Sunday, September 23, 2018

Mojo2 (#14) - New Design

If you are following along the design posts for Mojo - The Compliant Quadruped Walking Robot Dog - you have seen the development go through multiple iterations, the result of testing and trails.  Some trails have been very positive, such as better SEA design, others out right failures, such as the 'knobby knee' version.

Currently, the design calls for stronger servo motors to 'complete the step' in the walking gait. New servos call for stronger springs, which call for bigger SEA. This will also solve the issue of the weight of the robot consuming too much compression of the springs. The larger servos will also require a new Chassis/frame design.

the new chassis will be modular, which will allow for the width and length to be adjustable. Each 'leg component' will be a separate module that is bolted together with the other components. To make the robot wider a 'spacer' can be added between the components.

New Modular Leg Component Frame
SEA blue
The new Mojo will support larger springs. A new Serial Elastic Actuator (SEA) was created for the new larger springs. The SEA v5 version was reused, just scaled for the larger springs. In the picture, you can see the evolution from the "knobby knee" leg with the small SEA, to the new SEAs and Part of the new leg.

New SEA in Blue
Cable and Cam
on of the most frustrating parts of the original mojo build was the Cable & Cam system used to lift and pull the knee up. The first design used regular thread as the cable. The thread was connect to a 3D printed Cam on a servo. The thread ran over a printed 'post' on the hip, which was a decent design decision. Finally it was 'tied off' on a screw/or bolt connection at the base of the SEA (on the tarsal part of the leg). This build had constant problems with the thread coming off the Cam, and was difficult to have a fast connection on the leg.

Evolution of th Cam
The new design will use Fishing Line as the cable.  Fishing line is durable, smooth, and strong. The path will be the same, but will use "eye bolts" at the hip and leg. The option of a straw tube may be considered if the cable continues to come off of the connectors. teh ETH Zurich robot used a 'tensioner' system at the hip that has not been considered due to complexity at this time.
Mojo2 Leg Configured and ready for testing
Paws, feet, and other distal configurations
Mojo will need new feet. The new design will attempt to add some compliancy by using a expansion spring at the heel. Having a rubber sole for the foot will help in the gait. too soon for claws? :)

Film at 11...

No comments:

Post a Comment