Motors & Drives Information

Applications for linear motors are growing rapidly, but the one thing impeding even faster growth is engineers' lack of understanding of the devices. Simon Smith of Aerotech makes an introduction
Once the subject of futuristic speculation about maglev trains and super guns, linear motors have come of age as advanced and reliable alternatives for ball and roller screws for linear motion. Not only are the motors now affordable and practical, but the they are easy to control. Applications for linear motors are growing at a rapid rate, but the one thing impeding even faster growth is engineers' lack of understanding of the devices.

Without going into the basic physics, which are quite simple anyway, a linear motor is essentially a rotary motor that has been cut and made flat.

The forcer (rotor) is made up of coils of wires encapsulated in epoxy and the track is constructed by placing magnets on steel.

The forcer of motor contains the windings, Hall Effect board and the electrical connections.

The control for linear motors is identical as with rotary motors.

Like a brushless rotary motor, the forcer and track have no mechanical connection, i.e, no brushes.
Unlike rotary motors, where the rotor spins and the stator is held fixed, a linear motor system can have either the forcer or the magnet track move.

Most applications for linear motors, at least in positioning systems use a moving forcer and static track, but linear motors can also be used with a moving track and static forcer.

With a moving forcer motor the forcer weight is small compared to load, but there is the need for a cable management system with high flex cable, since the cable has to follow the moving forcer.

With a moving track arrangement, the motor must move the load plus the mass of the track.

However, there is the advantage that no cable management system is required.

Why use linear motors?

Linear motors overcome most of the disadvantages of the most commonly used ballscrews.

Ball screw systems are subject to screw wear and backlash and cannot tolerate high speeds or acceleration rates.

There is a temperature effect on the screw which reduces accuracy.

Ball bearings also reduce the smoothness in velocity and there is windup/compliance.

Because of wear, the characteristics change over time.

A linear motor directly converts electrical energy to linear mechanical force and is directly coupled to the load.

There is no compliance or windup, higher accuracy, and unlimited travel.

Today, linear motors typically reach high speeds, for example, 5msec, with high accelerations of 5g but can now reach up to 10g with 10msec velocity.

There is no wear, no lubrication and therefore minimal or no maintenance cost.

Finally, there is higher system bandwidth and stiffness.

A linear motor can be flat, U-channel, or tubular in shape.

The moving part of a flat motor, which is typically an iron core design, rides close to the surface of the secondary.

A U-channel motor's moving part rides within two rows of permanent magnets.

In all cases, the moving copper coil assembly is ironless, and thus has no attractive force, resulting in smooth velocities without mechanical disturbances.

The configuration that is most appropriate for a particular application depends on the specifications and operating environment.

Flat motors are often cheaper because they require fewer machined parts and magnets.

However, since the surface of the magnets is exposed, one limitation is that they cannot be used in environments that will be affected by magnetic flux that will "leak" out of the system.

In U-channel designs, this problem is significantly reduced since the magnets are contained within the motor casing.

As with any technology, there are always limitations and caution must be used to employ the correct solution in any application.

While cost was once a limitation over ballscrews, improved manufacturing methods and increasing volume, coupled with higher performance requirements of today's machine manufacturers, costs have reduced to be comparable with a typical ballscrew and motor alternative.

Indeed when cost of ownership is taken into account, in many cases a linear motor system may over time prove to be a considerably less expensive solution than the traditional screw alternative.

One area where a mechanical system will always be preferred is where a high load and hence high inertia is encountered.

Whereas a high torque to inertia ratio is beneficial in many instances, it must be considered that a linear motor is a servo system and therefore the usual inertia matching rules relating to motor and load must be applied.

There is none of the mechanical advantage inherent in a ballscrew and an unstable system running at high accelerations and velocity is a recipe for disaster! However, any reputable manufacturer or supplier would be pleased to aid in the sizing of a suitable motor, and should have a wide experience in applying the correct solution to each application.

Another disadvantage with linear motors is they are not inherently suitable for use in a vertical axis.

Due to its non contact operation, if the motor is shut down, any load that is been held vertically would be allowed to fall.

There are also no failsafe mechanical brakes for linear motors at present.

The only solution that some manufacturers have achieved is by using an air counterbalance.

Environmental conditions must also be considered.

Although the motor itself is quite robust, it cannot be readily sealed to the same degree that a rotary motor could be.

In addition, linear encoders are often employed as feedback devices and therefore care must be taken to ensure that the encoder is suited to the environment too.

That said, linear motors have been successfully employed in wafer dicing, an environment where highly abrasive ceramic dust has lead to the downfall of many supposedly more robust solutions.

Again, the motor supplier should be familiar with all the options, and would be pleased to offer advice in each case.