The latest design for linear motors takes advantage of a tube shape, which offers a force range of 8 to 32 N.

A motor turns, and through a series of belts, gears, couplings and ball screws, rotary motion changes to linear. For decades, designers have been configuring such mechanisms, essentially starting from scratch on each new project.

Now, thanks to advances in electromagnetic technology, there's an alternative. A new type of linear motor - tubular linear motors - promise to simplify design because they are nearly identical in form, fit, and function to ball screw and bull nut systems. These self-contained actuators are so similar in fact, that machines often need little or no modification to accommodate them.

Powerful simplicity

Tubular linear motors include an armature and a rotor. The armature, or thruster, consists of a single conductive wire cylindrically wound and encapsulated. The stator is a cylindrical assembly of sintered NdFeB permanent magnets arrayed in an onaxis North-South stack contained in an encasing tube. Without iron core elements, there is no cogging so motion is smooth.

The moving thruster does not actually ride on the stator. A relatively large air gap of about 1 mm separates the components. Such a gap is beneficial because it lowers alignment tolerances during installation. Supporting the thruster is an independent bearing system.

As with forcer platten motors, the single stator can independently control multiple thrusters.

Like almost all linear motors, tubular designs operate in servo mode. Analog or digital Hall-effect switches embedded within the armature render sinusoidal or multi-step trapezoidal commutation. Alternately, combining a magnetic or optical incremental linear encoder with a Hall-effect switch will deliver commutation, as well as position and velocity loop closure, and ensure correct phasing on power up.

Tubular motors are brushless, so they need little maintenance and have low EMI. Present versions offer continuous force exceeding 70 lbs, peak output of approximately 300 lbs, and maximum speed approaching 400 ips.

It's all in the symmetry

Much of their power output is due to their radial symmetry. All the magnetic flux intersecting the slider coils generates thrust. The arrangement of the thruster windings and flux pattern ensure that the current and magnetic fields are perpendicular. The result is maximum force.

The symmetry also balances the magnetic fields, reducing any attractive forces between the slider and stator. Lower forces ease installation and reduce loading requirements on support bearings. Tubular motors display attractive forces of several pounds, while conventional forcerplatten motors have forces of several hundred pounds.

Like all devices, there are limitations. For example, the size of these motors can be a drawback for some applications. Support for the stator assembly is only possible at the extreme ends. Therefore, sagging limits the stroke to approximately 80 in. with a 1.5-in. diameter stator. The motor thruster also has a profile height greater than that of other linear motors.

On the job

Regardless of their design type, linear motors are subject to similar application considerations. The relatively large gap between the thruster and stator makes tubular motors less prone to fouling by ferrous chips. Thus, they can operate in applications conventional linear motors can't. Even so, it's a good idea to use protective shields or some means of actively capturing and removing residues.

With a geometry that lets a tube-shaped armature ride over a rolled stator, all of the magnetic flux is usable for force or power.
Select figure to enlarge.

Horizontal motion applications are best for two reasons. First, when de-energized, linear motors lack holding force without the addition of a shaft brake. Secondly, these motors need some type of lifting force to go against gravity. They typically don't have the benefit of forcemultiplying mechanical ratios.

Early applications for tubular motors have focused on their positioning accuracy and speed. Semiconductor wafer fabrication, for example, relies on the sub-micron positioning available with these motors. In packaging and printing functions, it's the place-to-place speed that's important.

Here's an example of an application that required both functions. Researchers at the Max Plank Institute in Berlin wanted to automate biomedical sample spotting and gridding operations. A tubular motor was chosen to drive the multi-axis robot manipulators. The manipulators pick up samples and spot them onto 22 by 22 cm membranes. The robot grids up to 230,400 samples per membrane at a rate of 250,000 samples per hour.

But this is only the beginning for these devices. As developers refine the design and add more sizes, they will become an increasingly attractive alternative to the more established linear motor architectures.

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