Cassie Cal the Bipedal Robot Learns to Juggle
Thu, April 22, 2021

Cassie Cal the Bipedal Robot Learns to Juggle

Cassie Cal is a bipedal robot that can juggle a ball with its torso / Photo Credit: Alessandro de Leo (via Shutterstock)


Juggling is hard for humans but it doesn’t stop robots from learning it. Meet UC Berkeley’s Cassie Cal robot, a bipedal machine that consists of a torso, which acts as a platform to juggle the ball, wrote Evan Ackerman of magazine IEEE Spectrum. If Cassie can balance and juggle the ball at the same time, then it can do other things that “require dynamic multitasking.” The robot is from the Hybrid Robotics Lab at UC Berkeley, led by Koushil Sreenath, along with Katherine Poggensee, Albert Li, Daniel Sotsaikich, Bike Zhang, and Prasanth Kotaru.

Ackerman spoke with Albert Li via email. According to Li, their motivation for starting off with motion capture was to “address the control challenge of juggling on a biped without worrying about implementing the perception.” His team had a ball detector on a camera, which would mean they would not have to depend on the motion capture system. 

However, Li and his colleagues needed to mount the camera in a way “it would provide the best upwards field of view.” They also had to develop a reliable estimator. It was not possible for Li’s team to track the ball. Hence, they have to assume that their dynamic models were able to describe to the ball’s motion accurately enough until it bounces back into the air. When asked about how Cassie can juggle indefinitely, Li answered that are a few factors that affect how long the robot can sustain a juggle. While in simulation, the paddle “exhibits homogenous properties like its stiffness and damping.” 

Hence, there are some parts of the paddle that may be better for juggling. It results in “larger than expected error in a juggle” when a ball hits those parts. Since the paddle is small, it is possible for the ball to hit the edge, terminating the juggle instantly. 

It is also possible for large juggling errors to occur, causing Cassie’s feet to slip slightly, thereby changing its standing and stable position over time. Can Cassie juggle while walking? Li acknowledged that it is a more challenging feat to accomplish, considering it as a goal for future research.