So, no updates for a while. For a very good reason.

       I have been fighting with the design process. Really slogging away, forcing myself to endure days of staring at a screen making measurements and generally trying to animate the mechanics in my head. I had to create a whole new method of driving the joints, using cables instead of large cogs. This gives me the ability to make high ratio differences whilst maintaining the ability to make the parts printable. ( There is a minimum practical size to 3D printed gear teeth.)   

    This is specifically required for the upper arm Yaw mechanism which needs to be small form factor but high power and relatively low speed. I have gone through about 50 iterations of this mechanism, each time having to wait hours whilst my parts print.

   Each time I change the design I also need to redesign how I am going to get position feedback, which is actually quite difficult to design in pre-emptively.

   The first three large iterations of this joint required angles of the motors that made the upper arm far too large and heavy when using printed cogs, which is why I begrudgingly turn to cables.    

     I'm going to post here the final result. The skeleton. No drive motors as yet, or cables, but all the mechanisms are proven after painstaking testing. The test yaw I have here is surprisingly strong and uses heavy duty fishing line to transmit movement. The best part is that it almost looks like it could sit inside a persons arm. It also moves within a very similar set of angles, stopping very close to where our own arm yaw would stop in each direction. I can't wait to get the motors hooked up!!!

I am going to now post a picture of what the first iteration of the upper arm yaw looked like next to the current version. You can't really see it from the angle but the first iteration at the bottom of the picture is 12cm wide and weighs a whole lot  more than the current version.

I guess a final note, I should point out that each one of these joints is designed to hold a significant amount of weight. No-where near as strong as a person, or robot arm driven by dynamixels. But for the cost of each joint, these arms should hold an impressive amount of weight. Hopefully allowing them to interact with the world, carrying bottles, bags etc. If I get the fingers right ( I am thinking of having two sturdy fingers and a thumb) then I can easily see it holding an object of around 2 kilograms. The benefit of having a whole body is that many more motors can be brought into lifting movements, if enough control can be programmed in. I'm still struggling with IK of the legs, so I can certainly see these kinds of complex movements being an issue for me to program.

     Why design the arms before the abdomen? Because it is crucial to know how they will work before I design the bit that they sit in. My designs evolve rather than me sitting there and having exact measurements and angles drawn down. I have a rough idea, but nothing exact until I have been through a few iterations.