November 2013 The FABRICATOR 97
but they can do this in part because
they effectively transfer kinetic energy
into the floor. Conversely, palletized
systems that don’t effectively transfer this energy may work for some
material handling applications (with
additional setting time), but not one
involving precision movement.
A robot may be connected to the
foundation but still encounter problems if it’s too close to another machine that’s creating its own vibrations. Some get around this problem
by pouring a concrete isolation or vi-bration-dampening pad, which keeps
out externally generated vibrations.
But in this case, it is especially important to direct the robot’s kinetic
energy through the pad and into the
ground. If the robot isn’t grounded
properly, the isolation pad will trap
the vibrations the moving robot produces, which then have nowhere to
go but back up into the robot arm.
A robot arm must have a certain
amount of stiffness to move without
vibration. To achieve that stiffness,
robot arms for precision applications
may need to be built with extra mass,
which can absorb some of that energy and minimize this vibration.
A parallel-arm robot, with two parallel linkages near the base, aims to
achieve the same thing, without the
need to add mass to the rest of the
system (see Figure 2). This helps the
robot keep steady at the far reaches
of its work envelope. This also makes
it ideally suited for precise applications like laser welding.
Manufactured to absolute accuracy,
and external variables like payload
and vibration accounted for, a robot
now is ready to perform a precision
operation. But there’s one more element: software.
Velocity, acceleration, and deceleration all can affect the characteristics
of precision processes like laser welding. Today many run simulations not
just to ensure the application can access the workpiece where needed and
avoid obstructions, but also perform
the actual process to the level of accuracy required.
Say a laser cutting application has
the robot maneuvering around a corner. This requires the robot to decelerate and accelerate, but how will these
speed changes affect cut quality? In
laser processing, slowing down gives
more time for heat to build up.
CAD systems tailored for specific
robots integrate closely and, hence,
(see Figure 3). If the solid model shows that the robot will
need to cut around a tight corner, the software will slow
the robot down as needed. An offline robot program-
ming system then can take this information and write a
program that will instruct the robot to reduce laser power
in-process, while it moves around the corner, to maintain
cut quality throughout the contour.
A Different Robotic World
Decades ago buying a robot required a hefty financial
commitment. It was for the specialized application, like
high-volume pick-and-place, in which the savings far out-
weighed the high price tag. That scenario has changed en-
tirely. Many now may initially consider a robot for an ap-
plication because it’s much less expensive than alternatives.
As always, the choice depends on the application. If
you’re cutting hole after hole on flat surface after flat surface, a linear-drive system probably is your best choice. But
if you’re cutting various products, and perhaps retooling
your automation for another job down the road, a robot
may well suit your situation.
Douglas Hixon is a robotic welding and laser cutting specialist at ABB Inc., 1250 Brown Road, Auburn Hills, MI
48326, 248-391-8400, www.abb.com/robotics.