Tech Briefs Magazine - August 2023 - MD-25

As long as three of the robot's feet remain
in contact with the ground, it can
avoid tipping over. But if only one or two
feet are on the ground, the robot can't
easily correct for disturbances and has a
much higher risk of falling. This lack of
balance makes walking over rough terrain
particularly difficult.
" With current control methods, a quadruped
robot's body and legs are decoupled
and don't speak to one another to coordinate
their movements, " Manchester said.
" So how can we improve their balance? "
The team's solution employs a reaction
wheel actuator (RWA) system that mounts
to the back of a quadruped robot. With the
help of a novel control technique, the
RWA allows the robot to balance independent
of the positions of its feet.
RWAs are widely used in the aerospace
industry to perform attitude control on
satellites by manipulating the angular momentum
of the spacecraft.
" You basically have a big flywheel with a
motor attached, " said Manchester, who
worked on the project with RI graduate
student Chi-Yen Lee and mechanical engineering
graduate students Shuo Yang and
Benjamin Boksor. " If you spin the heavy
flywheel one way, it makes the satellite spin
the other way. Now take that and put it on
the body of a quadruped robot. "
The team prototyped their approach by
mounting two RWAs on a commercial
Unitree A1 robot - one on the pitch axis
and one on the roll axis - to provide control
over the robot's angular momentum.
With the RWA, it doesn't matter if the robot's
legs are in contact with the ground or
not because the RWAs provide independent
control of the body's orientation.
Manchester said it was easy to modify
an existing control framework to account
for the RWAs because the hardware
doesn't change the robot's mass
distribution, nor does it have the joint
limitations of a tail or spine. Without
needing to account for such constraints,
the hardware can be modeled like a gyrostat
(an idealized model of a spacecraft)
and integrated into a standard
model-predictive control algorithm.
The team tested their system with a series
of successful experiments that demonstrated
the robot's enhanced ability to recover
from sudden impacts. In simulation,
they mimicked the classic falling-cat problem
by dropping the robot upside down
from nearly half a meter, with the RWAs
enabling the robot to reorient itself mid-air
and land on its feet. On hardware, they
showed the robot's ability to recover from
disturbances - as well as the system's balancing
capability - with an experiment
where the robot walked along a 6-centimeter-wide
balance beam.
Manchester predicts that quadruped
robots will soon transition from being
primarily research platforms in labs to
widely available commercial-use products,
similar to where drones were
about 10 years ago. And with continued
work to enhance a quadruped robot's
stabilizing capabilities to match the instinctual
four-legged animals that inspired
their design, they could be used
in high-stakes scenarios like search-andrescue
in the future.
For more information, contact Aaron
Aupperlee at aaupperlee@cmu.edu;
412-268-9068.
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Motion Design, August 2023
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Tech Briefs Magazine - August 2023

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