  The search for non-Newtonian forces has been pursued following many different
paths. Recently it was suggested that hypothetical chameleon interactions,
which might explain the mechanisms behind dark energy, could be detected in a
high-precision force measurement. In such an experiment, interactions between
parallel plates kept at constant separation could be measured as a function of
the pressure of an ambient gas, thereby identifying chameleon interactions by
their unique inverse dependence on the local mass density. During the past
years we have been developing a new kind of setup complying with the high
requirements of the proposed experiment. In this article we present the first
and most important part of this setup -- the force sensor. We discuss its
design, fabrication, and characterization. From the results of the latter we
derive limits on chameleon interaction parameters that could be set by the
forthcoming experiment. Finally, we describe the opportunity to use the same
setup to measure Casimir forces at large surface separations with unprecedented
accuracy, thereby potentially giving unambiguous answers to long standing open
questions.
