Blackbird Bipedal Robot Uses Quasi-Direct-Drive OpenTorque Actuator for Movement

Foothill College computer science student Gabrael Levine is building a bipedal robot similar to Agile Robotics’ Cassie, but replaces the series elastic actuators with a quasi-direct-drive, giving the robot a more robust, back-drivable, and highly efficient method of walking. Levine’s Blackbird stands close to four-foot, weighs around 30 pounds, and costs less than $3,000 to build.

The robot is equipped with five MCP2515 CAN board, which are connected to a single ODrive board to power a series of brushless motors driven by a Raspberry Pi using a 400Wh battery. Blackbird’s legs were designed using carbon fiber tubes; several 3D printed parts, ODrive linear actuators, and a pair of OpenTorque actuators for locomotion.

Levine’s OpenTorque actuators are based on MIT’s Optimal Actuator design featured in the MIT Cheetah robot, which uses the largest possible motor that can fit on the robot, with the smallest possible gear reduction. These were designed using Herlea X8318S KV100 (Multistar 9235) brushless motors, THK RA8008 cross roller bearings, and a series of 3D-printed parts and assorted hardware (screws, pins, etc.).

The Blackbird’s controls are being developed using PyBullet, which includes model simulations with adjustable physics parameters, as well as reinforcement learning for implanting AI into robotics projects. The robot is still under development and is currently undergoing some hardware revisions to both the robot and the OpenTorque actuators. Levine explains, “Once the prototype Blackbird is built and walking (estimated February 2019)*, I’ll open source the design so anyone can build or modify it. I believe that hobbyists and researchers can hugely benefit from access to a low-cost, highly capable biped platform like the Blackbird.”

*He has since gone over this estimation.