Flow wrapping, resealable packaging, 3D modeling, and pick and place are just some typical linear-motor applications.

Linear motors are very quick and precise for positioning, but are also capable of slow, constant-traverse speed for machine heads and slides, as well as tool and part-handling systems. A variety of applications — laser surgery, vision inspection, and bottle and baggage handling — use linear motors because they are extremely reliable, require little maintenance, and improve production cycles.

Higher speed and force

Linear motors are directly coupled to their load, which eliminates a host of coupling components — mechanical couplings, pulleys, timing belts, ballscrews, chain drives, and rack and pinions, to name a few. This in turn reduces costs and even backlash. Linear motors also allow for consistent motion, precision positioning for hundreds of millions of cycles, and higher speeds.

Typical speeds attainable with linear motors vary: Pick and place machines (that make lots of short moves) and inspection equipment use linear steppers with speeds to 60 in./sec; flying-shear applications and pick and place machines that make longer moves use cog-free brushless linear motors for speeds to 200 in./sec; roller coasters, vehicle launchers, and people movers use linear ac induction motors to attain speeds to 2,000 in./sec.

Linear motors are simple, comprised of two main components: the stationary platform (or platen) with electromagnetic windings, and the moving forcer that may or may not contain permanent magnets. Speed capabilities depend on design.
Select figure to enlarge.

Another factor that determines which linear-motor technology is best: Force required to move the application load. The load or mass along with the application's acceleration profile ultimately determine this force.

Each application presents different challenges; however, in general, part-transfer systems use linear steppers with forces to 220 N or 50 lb; semiconductor, laser cutting, water-jet cutting, and robotics use brushless cog-free motors to 2,500 N; conveyor systems use linear ac induction motors to 2,200 N; and transfer line and machine tools use iron-core brushless motors to 14,000 N. Keep in mind that each application is different and manufacturer application engineers generally provide assistance at this specification step.

Other factors besides speed and force exist. For example, conveyor systems use linear ac induction motors because of their long length of travel, and the advantages of having a passive secondary without permanent magnets. Applications like laser eye surgery and semi-conductor fabrication use brushless cog-free for accuracy and smoothness of travel.

Basic operation

Linear motors operate through interaction of two electromagnet forces — the same basic interaction that produces torque in a rotary motor.

Imagine cutting a rotary motor and then flattening it out: This gives a rough idea of a linear motor's geometry. Instead of coupling load to a rotating shaft for torque, load is connected to a flat moving car for linear movement and force. In short, torque is the expression of work that a rotary motor provides, whereas force is the expression of linear motor work.

Accuracy

Let us consider a traditional rotary stepper system first: Connected to a ballscrew with a pitch of 5 revolutions per inch, accuracy is approximately 0.004 to 0.008 in., or 0.1 to 0.2 mm. A rotary system powered by a servomotor is accurate to 0.001 to 0.0001 in.