The use of linear motors in industries as diverse as packaging and semiconductor manufacturing is a testament to their suitability for a wide range of applications. In recent years, manufacturers have worked hard to remove the “mystique” that for so long surrounded these products and hindered their widespread usage, but linear motors still face several challenges to their adoption. For designers, engineers, and machine builders the difficulty starts with simply deciding, among all the types and styles available, which linear motor is best for their application.
Like rotary motors, the most common linear motors consist of a primary part, or forcer, and secondary part. And while there are many types of linear motors, the brushless ironcore and ironless designs are prevalent in automation and positioning applications. Each one has unique construction features and performance characteristics, and understanding these is the first step in selecting the most appropriate linear motor for a specific application.
Benefits of linear motors
Linear motors offer several advantages over belts, screws, and other drive mechanisms, including virtually unlimited lengths, low maintenance, and higher accuracy and repeatability. Since there are no mechanical transmission components—such as pulleys, couplings, and gearboxes—to introduce elasticity and backlash, the system’s accuracy and repeatability are determined by the controls and will not degrade over time. The lack of rotating or sliding components also means that linear motors are virtually maintenance free, with only the support bearings (linear guides) requiring periodic maintenance.
Linear motors can also provide unlimited travel by simply stacking magnet tracks end-to-end. (It’s important to note, however, that cable management may become the limiting factor for systems with very long stroke lengths.) And with the ability to use more than one primary parts on a single secondary part, systems can be built with multiple carriages performing independent movements, simplifying the system design and reducing space.
Ironcore linear motors
As their name suggests, ironcore linear motors are constructed with the coils of the primary wrapped around an iron core. The secondary part is typically a stationary, flat magnet track. Ironcore linear motors are characterized by their very high continuous force and ability to move large loads, which make them ideal for machine tool, injection molding, and pressing applications.
One downside of ironcore linear motors is an effect known as cogging, which is caused by the magnetic pull of the secondary on the primary as it moves across the magnet track. This pull creates a detent force that degrades the motor’s smoothness of motion. Manufacturers have various ways of reducing the cogging effect, but this should be carefully considered in applications such as printing, where smoothness of motion is critical to the quality of the end product.
Ironless linear motors
Ironless linear motors eliminate iron from the primary by using coils embedded in an epoxy plate. This reduces mass and enables them to achieve highly dynamic motion. Where iron core linear motors consist of a flat magnet track, ironless linear motors typically consist of a U-shaped magnet track, with two plates of magnets facing each other. This reduces heat dissipation and means that ironless linear motors have lower thrust forces than iron core motors. But their lower mass (due to a lighter primary part) gives them better acceleration capabilities and short settling times, making them ideal for precise, rapid movements.
Another benefit of ironless linear motors is that there are no attractive forces between the primary and secondary parts, since there is no iron in the primary. This makes them much easier and safer to assemble than ironcore linear motors. It also means that the supporting bearings do not have to be sized to accommodate the attractive forces, and will generally have a longer service life.
Despite their performance characteristics, there are two areas where linear motors are generally not suitable: vertical applications and harsh environments. In vertical applications, the non-contact operation of linear motors means that the load can fall during a power-off condition. Adding a counterbalance will provide a safety mechanism, but this adds mass, inertia, and complexity to the system. Harsh environments—especially those with metallic dust or shavings—present another challenge for linear motors, since debris can be attracted to the magnets and cause damage to the system. This is especially true for ironcore linear motors, where the magnet track is often exposed to the environment.
Feature image credit: Bosch Rexroth