Monday, October 21, 2019

Mojo3 - A Compliant Quadruped Robot Dog (#2) - Printed 'arm'

Well, it isn't quite an 'Arm', is it?  But I am calling it that.  In previous 'mojos' have named my leg parts by the bones in the body of a quadruped.  This iteration, I am keeping it simple. With a 'leg' (hip?) and a arm. of course it has hip points and knee points.  but we also have the mechanical part of the 'trapezoid' - the middle parts that make this a compliant system.

Below is the first draft of a compliant leg system in OpenSCAD.  Have drafted out the basic parts to get a feel for the shape. Most important is to start to visualize how the parts will fit together and start to move.  In this draft the parts are small, with a focus still on the "cheap" 9g servos.

Mojo3 - Robotic Leg - First Draft (OpenSCAD)

I have been thinking more on adding some compliance to the 'arm', with an additional spring that will take additional load off the feet and try to ensure more contact with the ground.  hopefully the servo will have enough pull to off set this compliancy when it is time to lift the leg.

In this iteration, i have simple spring in a slider, at the end of the 'arm'.  In a sense, this will operate more like a ball point pen. The first draft print, was enough to prove that the concept works.  I will need to add more surface to the fore-arm to contain the spring, however the slider and screw lock are fit for purpose and work well.

Mojo3 - Robot Leg-forearm - first draft

Next I will have to go find a small bearing and start work on the hip part of the leg.


  1. Looks like a solid component. I have to admit, I don't understand all the points and joints in the mockup but it's interesting to see your trial and error refinements.

    1. Thanks, the process of sketch-cad-print-iterate really exposes the little points that need corrects. I would have to say that I am happy with the process. It adds more 'fun' into builds.

      I have the first draft of the leg printed, I will add an animated GIF of the motion. I am trying to imagine the positioning of the 'serial elastic actuator' (big term, haha). But, this is easier to do when you can hold something and move it about. more to come, and you can see the trial-error progression.