I received a few days ago the new motors. Now I use 12 Volts, 90 rpm motors. You can see yourself in the picture the big different between the old ones (the little thing in front) and the new ones.
The new motors allow a fast movement while having a very good behavior at low speed.
I purchase these new motors directly in China here.
You can see them in action here:
The next step will be to use full rotation servos. This will ease the construction and keep the bill of materials shorter, therefore the cost of materials will be reduced.
But this involves writing software to control them... hope to be finish this week end!