mounted on a granite table using screws
at the center of the stages. The stages are
screwed onto a granite table. They are not
powered, so any position drift is entirely
the result of thermal expansion and the
granite. The relative position fluctuation of
170 nm every three minutes correlates perfectly to the temperature fluctuation of the
stages of 0.08 °C – a result of the air-conditioning system – and can be explained by the thermal expansion coefficients of aluminum ( 23 μm/m/°C) and of
granite ( 7 μm/m/°C) used in the setup.
Hence, the position drift is not caused by
the performance of the stages but by the
inhomogeneous materials in the setup.
But temperature changes also have
many other effects. For example, in a linear stage, the temperature of a steel lead-screw increases as it is cycled through its
travel, lengthening the screw by 10 μm/
100 mm/K change. If the position of the
stage is controlled by a shaft encoder or
the steps of a stepper motor, or if the
motor is turned off after the motion, the
screw will expand slowly, resulting in a
slow drift or in an error in repeatability if
the same position is approached many
times (Figure 2, bottom).
However, when a direct encoder is used
– e.g., a linear scale – the heating of the
screw does not affect stage position, and
high repeatability is achieved. Repeatable
positioning is the primary advantage of
direct encoders over shaft encoders or
open-loop steppers. Also, in steady position, a stage with a direct encoder will
correct for the length changes of the screw
as the screw cools down after the motion.
However, these corrections will be viewed
as position instabilities from the outside
world (Figure 2, top). The preferred behavior – continuous drift or correction –
depends on the application.
Dissimilar materials used inside a motion stage also can introduce significant
motion system errors. Figure 3 shows
schematically the cross section of a Newport FEM optimized U-channel design
used on the ILS and IMS series linear
stages (left) compared with a linear stage
with a box design (right). In the U-channel
design, the center of stiffness of the steel
parts (green) is in the same plane as the
center of stiffness of the aluminum parts
(blue). In the box design, the center of
stiffness line does not bisect all dissimilar
materials the same way, creating bimetallic bending that introduces a considerable
pitch error on the stage. In many applica-
Figure 2: The position
change of a vertical linear
stage with 45-kg axial
load is pictured after
some continuous motion
with motor on (left) and
motor off (right). With the
motor off, the stage position
drifts because of the heat
dissipation of the screw
after the motion. The stage
features a direct encoder
(not coupled to the screw)
with motor on, so the stage
position is continuously
corrected for. However,
this continuous position
correction yields to sudden
position changes rather
than a continuous drift.
Newport ILS/IMS U-Channel Design
Steel rails are centered in height.
Alternative Box Design
Center of Stiffness
Figure 3: The thermally
induced pitch of an athermal
Newport design is compared
with an alternative box
design.
Pitch at 20 °C*
Pitch at 30 °C*
Alternative
Design
130 µrad
300 µrad
ILS200
70 µrad
110 µrad
Thermal Induced Pitch* 17 µrad/°C 4 µrad/°C
* Stages mounted on four points on an adapter plate, no load
tions, pitch error is a major contributor to
positioning errors, particularly for long
travel, multiaxis positioning systems and
any application where the “point of interest” is far from the position-sensing device; e.g., the bearing plane.
Users should be cognizant of temperature, especially changes normally encountered in a lab or manufacturing environment, which can be the main cause of
errors in many high-precision motion
applications. Also consider that flatness
of the mounting surfaces, large distances
of the “point of interest” from the position
feedback systems, off-centered loading
and nonoptimum motion electronics can
introduce errors.
But that’s a different story.
Meet the authors:
Thomas Bartholomäus is director of product marketing at Newport Motion; e-mail: thomas.barthol
omaeus@newport.com.Beda Espinoza is senior
manager for product marketing at Newport
Motion; e-mail: beda.espinoza@newport.com.
