On a midsized robot, all these factors historically have created an average positioning error of ± 5 mm at the end of
the robot arm. But advancements in recent years compensate for these errors, which is why precision processes, like
laser welding, can be performed well with an articulating
arm. Now, average error on a robot with absolute accuracy
can be as small as ±0.5 mm.
Some robot link components today are manufactured
with a laser tracker, which measures the position of the
arm at specific joint locations. The robot is given the axis
position and moves to that point in space. The laser track-
er records the actual position and compares that to the
one that was programmed.
This is done well more than 100 times, measuring points
throughout the work envelope, with the robot working
at its maximum payload. This procedure—comparing
the difference between the actual and programmed position—helps construct an algorithm that produces the
offsets, which account for positioning errors and make the
entire robotic system much more accurate.
Payloads and Foundations
Of course, when integrating any precision robotic appli-
cation, you also need to consider factors external to the
actual robot arm. For example, the
robot payload—like the laser pro-
cessing head—should not exceed
half the maximum payload amount.
A laser head that’s 7 or 8 kg should be
put on a robot arm that can handle
twice that amount, or at least 16 kg.
The payload measurement reveals
the maximum weight the robot can
move. The accuracy is in the static position—at the start and end points—
which is all that really matters in most
material handling applications. But at
or near payload, the positioning accuracy between those start and end
Think about picking up a sturdy
pen with a large boulder attached to
it, and try drawing a circle with it. You
could physically move it in a circle,
and start and end at the same place,
but the circle wouldn’t look good at
all. Now remove the rock, and you
just have a sturdy pen remaining. The
weight of the pen makes it easy for
you to press it against the paper and
move it to produce a good-looking
circle. Replace it with a really lightweight pencil, and you find that it’s
so light that it just doesn’t fit well in
your hands, so again, the circle isn’t as
The same thinking applies to robotic payloads for precision applications.
When thinking about payloads—
processing heads, cables, hoses, and
the like—you need to aim for a happy
medium, not too heavy and not too
light. This includes upper arm load,
or attached load, such as dress packs
and other auxiliary equipment. When
possible, reduce or eliminate as much
attached load as you can.
A robot’s concrete foundation also
plays a role here. These days more
robots are being integrated as part
of palletized cells, units that can be
disconnected and moved with a fork
truck to other areas of the plant. This
can make automation truly flexible,
but you still need to account for the
physical attributes of a robotic system, including its need for a good
foundation. Industry standards, like
those from the Robotic Industries
Association, address this.
A robot’s mass is much less than that
of a linear-axis system, so the foundation requirements aren’t as stringent,
but robots still need to be able to
transfer enough energy into the floor
to absorb vibration. If the concrete
can’t absorb that energy, it will oscil-
late back up into the robot arm.
Certain palletized systems work
very well for precision applications,
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