The advancing requirements for dynamics, accuracy, and synchronization have also led to new approaches in linear drive technology. Similar to torque motors as rotary direct drives, linear direct drives have been developed where the load is directly coupled to the moving part of the motor without mechanical transmission elements. This rigid mechanical coupling offers the same advantages as in rotary direct drive technology and opens up completely new possibilities for machine concepts.
In contrast to linear systems driven by rotary motors (see Actuators), linear motors do not have a limitation on the travel length. The stationary magnetic assembly can be assembled from standard profiles of any length. Since the moving winding head can be used for any distance, there are no performance losses depending on the travel length.
On the other hand, spindle systems have critical speed limits, and inertia increases with the length of the travel path. Speed limitations, high inertia, and low stiffness are the main disadvantages of other systems compared to linear motors for longer travel distances.
Linear motors in principle work as rotary motors do. Just imagine you cut a rotary motor and bend it until it is flat. A linear motor consists only of two components: the winding (forcer) and a steel plate with the magnets mounted (magnet assembly). The copper windings are embedded either into an epoxy or iron core and carry all the motor current.
The magnet assembly consists of rare-earth magnets mounted in alternating poarity on a rigid steel base plate. They produce a magnetic field perpendicular to the base plate. Current flowing in the copper windings produces the Lorentz force F = I x B which can be used for acceleration of the load inertia.
The Forcer normally is connected to the moving part of the machine whereas the magnet assembly is fixed to the stationary part of the machine. The airgap between forcer and magnet assembly is typically 0.6mm, but can vary by +/-0.3mm without causing a dramatical loss of performance.
For ironless motors the coils get embedded into epoxy material. These motors are very well suited for very smooth linear motion. There is no attraction force between forcer and the u-shape type magnet assembly and the forcer. For iron core motors the coils are wounded around a laminated steel core. The steel is used to concentrate the magnetic flux, so that the force density is higher compared to ironless motors. Due to the iron there is a significant magnetic attraction force between the forcer and the magnet assembly which can be used as bearing preload. Like in conventional rotary motors iron core linear motors have also some cogging.
A special version of linear motors are the so called "Voice Coil Motors". These are single phase linear motors for short stroke and low force applications. They are used in optical systems and in semiconductor applications for fast positioning of low weight loads; they can also be found as actuators in critical valve applications, where the control of the hysteresis is important.
For the commutation, the digital speed control and the positioning operation of our linear motors we use displacement measuring systems or sensors. You can find more information under sensors.