Linear motor shootout

DN Staff

February 15, 1999

13 Min Read
Linear motor shootout

The myriad motor designs, intriguing new product introductions, and recent company acquisitions, however, can make it a tough technology to keep up with. With that in mind, Design News searched out as many major suppliers of linear motors as we could find--14 in all--and asked them to provide information on their companies and motor designs.

We kick off the article with a perspective from the field, followed by a brief writeup from each company describing what makes their designs unique, and wrap up with a rundown of who makes what in a comprehensive table.


Growth ahead, and a few challenges

Mark Sabine, President, U.S. Automation; and Ron Van Bruwaene, Sales Application Engineer, MS Technologies

What's driving the sales surge in linear motors, and what will it take to grow the market even more in the future?

Right now what we see happening with linear motors from the applications engineering side is your classic domino effect. Once one company starts using the technology, suddenly the rest of industry is scrambling to keep up. That's certainly true in the electronics, machining, and computer peripherals industries today. Linear motor technology is leapfrogging to other industries as well.

Simply put, linear motors afford lots of benefits--and that in big part is driving the expansion of the market. Obviously speed is one main advantage. The technology is ideal for applications that require high position accuracy and repeatability. Using linear motors, designers can also gain more travel in the same envelope--they can even have two different motors go to the same area.

Although the market is expanding, one of the factors limiting faster growth is the fact that linear motors are not a direct replacement for rotary motors. There are special considerations, such as the need for high flex since the motor is moving, whereas with a rotary motor it is typically tied to a ball screw or belt. Typically, there are also many auxiliary products, such as cable carriers and linear bearings, that all have to come together in a new application.

And unfortunately, a design engineer can't go to a catalog and pick out exactly what he or she needs today, as is the case with other types of motors. The solutions are very application-dependent, and the industry is changing so rapidly and there are so many refinements to the designs that it can be tough for design engineers to keep up with it all.

In the future, however, as more people gain experience with linear motors, the technology should become easier to use. That will help grow the market, as will the decrease in price that comes with economies of scale and the innovative technologies that continue to hit the market.

Sabine and Van Bruwaene are linear motor experts working for Automation Solutions International, a partnership of high-tech distributors that offers active design and application engineering support for a wide range of motion control products.

Perspective From the Vendors

Aerotech U-channel linear motors

Aerotech's family of "U-channel" brushless linear servomotors is direct drive and consists of only two non-contacting parts--a moving forcer coil and a "U-channel" rare earth magnet track. The forcer coil resides inside the magnet track. This design eliminates backlash, windup, wear, and maintenance issues associated with ball screws, belts, and rack-and-pinions. Travel lengths are unlimited due to Aerotech's unique "stackable" magnet track system with symmetrical mounting-hole pattern. A non-magnetic forcer eliminates cogging and magnetic attraction to allow extremely smooth motion and very tight velocity and position control. Aerotech also offers a line of flat linear motors. The BLMF series has no magnetic attraction, making it ideal for scanning and other "smooth velocity" applications.

Anorad moving magnet linear motor

Anorad's Lightning MM-NS (moving magnet no steel) linear motor features magnets in the slider, which moves over a stationary coil. A switching circuit built into the stage only energizes coils directly below the magnets. The primary advantage? More force from the same motor size, with greater continuous output because a stationary coil is simpler to force-cool. This innovative design approach has several other major advantages over conventional, moving-coil motors. The design uses fewer magnets than other designs, reducing cost and eliminating ferrous material attraction. The fact that there are no exposed magnets means that ferrous material won't collect on the track. Lightning delivers greater accuracy, speed, and acceleration for about the same cost as a ball screw or belt drive.

Baldor/Normag cog-freelinear motor Baldor manufactures three of the four different permanent magnet brushless linear motor technologies--ironcore, ironless, and slotless. The cog-free brushless linear motor features a moving encapsulated coil assembly and a double-sided permanent magnet secondary. The moving primary has no laminations in its structure, reducing its inertia. The stationary secondary consists of neodymium magnets mounted in a U-shaped assembly providing the high force ratings per unit area. This low-inertia, cog-free design provides accelerations to 10+g and velocities greater than 200 in/sec. The coil assembly exhibits no attractive forces between the primary and secondary. It provides cog-free linear motion with no reluctance, end effect, or drag force exhibited. Reduced velocity and force ripple make it ideal for contouring applications.
California Linear Devices tubular linear motor The California Linear Devices tubular linear motor looks like a conventional motor and is as simple as a conventional rotating motor, but provides all of the results of the more complex linear motor systems on the market. The motor has an armature which contains the permanent magnets to provide the high force levels. The armature slides into a stator assembly containing the electromagnetic coils, which are commutated to drive the motor. There are no gears, ball screws, etc. The electrical drive is a three-phase, dc drive, sinusoidally commutated unit. The advantages are multiple: one moving part; only two wearing parts; no backlash to affect linear positioning accuracy; programming directly in linear units; integral bearings; and combined controller, driver, and power supply.
GE Fanuc linear motor Providing one of the highest ratings available in the industry, the 15,000N linear servo motor from GE Fanuc is ideal for high-force applications, eliminating the need to group multiple lower-force linear motors to achieve the same results. The new linear servo motor enables end users to achieve higher cutting speeds and accuracy as well as lower non-cutting times for improved throughput and increased overall machine productivity. This high-force direct drive is used in place of ball-screw drive systems to achieve higher performance and better dynamic accuracy. With the elimination of mechanical linkages, linear motor drives can have much higher gain factors and provide less servo lag. This translates to higher net part tolerances and finer surface finish.
Industrial Devices Corp. tubular linear motor The LD series tubular linear motors and LM series tubular linear modules, licensed from Linear Drives Ltd. (Essex, U.K.), are unique technologies designed to be a near drop-in replacement for a ball-screw system. One of only two working parts, the thrust rod is a stainless steel tube that contains cylindrical neodymium magnets spaced along its length and is typically stationary. The thrust block is typically the moving element and contains the motor coils and encompasses the thrust rod. The fact that the motor is completely symmetrical yields negligible attractive and repulsive forces between the block and rod, even though the block contains iron (to boost the force output). Another advantage is that a tubular motor uses the full available flux from the magnets to generate thrust.
Indramat permanent magnet linear motor Indramat offers two types of high-performance linear motors: synchronous (rare-earth permanent magnet) and asynchronous (induction) in more than 50 models. The synchronous linear motor has a series of rare-earth magnets mounted as a secondary and a primary with a three-phase winding. Generally, the fixed length primary moves on the secondary, which can be composed of multiple segments. Standard traverse lengths of more than 30 ft have been implemented. Indramat linear motors are manufactured for highest efficiency (1.72 N/W) and force density, allowing use of high-voltage (750V) drive controllers. Induction linear motors do not use magnets in the secondary, allowing for easier installation and maintenance, but one trade-off is lower force density. All motors are fully encapsulated, with no exposed epoxy material.
Kollmorgen's ironcore and ironless linear motors Kollmorgen offers both ironcore and ironless linear motors. Ironcore motors have coils wound on silicon steel laminations, to maximize the generated force, with a single-sided magnet way. Using a patented electromagnetic design, Kollmorgen's Platinum DDL linear motors have the highest rated force per size, a high-Km motor constant (equals low thermal losses), and low cogging forces without the need for skewing of the magnets. The high thrust forces possible with these motors make them ideal for accelerating and moving high masses, and maintaining stiffness during machining or process forces. Ironless motors have no iron, or slots, for the coils to be wound on. Therefore, these motors have zero cogging, a very light mass, and absolutely no attractive forces between the coil assembly and magnet way. These characteristics are ideal for applicationsrequiring very low bearing friction, high acceleration of lighter loads, and for maximizing constant velocity, even at ultra-low speeds. The modular magnet ways consist of a double row of magnets to maximize the generated thrust force and to provide a flux return path for the magnetic circuit.

MFM's moving-coil linear motor The Hercules series linear motor from MFM Technology is a unique moving-coil design based on the ironless "zero cogging" principle. The Hercules winding design maximizes the amount of copper over the standard ironless coil arrangements, thereby achieving high motor constant. By adapting the MFM patented, two-wire brushless technology, power is obtained from the bus voltage of the drive amplifier to power the integrated-fan cooling system. This cooling system allows the motor to provide more force per unit volume without exceeding the thermal design parameters of the coil. This unique thermal management provides performance comparable to water-cooled designs for demanding loads and profiles without expensive, bulky, unreliable water-cooling components. For smooth operation, these motors can be driven by a series of sinusoidal amplifiers.
MTS Automation ironless core linear motor Due to the recent acquisition of Airex Corp, MTS Automation--Custom Servo Motors now offers a complete line of brushless linear servo motors. Unlike ironcore motors, MTS Automation linear motors utilize an ironless core that greatly reduces motor cogging and provides increased motor smoothness. The motor itself has no moving parts and is built to attach to the customer's bearing system. Through patented machine-winding technology, MTS Automation is able to produce linear motors with exceptional electromagnetic characteristics, including highest force-to-coil-weight ratio, peak forces to 250 lb, and continuous forces to 80 lb, as well as high copper fill and excellent thermal performance. The standard Solution 2000 Series motors consist of wire coils connected as three phases that interact with an alternating magnetic field to produce force in either direction. The very strong magnetic field (10k Gauss) is created using high-temperature, rare-earth magnets attached to highly permeable magnetic backiron. The magnet structure is designed to maintain high mechanical stiffness which allows flexibility in mounting.
NSK ironcore slotted linear motor NSK designs and manufactures ironcore slotted linear motors. There are two basic types available--moving coil and moving magnet. All of the linear motor models include NSK linear guides, which provide an extremely stiff and robust mechanical system. NSK has also developed the K1 lubrication system--an oil-impregnated resin that allows completely maintenance-free operation. The linear motor designs are unique in that they provide extremely low-force ripple or "cogging." This is accomplished by optimizing the skew angle of the magnets, as well as the large number of partitioned slots. More slots-per-pole minimizes the cogging effect. High thrust is obtained by using high-energy, layered neodymium magnets arranged with a very small pitch between magnets, providing high thrust at low speeds.
Parker Hannifin (Compumotor) slotless linear motor Parker Hannifin's SL series slotless linear servomotor product line is the end result of engineering efforts from the Compumotor and Daedal divisions and Parker's Motor Design Center. This "hybrid" design offers many of the same advantages of the more traditional ironcore and ironless designs with some unique advantages. Manufacturing techniques employed in Parker Hannifin's slotless rotary designs transfer easily to linear technology, providing competitive prices and minimizing lead times. The simplified winding design, a laminated backiron and epoxy-filled aluminum housing, completes the basic motor. Contrasting ironless technology, equivalent force outputs are achieved with a single magnet bar, dramatically impacting system cost. Attractive force of the forcer to magnet rail, as well as cogging or detent force, is lower in a slotless design, simplifying the design of linear bearings.

Sulzer Electronics Sulzer Electronics' LinMot(R) motor technology, introduced in Europe in 1997 and available today through U.S. distributors, is based on the moving magnet, two-phase, synchronous principle. Position sensors and bearings are integrated into the motor itself. The mechanical design is similar to pneumatic cylinders (small in diameter) and therefore easy to integrate into machines. Because of the integrated design (only two parts--a slider and stator, as sensors, sensor-electronics, and bearings are already assembled in the motor casing), the motors can be used in harsh industrial environments. Together with the controller units (single-, two-, and four-axis controllers), which include amplifier and PLC (closed loop), LinMot(R) offers a "plug-and-play" servo-system.
Trilogy slotless, ironless linear motor Trilogy invented the slotless, ironless linear motor comprised of tightly packed windings formed into an I-beam shape and sealed with a hard-epoxy shaft for protection. Trilogy's design strength is due to two specific features. First, overlapping windings allow 15 to 20% more turns in the motor than in non-overlapping designs. This adds to motor efficiency. Second, a forming fixture presses the windings into an I-beam-shaped "brick" with a flat planar mounting surface. This surface mates to an aluminum attachment bar for superior heat removal by maximizing the surface contact area. The crucial key to successful linear motor applications is to first minimize the amount of watts required, and then to easily remove those watts once they are generated.

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