Laughing Squid made this interesting post on a six-legged robot that can turn into a ball.
As you can see, the sphere sections are reinforced using some T-shaped aluminium brackets for supporting and holding the plastic (PC) sphere sections. This made a huge difference when it comes to rigidity. On MKI I struggled with the fact that the plastic sphere section was to flexible and I had to reinforce the edges with ribs. The problem was the ribs kept getting loose (the PC glue wouldn’t hold), so I had to do even more improvements. On MKII the ribs isn’t really needed, simply because of the aluminium brackets that is shaped to match the inner part of each sphere section perfectly. I did spend some hours in the workshop fine-tuning the bending tool to make the bracket fit.
The main challenge with MKI was that it wasn’t able to roll in a straight line of path, only in a curved direction. I believe the main reason was the constrained motion of the upper sphere sections. Having only one DOF (degree of freedom), or only one servo motor for each of the six upper sections made it very challenging. It might be possible to make a more complex linkage system that gave the upper sections a better motion when rolling. Not sure how though..
I therefore decided to use two servo motors on each of the six upper sections. Having 2 DOF would make it easier to control, so that the upper and lower sections move in a symmetric motion.