There are two main types of linear motors: iron core and ironless, depending on the construction of the motor’s primary part. Iron core designs produce very high thrust forces and are often found in machine tool and heavy duty transport applications. Ironless linear motors, on the other hand, have a lower mass and can produce highly dynamic movements, making them popular in packaging, semiconductor, and general automation applications.
Cutaway view of an ironless linear motor
Image credit: Aerotech Inc.
Ironless linear motors are so named because the primary part (analogous to the stator of a rotary motor) is made of windings wrapped around laminated steel and embedded in epoxy resin. This construction method eliminates iron from the primary part, so unlike iron core motors, there are no attractive forces between the primary and secondary parts that can cause cogging and make assembly difficult. The absence of cogging, or detent forces, enables them to travel with virtually zero force and velocity ripple, meaning that they can travel not only at very high speeds with excellent positioning accuracy, they can also travel at very slow speeds with typically less than 0.01 percent speed variation.
The attractive forces found in iron core designs also make assembly difficult and add significant load to the support bearings. Since ironless linear motors don’t experience these attractive forces, handling and assembly are much easier and less dangerous. Ironless designs can also use smaller support bearings, whether profiled rails or air bearings, since they don’t have to support the forces caused by the attraction between the primary and secondary.
The secondary part of an ironless linear motor is often U-shaped, with two magnet plates facing each other, separated by a spacer. The primary part (also referred to as the forcer) travels in the space between the magnet plates. This somewhat enclosed design makes heat dissipation more difficult and limits the forces that ironless motors can produce. But ironless designs have lower mass, which lets them achieve very high acceleration rates and short settling times. Since there are no mechanical linkages to introduce backlash, ironless linear motors are ideal for applications where very high precision is required, with positioning accuracy limited only by the feedback device.
Ironless linear motors are sometimes referred to as air-core, since the windings (primary part) are located in the air gap between the two magnet plates.
High-precision positioning stages often incorporate an ironless linear motor, profiled guide rails, and an optical encoder.
Image credit: Primatics Inc.
While they are used in a wide variety of applications, linear motors are not inherently suited for vertical applications, since they’re non-contact (the only friction in the system comes from the linear support bearings) and have no built-in braking mechanism. When mounted vertically, if power is lost, the primary part and load will fall, so a mechanical counterbalance is required for safety.
Linear motors do, however, have two unique performance features when compared with other drive mechanisms. First, multiple magnet tracks can be mounted end-to-end to produce virtually unlimited stroke lengths. Another defining feature of linear motors is their ability to produce coordinated but independent motion by mounting multiple primary parts on the same secondary. Linear motors are also suitable for use in cleanroom environments, since particulate-generating components are typically limited to support bearings and cable management, both of which are available in cleanroom-suitable variations.
Great article!!!