Moore's Law, famous for predicting exponential gains in semiconductor chip density, has long endured thanks in large part to advances in linear motor performance. Driven by semiconductor industry requirements, linear motor manufacturers have increased precision, reduced prices, developed multiple motor types, and simplified integration into automation equipment.
Today's linear motors can provide 20g peak acceleration and 10 m/sec velocity, deliver unmatched dynamic agility, minimize maintenance, and multiply uptime. They've moved beyond specialized semiconductor industry usage to provide advanced performance in hosts of applications and industries. In fact, with ten times the speed and ten times the operating life of traditional ballscrews, linear direct-drive technology is often the only feasible solution for modern productivity-enhancing automation.
For applications that require high acceleration and deceleration, linear motors can provide the dynamic stiffness, fast settling, and short distance moves not easily achieved with other technologies. Conventional mechanical positioning systems are dynamically limited by hysteresis, backlash, and wear. Similarly, pneumatic actuators suffer from piston mass and piston-cylinder friction, as well as air compressibility and its subsequent effect on servo control complexity. By shedding the mass and inertia of these conventional devices, linear motors can offer excellent dynamic stiffness.
What's more, by directly creating drive force, they can achieve closed-loop bandwidths that alternative positioning mechanisms can't even approach. The absence of cascaded mechanical linkages eliminates positioning uncertainty (slop) and mechanical resonances. As a result, linear motors and actuators are able to take full advantage of modern controller performance tuned for high loop gain, optimizing bandwidth, settling time, and recovery from transient disturbances.
Linear motors and actuators also excel in making millimeter-range moves that typically operate in the static friction zone. Their low mass and minimal static friction minimize 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.
Linear motor types
Motor manufacturers typically specialize in one or another of three basic linear motor configurations: Flat bed, U-Channel, and Tubular. Each motor has unique benefits and limitations. Drawbacks specific to one motor type can often be sidestepped by using either of the two alternatives.
Flat bed motor
Flat bed motors, while offering unlimited travel and the 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 to support the additional load.
U-Channel motor
The U-Channel motor with its ironless core offers low inertia, hence maximum agility. However, the forcer's load-carrying magnetic coils travel deep within the U-Channel frame, restricting heat removal.
Tubular motor
Tubular linear motors are thermally efficient and can provide drop-in replacements for ballscrews and pneumatic positioners. The motor's permanent magnets are encased and protected in a stainless steel tube (thrust rod). Without additional support, load travel is limited to 2 to 3 m depending on thrust rod diameter.
Information provided by Copley Controls. For more information on linear motors, visit www.motionsystemdesign.com.