  Compliance in actuation has been exploited to generate highly dynamic
maneuvers such as throwing that take advantage of the potential energy stored
in joint springs. However, the energy storage and release could not be
well-timed yet. On the contrary, for multi-link systems, the natural system
dynamics might even work against the actual goal. With the introduction of
variable stiffness actuators, this problem has been partially addressed. With a
suitable optimal control strategy, the approximate decoupling of the motor from
the link can be achieved to maximize the energy transfer into the distal link
prior to launch. However, such continuous stiffness variation is complex and
typically leads to oscillatory swing-up motions instead of clear launch
sequences. To circumvent this issue, we investigate decoupling for speed
maximization with a dedicated novel actuator concept denoted Bi-Stiffness
Actuation. With this, it is possible to fully decouple the link from the joint
mechanism by a switch-and-hold clutch and simultaneously keep the elastic
energy stored. We show that with this novel paradigm, it is not only possible
to reach the same optimal performance as with power-equivalent variable
stiffness actuation, but even directly control the energy transfer timing. This
is a major step forward compared to previous optimal control approaches, which
rely on optimizing the full time-series control input.
