A study of spiders

The decision of copying spiders in the physical design of the robot has spawned some extra focus to the project.

Here are some you-tube videos showing spiders moving in slow motion:

 

And here a study of the degrees of freedom found in the legs of a spider, summarized elegantly in the two shamelessly borrowed figures below:

Range of freedom in XZ plane (Z is up)

Range of freedom in XY plane (ground)

Cobalt blue tarantula

Lately I have pondered (in-between the bouts my extremely demanding and satisfying job) about how to model the physical layout of the robot.

I have proclaimed it to be a hexapod from the start. This idea seemed like the most logical at the time. 4 is too few and 8 is too many. Having an odd number of limbs leeds to more complex gaits. Also, keeping the number of movable parts per limb low would reduce the cost and complexity of each limb.

However now I am not so sure any more. I personally like spiders better than insects and spiders have 8 legs. Also, the number of moving parts per limb in a spider (and in insects too for that matter) is much higher than 3. Evolution is the best mechanism to achieve optimization there is, and it has been working for over 4 billion years to perfect the spiders we see today. It follows thusly that one would be unnecessarily stubborn/proud not to at least observe, analyse and learn from the way spiders are.

Anatomy of a spider, for study before deciding on the anatomy of a robot.

What's in a spider anyways?

So I decided  to go all in, and find the perfect spider species to use as a template for the design of the robot. I ended up with the cobalt blue tarantula as the best  candidate for the following reasons:

• It is strikingly beautiful both in color and proportions. I firmly believe that aesthetics play a bigger role in design than merely pleasing one's eye.
• It is not too specialized. Many spiders have specialized into niche markets that make them unfit as a model to a "general purpose" robot. It is a solitary hunter and so is my robot.
• It has an well balanced anatomy that would be relatively easy to replicate.
• It has a temper similar to that which I want to give the robot (shy but aggressive).

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).

Robot leg anatomy

Here is (mostly for my own reference) a simple glossary for the parts that go into a very simple robot leg, with illustration.

The leg is divided into 4 parts:

1. Hip (Coxa)
2. Thigh (Femur)
3. Shin (Tibia)
4. Foot (Tarsus)

I prefer simplicity so I will use the names in English, while others will prefer the names in Latin.

In my robot, there is no Foot, I just included that for completeness. Also this referece lacks any mention of joints or muscles (actuators). I simply lists the bones or "links".

Anatomy of robot legg showing Hip, Thigh, Shin, Foot (Coxa, Femur, Tibia, Tarsus).

I used wikipedia and this site for inspiration and reference: http://bugs.osu.edu/~bugdoc/Shetlar/462/462InsectMorphology/Morph03.htm