Motors and Drives
4 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. The option to use linear motors is often overlooked, so here Andy Parker-Bates looks at the features and benefits of the technology.
4 Les constructeurs de machine doivent souvent créer un mouvement linéaire pour lequel le choix par défaut est généralement l’utilisation d’un moteur rotatif et d’un mécanisme pour convertir la sortie en linéaire. L’option qui consiste à utiliser des moteurs linéaires est souvent négligée, Andy Parker-Bates a donc regardé de plus près les fonctions et les avantages qu’offre cette technologie.
4 Maschinenkonstrukteure müssen oft eine lineare Bewegung erzeugen und dies erreichen sie meist durch die Verwendung eines Drehmotors in Verbindung mit einem Mechanismus, der die Leistung in eine lineare Bewegung umwandelt. Die Möglichkeit, einen Linearmotor zu verwenden, wird oft außer Acht gelassen, aber Andy Parker-Bates stellt hier die Merkmale und Vorteile dieser Technik vor.
Linear progression: using linear motors
L
inear 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.
Classifications
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.
Fig. 1. Linear motors provide the best solution in applications where speed, acceleration and precision are required.
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