Sunday, July 8, 2018

Mojo - A Compliant Quadruped Robot (#7) - SEA v5

Hi all, back to work on the walking robot after a short break!

Third time is a charm!  I left on break knowing that the servos on Mojo-bot were not going to be strong enough to pull the knee joint in.  That would leave me with only a few easy options.  
  • Use stronger servos - the 9g micro servos are cheap, but really limited on torque. My preference is to stay with the cheap servos for now.  The next grade up is a substantial cost increase.  The current set I have now are around 2 bucks a piece.
  • Mini-Mojo - revamp the whole design to be much smaller.  This option would yield a smaller bot, that is perhaps in the strength range of the servos.
  • Re-Design the Serial Elastic Actuators (SEA), Again!  - this will be the 5th Design version. But, if I went with Mini-Mojo a new design would be required again.
The SEA is by far the most complex component of the robot. I am envious of EPFL because it looks like their SEA is easy.  But, some internet searching turned up no commercial version. Since I am using re-purposed printer parts for my robot, I decided this might be the best option.  I have 4 lighter weight springs that would work better for the servos. And, I could improve the transitional forces on the SEA by imbedding a metal rod from the printers into the core of the linkage.

Off to the Drawing board!

I decided to revisit the design with a piston/plug that is inside of a forked chamber. Because I am using OpenSCAD with variables, I could keep my existing design doc, and just make modifications. Instead of using a "Fork" from my very first version, I would create a cylinder that could be glued to the connector.  This would require I put a "plug" at the end of the piston - also gluing together.  The rod would be embedded into one connector, allowed to freely ride inside the other connector.  This would provide good translational support. I just need to be careful with the friction of the plug inside the cylinder.

SEA v5 - First Attempt

The first designs looked like the above.  It took three iterations of printing/assembling to dial down all of the parameters. I needed to increase the cylinder wall size, that broke just extracting from the print bed. Adjusting the diameters, changing distances, adding additional expansion slots, and counter sinking the plug, etc.

Parts for the SEA v5

The 3rd variation on the 5th design, started to actually work. The tolerances were tight enough that it did not require any glue.  but I will add a drop before I start kinematic tests.

New SEA v5 on top, vs SEA v3
The new design is much easier to compress.  It has almost negligible translational (side to side) movement, which is very important in this application. The resistance due to friction seems to be manageable (we will see in testing).  Also, the new design has more range, going from 8mm of compression travel to 10.7mm.  This will work better to bend the knee.

I will have to comment, that the old design had a better 'esthetic'.  :)  the big spring looks impressive - but proved too much for the little servos!  Now to print the other three out, put them on the robot and get it twerking again! :)

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