Heat &
Motion
DUKE IT OUT
Figure 1: Depicted here
is the relative position
change between two
linear stages mounted
on a granite table. Note
that the stages are not
powered. The position
change is entirely the
result of the different
thermal expansion
coefficients of aluminum
and steel.
Temperature
changes
hold the
key to
precision
positioning
applications.
BY THOMAS BARTHOLOMÄUS
AND BEDA ESPINOZA, NEWPORT CORP.
Tmost motion applications is
temperature or, more precisely,
temperature change. Temperature
alone affects the accuracy of any
he dominant cause of error in
position measurement, and this error –
linear – is either irrelevant or can be compensated for easily. However, temperature
changes cause drift and affect repeatability
or reproducibility, very real concerns for
most high-precision motion applications.
It is much more difficult to control or
compensate for this.
Any experimental setup or machine
is made of materials such as steel, granite
or aluminum, all of which inherently have
different linear thermal expansion coefficients. Because any position is always
measured between two points, one typically
a stationary reference – for instance, from
an optical table or an overhead granite
bridge – and the other on a motion stage,
the dissimilar thermal expansion of different materials in the setup causes length
changes between the two points, eventually
seen as drift or error in reproducibility in
the application. Therefore, the position of a
motion stage may be perfectly stable and
accurate, but the position relative to the reference may change because of nonuniform
material expansion within the setup.
A perfect correlation
To illustrate this point, in Figure 1,
the distance between the edges of two
aluminum linear stages is monitored with
a laser interferometer. The stages are
