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| Photo Credit: Bayley Wang |
This project has been a continuation of the Hobbyking Cheetah project, but for research rather than out of my pocket. Here's my thesis on the actuator and robot design. I designed and built all the all the hardware, both mechanical and electronics (with some design and fabrication help from Alex), and Jared wrote pretty much all the software and high-level control running the robot, including the Convex Model Predictive Control for locomotion.
The design principles behind the robot are very similar to Cheetah 3: High torque electric density motors, low gear ratio, efficient transmissions to minimize friction and reflected inertia, lightweight legs with motors placed to minimize leg inertia. A few things are different, though. Mini Cheetah has 12 identical modular actuators with built-in motor control, gearbox, and support structure. The electric motors are off-the-shelf and very cheap, unlike Cheetah 3's custom designed motors. Mini Cheetah has even more range of motion than Cheetah 3 at the hip, so it's able to point it's legs completely sideways. This means that it should be possible to make the robot land feet-first regardless of what orientation it falls in.
I finished the hardware for the robot last May, but the publication cycle is kind of slow, so I wasn't able to put up any info about it before. There's going to be a paper about the robot in ICRA 2019 in Montreal, so the robot and I will both be there.
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Jared and I (mostly Jared) did the backflips for our final project for Underactuated Robotics last spring. There are some more details about how it works in the thesis, but basically we did a nonlinear trajectory optimization offline, and just played back the resulting joint torques on the motors, with a little bit of joint position control to the optimized trajectory on top. Our simulation of the dynamics was accurate enough that it worked on the first try in the hardware.
