Sunday, October 27, 2019

Mojo3 - A Compliant Quadruped Robot Dog (#3) - Printing/Iterating Design

The first draft of the leg is starting to take shape. Each of the initial parts have been printed.  I have assembled them on paper, CAD (OpenSCAD), and now in 3D printed PLA.  It is interesting to see how well they align to the initial paper sketch.

Mojo3 - printed leg covering original sketch

Side by side, the first printed parts are next to the original conception. The upper right piece is the 'leg'.  The leg has a bearing in the top to transfer the weight of the robot. there is a 'control arm' that will connect to the servo. At the base are two connectors, that allow the 'arm' to move up and down. I call the connectors 'trap' - short for trapezoid, the shape created from the four fastened points. Finally, the 'arm' is made of the base, connected to the trap, and the fore-arm, which will touch the ground.  The fore-arm can slide on arm-base. There is a spring in the fore-arm that will absorb some of the motion of the leg and ensure good ground contact.

Mojo3 - Leg first draft print, next to original sketch.
What is missing?  the foot and claw.  Every Totally Not Evil Robot needs to have a claw, right?  ;)

Thinking about the next step of the development, there will need to be a 'Serial Elastic Actuator' (SEA).  This SEA is basically a spring, that provides a 'compliance' with the ground, but allow for the arm to be pulled up, allowing for the leg to lift and be moved forward.  (Here is a link to my previous designs SEA v5.) 

From the design of the fore-arm, above, it occurred to me that there may be a simpler design option for the SEA.  In essence, the fore-arm is a SEA.  The next phase of design will have a new SEA that is attached from the arm-base to the hip of the leg.  below you can see this new component in red on the CAD design. Let us see how this works.

Mojo3 - potential new style of SEA
Next steps are to print and test it out...

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.

Monday, October 7, 2019

Mojo3 - A Compliant Quadruped Robot Dog (#1) - Back to Work!

Time to get back to work!  I have moved my home and office and started a new business.  time now to stop playing 'Factorio' in my spare time, and time to start building new robots!!  Afterall, the Totally Not Evil Robot Army isn't going to conquer the galaxy by themselves (yet)!

Over the summer there have been a few developments worth mentioning (look them up!).  Such as Boston Dynamics starting to offer their Spot robot for commercial purchase.  Not as favorable, some robot shops in Bristol, England have gone out of business. But, on the upside, the good people at ETH Zurich Autonomous Systems Lab are still producing awesome robot projects.  The community is continuing with things like this past weekend's Toulouse Robot Race. Plus many new videos on YouTube:  Oracid1 has a good quadruped playlist, and even James Bruton found the benefits of compliance in quadrupeds! ;)

To get back into the design phase, I am planning on starting mark 3 of Mojo, the compliant quadruped robot dog. Hereby to be called Mojo3 (so original!)  In the next design, Mojo3 will improve off the systemic issues of Mojo2.  Primarily, I will remove the servo from the direct load bearing connection between the leg and the body. With basic common engineering sense, I will use a bearing for this connection, and utilize a cam system to move the leg. 

Next I will incorporate the knee flexor servo into the primary leg servo, attempting to remove a servo (x4) from the design. This should again use a cam system to translate the leg motion into the up-down motion of the knee flex.  I will reduce the weight of the robot, Mojo2 was 263g (seems light) without the battery. This is due to the heavy servos. I will attempt to use the 9g servos again, lets see. compact and agile will be the plans, along with off the shelf components.

Here is the first drawing of Mojo3!  Can you see the various components??

Mojo 3 - Compliant Leg sketch-draft
Next we have to spin up the CAD design!