  We propose a mechanically simple and cheap design for a series elastic
actuator with controllable stiffness. Such characteristics are necessary for
animals for running, jumping, throwing, and manipulation, yet in robots,
variable stiffness actuators are either complicated or mimicked at low
bandwidth through feedback controllers. A robust and simple design is needed to
build reliable and cheap robots with animal capabilities. The key insight of
our design is attaching torsional springs to timing belts to create a variable
stiffness linear spring. In an antagonistic pair, varying the distance between
motor and joint then varies the actuator stiffness. We build a prototype of our
proposed actuator, show the theoretical behavior matches the experimental
characterization, and demonstrate an application to robotic one-legged hopping.
