Flexipod:
A Legged Soft Robot Platform for Dynamic Locomotion
Abstract—We present an open-source untethered quadrupedal soft robot platform for dynamic locomotion (e.g., high-speed running and backflipping). The robot is mostly soft (80 vol.%) while driven by four geared servo motors. The robot’s soft body and soft legs were 3D printed with gyroid infill using a flexible material, enabling it to conform to the environment and passively stabilize during locomotion on multi-terrain environments. In addition, we simulated the robot in a real-time soft body simulation. With tuned gaits in simulation, the real robot can locomote at a speed of 0.9 m/s (2.5 body length/second), substantially faster than most untethered legged soft robots published to date. We hope this platform, along with its verified simulator, can catalyze the development of soft robotics.
Paper video
Material
The flexible material used for printing the soft body and legs is a flexible TPU(Cheetah flexible filament, Ninjatek). Although the filament has a shore hardness of 95A, it is possible to achieve a lower hardness by varying the infill density and the flow rate (the percentage of the material extruded).
(a) Shore hardness vs. flow rate and infill density; (b-c) 3d printed cross-section of (b) gyroid 20% infill density and 90% flow rate, and (c) with 16% infill density and 80% flow rate.
Body design
The body is printed with the flexible material. 96 vol. % of the 3D printed parts of the robot are soft, making the robot is 80 vol.% soft.
Structure of the soft robot: (1) Soft main body; (2) Camera; (3) Electronic components; (4) Li-Po battery; (5) Bearing; (6) DJI M3508 brushless DC motor enclosed in a motor shell; (7) Soft leg.
Soft leg
The leg’s hollowed structure makes the it flexible and impact resistant. To demonstrate its damping effect, we dropped the Flexipod from as high as 2.0 m height. At impact, the soft legs were compressed, and the body was bent inward to absorb the impact. Thus, the Flexipod is able to recover from the fall on its own.
Flexipod soft leg and demonstration: (a) Soft Leg, leg coupler and bearing; (b) 3D model of the leg assembly showing its internal structure; (c) chronophotograph of the Flexipod dropped from 2.0 m.
Simulation
We developed a soft robot simulation environment based on the Titan – CUDA accelerated massively parallel asynchronous spring-mass simulation library. We extended the Titan library with a rotational kernel for contact-coupling of soft bodies. The Flexipod simulation achieved 10^9 spring evaluations per second on a consumer Nvidia 2080Ti GPU.
Locomotion patterns
(a) bounding gait, (b) pace gait, (c) turning, (d) backflip
More backflips
Locomotion in the wild
People
Boxi Xia, Hongbo Zhu, Zhicheng Song, Yibo Jiang, Jiaming Fu, Hod Lipson
Creative Machines Lab at Columbia University