There are several considerations when selecting a direct drive linear actuator for a specific application, such as the required speed, force, motion profile, available physical envelope, and environmental factors. This means the chosen actuator solution must produce the load force and speed requirements, fit in the available space, and perform as required under the subjected environmental conditions of the application.
A linear actuator is part of a total mechanism that provides motion to a load – to move it, machine it, lift it, inspect it, etc. The linear motor is the muscle of the linear actuator that provides the necessary force and speed (required load point) to perform a given function. The fastest and most reliable way to determine these needs is with a motor sizing tool that calculates the load points required by the motor and analyzes information about the load, transmission elements, and motion profile to select a motor from the database that matches the parameters. As the initial load points are determined, the sizing tool checks for the optimum solution and narrows the motor choices based on the required force, speed, inertial ratio, and associated margins acquired from the motor ratings.
A linear motor’s physical size is mostly determined by its continuous force producing capability. All linear motor designs have the option of either moving the forcer with the magnet track fixed or moving the magnet track while the forcer is fixed. Linear motors are available in iron core and ironless (u-channel) designs.
An iron core design, with its high flux producing capability enabled by the back iron in the forcer, is ideal to achieve higher thrust forces. Iron core motors operate on a flat magnetic way and tend to have some force ripple due to cogging. An ironless design with its lower mass construction is appropriate for quick accelerations. Ironless motors operate in a u-channel design with magnets on both sides of the forcer and have zero cogging.
Direct drive linear actuators are sold in two basic configurations – as a motor coil and magnet way set designed to be integrated into the application mechanism or in prebuilt linear actuators or stages. In the kit version, a machine designer has complete control over how the linear motor will integrate within the machine. The designer needs to consider the mechanics required to mount the motor, feedback selection and placement, cable management, and bearing selection. In a pre-built linear actuator, all mechanics to support the linear motor are predetermined so the actuator can be quickly integrated into the machine. Typically, prebuilt linear actuators have a limited selection of force and stroke lengths available, while the kit approach offers considerably more flexibility.
Linear motors have the same environmental and temperature considerations as any servo motor. An important consideration of a linear servo motor is the potential accumulation of metal or other particulates on the magnet way that may cause potential damage to the forcer or magnets. In machining applications, a bellows or similar covering can help protect the magnets and air gap from accumulating unwanted debris.
Careful consideration of the motor space requirements and environmental conditions, in addition to the speed and force requirements, will lead to a successful selection of the proper direct drive linear actuator.
