The accuracies of rotary and linear motors range widely — but only when quantifi ed at the smallest of scales. The typical rotary stepper can still position accurately to within the diameter of a human hair; a servo improves this by a factor up to 80 times, while a linear motor can improve this up to 500 times — to a few nanometers.

Brushless cog-free motors have a coil assembly in the moving forcer without steel laminations. The coil consists of wire, epoxy, and nonmagnetic support structure. This unit is much lighter in weight. The basic design produces a lesser amount of force, so additional magnets are inserted onto the stationary track (aid to increase force) and the track is U shaped with magnets on each side of this U. The forcer is inserted into the middle of the U.

These motors are suited for applications that require smooth operation without magnetic cogging, such as scanning or inspection equipment. Their higher accelerations are useful in semiconductor pick and place, chip sorting, and solder and adhesive dispensing. These motors are designed for unlimited travel.

Linear steppers have been available for a long time; the moving forcer consists of laminated steel cores precisely slotted with teeth, a single permanent magnet, and coils inserted into the laminated core. (Note that two coils result in a two-phase stepper.) This assembly is encapsulated in an aluminum housing.

The stationary platen consists of photochemically etched teeth on a steel bar, ground and nickel-plated. This can be stacked end-to-end for unlimited length. The motor comes complete with forcer, bearings, and platen. The attractive force from the magnet is used as a preload for the bearings; it also enables the unit to be operated in an inverted position for a variety of applications.

Ac induction motors consist of a forcer that is a coil assembly comprised of steel laminations and phase windings. The windings may be either single or three-phase. This allows for direct online control, or control through an inverter or vector drive. The stationary platen (called a reaction plate) usually consists of a thin layer of aluminum or copper bonded onto cold roll steel.

Once the forcer coil is energized, it interacts with the reaction plate and moves. Higher speeds and unlimited travel lengths are this design's strengths; they're used for material handling, people movers, conveyors, and sliding gates.

New design concepts

Some of the latest design improvements have been implemented via reengineering. For example, some linear stepper motors (originally designed to provide motion in one plane) are now reengineered to provide motion in two planes — for X-Y motion. Here, the moving forcer consists of two linear steppers mounted orthogonally at 90° so that one provides X-axis motion, and the other provides Y-axis motion. Multiple forcers with overlapping trajectories are also possible.

Linear steppers are mechanically robust and are not damaged by stalling or wear.

In these two-plane motors, the stationary platform (or platen) utilizes new composite construction for strength. Stiffness is also improved, so deflection is reduced by 60 to 80% compared to previous production models. Platen flatness exceeds 14 microns per 300 mm for accurate movement. Finally: Because steppers have a natural attractive force, this concept allows the platen to be mounted either face up or inverted, thus providing versatility and flexibility for applications.

Another engineering innovation — water cooling — extends the force capability of linear ac induction motors by 25%. With this capability extension, as well as the benefit of unlimited travel length, ac induction motors provide highest performance for many applications: amusement rides, baggage handling, and people movers. Speed is variable (from 6 to 2,000 in./sec) through adjustable speed drives presently available in industry.

Yet another motor includes a stationary cylindrical housing with a linear moving part to provide motion. The moving part may be a rod consisting of copper-clad steel, a moving coil, or a moving magnet, like a piston within a cylinder.

Select figure to enlarge.

These designs provide the benefits of linear motor plus perform similar to a linear actuator. Applications include biomedical colonoscopies, cameras with long shutter actuators, telescopes that require vibration damping, lithography focusing motors, generator switch gears that throw breakers to put generators online, and food pressing — as when stamping out tortillas.

Complete linear motor packages or stages are suitable for positioning payloads. These consist of motor, feedback encoder, limit switches, and cable carrier. It is possible to stack stages for multi-axis movement.

The electromechanical operation of rotary and linear motors is the same. For the latter, load is just connected to the fl at mover providing linear movement and force.
Select figure to enlarge.

One advantage of linear stages is their lower profile, which allows them to fit into smaller spaces compared with conventional positioners. Fewer components make for increased reliability. Here, the motor is connected to regular drives. In a closed-loop operation, the position loop is closed with a motion controller.

Again, besides stock products, custom and specialty designs abound. In the end, it's best to review equipment needs with an application engineer to determine the optimum linear product suited for application needs.

Motion System Design salutes the author of this article, John Mazurkiewicz, for his contributions to the motion industry. Mazurkiewicz retired from Baldor Electric Co. last year. For more information on linear motors, visit www.baldor.com or email motion_support@baldor.com.