limited to 1 micron per 100 mm. However, this can be
improved significantly with modern controllers if calibrated
and compensated for with look-up tables or polynomial error
Rotation stages comprise a platform that rotates relative to
a base. The platform and base are joined by some form of
bearing which restricts motion of the platform to rotation about
a single axis.
Various motors and drive principles can be employed, from
stepper-motor driven worm gear designs to direct-drive closed-loop torque motors. Low-profile piezomotor stages provide
self-locking capabilities with zero jitter and drift and requiring no
holding current at rest.
Precision motorized rotation stages are used in applications
such as fiber-optical alignment, semiconductor inspection,
biomedical applications and X-ray crystallography.
Air-Bearing Rotation Stages use a thin film of pressurized
air to provide an exceedingly low friction load-bearing interface
between surfaces. The two surfaces do not touch. As they are
contact-free, air bearings avoid the traditional bearing-related
problems of friction, wear, particulates and lubricant handling,
and offer distinct advantages in precision positioning and in
high-speed applications, where eliminating backlash and static
friction are critical.
Typically used for the highest precision and smoothness of
motion/velocity, air-bearing rotation stages deliver ultra-low
runout and wobble, as well as extremely high resolution and
repeatability. Pitch, yaw, roll on the order of 1 arc second are
feasible. The absence of friction eliminates backlash and gives
the air bearing stage high repeatability.
Goniometers are instruments that either measure an angle
or allows an object to be rotated to a precise angular position.
They are often used in crystallography and in X-ray diffraction to
rotate the samples, and are also useful in (fiber) optic alignment
A positioning goniometer or goniometric stage is a device
used to rotate an object precisely about a fixed axis in space. It
is similar to a linear stage, however, rather than moving linearly
with respect to its base the stage platform rotates partially
about a fixed axis above the mounting surface of the platform.
Positioning goniometers typically use a worm drive with a
partial worm wheel fixed to the underside of the stage platform
meshing with a worm in the base.
In order to achieve precision at the micron and sub-micron
level in multi-axis motion applications, hexapod parallel
positioners have become popular. Hexapods, six-legged
parallel-kinematic mechanism (PKM) motion systems, effectively
reduce the footprint, and moving mass of a traditional serial
kinematic stacked-stage positioning system while increasing
stiffness and responsiveness. This, together with the arbitrary,
Hexapods, in their most common form consisting of two
platforms, a fixed-base platform and a second movable
platform, which are connected and supported by six
independent legs (struts or links) that expand and contract
in parallel. A similar 6-axis design, called SpaceFab (see
Figure 5), is based on a top platform connected to three XY
linear stages with three passive struts. It provides similar
performance to a hexapod, yet allows for a lower profile and
longer XY travel ranges (with reduced angular motion).
The six degrees of freedom enable the secondary platform to
move in three linear directions, lateral (X) and longitudinal (Y),
vertically (Z), and the three angular directions (pitch, roll and
yaw), by the legs. Thus, hexapods can perform manipulations
that encompass total freedom of motion in a relatively compact
space, with high stiffness and without moving/sweeping cables
that can break and foul.
VACUUM COMPATIBLE POSITIONERS
Vacuum applications are increasingly more important
for many fields of research and industrial manufacturing.
Requirements span from vacuum levels from 10-3 mbar
to 10-9 mbar. While piezoceramic drive units can easily
be modified for extreme vacuum, an increasing number of
motorized positioning devices are also being incorporated
successfully into vacuum applications where long travel and
high precision motion are needed.
Positioning systems specifically developed for vacuum
operation must meet a number of criteria. Because of limited
space within vacuum chambers, the selection of suitable
components is crucial for the vacuum compatibility of a
positioning system. The body of the positioning device must
be designed for placement in closed compartments, to avoid
outgassing. Holes, as well as screws, need to be vented, and
a reduction of the surface is desired. Air pockets, such as in
under mountings, must be avoided as they considerably delay
pump-down to target pressure or even make generating a
stable vacuum impossible.
The article was written by Stefan Vorndran and Scott Jordan
from Physik Instrumente L.P. Vorndran is Vice President of
Marketing, while Jordan is Senior Director of NanoAutomation
Figure 5. Miniaturized 6-axis parallel-kinematic SpaceFab positioning system.