2014-07-23

Body material and structure

I have been bending my brains on the matter of how to build the robot body. There are so many options for materials, structures and methods each with their own benefits and downsides. So after contemplating for a while I realized I needed to summarize what parameters are important for my robot before proceeding.

Toilet paper roll robot


I want the robot to be robust to the point that it can roam and tumble about in unknown terrain without wearing down or breaking. This includes the occasional fall and crash.

I want the robot to be energy conservative. This means indirectly that it should be relatively light, so it does not have to drag around a lot of unnecessary mass.

I want the robot to be very flexible in its degrees of freedom, acrobatic if you will, so its limbs may be very slender with long ranges of motion.

I want the robot to be precise in its motion and sturdy. A lot of this comes from the ability of its body to remain composed in any configuration, and not wobble or bend.

It would be beneficial if it is easy to work with the chosen materials, and that they are not prohibitively expensive to acquire, so that I can play with prototypes before deciding on a final design.

In the far end of the spectre, I want it to be hard to detect on radar, have a neutral heat signature and resist heat,  shrapnel and projectiles from conventional weaponry. OK, so I watched too much mech anime.

So what options exist?

carbon fibre profiles


We could use Aluminium, Stainless Steel, Carbon fiber, Glass fibers, Plastics, Nylon, Wood, ...

We could create a cage from joined profiles.

We could combine cage elements with wires and cloth to create intricate shapes with  "skin", as is common in glider aircraft design.

We could simply make an empty hull by welding or bonding plates.

We could cast solid parts.

The idea I ended up with that appealed the most to me is to build a cage from carbon fibre tubes, bonded with strong adhesive to custom machined aluminium end pieces that provide all the necessary contact points such as bearing hinges for the joints, ball joints for actuator connections and ground contact shoes.

The idea came when I was browsing alibaba.com (again), and I found a bunch of suppliers of carbon fibre profiles. Next I found a tutorial on how to work with carbon fibre as a material, which seemed pretty straight forward.

Carbon fibre is really strong and light when it is used properly, but using it properly can be a challenge. You cannot simply drill a hole and fit it with a bolt like you would do with a steel tube. The carbon fibre tubes require that the strain is applied evenly over a larger area, and to do this we need to bond the tubes to a ferrule of some sort. If this ferrule is made from machined aluminium that in turn is bolted to whatever we need then we are set.

perspective illustration of carbon fibre tetrahedron robot limb cages

When thinking in "cages", the design for limbs is self evident. We simply construct oblong tetrahedrons cage configurations for each limb. The knee joint is simply a double bearing hinge that connects two limbs along axis of the knee joint.

The thigh joint is more complex, it could be solved by creating a ball joint in a single point, or by introducing a third intermediary limb to separate the two axis of the motion into two separate hinges similar to the knee joint, that are situated perpendicular to each other

In the illustration above, the ground contact foot is marked A, the ball joints for actuator connection are marked B, the knee joint is marked C and the thigh joint is marked D (in this illustration it is simply a ball joint for simplicity's sake).


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