Wednesday, February 21, 2001
Though small, the linear-motor slice of the overall motor-market
pie is sizzling. While linear motors continue to expand into new areas, one
factor limiting growth is the complexity of integrating ancillary components.
It's not uncommon for the added cost and time of aligning and assembling rails,
slides, bushings, and feedback devices to offset any gains in speed,
acceleration, and accuracy.
But three linear motor suppliers have recently introduced
"self-contained" linear motors. By integrating magnets, coils, bearings, and
feedback into a single device, these new designs promise to make linear drives
easier and less costly to use.
All designs employ a magnetic rod surrounded by a column of
current-carrying coils. This approach yields greater force per volume than flat-
or channel-type linear motors, because the coils act on the entire (360-degree)
surface of the moving member. And all could be considered throughput- and
accuracy-enhancing replacements for more conventional pneumatic,
electromechanical, and hydraulic systems. But in terms of size, force output,
and application areas, each appears to be targeting its own niche, although
there is some overlap.
With prices starting at $600/axis, LinMot's small-diameter (23-
and 37-mm) cylindrical design makes it a strong contender in the pneumatics
arena. Achieving forces up to 45 lbs, LinMot(r) P drives target cost-sensitive
applications involving high throughput and light loads that don't require
micron-level accuracy. To keep size and cost down, the design uses internal
Hall-effect positioning sensors to achieve 0.004-inch linear resolution. For
more information visit www.linmot.com.
The ThrustTube™ (www.thrusttube.com) from Linear Drives Ltd.
exploits a simple patented design based around a tubular magnetic thrust rod and
a moving thrust block that carries a set of thrust-generating, circular armature
coils. Without forced cooling, the design can achieve 292-lb forces. Its speed
ranges from a few microns/sec to greater than 10 m/sec, with up to 20g
accelerations. With the addition of a linear scale, it achieves micron-level
accuracy.
Providing up to 750-lb peak force (with 30A excitation) over a 2-
to 12-inch stroke range in 2-inch increments, the 5020-4 direct drive linear
tubular motor from California Linear Device (www.calinear.com) is a contender in the
low-end hydraulic arena. The compact linear servomotor develops a maximum
frequency (accelerating and decelerating) response of 60 Hz over a 1/8-inch
distance. With a 50-inch/sec top speed, its sensing probe extends the length of
the motor running through the center to achieve positioning accuracy of
0.001-inch.
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