More than half of all linear motors go into applications for machine tools or the manufacture and assembly of semiconductor components and electronics. It’s no wonder: Linear motors are the precise but generally costly option to which other linear devices are compared. In fact, applications for these relatively new motion components also include those that need fast and precise positioning, or slow and extremely steady axis traversing. More after the jump.
Depending on the type, linear motor speeds range from a few inches to thousands of inches per second. They’re capable of unlimited strokes and (with an encoder) accuracy to ±1 μm/100 mm. For this reason, a variety of medical, inspection and material-handling applications use linear motors to boost throughput.
Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive. So, they avoid the gradual wear of traditional rack-and-pinion sets. Linear motors also avoid the drawbacks of rotary motors with belts and pulleys for translation … more specifically, limited thrust because of tensile-strength limits; lengthy settling times; belt stretching, backlash and mechanical windup; and typical speed limits of 15 ft/sec or so. In the same way, linear motors avoid lead- and ballscrew efficiencies (of 50 and 90%, respectively), as well as whip and vibration. They don’t force engineers to sacrifice speed (with higher pitches) for lower resolution, either.
Multi-axis stages that use linear motors on each axis are more compact than traditional setups, so fit into smaller spaces. Their lower component count also boosts reliability. Here, the motors connect to regular drives, and (in servo operation) a motion controller closes the position loop.
Linear stepper motors deliver speeds to 70 in./sec, suitable for relatively quick-acting pick-and-place and inspection machines. Other applications include part-transfer stations. Some manufacturers sell twin linear steppers with a common forcer to form X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few nanometers for every hundred millimeters to output accurate moves.
Some cost-sensitive applications benefit from hybrid linear motors, as they have inexpensive ferromagnetic platens. Much like linear stepper motors, they vary magnetic saturation from the platen to shape opposition to magnetic flow; feedback plus a PID loop with positioning control helps the motor output servo-grade performance. The only catch is that hybrid motors have limited output and exhibit cogging from coupling between the forcer and platen. Two solutions here are phase-teeth offset or driving to get partial saturation of platen teeth and sections of forcer teeth. Some hybrid motors also use external cooling to boost output during continuous operation.
Linear ac induction motors that run to 2,000 in./sec work for people movers, roller coasters and large aerospace applications. General-purpose types can move a few inches to 150 ft/sec or faster. Cylindrical linear motors have steel rods and a moving coil or rods filled with stacked magnets, so work in myriad machines that need quick and accurate strokes. In a similar way, ironless-core (or air-core) linear motors output up to 3,000 N and speeds exceeding 230 in./sec. These capabilities make this linear-motor subtype indispensable in long-stroke pick-and-place applications, flying-shear setups, and laser and waterjet cutting.
Linear ac synchronous motors can output 7,000 N or more. Some use water-cooling to boost force output—enough to let the motors drive large baggage handling and amusement-ride axes. Iron-core motors are also suitable for select machine tool and robotics applications.
Just consider one specific robotics use: KUKA Systems North America recently began selling PULSE carrier conveyance systems for automotive car-body assembly lines. The design from KUKA in the U.S. uses linear synchronous motors from MagneMotion to move body sections through robotic workstations of assembly lines as other machinery does joining.
PULSE setups are flexible and 30% faster than conventional friction-based transfer systems. That lets plants get more use out of process equipment (and reduce production-line footprint). With solid-state linear motors, the PULSE lines have fewer components (sensors, connectors and cables) that wear and fail. Quick-stop axes boost safety. One PULSE line handles up to four models and adding functions to handle a second model is cheaper than with a conventional transfer system.
Film wrapper uses direct-drive motors
Getting consistent packaging despite constant product variations is a challenge. Alpenland Maschinenbau (ALPMA) aimed to overcome it with its new MultiSAN film-wrapping machine, which packages round and rectangular soft cheeses, karospar-wrapped cheeses, and cheeses shaped in cylinders and half moons.
“Some cheese makers today only make a few cheeses, but change production every few hours to produce custom orders by the truckload,” said Helmut Eitermoser, MultiSAN designer. The reasons for smaller batches are like those in other industries: shorter product life cycles, customer-specific product requirements and more product variety.
Until now, cheese makers used packaging machines in which a mechanical master shaft studded with cam drives the synchronous motion of individual mechanisms.
“These cam machines work for packaging just one product type, because they’re precise, cost-effective and durable,” said Eitermoser. These can even package similar shapes—round and half-moon—and different sizes. But this takes compromise, as the packages for each product version are suboptimal. It’s impossible for cam-based machines to wrap several different products in a folded or wrapped package with rapid changeovers. Such tasks take reconfiguration or even machine replacement before product changeover.
For the MultiSAN, the ALPMA design team used direct-drive technology with a virtual master shaft and electronic cams to completely replace mechanical versions.
The envelope folds under the cheese; the machine makes them with four LinMot linear motors horizontally mounted … each driving a shutter blade through a linkage. Once the machine wraps the cheese in film and fixes it in place on a round plate with a gripper, the shutters press the protruding film together on the bottom of the product in rapid succession. That makes the fold pattern. Another linear motor pushes the cheese off the round plate and onto a conveyor.
For this application, the design team initially suspected that only linear motors would have the required power density. “Tests showed that rotary servomotors are unsuited for this task because they take up too much space,” said Eitermoser. Instead, he chose PS01-23x160H-HP-R motors for the MultiSan. The motors have 23-mm (stator) diameters not including the plug connection on the end—but can still output peak force to 130 N when coupled with an E1130-DP-HC controller. Matching sliders come in versions for strokes from 20 to 780 mm. The version in the MultiSAN has a 120-mm stroke.
The space-saving form factor of the linear motor also lets the new design take up the same floor space as comparable mechanical-cam-based versions … and cycle times of the two machine types are the same.
But differences abound. To create a modular construction, the ALPMA design team combined the mechanical unit with shutter drives for a subsystem working several machine functions. It is designed to function as a heat sink for the linear motors as well. These machine features help get product with consistent and repeatable packaging.
“We’ve also leveraged the linear motors’ sensitivity and direct reactions to give operators a way to address product-consistency variations,” said Eitermoser. So, onsite parties can set their own parameters for various consistencies, and the machine operator can adapt machine settings to product changes with a push of a button.