Linear Motors and Actuators Meets Automation Needs
- Tuesday, 01 April 2008
Linear motors and actuators are now cost-competitive with ball screws and belt drives and offer distinctly superior agility and bandwidth for advanced positioning applications. New micromotors and actuators are helping to automate tasks not previously feasible. Direct linear drives are increasingly replacing servo-controlled pneumatic cylinders, contributing reliability and controllability, free from the cost, noise, and upkeep of air compressors.
Driven by semiconductor industry requirements, linear motor manufacturers have steadily increased precision, reduced prices, developed multiple motor types, and simplified integration into automation equipment. Modern linear motors provide 20g peak acceleration and 10-meters/second velocity, deliver unmatched dynamic agility, minimize maintenance, and multiply uptime. They have moved beyond specialized semiconductor industry usage to provide advanced performance in hosts of applications.
With ten times the speed and operating life of ball screws, linear direct drive technology is often the only solution for productivity-enhancing automation.
The dynamic performance of conventional positioning mechanisms is limited by lead screws, gear trains, belt drives, and flexible couplings, which produce hysteresis, backlash and wear. Similarly, pneumatic actuators suffer from piston mass and piston-cylinder friction, as well as air compressibility, which produces servo control complexity. Linear motors and actuators shed the mass and inertia of conventional positioners, and freed from these fundamental limitations, provide unequalled dynamic stiffness.
Direct drive force creation enables linear motors and actuators to achieve closed-loop bandwidth unavailable with alternative positioning mechanisms. The motor and actuator are able to take full advantage of modern controllers. These controllers are tuned for high loop gain operation, achieving wide bandwidth control, fast settling, and rapid recovery from transient disturbances.
Linear motors and actuators excel in making millimeters distance moves that operate in the static friction zone. Their low mass and minimal static friction minimizes the drive force necessary to start travel, and simplify the control system’s task in preventing overshoot when stopping. These attributes enable direct drive motors and actuators to scan microscope slides, for instance, and chart the X-Y locations of artifacts only millimeters apart.
Applications requiring rapid repetitive motion can exploit the linear actuator’s high bandwidth to double the throughput of ballscrews or belt drives. Machines that slice rolls of material to length (paper, plastics, even diapers) maximize throughput by operating without stopping the material flow. To cut on the fly, such machines accelerate the cutting blade to synchronize with material flow, travel at material speed to the cutting location, and then initiate the cut. After cutting, the blade is returned to to its starting point to await the next round-trip cutting cycle.
Linear motor types
Three basic linear motor configurations are available: flat bed, U-channel, and tubular motors. Each motor has intrinsic benefits and limitations.
Flat bed motors, while offering unlimited travel and highest drive force, exert considerable and undesirable magnetic attraction between the load carrying forcer and the motor’s permanent magnet track. This attraction force requires bearings that support the extra load.
The U-channel motor, with its iron-less core, has low inertia, hence maximum agility. However, the forcer’s load carrying magnetic coils travel deep within the U-channel frame, restricting heat removal.
Tubular linear motors are rugged, thermally efficient, and the simplest to install. They provide drop-in replacements for ballscrew and pneumatic positioners. The tubular motor’s permanent magnets are encased in a stainless steel tube (thrust rod), which is supported at both ends. Without additional thrust rod support, load travel is limited to 2 to 3 meters, depending on thrust rod diameter.