Summary of direct-drive motors

LinMot PR01 linear-rotary motors come in myriad sizes for different force and torque outputs. They integrate a linear motor with a rotary torque motor. Controls independently command the two.

A direct drive motor is any motor — rotary orlinear — in which the load is connected directly to the motor,without mechanical transmission elements such as gearboxesor belt and pulley systems. In other words, the motor directlydrives the load.

Regardless of its design, a direct-drive motor has the benefit of eliminating mechanical components that can introduce backlash or compliance and degrade positioning accuracy and repeatability. The elimination of mechanical connections also reduces load inertia and allows more dynamic moves (higher acceleration and deceleration rates with heavier loads) with less overshoot and oscillation. Direct drive motors also have lower noise production than conventional motors, which is important for noise-sensitive applications, like those in the medical and laboratory industries.

Without additional transmission elements, direct drive motors tend to be more compact than traditional motors, making them easier to integrate into machines and systems with tight spaces. And with fewer mechanical components (often, the only wear components are linear guides), maintenance is reduced and mean time between failures (MTBF) is increased.

Rotary direct drive motors are used to drive goniometers, gimbals, rotary tables, and SCARA and 6-axis robot arms. Many designs have a center bore, which allows electrical cables and pneumatic lines to be routed through the center of the motor. Linear versions are used in numerous automation applications, including packaging machines that require rapid strokes on a continuous basis, machine tools that require extreme positioning accuracy and high load carrying capability, and semiconductor manufacturing equipment that requires ultrasmooth and precise motion.

Shown here are iron-core linear-motor options from Chieftek Precision USA.

LINEAR MOTOR SUBTYPES

Linear motors include a stationary platform that industry calls a platen or secondary (with electromagnetic windings) and a moving forcer or primary that sometimes includes permanent magnets. Linear motors can make fast and precise moves for positioning or move slowly and steadily for material processing. Visit linearmotiontips.com/linear-motors for more on this.

Linear-motor speeds range from a few inches to thousands of inches per second. The motors deliver unlimited strokes and (with an encoder) accuracy to ±1 μm/100 mm. Myriad inspection, medical and material-handling applications use linear motors to boost throughput. As is the case with their rotary counterparts, linear motors use common drives and motion controllers. Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive.

So linear motors avoid the drawbacks of a rotary motor with a belt and pulley for translation — lower thrust because of tensile-strength limits; longer settling times; belt stretching, backlash and mechanical windup; and typical speeds to 15 ft/sec or slower. They also avoid the gradual wear of traditional rack-and-pinion sets ... lead and ballscrew efficiencies (usually around 50 and 90%) and whip and vibration. Plus they don’t force engineers to sacrifice speed (with higher pitches) for lower resolution.

In linear motors, magnetic flux through the airgap area is proportional to magnetic and electrical loading ... the vector quantity of flux lines between platen and forcer. Engineers express this value in Tesla or Gauss. Typical airgap flux densities range from fractions to a few Tesla.

Linear stepper motors are an established design with a toothed forcer of laminated steel cores wound with coils. Toothed platens mount end-to-end for unlimited travel. Thrust originates from reluctance force. Linear steppers deliver speeds to 70 in./sec — useful in inspection machines or parts transfer. Some manufacturers pair twin linear steppers off a common forcer to work as X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few μm for every hundred mm to output accurate movement.

Now consider hybrid linear motors which usually have ferromagnetic platens. Those with steel platens move to 3 m/sec; those with laminated platens move faster. Much like linear steppers, they vary magnetic saturation to shape opposition to magnetic flow ... so thrust originates from reluctance force. Feedback and controls make for servo performance. Key to hybrid linear-motor performance is a yoke on the platen that makes paths through which flux travels and closes flux loops between platen teeth and forcer. Hybrid-motor drawbacks are limited output and cogging from reluctance coupling between the forcer and platen. Two setups are phase-teeth offset or driving to get partial saturation of platen teeth and sections of forcer teeth. Here, the drive only magnetically saturates working teeth sections. Some hybrid-core motors also use external cooling to get more output during continuous operation.

Linear ac induction motors that run to 2,000 in./sec work for people movers, rollercoasters and large aerospace applications. General-purpose types can move a few inches to 150 ft/sec or faster. Linear ac synchronous motors are either iron-core or ironless-core motors. Ironless-core linear motors have an epoxy forcer plate holding copper coils. This forcer moves in a U-shaped magnetic platen to output up to 3,000 N and speeds exceeding 230 in./sec. These cogfree linear motors are lighter motors with potentially unlimited travel and quick acceleration — though their main benefit is smooth output. Their speed is helpful in flying-shear applications and long-stroke pickand place machines in semiconductor fabrication and elsewhere. Other applications exist for waterjet and laser cutting and robotics tasks.

Iron-core motors have slotted steel lamination stacks (insulated to reduce Eddy currents) to output 7,000 N or more. The forcer coil setup includes these steel laminations and windings in a single or three-phase configuration. This allows for control directly from a line or through a drive. Some such linear motors use water cooling to boost force output — for heavy uses as on baggage handling and amusement-ride axes. Iron-core motors are suitable for some machine-tool applications as well.

Cylindrical linear motors are sometimes called tubular linear motors. These have steel rods and a moving coil or rods filled with stacked magnets. With the same footprint as a lot of linear actuators, these offer high stiffness and other advantages over other linear-motor designs … and work in myriad machines that need quick and accurate strokes.

HOW DO IRON-CORE LINEAR MOTORS WORK?

Linear motors are often classified as either ironless or iron core, referring to how their primary parts are constructed. Ironless linear motors have a primary of windings embedded in epoxy resin. Iron-core linear motors have windings mounted in an iron lamination stack.

For both motor types, the number and length of the windings determine how much force the motor can produce. Iron-core motors have a force density (force per working area) up to twice that of ironless motors. So to produce a given continuous force, an ironless motor must be twice as large as a comparable iron-core design. Force here is the product of current through the windings and the flux density in the primary: F = I x B.

Ironless linear motors have a primary that’s embedded in resin, so there’s no magnetic attraction between the primary and secondary ... so forces these motors can produce are smaller than those produced by iron-core designs. For iron-core linear motors, magnetic attraction between the primary’s iron and secondary’s permanent magnets allows high force output — though this attractive force also creates cogging.

Cogging is detent force that the motor experiences when the steel laminations of the coil cross the magnets of the secondary. This force degrades the smoothness of movement and can be significant ... making iron-core motors less desirable for applications that need extremely smooth motion. That said, some manufacturers have developed methods to reduce the effects of cogging — by skewing the magnets of the secondary part (which eases the change in attractive forces as the primary moves across the magnets) or by using feedback and controls to compensate for the effects of cogging.

In addition to their ability to produce very high thrust forces, iron-core motors also dissipate heat thanks in part to their relatively open design. But this design also leaves them susceptible contamination — particularly metallic chips or flakes that attract to the permanent magnets and cause damage. Another factor that contributes to the ironcore linear motor’s good heat dissipation is the fact that their primary is mostly metal (as opposed to the epoxy enclosure of an ironless design) which acts as a heat sink.

When designing a linear system using iron-core motors, account for the attractive force between the primary and secondary parts, as that’s essential to properly sizing the support bearings. Linear profiled rails are the most common guide systems used with linear motors, although air bearing systems are sometimes used. Because air bearings need a preload, the attractive force between the windings and the magnets is beneficial for an iron-core motor using air-bearing guides.

With high continuous forces and good heat dissipation, iron-core linear motors are suitable for pressing, molding, and machining applications. They also excel at high-speed testing that requires the application of high forces or pressures.