Sunday, June 10, 2018

Mojo - A Compliant Quadruped Robot (#4) - Back to the Spring

As it turns out a couple of tries at the "spring" linkage is not working out.  In both cases the chopstick I was using to hold the linkage together was at the same time too long and too short.  Long enough to hold the construction together, is too long for allowing compression. But, short enough for compression results in the piece falling apart. back to the drawing board!

My basic requirements:
  • be able to compress a 25mm spring down to 10mm
  • not capable of pulling apart
  • maintains it's x and y stability (keeps from bending)
  • low friction
  • easy to print
  • easy to construct (minimal glue and clamping)
  • easy to connect to the leg

Taking another close look at the EPFL leg, I see a fork on one side, and rod/piston holding the spring, and a coupling that allow the spring to compress and the piston to slide up.  Sketched out it should look something like this:
EPFL Version top, drawing of new Serial Elastic Actuator bottom

once I go through the CAD it the parts would look like this:

Parts drawn in OpenSCAD

the fork is printed flat, the Piston/post vertical, and the end cap vertical.  As I was designing it, I realized that I need to add the spring to the construction, so the post would have to be separate from the end cap and glued on, once the spring is added.  The ring at the bottom of the post is not physically touching the post - I hope it prints!

I will need to glue the post to the end cap, after the spring is added then glue the fork to the ring.  that is two (three) really big points of failure under stress, so there may be a few more iterations to reduce this risk.

My attempts to build this pointed out a few more flaws.  Conceptually, it should all work together, however the implementation is proving to be problematic - too many glue points where the glue will be the only thing keeping the pieces together.  The resulting engineering is undesirable. It will not be very reliable and will have premature failure.  Back to the drawing board!

This new design has only two parts and no glue!  The design is based on the shock absorbers on my "Hello World" Rover. the two pieces slide together and 'click' into place.  The spring will site between the two.  It should provide the stability and motion needed, although friction will be a problem (especially true since the printing process leave horizontal ridges).  For all the work put into this part this weekend, I think it will do.  :)

After a few iterations on the printer and tweaking the design a bit, I have a working linkage with 7.5mm of compression.  Now, I have to think if my servos can manage this.  Good engineering would have worked out all the forces before hand. But, for this hobby round, I am just tinkering.  The springs are from recycled printers, I have a few to choose from. On the enhanced round I might consider different characteristics such as the amount of travel vs the force.  But, this will do for now - I have 3 more for a complete set.

Learnings this round: a lot of 3D Printing and 3D Part design thinking. Difficulties in getting a high importance but complex part to be built.

next up ...  servos!

No comments:

Post a Comment