Linear Motors Hit Their Stride

July 18, 2005

8 Min Read
Linear Motors Hit Their Stride

Once considered expensive and exotic, linear motors are at last becoming valuable workhorses in a variety of OEM applications.

Besides delivering nanometer precision in the semiconductor and machine tool fields, linear motors are now meeting the needs of design engineers in applications that range from medical equipment to packaging, printing, and even metalworking.

As demand increases, the costs of these motors are also moderating. "Prices have been coming down by 10 to 15 percent annually over the last couple of years," notes California consultant Dan Jones of Incremotion Associates. He adds that the ready availability of less costly rare-earth magnets from China—a key component in linear motors—is a big factor in making such motors more affordable.

What's new in linear motor products and applications? Design News asked for updates from some of the country's leading suppliers. Here's their assessment of what's driving the rising interest in these motors.

Anorad: Eliminating the Bottlenecks

One of the pioneers in linear motors, Anorad sees growing use of the motors in mainstream industrial applications, such as packaging, food processing, and form/fill machines. The company recently introduced its LC line of economical iron-core linear motors specifically targeted for such applications. Offered in six frame sizes (widths), with each frame size available in six different lengths, the new motors offer instantaneous force ratings from 44 lbs to 2,100 lbs.

Anorad's LC series of linear motors targets mainstream industrial applications, such as packaging machines.

Recently, Anorad supplied LC-50 motors, in 200 and 400 frame lengths, for two axes in a packaging machine that formerly relied on air cylinders. Since this application was a retrofit, the integrator developed a custom assembly that could bolt right into place. The system used an off-the-shelf magnetic linear encoder for position feedback with hall motor commutation. All axes, including several rotary servos, were controlled by a Rockwell Automation ControLogix platform using a SERCOS interface and Ultra 3000 servo drives.

Application engineer Claude Chirgnan notes that the integration, which the customer described as a "great success," was accomplished in just five days. Moreover, productivity increased as the linear motors delivered high acceleration, speed, and greater accuracy while curbing downtime from paper jams and other problems.

Aerotech: Compact Answer to Precision Process

Aerotech reports that use of linear motors in positioning stages has increased significantly in recent years due to the faster speeds, higher accuracies, and tighter repeatability that such motors provide over traditional ballscrew drives.

The company's BLMC line offers an ironless, brushless linear motor that provides a high force constant in a small package. The benefit of the ironless design is that the forcer coil does not contain steel laminations, which can produce cogging as the forcer passes over the magnetic track. What's more, the brushless feature eliminates the torque ripple associated with mechanical commutators. Both characteristics make the BLMC fit for applications that require smooth velocity and accurate position control, say Aerotech engineers.

One application that demonstrates the advantages of the BLMC motor is the manufacture of flat panels. This process features a drum-writing technique, which prints features directly onto a cylindrical surface with a mechanical system that looks like a high-tech lathe. The drum mounts horizontally to a specialized rotary axis with a corresponding linear axis adjacent to it. This writing method produces micron-level features by moving a laser source at constant speed, well below 100 microns/sec, relative to the drum. Due to the size of the work piece and the quantity of features, the total time to process a part is greater than 48 hrs. Aerotech engineers note that a linear motor is essential to maintaining tight dynamic accuracy at such low speeds over such a long time period.

Parker-Trilogy: Lower-Cost Iron-Core Solution

Industry giant Parker-Hannifin gained a foothold in the linear motor market with its December 2004 acquisition of the Texas-based Trilogy Systems. Parker-Trilogy's iron-core motors incorporate patent-pending anti-cog technology.

Parker-Trilogy's R7-2A iron-core linear motor, part of a line with patent-pending ant-cogging technology.

Traditionally, iron-core motors would include skewed magnets or laminations to reduce cogging, explains Product Manager Jack Marsh. However, that approach can reduce the motor's efficiency. By incorporating separate mechanical features into the motor that produce equal and opposite cogging forces, the company's new technology uses destructive interference to eliminate the cogging due to the lamination assembly. This innovation has made iron-core motors suitable for applications that traditionally would require a more expensive ironless solution, Marsh says.

Among recent applications, a large semiconductor equipment manufacturer needed a linear stage to position a robot weighing more than 600 lbs. The application required that the robot be able to move and settle 2m in about 1 sec. Parker Automation supplied a positioner driven by an R16 iron-core motor, using an LME5 magnetic encoder. Engineers also chose a Compax3 drive/indexer to position the robot and for communicating with the system controller via I/O. Result: a cost-effective, low-maintenance solution implemented in a short time frame.

Parker offers sizing software, which draws on an empirical database, to predict temperature increases in linear motors and prevent failures from excessive motor loads. In addition, the company has white papers on motor sizing and basic linear parameters. The links for these online papers are:

http://rbi.ims.ca/4412-500

http://rbi.ims.ca/4412-501

Baldor: Continuous-Duty Workhorse

Baldor is touting a new linear motor design that it believes combines the performance of traditional brushless linear servo motors with the cost advantages of open-loop linear stepper motors. Called the "HyCore," the new motor offers the closed-loop benefits of zero backlash, high efficiency, unlimited travel, fast velocities, and high acceleration. The HyCore's forcer component consists of a three-phase servo motor winding controlled via closed loop using feedback from an external encoder. The stationary platen contains no magnets, so it can be easily cut to length to suit the application.

The "HyCore" linear motor from Baldor offers the closed-loop benefits of zero backlash, high efficiency, unlimited travel, and fast velocities.

Application engineer John Mazurkiewicz says the HyCore is especially cost-effective in applications requiring a long stroke or high continuous duty—often as a replacement for ballscrew and belt-pulley actuators. One such example is in materials handling where linear ac induction motors move carts containing product along tracks. The ac induction plate is positioned on the bottom of the cart, while the primary coil assembly is situated along the track. When ac voltage is applied to the coil windings, a traveling magnetic field is created. This induces current in the reaction plate, which in turn creates its own magnet field. The interaction of the two magnetic fields generates the force and the direct linear motion.

Baldor engineers believe that there are many applications, now using fluid power, which could migrate to linear motors with considerable energy savings. However, linear motor performance capabilities will need to expand beyond their current ceilings for continuous and peak force.

Danaher Motion: Speed and Precision

Where speed is essential, there are few rivals to the linear motor, says Lee Stephens, systems engineer for Danaher Motion, which offers both ironless and iron-core designs in its Platinum DDL (direct drive linear) line from Kollmorgen. One such example is an application in which molten metal must be sheared at the rate about 6 m/s and completed in 250 msec. Previous solutions included a hydraulic cylinder pair and opposing blades as well as a cam system connected to a reciprocating axis.

Danaher Motion offers both iron-core and ironless options in its Platinum DDL (direct drive linear) line.

By substituting a linear motor design, the customer eliminated setup and maintenance on cylinders and cams. The linear motor also made the system more robust. Since the flow of the molten metal is not consistent throughout the process, operators need to adjust flow rates to get enough material through the orifice prior to the cut. In the reciprocating axis design, scrap would result as the flow rate changed or was adjusted. With a linear motor, however, the cut can be held off until enough material has flowed through the orifice, resulting in consistency of length without flow control.

Stephens also cites a precision measurement application, which required maintaining a tight tolerance of parallelism between two bearings along a 10-ft axis. With an ironless linear motor, the system can be as accurate as the placement of the bearings allows, Stephens explains The repeatability of the linear solution also permits mapping that would not change relative to speed or wear as with a ballscrew.

Such examples from Danaher and other leading suppliers demonstrate the potential of linear motors in demanding motion control settings. As OEM engineers become more skilled in integrating these motors into their designs, and as prices continue to drop, such applications are sure to multiply.

ONLINE LINEAR MOTOR RESOURCES

Anoradhttp://rbi.ims.ca/4412-502

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