Tech Briefs Magazine - May 2024 - PIT-17

pitch errors, require regular maintenance
and lubrication, and can exhibit wear
over time. Plus, they are connected to a
drive shaft via a coupling, resulting in
windup-related errors. Direct-drive mechanisms
not only eliminate such errors but
also require virtually zero maintenance
and achieve higher speeds and smoother
motion. Direct-drive stages are advantageous
in photonics applications because
the high speed and smooth motion contribute
to increased throughput and quality,
resulting in a lower ownership cost
over time. Ultimately, multi-axis systems
can use both direct- and indirect-drive
stages, with the former allocated to the
most critical-performance axes and the
latter reserved for less critical supporting
or adjacent motion.
Serial- and parallel-kinematic architectures
are a key consideration in selecting
positioning mechanics. In serial-kinematic
arrangements stages are
stacked, with each stage's orientation
corresponding to a direction of motion.
Parallel-kinematic architectures, such as
hexapods or Stewart platforms (shown
in Figure 4), use multiple actuators in
parallel to position a single platform.
Hexapods require six linear inputs - one
for each strut - and kinematic transformations
transpose these inputs into
three linear and three rotational outputs
of the moving platform.
Both architectures facilitate three-dimensional
programming and virtual
pivot-point rotations. Serial-kinematic
arrangements offer intuitive visualization
and straightforward programming,
and they can be more accurate and repeatable
at the functional point. They
also offer superior design modularity
and greater efficiency in 6-DOF use
cases. In contrast, parallel-kinematic architectures
can provide higher stiffness,
smaller form-factors and clearer payload
access. Because most hexapods are
screw-driven, they tend to be slower than
direct-drive, serial-kinematic architectures.
Choosing the optimal architecture
depends on the application's priorities.
An experienced motion supplier can
help guide this decision.
Controller Technologies and
Selection
The controller choice and setup are
equally as important as stage selection.
Deciding between pulse-width modulation
(PWM) and linear power-stage amplifiers
is key. Linear amplifiers offer
extremely low noise and sensitivity to
electromagnetic interference, making
them ideal for nanometer-level stability
and minimum incremental motion.
However, they are larger and costlier
than PWM drives. While PWM drives
are smaller and more economical, they
can exhibit switching noise. Opt for a
controller architecture in which linear
and PWM drives can coexist within a
system.
Trajectory and servo feedback rates
are another important consideration. A
high trajectory generation rate, on the
order of 20 kHz or faster, helps to facilitate
high speed and precision. Slower
trajectory rates result in having too few
points to fully define the desired path,
resulting in dynamic position errors.
Remedies include operating at a slower
velocity or increasing the trajectory rate.
Some controllers even allow for a spline
interpolation between trajectory points
to further minimize the following error.
Choosing a controller that offers automated
alignment algorithms is an enormous
benefit to throughput. These algorithms
are especially useful for identifying
first light and then efficiently searching for
peak power transmission. Many different
alignment techniques exist, such as spiral,
raster, hill-climb scans and more. Leading
motion suppliers can assist with choosing
and optimizing a search routine for your
unique process. Still, alignment algorithms
can only be as precise as the stages
they're controlling.
Working with Motion
Suppliers
Figure 4. Hexapods are parallel-kinematic devices that use six actuators working in parallel to position a
common platform. (Image: Aerotech)
Photonics & Imaging Technology, May 2024
Defining a motion system for optical
and photonic alignment applications
requires working closely with a supplier
who understands your technical and
commercial challenges. A proficient
supplier asks detailed questions to evaluate
your priorities and offers multiple
solutions with clear tradeoffs, helping
you make informed decisions. A supplier
who recognizes your needs and is invested
in your success will likely present
a framework of multiple solutions, especially
if the tradeoffs are nuanced. Do
not hesitate to ask how a supplier has
addressed similar motion challenges in
the past, and be sure to request relevant
test data and performance plots to reduce
technical risk. Assess a supplier's
ability and experience to help you transition
from the lab to the fab. Scaling
up in a technically and economically
sound manner is critical but often overlooked.
Furthermore, consider the sup17

Tech Briefs Magazine - May 2024

Table of Contents for the Digital Edition of Tech Briefs Magazine - May 2024