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Linear Shaft Servomotors

Linear Shaft Servomotors

Linear shaft servomotor technology provides three basic benefits: It is simple, consisting of only a magnetic shaft and a forcer; motors are high precision, taking advantage of its ironless design to offer zero cogging and complete stiffness; and being a non-contact design, the shaft motor's non-critical air-gap eliminates variation in force over the entire stroke of the device.

But now, the trends and focus is on new product variations that meet specific application needs and exploit the advantages of the fundamental technology compared to other linear motor solutions.

"What we did differently with the technology than all the other cylindrical linear motor manufacturers is that they've always tried to adjust the windings," says Jerame Chamberlain, Linear Shaft Motor Product Manager for Nippon Pulse. "The concept was that the magnets are the magnets, and it's just a matter of changing the type of laminations and the winding type, lower or higher inductance windings, to get more power out of the motor."

He says the design of Nippon's shaft motor comes from a completely different perspective. Instead of looking at how to adjust the windings, the design focuses on increasing the strength of the magnets. The basic formula for the force a motor can provide is "force equals electrical current times the magnetic field." Increasing the magnetic field decreases the amount of current required by the motor, and using less power means less heat is going into the device.

"We've taken the same concept as the U-shaped linear motor coreless winding and shaped it into a cylinder around the magnets and a much stronger magnetic field," Chamberlain says. "It is a much stiffer design, about one hundred times stronger than U-shaped motors, plus there are no more Eddy currents, heat or cogging issues. The other design concept that makes it unique is the fact that all of the windings cross the magnetic field at a ninety degree angle. Unlike traditional U-shaped motors current is flowing in directions that oppose the direction of travel."

An inherent advantage of the technology is that the design of the motor makes the air gap non-critical because the magnet is in the center, which makes alignment and installation of the device very simple to do. The coil completely surrounds the magnet, so force is the net effect of the magnetic field.

The linear shaft motor provides a two-fold benefit for the ultra-high-precision sub nanometer to high picometer positioning. In those types of environments, users want the feedback right in the center of the gravity of the device to get truly accurate positioning. But you also want your motion and the power creating the motion right at the center of gravity of the device, and both pieces can't be in the same location.

By putting an encoder in the center of gravity, and because the air gap is completely non-critical and the motors will always make the same amount of force, two motors can be spaced an equal distance off the center line of the stage. Competing solutions often require two sets of encoders and two servo drives to provide this functionality.

The linear shaft motor also doesn't require all of the magnetic pitches in the system to be 25-, 30- or 60-mm long, the standard for linear motors in the marketplace. This makes the parallel drive system possible because the magnetic pitches are orders of magnitude longer than that, reducing the sine error on any mounting irregularities between the two drives.

Designed for ultra-high-precision positioning markets, this capability is a huge advantage for gantry system builders designing glass cutters or laser engravers. In the past, systems might have had two ball screws connected through a chain, two different motors driving separate ball screws using two different controllers that would electronically be connected together, or even two linear motors with encoders electronically connected together with two drives. Now that can be done with two shaft motors, one encoder and one amplifier.

Another key advantage of the linear shaft motor is that its continuous current is based on just the motor in free air, absolutely no heat sinking and no movement. Independent testing has documented that an aluminum heat sink about a quarter of inch thick and three times the surface area of the motor increases the heat dissipation of the motor so the current through the motor can be increased by a factor of 40 percent.

The cylindrical design of the shaft motor is also built with higher-grade-quality products than most linear motors. It's the only linear motor with a class H winding, which allows up to an 185C temperature rise in the windings.

Nippon Pulse recently introduced its new L427 series that features a 5-mm air gap between the forcer and magnetic shaft. With a 5-mm air gap between the motor's forcer and shaft, users have more flexibility when machining their device to level. It is also an optimal solution in environments where there is the potential for buildup of debris on the shaft because the air gap will prevent the motor from jamming and increase the time required between cleanings.

Other new product developments include low-profile, wider stages that Chamberlain says have the smallest dead zone of any linear motor stage on the market. And for customers that don't need a longer magnetic pole pitch because they are not implementing a parallel design, new high-density, short linear motors reduce the overall length by shrinking magnetic pole pitch

Linear Shaft Servomotors
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