If you think belt drives have too much slop for your
precision linear motion applications, think again. Bell-Everman Inc., a specialist in
high-accuracy motion systems, has come up with a new kind of belt drive for
applications that usually call for pricier linear motors, top-quality ball
screws or rack-and-pinion drives.
conventional belt drives, this ServoBelt actuator has a moveable carriage that
rides atop a linear rail. But that's where the similarities end. The
ServoBelt features a patent-pending design based on a pair of toothed,
steel-reinforced polyurethane belts. The lower belt is static and bonded to a
custom aluminum extrusion, while the upper "active" belt runs through a set of
rollers within the carriage.
Along most of the rail length, the two toothed belts engage
one another. In the carriage, though, the belts zip apart with the upper belt
running over a servo-driven, toothed roller. This roller runs along the upper
belt to move the carriage. A set of idlers, meanwhile, helps guide the upper
and lower belts as they separate and re-engage as the carriage passes by. For
ease of integration and configurability, Bell-Everman builds most of the
drive's chassis and structural elements from off-the-shelf linear ball rails
and aluminum t-slot framing from Bosch-Rexroth.
Everman, Bell-Everman's chief technical officer and a principal in the firm, likens
the new drive to a rack-and-pinion whose rack happens to be a toothed belt.
"But this drive is zero clearance," he says, explaining that the conformance of
the belt teeth to the drive roller teeth eliminates a potential source of
backlash found on traditional belt or geared drives.
design of the belt mechanism also means almost all of the belt – all but
three inches or so – remain free of tension as the carriage goes on its way.
"Only the portion of the belt between the rollers are under tension. There's no
stretching anywhere else," says Everman. Keeping the belt mostly free from
deformation contributes to a uniformly high stiffness that belt drives usually
lack, and Everman says the ServoBelt is at least 10 percent higher
than a conventional belt drive.
This stiffness gives ServoBelt a dynamic response and speeds
associated not with belt drives but with more expensive linear motors and
top-quality mechanical drives. Everman says, for example, the
ServoBelt settles 10 to 20 percent faster than a conventional belt drive after
a move. "It doesn't have the springiness that makes standard belt drives hard
to tune," he says. As for speed, the
standard ServoBelt offers acceleration of 4 g and speeds up to 4 m/s. An
optional bearings upgrade will increase the speed limit to 10 m/s, which is
well into linear-motor territory.
is far more accurate than a conventional belt drive and many rack drives too.
Everman says the ServoBelt has an absolute bi-directional accuracy of
+/- 60 microns and a unidirectional repeatability of +/- 10 microns. "A
traditional belt drive's accuracy and repeatability would be measured in hundreds
of microns," he says. With an optional linear encoder, the ServoBelt gets
even better. Everman says it can be configured to achieve "sub-micron" accuracy
levels previously possible with linear motors or the very best ball screws.
ServoBelt covers a standard travel range from 0.15 to 50m. Between its
five servo motor sizes and different rail options, the system can handle peak
forces to 880N and continuous to 300N. Everman believes the ServoBelt will be
most attractive on large format machines
and material handling systems used in the packaging, wood working, metal
cutting and aerospace industries. Most of these machines will likely
require travel distances greater than 2m, benefit from independent carriages
on the same axis and have accuracy levels that would have necessitated linear
motors or high-end rack drives in the past. "What we've done is come up with
something that performs like a linear motor with a cost-per-foot that's closer to
a belt drive," says Everman.