Automation
Stiction: Resistance to the start of
motion (also known as static friction or
breakaway friction) – exacerbates screw
backlash effects and may cause
pitching, rather than linear, motion.
Overshoot: Because mechanical
bearings have some stiction to
overcome, often a large force (relative
to the commanded move distance) is
required to overcome this artifact. If the Figure 2. Ball-screw-stage minimum
Issue II 2009
Figure 1: Typical capacitive probe test force is slightly too large, the stage can incremental moves (100 nm moves
square4
overshoot its move by some amount attempted)
.com
linkages (spur gears, etc.) in the stage’s lower than the minimum achievable
drive train. All of these culprits consume move distance. Because the stiction
some small amount of the motor shaft’s must now be overcome in the opposite
rotation before completely engaging direction, the end effect is the same,
and turning the ball or lead screw. although the error has changed signs.
While most of the above ‘motion
sinks’ are present in screw-driven While many stage designs suffer from
oasiasemiconductor
stages, direct-drive systems also may some or all of the mentioned effects,
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have non-idealities that can corrupt some, such as air bearing linear stages,
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their motion. The fact that a feedback do not. As the bearing surface is
device (typically a linear encoder in virtually frictionless, stiction is a non-
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direct-drive systems) is not exactly at issue. Furthermore, overshoot is rarely a
the work point (carriage) creates an factor as an air-bearing stage will be Figure 3. Ball-screw-stage minimum
error. While the motor might have able to respond to all but the smallest incremental moves (200 nm moves
accomplished some small incremental corrective force commands. Lastly, attempted)
move, the tabletop may or may not skidding will not manifest because no
have followed the same motion profile. rolling elements are present.
Parasitic angular motion (pitch, roll,
yaw), along with offsets between the Screw-Driven Stages
encoder and work point (causing Abbe As noted above, ball screw and lead-
errors), and linear encoder screw-driven systems have some
misalignment with direction of travel all inherent non-idealities such as screw
can degrade fine stepping ability in backlash and coupling compliance,
linear motor axes. among others. To quantify these error
contributions, a series of small moves
Bearing Effects was attempted by a precision ball-
In both rotary motor driven and direct screw-driven-stage (screw pitch of 2 Figure 4. Ball-screw-stage minimum
drive stages, bearing non-idealities can mm/revolution), with the stage’s rotary incremental moves (500 nm moves
degrade performance and impact the motor encoder used for position attempted)
stage’s minimum step-size capability. feedback. The encoder output an
amplified sine signal format that was
Skidding: Lightly loaded ball interpolated using encoder
bearings have a tendency to skid rather multiplication technology (along with
than roll during initial moments of a controller quadrature) such that the final
commanded move. This represents a linear resolution was 10 nm per
different motion (as seen by a rotary interpolated machine count. The
motor/drive screw combination) controller faithfully commanded and
compared to traditional rolling motion, plotted each move based on the
but also can cause wear on the balls feedback from the rotary encoder.
themselves, which will in turn adversely However, independent verification of
affect bearing performance. the move was obtained by using
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