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
"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
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