AcroMonk: A Minimalist Underactuated Brachiating Robot
In IEEE Robotics and Automation Letters, IEEE, volume 8, pages 1-8, Jun/2023.
Brachiation is a dynamic, coordinated swinging maneuver
of body and arms used by monkeys and apes to move
between branches. As a unique underactuatedmode of locomotion,
it is interesting to study from a robotics perspective since it can
broaden the deployment scenarios for humanoids and animaloids.
While several brachiating robots of varying complexity have been
proposed in the past, this letter presents the simplest possible
prototype of a brachiation robot, using only a single actuator and
unactuated grippers. The novel passive gripper design allows it to
snap on and release from monkey bars, while guaranteeing well
defined start and end poses of the swing. The brachiation behavior
is realized in three different ways, using trajectory optimization via
direct collocation and stabilization by a model-based time-varying
linear quadratic regulator (TVLQR) or model-free proportional
derivative (PD) control, as well as by a reinforcement learning (RL)
based control policy. The three control schemes are compared in
terms of robustness to disturbances, mass uncertainty, and energy
consumption. The system design and controllers have been opensourced.
Due to its minimal and open design, the system can serve
as a canonical underactuated platform for education and research.