Mojo3 - A Compliant Quadruped Robot Dog (#4) - SEA thoughts and more leg design

The Serial Elastic Actuator (SEA) is the key to this type of robotic quadruped design. This component acts as a 'compliant' spring to absorb and re-energize the through the motion of the gait. In this application, a servo will pull (compress) the spring while lifting the leg to move the leg forward. And release the spring on the hindward motion of the leg.

Here is the current-draft printed:

Mojo3 Leg first printed draft

The design of the SEA must be as frictionless as possible. It must smoothly compress and expand. Finally, the servo must be strong enough to easily compress the spring/actuator. It is my goal with Mojo3, to be able to use common springs (found in ball point pens) and cheap 9gram, 5V servos, like MG-90.

After the first printed draft I have been having some design thoughts and challenges. I like the simple nature of this version of the SEA as compared to Mojo2. This is much easier to print and assemble.  However, I will need to iterated a few more times to reduce the friction and avoid the "bend" (non-alignment) of the parts of the actuator.

Moving the servo:  I wanted to avoid having 2 servo motors to control the leg.  This can be achieved by making a 'mechanism', with a set of gears and linkages, to have a single servo move two different components. While I believe this is possible, I am starting to see benefits of a two servo solution.  1) reduce the overall friction of the system, simpler, and less likely to become fouled. and 2) the SEA servo is going to need as much strength as possible for its duty cycle.  Given these thoughts, I am going to focus on a SEA servo design with the servo 'in' the leg and not on the chassis, as was the design in Mojo2.

Here is a current picture of the parts going into the leg assembly:

you may be able to see the initial leg chassis that I have drafted out to hold the leg bearing and connection to the primary drive servo.  There are a collection of initial drafts and test prints that are not connected to the leg/chassis.

Current print - leg and assembly (can you see the two servos?):

Mojo3 - Compliant Quadruped Robot 'dog' leg - 3Nov2019

next steps will be to make a more awesome SEA.  ;)

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...

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.

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!

Sabbatical Almost Over

Hi all, 

I will be shortly returning from a break.  It has been a busy summer with travel, a new company, a move to the city, and more.

I am setting my Totally Not Evil Robot Army Labs, it will be returning shortly. 

I have also noticed that my blog has been 'picked up' by Ukrainian Search Engine Optimization groups (nefarious or not?). Most of the blogs are coming from random and not so random sites mostly in the Ukraine and Russia.  Welcome I guess?

In the meanwhile, if you are in Europe in October, the next annual Toulouse Robot Race being held.  I do not believe I will make it this year! :(

bis bald!