  Actuation remains a signifcant challenge in soft robotics. Actuation by light
has important advantages: objects can be actuated from a distance, distinct
frequencies can be used to actuate and control distinct modes with minimal
interference and signifcant power can be transmitted over long distances
through corrosion-free, lightweight fiber optic cables. Photo-chemical
processes that directly convert photons to configurational changes are
particularly attractive for actuation. Various researchers have demonstrated
light-induced actuation with liquid crystal elastomers combined with azobenzene
photochromes. We present a simple modeling framework and a series of examples
that studies actuation by light. Of particular interest is the generation of
cyclic or periodic motion under steady illumination. We show that this emerges
as a result of a coupling between light absorption and deformation. As the
structure absorbs light and deforms, the conditions of illumination change, and
this in turn changes the nature of further deformation. This coupling can be
exploited in either closed structures or with structural instabilities to
generate cyclic motion.
