A tubular linear motor consists of two basic parts, a stator and a shaft. A stack of magnetic rings makes up the shaft which slides freely back and forth inside a column of coils arranged in the form of a three-phase stator.

Form, fit, and function, the engineering mantra made public in the 1990s, is usually applied in reference to consumer goods such as cars and cell phones. But the concept also holds in the design of motion systems, particularly with respect to linear actuators.

The use of servo control has, in a sense, spoiled industry. Now, everyone expects the same sort of improvements along the “linear axis” that they’ve become accustomed to on the “rotary axis.” Never mind that the available envelope of space keeps shrinking. If you’re a “good engineer,” you’ll find a way to actuate that big, clumsy load more quickly and precisely than before – so they say.

It is precisely for this reason that researchers are delving ever deeper into the realm of electromagnetics, electronics, and control dynamics, hoping to nudge the technological envelope just enough to satisfy the demands of industry one more day. So far, one of the more promising developments is a device in the form of an electric motor, but with the fit and function of a pneumatic or hydraulic cylinder.

New twists

To see one of these mechanical specimens in the lab, you’d think you were looking at an ordinary electric motor with, perhaps, a somewhat fat rotor. But when the rotor fails to turn and instead begins to extend smoothly along its axis, your instincts are likely to take over, telling you there’s a pressure cavity somewhere inside filling with fluid. Most people are inclined to believe the mechanism is hydraulic until they see the actuator run full bore, cycling back and forth at more than 100 in./sec.

It’s that kind of dynamic response that makes electromagnetic actuation a natural for linear motion. It’s also the reason why researchers have spent years experimenting with rotary motors, manipulating copper, iron, and magnets like pieces of a Rubik’s cube. Retracing the twists and turns from rotary motor to linear actuator is, in fact, the best way to get a handle on the construction and operation of today’s moving-magnet, or tubular, linear motors.

Imagine starting with a brushless dc motor. After removing the rotor, you open it up and lay it flat. (If you stopped here, you’d essentially have a conventional linear motor, but you’re not done yet.) Next, you wrap the sides together – which were originally at opposite ends of the rotor – curling the assembly into a tube. The permanent magnets, initially oriented along the length of the rotor, now form a stack of rings of alternating magnetic polarity.

With a little modification, a permanent magnet rotor can be transformed into a magnetic shaft like the kind used in tubular linear motors.
Select figure to enlarge.

In a tubular motor, the magnetic shaft rides on an integral bearing system, two sliding-contact bearings incorporated in the endbells. The shaft is suspended in, and passes through, a column of currentcarrying coils held in slots along an iron core. A dozen or more coils separated by poles or teeth are arranged by phase (A, B, and C) in repeated succession down the length of the stator. The electromagnetic circuit thus formed bathes the entire surface area of the shaft in magnetic flux, achieving maximum linear force per unit of volume.

Naturally, the more powerful the magnets – neodymium-iron-boron is the standard today – the more force the actuator can produce. Force is also a function of the length and diameter of the motor. Usually, it’s easier and more economical to change the length of the motor, rather than the diameter. In fact, in the lab as well as in the field, motors are frequently assembled in pairs, end-to-end, to double force output.

There’s another advantage in increasing the length of the motor. The longer the shaft, the longer the stroke. Although any length is feasible, a 12-in. stroke is usually plenty for most applications.

Your choice

New technologies like tubular linear motors typically emerge to fill a gap or shortcoming in existing art. In this case, the gap (in linear actuation technology) is fairly sizable.

The fitness of a linear actuator is primarily a matter of accuracy, force, footprint, programmability, environmental considerations, and of course, cost. There are other considerations as well; but the point is, a linear actuator must satisfy several (often-conflicting) requirements. It’s no surprise that so many actuation technologies exist today – hydraulic, pneumatic, mechanical, electromagnetic – and it’s a safe bet it will be that way for years to come.

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