Motors & Drives
Re-define expectations for the future of linear motion
Jon Severn looks at some of the latest developments in linear motors to see how they compare with conventional technologies.
Jon Severn examina algunos de los últimos avances en motores lineales para ver cuáles son las diferencias respecto a las tecnologías convencionales.
Jon Severn schaut sich einige der neuesten Entwicklungen bei Linearmotoren etwas genauer an, um herauszufinden, wie sie im Vergleich zu konventionellen Technologien abschneiden.
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Fig. 1. SMAC’s LAR31 series moving coil actuators are unusual in that they feature a vacuum-through-shaft
n any application for linear motion there are design considerations relating to speed, acceleration, positioning accuracy, size, complexity, maintenance requirements and, of course, cost. Selecting the right technology for the task is often far from simple and, historically, tradeoffs have been common: while one type of linear motor or actuator might excel in some respects, it might involve compromises elsewhere. However, newer breeds of linear motors and actuators mean that there are often products available that meet more of the application requirements, with fewer compromises necessary.
Te arguments for and against different
technologies have often seen engineers moving away from hydraulic or pneumatic actuators in favour of electromechanical alternatives. Te days when electromechanical options could not compete with hydraulic actuators in terms of force, or pneumatic actuators in terms of speed, are largely behind us. Furthermore, suppliers of electromechanical products argue that their offerings are simpler to implement and easier to control, with none of the mess or fuss associated with fluid power.
Te latest innovations are redefining
expectations for linear motion. While screw drives and belt drives rely on converting the output of a rotary motor into linear motion, linear motors offer direct linear actuation. Te linear motor, in essence, is a conventional rotary electric motor unwrapped.
Today’s market for linear motors is
due, in part, to the success of the tubular linear motors that can be traced to British engineer Hugh-Peter Kelly, who, in the 1980s, designed a tubular motor with permanent magnets enclosed in a sealed tube. Tis concept was brought to market by Linear Drives (now Copley Motion Systems). Te patented permanent magnet arrangement induces a
sinusoidal response in the coils that are enclosed in a square-profiled body.
Importantly, machine builders could control
these linear motors using standard sinusoidal servo drives as found in conventional rotary motor-based motion control systems. Tubular linear motors are more rugged than
conventional linear motors, which enables them to be used in industrial environments including food packaging and machine tools. Te tubular construction protects the permanent magnets from the external environment and automatically balances attractive forces so that the motor is easier to integrate into machine designs. Tese motors operate at speeds of 5-9m/s with high rates of acceleration for dynamic motion control. Copley Motion Systems took tubular motor design a further stage in 2005 with the Servotube, which integrated the position sensing electronics within the motor body. Development of the Servotube has continued, with the current generation of motors combining compact dimensions with high speed and accelerations of up to 25 ‘g.’ Te company says that its STB11 motors, for example, can provide ten times the speed and ten times the life of ball screw actuators, making them suitable for boosting productivity in high-speed point-to-point positioning applications. Most recently, the company has introduced a stainless steel version of the Servotube, aimed at hygienic applications requiring frequent wash down (Fig. 1). Electrically, the Servotube Hygienic operates from a three-phase 600V supply and incorporates a digital incremental encoder and commutation output for use with most standard digital servo drives. It is available in two models that both offer ±400 micron position accuracy and 25micron repeatability. Te XHB3804 and XHB3810 provide 744 and 1860N peak force capability and are capable of speeds of 4.7 and 2.6m/s, respectively. Both models can be water-cooled, providing up to 70percent extra continuous force for applications requiring high work rates or those with high ambient temperatures. Further innovations in tubular linear motors have come from Festo, which is rapidly establishing a broad range of electrical and electromechanical motion options to complement its better known pneumatics
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