Monday, October 15, 2018

Mojo2 (#18) - First Successful Walk

Mojo can now walk!

Numerous testing and enhancements including new feet/"paws", stronger 'tibia', and gait positioning have led to the first successful walk.  See the video below.

Leading up to the successful walk, here are the issues that had to be resolved.

Back side of Mojo2 - right hind leg is translationally forced inward.
The new button - switches between 'walk' and 'stand' is very useful!

New tibia, stronger with larger hinge
one of the biggest issues was the amount of "translational" movement of the leg.  This means that the leg was able to be bent under the robot very easily by the forces put on it. It is caused by the looseness and flexibility in the leg and its joints.  tightening the screws holding the linkages together helps - but increasing the size of the hinges would help more. In addition, creating a tighter screw hole will decrease this flex.  In the process of doing this, it became evident that the 'tibia' linkages were highly flexible - they twisted quite easily. The fastest fix to this issue was to make the tibia thicker and reprint the linkages.

New Tibia - larger, much less torsional flexibility - same size 70mm
With all (8) new tibia installed, the leg structures are significantly more ridged. this prevent the bending inward as seen in the first part of the video.  you can easily spot the new tibia, since they are now red instead of back. 

New Paws with a more compliant capability
The Paws have been replaced in the latest video sequence. you may notice that the robot is resting on the new compliant, spring loaded, paws.  This should add to the compliancy of the robot as well as provide more traction to the pull of the leg.  However, it is evident in later tests, that the compliant paw requires the leg to lift more in order to clear the ground surface.  This is a problem yet to be addressed.

New Distal "paws" or feet, now compliant with effective springs.
Gait Sequence
The gait sequence is actively being adjusted. In the latest runs, the hind legs have been moved back by 10 degrees.  This new posture has improved the stability, and may have been directly responsible for the first successful steps.  However, there are many many variable in this dynamic problem. Tuning one or two often leads to another instability.

In the process of adjusting the tibias, the SEA on the rear hind leg broke. I printed out new one and used a different spring.  The k-constant value of the springs varies by spring. This creates another uncertainty in the tuning of the legs. Having a reliable means to determine the 'k' of the springs would help in creating a consistent behavior across each leg. An adjustable SEA would do wonders.

Broken connector at top of SEA

Getting it to repeat
To be honest, some minor adjustments after the successful test, have created a newly unstable robot. Tightening some of the joints can result in the robot starting to 'moon walk'. This illustraits how sensitive the robot currently is to small changes. More improvements into tuning and adjustment will be needed to start getting consistent results.  But that is what makes this fun.  Right? :)

Another failure: the eye screw suffered metal fatigue being screwed in to a small hole. This can not be removed, thus requiring a new part to be printed.

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