

PM

"

to 3,000

7,000

4

30

Force to 3,600 N w/air cooling

Industrial Devices Corp.

LM Series

PM

to 1,850

to 358

to 5 (b)

5 (b)

Partners with LDL; Self-cooled; Force to 487 N at 2 m/s; P; T

Kollmorgen

Platinum IL

PM-IL

Unlim.

450

540

>8

110

No mag attraction; 0 cogging; G

Platinum IC

PM-IC

"

7,500

12,000

>6

35

High mag attract preload; G

MTS Automation

MaxPlus Series

PM-IC

"

20-250 (s)-(d)

20-360 (s)-(d)

5

>10

0 mag preload; High force/coil mass ratio; Eff. therm. performance

Mitsubishi

Linear Motor

PM

to 1,000

to 60

to 400

2

5

Pos, vel, & force control modes

NSK Corp.

Megathrust

SR

to 30,000

1,200

1.8

2.5

No perm magnet; High resolution

PM Series

PM

to 2,000

750

2

9

High servo stiffness

Parker Hannifin/Compumotor

SL Series

PM-SL

Unlim.

to 650 (s)

1,200

5

11

Minimum attraction force; Cost-effective

Rexroth

LSF Series

PM

"

(x)

7,800

10

10

High peak speed & acceleration

Indramat

LAF Series

IND

"

"

4,000

3

10

No PM or perm magnetic field

Sulzer Electronics

LinMot-P Series

PM-MM

to 1,200

55

100

3

24

Unit includes bearings, sensors

THK America

M1A2

PM

Unlim.

to 432

to 540

>5

>5

Complete positioning system; PX

Trilogy Systems

LM410

PM-IL

"

to 600 (d)

1,000

5

10

Modular tracks; No attract force



Linear motor terms in brief

Direct-drive linear motors fall into two basic classes: permanent magnet (PM) synchronous (or brushless), and asynchronous linear induction motors (LIMs). Brush dc linear motors, which made this technology practical over 20 years ago, exist but are not the focus today. Variable-reluctance linear motors are also receiving some attention.

PM brushless motors abound in various subclasses, such as the moving coil and moving magnet types (see diagram). Other PM motor varieties include:

Ironcore , where steel laminations are added for increased flux to develop higher thrust forces per frame size.

Ironless refers to a core containing only copper coils (and epoxy encapsulation). Smooth "cog-free" motion is produced since no attractive force exists between coil and magnet—but at the cost of lower force output.

Slotless refers to a special design of steel laminations where the windings go through holes in the stator rather than slots. The result is a smoother surface facing the magnet. This design also reduces cogging by eliminating variation in attractive force.

Tubular linear motors roll up the unit about an axis parallel to its length. In one style, an outer thrust block carrying the motor coils envelops and moves along a stationary thrust rod that houses magnets. Another style incorporates magnets in a central rod that moves relative to an outer stator member. See next sidebar for product examples. Travel is limited since the thrust rod must be supported at both ends (or at one end for the moving-rod version).

Configurations galore

Linear motors exhibit a rich variety of shapes. A novel "tubular" motor variant from Linear Drives Ltd. (LDL, Basildon, Essex, U.K.) consists of a thrust block that moves along a stationary rod support element. Benefits include optimal use of magnetic flux generated because current always cuts the radial flux path at right angles; self-cooling as the motor moves along; and force rating that increases with motor speed. Limited travel length and force range are the downside. Industrial Devices Corp. (Petaluma, Calif.) has exclusive production/distribution rights for LDL tubular motors in the U.S., via a limited partnership.

California Linear Devices' (Carlsbad, Calif.) tubular motor reverses the arrangement with a central armature containing permanent magnets as the single element that moves relative to the outer stator coils. The unit resembles a traditional rotary motor and produces peak forces in the 1,555-6,225 N range.

And rotary motion isn't beyond these motors' capabilities, as in Anorad's linear motor coil and circular magnetic assembly combination that serves as the motion element (photo). It was used as a cost-effective solution versus a rotary torque motor. For more details, see Online Extra at



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