Accounting for Errors
Technically speaking, every physical positioning system has errors. ;e track
on a linear system is machined to a precise tolerance, but there still is a tolerance plus or minus some amount (usually with a lot of zeros to the right of the
decimal point). ;e drives that send the processing head along the track also
have slight positional errors. It’s just the nature of anything manufactured. No
matter how precise the manufacturing method, nothing is absolutely, perfectly
accurate.
Figure 2
;ese robots have parallel linkages that minimize vibration without adding mass to the
rest of the system.
Figure 3
CAD systems tailored for the robot can develop and simulate programs to ensure process consistency, adjusting speed and laser power for the application at hand.
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What really makes a linear system so accurate isn’t the fact that it lacks positioning errors, because, again, nothing is perfect. It’s the fact that the errors
don’t compound or stack up. An error in one axis doesn’t a;ect the errors in
the other axes.
In a typical 5-axis prismatic system, each axis is linear except for the last two,
which are rotational. A processing head can move to a certain point on the X
and Y plane that may be just a hair o;, but that inaccuracy won’t a;ect the
head’s ability to move down to the workpiece in the Z direction. ;e XY error
still is there, but it has no practical e;ect on the Z position, at least in most
applications sheet metal fabricators deal with. ;e last two rotational axes do
compound, but this occurs right at the workpiece, so the compounded error
doesn’t add up to very much at all.
An articulating arm doesn’t operate this way. On a 6-axis robot, the error of
each axis contributes to the overall positioning error of the arm. A minute error
in the first axis, near the base, compounds with all the errors of the remaining
five axes. By the time it reaches the sixth axis at the end e;ector, that error can
be significant.
And, again, anything manufactured has a tolerance too, and this includes every link in a robot arm. If one joint’s radial position is o; a few arc-seconds, and
the attached link is a little longer or shorter than nominal, that will magnify the
error. So the machining tolerances of the links play a role, as does the location
of the joint within the link.
Another error is deflection of the arm; that is, the bending of the arm during acceleration and deceleration. And then there’s an error called compliance, which deals with gearboxes giving a little bit, also most prominent when
changing speed.
Decades ago buying a robot required a
hefty financial commitment. It was for
the specialized application in which the
savings far outweighed the high price tag.
That scenario has changed entirely.
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at FABTECH
Booth #S3540
