The Fabricator - November 2013

Attaining Accuracy

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 points suffers.

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 good-looking.

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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