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Understanding linear motor technology

Understanding linear motor technology
The linear motor seems to be coming of age. The concept has been known for decades, but until recently they were too expensive or too delicate for most applications. However, with rare earth magnet prices coming down and demand for high speed, high precision going up, linear motors are set to become far more mainstream, says Andy Parker-Bates of Parker Electromechanical

Linear motors are based on the same scientific principles as their everyday counterparts, rotary motors: relative movement of a magnetic field and a metal component inducing an electric current. Indeed their inventor, Professor Eric Laithwaite, described them as "ordinary motors spread flat." Almost certainly the most significant thing about a linear motor is that the forcer (the moving part, equivalent to the rotor) floats in a magnetic field above the platen (stator or track). As such, there are no frictional forces, so it can achieve incredibly high acceleration, velocity and deceleration.

Almost as significantly, the forcer can achieve precision positioning anywhere along its length of travel. These two attributes open up a world of applications that are virtually impossible with more traditional mechanical linear actuators. Many lay-people associate linear motors with maglev trains. In fact, linear motors were originally designed to drive the shuttle through weaving machines in the textile industry. Today however it is the production industries that are adopting the technology at some speed.

There are several variations on the basic design of linear motor, each with its own advantages and applications fields. The most common ones are called tubular, flat and U-shaped (or channel). In all of them the forcer contains the coils, encapsulated in epoxy resin, while the platen holds a row of powerful magnets plus linear tracks or bearings to keep the forcer precisely aligned.

A few other components are also needed: Hall effect transistor, thermistor, linear encoder and electrical connections (each has an equivalent in a rotary servomotor). These generate the commutations of the magnetic field to drive and control the motor's actions.

The lack of physical connection between the forcer and platen means that speeds of over 40m/s, accelerations exceeding 200m/s2 and infinite travel are achievable, although 5m/s, 50m/s2 and 5m are more typical performance figures in the real world. As there are no brushes there is no brush wear, no friction, and no need for lubrication. Also, issues such as the backlash and windup of linear converter mechanisms are avoided, so positioning accuracy and repeatability are higher. 

Tubular: In a tubular motor the forcer is cylindrical and the platen is a round bar (containing the all-important magnets), which passes through the platen; this configuration was the first to find regular commercial applications. It does, however, suffer from the potential for the magnetic flux field to leak into the surrounding environment, which can limit its application, while there is also the risk of items becoming trapped under the forcer. 

Although the bar-like platen appears strong and robust, it is in fact hollow. As it can only be supported at its ends, it tends to bow in the middle, so if the platen is too long the degree of bow will cause the forcer to grind to a halt.

Channel: A U-shaped channel linear motor uses two facing tracks of magnets, between which the forcer runs. The tracks are connected along one edge by a linear bearing, and in most cases the forcer actually runs on this so that its load bearing capacity is increased significantly. One of the disadvantages of this design is that heat can quickly build up within the channel. However, forced air cooling is relatively easy to install and resolves the problem. This type of device is often used in long travel applications, as the tracks can simply be joined end to end almost infinitely.

Flat: A flat linear motor is similar to a channel unit, but uses only one track laid horizontally so that the platen rides above it. This is relatively cheap, simple to install and maintain, and also allows better heat dissipation. There are at least three variations on this basic theme: slotless ironless, slotless iron and slotted iron. In the first type the forcer has no iron components (using aluminium instead), which means there is no cogging, or stepping of the platen as it passes from one platen magnet to the next. This gives a smooth motion but only limited motive power.

A slotless iron motor incorporates iron laminations in the platen, which enhances the magnetic flux and increases the motive power. However cogging also becomes apparent as a stepping characteristic is induced into the platens movement as it passes from one magnetic field to the next. In a slotted iron motor, the forcer contains extra iron to increase the flux and power (and cogging) even more. These are often used to replace mechanical linear actuators.

In application, linear motors are increasingly replacing belt drives, screw drives and rack and pinion mechanisms. The advantages include improved dynamic performance, lower maintenance, reduced noise, easy programming and reprogramming. The biggest drawback is cost, but with the price of the components in a linear motor falling in recent years, combined with increased usage, unit prices are changing quite markedly. Also, end user attitudes are changing from an emphasis on initial purchase price to total life costs, and the minimal maintenance of a linear motor compared with a complex mechanism can change the balance of that equation. 

Linear motors will never be a universal solution for all linear motion requirements, yet they are becoming usable for a far wider range of applications. Therefore, sensible machine builders and design engineers should always at least consider their adoption - and some expert advice will never go amiss.
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Parker Hannifin Ltd (Poole)

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