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Blackbird 2 CAD Sketching

A project log for Blackbird Bipedal Robot

A low-cost, high-performance bipedal walking robot

Nathan BoydNathan Boyd 09/27/2019 at 09:168 Comments

With the end of the year coming around, we thought it would be fitting to share some CAD outlines for a new blackbird model. While we haven't officially started working on Blackbird 2, we wanted to show some very very very rough outlines for the future designs being considered. Blackbird 1 was nice because we really put a focus on 3D printed parts, modularity, and commercial off the shelf (COTS) products. 

Blackbird 2 will be a far more customized solution, pursuing a more "Design for Manufacurability." It will be composed of easily accessible and manufacturable materials such as molded plastics, carbon fiber, and aluminum. We are also currently working on an entirely custom backdrivable pancake motor solution that will be used for extremely high torque and repeatability (which will outperform OpenTorque in almost every way for a small price increase). These new motors will be the bread and butter of the new design, allowing it to do highly dynamic maneuvers like jumping. Since control is the heart and soul for locomotion, the new design will focus more on lowering the leg inertia and moving the center of mass to a more controllable position.

The open hip design (above) is probably our leading design at the current moment due to it's flexibility and high degree of freedom. By keeping an open design, each leg has an insane ab/ad range and can yaw full 360 if desired. The design stacks all three of it's leg motors (hip, knee, and foot) on the hip of the robot and will rely on wiring/belts to transmit torque to the appropriate joints. This reduces the inertia of the leg and allows for better leg trajectory tracking. Some downsides of the design is the location of the center of mass, which will be higher from the ground because the legs are mounted underneath. Depending on the strength and weight of the legs, the simple block of support above the motors may not be enough to prevent the body from twisting/bending. 

A second closed hip design was inspired by the design of Atlas, by Boston Dynamics. It's essentially the same things as the open hip design, except with a fully enclosed body to help support the motors. Center of mass will be lower, since the battery could be mounted lower on the body, which is better for balance. The hips are mounted off-axis from the yaw joint, which gives the foot an arched trajectory while yawing. This, may be an advantage because it could contribute to the stride phase while walking and accelerate the body faster. Ab/ad is smaller compared to the open hip design, and can only reach to roughly +-60 degrees before making contact with the body. The thought was that we could also integrate small motors into the foot to handle its smaller loads. If the integrated motor design doesn't work out, the motor can get stacked at the hip and transferred to the foot in a similar fashion to the open hip design. 

The next major design steps is going to be refining the transmission system in the legs and conducting FEA analysis on the joints/body. Since there may be some material and linkage based non-linearities in the Free Body Diagrams, a fair bit of the analysis will come down to physical testing. 


Discussions

Josh Pieper wrote 10/03/2019 at 16:52 point

Do you have an estimated mass, torque, and speed for your pancake variant?  Roughly the same as the OpenTorque?  I'm getting close-ish myself to a more production ready version of my moteus actuator with 40Nm at ~430g that is only around 35mm thick.  I'd be happy to send some pre-production units your way if you wanted to experiment with them.  (I don't know how close you actually got to the 80Nm from OpenTorque or not, so what I have may not be enough).

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Gabrael Levine wrote 10/03/2019 at 19:15 point

We’re aiming for 80 Nm continuous and 160 Nm peak at 1700g, but this is of course subject to change as we refine the design. The continuous torque may end up being significantly higher, depending on how good we can get the heat dissipation to be.

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Nathan Boyd wrote 10/04/2019 at 02:46 point

Heat Dissipation, max torque, and how robust we really want to make it will be some of the design decisions we have to figure out. Currently our design is fully enclosed, which adds a fair bit of weight to the system. Beefing up extra mechanics associated with the motor to take higher loads also add weight. I'm not surprised by the weight of your actuator, since it's essentially an equivalent to the one designed by Ben Katz for Mini Cheetah. Ours is currently being optimized differently. I'd love to try out some of your motor pre-production models if possible and put them through some testing to see how they perform. 

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Josh Pieper wrote 10/04/2019 at 15:38 point

Cool, I'll keep you in the loop and keep following your developments!

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Hexastorm wrote 10/03/2019 at 09:43 point

The pan cake motors look a bit like an Archimedes drive I have seen from the TU Delft https://www.youtube.com/watch?v=5il6n6GIyQE . I am not sure about the business case for the legs, although they are technically amazing, but the actuator could be interesting.

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Larock1234 wrote 10/03/2019 at 09:28 point

Amazing to see the progress of your project! Will the new pancake motors be open-source as well?

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Nathan Boyd wrote 10/04/2019 at 02:56 point

The current plan is to not open source the motors (currently patent pending), especially since there will be some involved machining to produce one. We are currently thinking about releasing Blackbird 2 as a kit and its motors as individual products, but that depends on a lot of factors. I think we will only open source the motor if we are confident that selling it independently would not be mutually more beneficial. 

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Larock1234 wrote 10/04/2019 at 05:16 point

Thanks for your answer. Ok, totally understandable. I am glad that you are planning to sell them tough!

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