Friday, 4 July 2014

Quadruped

posted 2 Apr 2014 03:46 by David Taylor   [ updated 2 Apr 2014 03:47 ]

Now that my printer is up and running I've started experimenting with a couple of things and also looked at improving some aspects of it. More on that in another blog post coming soon.
 
I just wanted to make a quick mention of a design I have recently uploaded to Thingiverse.
 
At my local hacker space (House4Hack) one of the guys brought along a little quadruped that he printed from thingiverse and showed off his kinematics. I was quite impressed. However, given that it only had 2 degrees of freedom per leg, the kinematics weren't that great.
 
Various quadrupeds on YouTube look way better, but all of them had 3 degrees of freedom per leg. Each of them however had quite chunky legs because the servo's were located in the leg. The 3rd degree of freedom also made the legs quote a bit longer, creating even more torque for the servo's to manage. This got me thinking:
  • This makes the servo the pivot point. Is it a good idea to load weight onto an off-centre pivot?
  • Can the legs be made thinner or smaller?
  • Can the legs be cable driven like muscles are attached to human bones?
This was enough to give me an excuse to fire up my favourite 3D modelling app and out popped my version of a leg which I posted onto Thingiverse.
 
Below is an image showing 4 legs configured into a quadruped, but I gues nothing stopps you from building a hexapod or more...
 
The design places all servos inside the body. The hip is also made to be modular so that you can use servos you like. I've also not included the body skeleton as this would depend on your servos and design.
Gineer's Quadroped
 
This is my design for a quadruped. Most 3 Degrees of Freedom (DoF) legs also have 3 parts to the legs. I wanted to see if it was possible to get 2 DoF from one joint.
Design Decisions:
  1.  I then decided to go with a cable drive and keep the servo's inside the body, rather than make them part of the leg as most others have.
  2. Make it modular so that you can attach what ever size servo's you want, to it. I have not included the central square into the downloadable files as this would depend on the size of servos you choose to use as well as what and where you would like to mount any electronics and/or sensors or cameras you want to include.
  3. The two part leg should print pre-assembled. There is 0.2mm gap so even a normal self build 3D printer should be able to print this. I sliced this with KISSlicer64 but be sure to enable "Avoid Crossing Perimeters" in your slicing program. I found that Slic3r does not slice this thing right and screws up the design. Let me know if you have better luck.
The first part of the leg is just over 120mm long and the second part is about half that, so it should just fit onto a 200mm x 200mm heat bed with a couple of mm to spare or you could rotate it 45 degrees if it's to close.
 
Download it, print it, and let me know...
 
I would love to hear feedback if anyone finds any bugs or if you have any bright ideas.

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