Automation
performance applications as fiber Further Evaluation
alignment, Bragg grating production, While several major design choices have
and laser micromachining. been contrasted, a number of other
design options were not detailed or
Testing Effects and Choices were deliberately set aside for the sake
Note that some system effects are not of comparing a few common
specifically addressed by the described technologies. For instance:
tests. For instance, environmental
effects can provide a substantial impact circle6 Might the screw stage’s Figure 8. Linear motor stage minimum
on motion system performance. A performance have improved incremental moves (50 nm moves
carefully controlled (with respect to with the addition of a attempted)
both vibration isolation and secondary linear encoder scale
temperature) test chamber is required along its travel? If so, by how
to make a true comparison of stage much?
motion. Otherwise, the step size circle6 What impact does the relative
measurements could easily be size of any stage have on its
corrupted by thermal expansion and ability to make very small
ground floor vibration. moves? Will a very small screw
In fact, this is even more important driven stage perform much
in the context of the final application better than a larger version?
than as a testing consideration. If circle6 Ball-screw-stages have been Figure 9. Linear motor stage minimum
temperature effects cause a 2-micron measured, but what about lead incremental moves (100 nm moves
shift in the stage carriage position screw-driven slides? attempted)
(which may or may not be detected by circle6 All of the above testing
its encoder), the work piece mounted to measured moves made by
28
the tabletop will shift as well, which stages driven by brushless System resolution (high line-count
could have critical consequences in, say, servomotors. Would stepper encoders and interpolation) does not
www
the final dimensions of a micro- motor driven stages perform necessarily allow very small step size.
.eur machined part. The testing performed differently? The ball-screw example above
oasiasemiconductor
during this study used a temperature circle6 A number of encoder choices illustrates that some drive technologies
controlled room and had the stages exist for both rotary and linear may require tens or hundreds of counts
mounted on an air isolated granite test motor stages. It is natural to before a move can be made repeatably.
plate. assume that a finer pitch, more Minimum step capability is
Similarly, amplifier technology has accurate scale will offer better direction dependent. Although this may
not been considered. Linear amplifiers step size capability, but how seem obvious for some drive
.com
will offer smoother motion as they do great is this improvement? technologies (such as screw based),
not display the same switching noise circle6 How do alternative drive (such even the direct drive example
(often at 20 kHz) as pulse width as piezoelectric) or bearing (such demonstrated some lost motion during
square4
Issue II 2009
modulated (PWM) drives. This can as air) technologies compare to direction reversals. To completely
manifest as greater in-position dithering those outlined above? remove these types of artifacts a
that could easily ‘swamp’ small step noncontact bearing must be used.
moves. Additionally, PWM drives can Although the data shown above is Any applications should be
exhibit a ‘dead band’ effect. This occurs valuable in its own right, more testing approached carefully and with an
at direction reversals, where the encompassing some or all of the above appreciation of the difference between
commanded signal changes signs. A points will allow a more complete and resolution and step size. Some coarse-
linear drive can smoothly transition to a thorough comparison. positioning requirements will not
different direction, while the PWM require fine stepping capability while
source will experience some finite Conclusions others will.
amount of time during which no current A cursory view of the above data allows Although the results shown by this
is available through the output. a realistic comparison among some very study detail some contrast among
This will display as delayed or lost common drive technologies to be made various stage families’ performance,
motion control in a practical system. with respect to minimum incremental applications will ultimately dictate stage
Linear amplifiers were used in the move capabilities. However, a more choice. Thoughtful drive and bearing
above testing, which allowed the considered view of the graphs shown in choices will only enhance the chances
resultant data to most accurately this article exposes other important of success in applications requiring
portray stage capability. comparisons to be considered. short, repeatable, and precise moves.
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