Direct-drive linear motion expands its applications
Direct-drive linear (DDL) motion technology had its start in the machine tool industries: high-quality machining, honing, grinding, punching, and laser cutting. Linear motor systems have expanded into gantries/material handling, flying cut-off equipment, metal forming, assembly shuttles/conveyors, and food-processing machinery and other applications. See photos, supplier table, links to related stories.
Hegla GmbH & Co. KG (Beverungen, Germany) employs IndraDyn L linear motors from Bosch Rexroth to improve throughput on its Galactic glass-cutting machine. DDL motors provide 140% greater x-y axis acceleration than the prior standard rotary servo motor and rack design.
There is general agreement that direct-drive linear (DDL) motion technology had its start in the machine tool industries—in applications such as high-quality machining, honing, grinding, punching, and laser cutting. Linear motor systems then expanded into gantries/material handling, flying cut-off equipment, metal forming, assembly shuttles/conveyors, and food-processing machinery, according to Bosch Rexroth, Electric Drives and Controls Div.
“Initially, mostly high-end machines contained linear motors but today more and more mainstream and‘economy machines’ take advantage of DDL motor benefits,” says Karl Rapp, industry sector manager, Automation & Machine Tool at Bosch Rexroth .
A typical example of process improvement due to DDL motion is in orbital grinding of crankshaft pins. Previously, each pin had to be centered and ground individually, but in the new process, the crankshaft rotates around its center, explains Rapp. “The x-axis linear motor follows the pin using its high dynamic and static stiffness and maintains roundness accuracy of below 1
Siemens’ 1FN6 Series brushless synchronous linear motor offers maximum thrust force range of 900-8,080 N (200-1,802 lbf), depending on the model. For greater system economy, multiple primaries (sliders) can run on the same magnet-free secondary track while moving in the same or opposite direction.
At Siemens Energy & Automation , expansion of direct-drive linear motion technology is seen to have reached well beyond machine tool applications. Jeff Gerlach, Siemens E&A consulting business developer, summarizes the newer usage areas as packaging and automated sorting machines, as well as high-speed conveyor systems. DDL motion can be especially cost-effective for conveyors due to much higher speeds developed than with belt- or roller-type arrangements. The company is pursuing these applications with its latest-generation brushless PM synchronous linear motor, the 1FN6 Series (see main article).
Rockwell Automation likewise notes DDL motion systems moving into many of the above applications—along with printing, pick-and-place assembly processes, and electronic packaging. As design and manufacturing innovations continue to drive down cost of direct-drive linear motion systems, they will see still wider application.
Additional direct-drive linear motion system companies
A relatively large number of manufacturing companies supply direct-drive linear (DDL) motion systems, despite significant acquisitions that have occurred in this arena in the last few years. The following table is a non-exhaustive list of DDL motion system suppliers, beyond those covered in the main article.
Baldor Electric Co.
California Linear Devices
Copley Controls Corp.
GE Fanuc Automation
Oswald Elektromotoren GmbH
THK Co. Ltd.
Definition of DDL motion technology, here and in the main article, excludes rotary motors direct coupled to a ball screw.
Linear motor types and terminology
As mentioned in the main article, the permanent magnet (PM) brushless synchronous motor dominates today’s direct-drive linear (DDL) motion systems. However, this linear motor type has several variations and subclasses.
Moving magnet type contains permanent magnets in the primary and coils in the secondary. This allow for simpler design with stationary power cabling and easier integration of the feedback device. Moving coil reverses the linear motor structure. Motor coils are in the primary section and the secondary has the magnet track. It means that more permanent magnets are required (especially for long traverse) and the need for power cables to move with the primary and feedback integration tend to be more complex.
Ironcore refers to adding steel laminations to the magnet track for increased flux to develop higher thrust forces per frame size. Further, single magnet track and dual magnet track variations exist. The latter has the advantages of balancing the high magnetic forces developed between the primary coil and the magnet track.
Ironless refers to a primary 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 magnets. This design also reduces cogging by eliminating variation in attractive force.
Tubular linear motors roll up the flat linear structure 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. Travel is limited since the thrust rod must be supported at both ends—or at one end for the moving-rod version.
— Frank J. Bartos , P.E., Consulting Editor
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