Machine builders often have to create linear motion, for which the default choice is generally the use of a rotary motor and mechanism to convert the output to linear.
Linear motors are often described as 'unrolled rotary motors'. This is a good starting point, because it highlights the fact that the underlying principals are exactly the same for both types of motor. This comparison is actually attributed to Professor Eric Laithwaite of Imperial College, London, whose pioneering work in the 1950s and 1960s was key to the development of today's maglev monorail trains.
Linear motors convert power directly into linear motion, without the need for belt drives, ballscrews or other mechanisms. They are usually simple, reliable and robust, and are able to provide accurate speed, motion and positioning over millions of cycles. While it should be noted that they are not always suitable for every application, their main benefits can be listed as follows:
- Highly accurate and repeatable.
- No backlash, slack, play or wind-up as is often found in mechanical systems.
- High acceleration, deceleration and highly dynamic speed control.
- Reliable and robust; only two parts, neither of which wears due to the air gap between them.
- Easy installation due to modular nature of magnet pieces.
A common confusion over linear motors is that there are different ways to describe the component parts. So let us use 'platen' for the unrolled stator, and 'forcer' for what was the rotor or coils; usually the platen is stationary and the forcer moves above it, although like a drum motor this configuration can be reversed.
Different type of industrial linear motors can be compared to AC induction motors, DC motors, steppers, servos and synchronous motors. The most common type is essentially a brushless servo motor laid flat.[Page Break]
Linear motors are often classified by the type of forcer, i.e. an iron core or ironless. An iron core motor has coils projecting down from the forcer that react with the electromagnetic field generated by the platen to create motion. Unfortunately, as the coils pass over the north and south poles of the platen a stuttering or 'cogging' motion can result, which is unacceptable for precision positioning tasks. To reduce this affect a laminated or slotless forcer can be used, although there will always be some degree of cogging. For precision motion an ironless, motor is best. This provides smooth motion, higher speeds and greater energy efficiency. Ironcore motors are usually used where higher forces are required and where their lower precision and greater velocity ripple (due to cogging) is not a problem.
Many linear motors have a flat magnet track; if strong thrust, precise positioning and smooth motion with lower velocity ripple are required then a channel design can be used. This uses an ironless forcer running in a U-shaped platen with rows of magnets on both inside vertical faces.
Manufacturers of linear motors can alter the performance of their units in a number of ways. These can include: the use of different strengths and sizes of magnet, or by skewing them at angles and overlapping them; by using different gauges of wire in the coils, enlarging or reducing the size of the coils; by adjusting sizes, shapes and weights of the platen or forcer. The result is that linear motors can be optimised for a great many applications; potential users are well advised as a first step to seek expert advice on whether to use a standard or bespoke product.[Page Break]
Let's get practical
In theory, the forcer of a linear motor moves above the platen, separated from it by a tiny air gap. This is an attractive proposition because there are no frictional losses or wear, plus built-in tolerance and self-adjustment for misalignment. In practice, however, a bearing is usually added to carry the load being moved by the motor.
To ensure that linear motors function efficiently and reliably for long periods of time it is important that bearings are sized to carry the required loads, and that each motor is mounted to a solid base. The primary requirement is that the platen needs to be completely flat and rigid, otherwise performance will be significantly degraded.
Thermal management also needs to be considered. Although rotary motors will generally run hot rolled out linear motors are better able to dissipate the heat they generate. However, it should be noted that the forcer tends to heat up and this may affect the payload, in which case some form of insulation will be required.
A defining issue with linear motors can be cost, compared with rotary conversion alternatives. Linear motors require expensive rare-earth magnets, which means costs increase with stroke length. Iron core motors need fewer magnets than ironless, but of course cogging may become an issue.
Currently, linear motors are competitive up to a few metres of stroke length, after which the benefits of belts, screws or rack and pinions begin to build markedly. Another practical aspect related to stroke is cable management. Power has to be supplied to the forcer throughout its entire travel; flexible cables and supporting cable chains will have to be funded - often at notable cost. (One way around this is that it's sometimes possible to fix the forcer so that it is stationary and release the platen so that it can run back and forth over it.)[Page Break]
Motors in parallel
Another limiting factor can be peak power. The most cost effective approach in engineering and machine building is always to use standard products rather than bespoke. If you need to move a large load it is possible to use two or more motors together.
For best results you would bolt them all together and commutate them as a single unit. The effect would be a straight multiple of the motor power times the number of motors (less a small efficiency loss).
Looking at the thrust output of a linear motor compared with a screw or other mechanism, it is notable that there is a drop in power. So we can conclude that linear motors are probably not the best answer for power hungry applications, but they score heavily on acceleration and precision positioning, and also on being compact in size, robust and reliable.
Linear motors are not generally used in aggressive applications, or where there is a high level of contamination. Basically the platen needs to be kept clean along its entire length and the electronics need to be protected from the ingress of moisture and dust.
When describing the construction of a linear motor it was noted that the platen is basically a long row of powerful magnets. Although the emitted magnetism is not too large, their suitability for applications such as medical magnetic imaging scanning machines should be considered carefully. General industrial applications, such as machine tools, where swarf and metal chips abound would require a linear motor to be shielded from the main environment.
So where do linear motors provide the best solution? In essence, in applications where speed, acceleration and precision are required; these can be extensive, ranging from high volume semiconductor manufacture, to digital printing and biomedical systems, and from LED and LCD production, to inspection, testing, and sampling, in both manufacturing and research.
Practical industrial linear motors have been around for 20 years or more, during which time they have found many niche applications rather than become a mass market product. This can be attributed mainly to their price and to a lesser extent to some of their limitations.
But there is now a steady increase in their take up. We can certainly expect to see them used more widely in future. It may be that one day soon something will happen to open the flood gates and they will become far more commonplace in a short space of time.
Andy Parker-Bates is with Parker Hannifin Ltd, Warwick, UK. www.parker.com