Automation
Overall, though, very small motions were commanded
and realised from the linear motor stage. With some system
changes (finer resolution, for instance), 10 nm or even 5 nm
steps could likely be made in practice
capacitive gauges mounted close to a drive strategy if a suitable ‘run up’
target carried by the stage (see Fig 1). distance is prescribed and only
Issue II 2009
Figures 2 through 5 display this unidirectional moves are required.
square4
series of moves. Although the
Direct Drive Mechanical
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referenced (in each graph) move was
commanded, initial moves in a given Bearing Stages
direction only caused the stage While screw-driven systems display
tabletop to translate a small portion of noticeable motion sinks, direct drive
the desired distance. In other words, (linear motor) linear stages also have
while the motor spun an amount some inherent parasitic effects. Similar
commensurate with the commanded Figure 5. Ball-screw-stage minimum to the screw stage testing detailed
oasiasemiconductor
distance, the desired move was not incremental moves (1000 nm moves above, a series of small moves was
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accomplished. After a few commanded attempted) commanded of a direct-drive stage
www
moves, this hysteresis has been whose linear encoder provided a
overcome, and the desired moves are fundamental pitch of 20 microns. Once
27
realized in their entirety. again, the encoder output was of
The larger moves (Figures 4 and 5) amplified sine signal format and was
appear somewhat less susceptible to interpolated using encoder
these effects. Stated more accurately, multiplication technology to yield a
the wasted motion of the motor is less resolution of 10 nm per machine count.
significant compared to the magnitude Capacitive probe measurements
of the commanded move. Although produced graphs shown in Figs 6 - 9.
even smaller (10 nm through 50 nm) Figure 6. Linear motor stage minimum Note that although very small step
moves were attempted and plotted, the incremental moves (10 nm moves size is outstanding compared to the
stage tabletop manifested no attempted) ball-screw-driven stage, some parasitic
appreciable motion in response to the motion is still wasted on the return
commanded move, and essentially only ‘staircase’ of moves. This error is due to
floor/background noise was gathered. bearing non-repeatability. The error
The graphs clearly show hysteresis distance was also too small for a
at each directional change. In the corrective move to reliably make or for
smallest move depicted (Figure 2), the the controller to detect, recall that
coupling and screw windup and system resolution is 10 nm, compared
backlash are never fully overcome to a parasitic error distance of 1-2 nm.
before the test regimen began Simple stage tuning artifacts cause the
commanding reverse motion steps. Figure 7. Linear motor stage minimum minor positional overshoot seen at the
Even in the largest commanded moves incremental moves (20 nm moves start of some of the plotted moves.
(1 micron steps shown in Figure 5), attempted) Overall, though, very small motions
some hysteresis is apparent. were commanded and realized from the
Although the graphs might rule out linear motor stage. With some system
this family of stages for applications tens of microns or millimeters) can take changes (finer resolution, for instance),
requiring small, bi-directional, advantage of the relatively good 10 nm or even 5 nm steps could likely
incremental moves, large classes of accuracy and repeatability of these be made in practice. This ability allows
applications are routinely served by this stages, as well as their cost relatively cost-effective, direct drive,
technology. For instance, general effectiveness. Even tasks requiring very mechanical-bearing stages to be used
positioning (point to point moves of small moves might be able to utilize this in such high precision and high
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