Linear motion is central to many moving machines, and the direct-drive nature of linear motors can simplify overall machine design in these applications. Other benefits include improved stiffness, because linear motors are fixed directly to the load.

Integrating these motors (and the peripheral components they require) can seem daunting, but the process can be broken down into five simple steps. Following this step-by-step process allows machine and robot builders to reap linear-motor benefits without extraneous effort or complexity.

Determine motor type

The first step is to select the linear motor from the types available.

Iron-core motors

Iron-core motors are most common, and suitable for general automation applications. Iron-core refers to this motor's coil construction, which consists of iron-core laminations. A typical configuration consists of a single-sided stationary magnet track and a moving motor coil or forcer. The iron core maximizes the generated thrust force, and creates a magnetic attraction force between the coil and magnets.

This magnetic attraction force can be used to effectively increase the rigidity of the linear guidance system by preloading the linear motion bearings. Magnetic preloading can also boost the system's frequency response by improving deceleration and settling.

On the other hand, the attraction force must be properly supported by increased load capacity from supporting members and linear bearings. This may degrade the machine's mechanical design freedom.

A second iron-core linear-motor configuration consists of a pair of stationary magnet tracks placed on either side of the moving coil. This patented construction negates the effects of magnetic attraction while delivering highest force per cross-sectional area. The balanced design reduces bearing load, allowing use of smaller linear motion bearings and decreasing bearing noise.

Ironless motors

Ironless linear motors also exist; these motors have no iron in their coils, so there's no attraction between the motor members.

The most common ironless type is the U channel: Two magnetic tracks are joined to form a channel in which the motor coil (or forcer) moves. This motor is ideal for applications requiring low velocity ripple and high acceleration. The zero-attraction force and zero-cogging nature of the ironless construction minimizes torque ripple; acceleration is increased because the coil is relatively lightweight.

A second ironless configuration is in the form of a cylinder. Magnets are stacked inside a stainless steel tube, and the motor coil moves around the cylinder. This configuration is suitable when replacing ballscrews, as it produces much higher speeds and positioning accuracy in roughly the same envelope.

Coil sizing and track length

No matter the configuration, all linear-motor coils should be sized to application requirements: applied load, target move profile, duty cycle, accuracy, precision, service life, and operating environment. Tip: Enlist technical support from linear-motor manufacturers and sizing software (which is often free) to select the best motor type and size for a particular application.

Magnet track sections are offered in several lengths and can be stacked end-to-end to achieve the target travel length, with the total magnet length being virtually limitless. To simplify design and reduce costs, it's best to use the longest length magnet track sections available from the manufacturer.

Decide on an encoder

The second step when designing a linear motor system is selection of the linear encoder. The most common are incremental linear encoders with optical or magnetic read head sensors. Select an encoder with the required resolution and accuracy for the application, and one that is suitable for the machine environment.