Sunday, November 3, 2019

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.  ;)

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