  Based on the idea of variable stiffness mechanisms, a variety of such
mechanisms is shown in this work. Specifically, 2-DOF parallel kinematic
machines equipped with redundant actuators and non-linear springs in the
actuated joints are presented and a comparative overview is drawn. Accordingly,
a general stiffness formulation in task space of all mechanisms is given. Under
fixed geometric parameters, optimization of task space stiffness is carried out
on the designs comprising all kinematic solutions. Finally, a stiffness metric
is introduced that allows a quantitative comparison of the given mechanism
designs. This gives rise to design guidelines for engineers but also shows an
interesting outline for future applications of variable stiffness mechanisms.
