So here is my first thread around here, and what I will talk about is my current tachikoma-inspired 4DOF quadrapod.
On this project, I work with my photographer/blacksmith/tachikoma-fan friend : http://landrycollet.fr/
Since the Ghost in the shell series release (6 years ago here), I have been a huge fan of the tachikoma. When Tomotaka Takahashi built a robot tachikoma for the release of the Ghost in the Shell: S.A.C. Solid State Society film, I was thrilled. But I had unrealistically high expectations, so I felt a little disappointed. Sure it is cute as hell, but the legs are 3DOF only (vertical-horizontal-vertical axes, not the typical vertical-horizontal-horizontal axes) and it did not walk as well as I hoped. Also, it doesn't have functional wheels.
Fast forward two years ago when I came across this robot : http://www-robot.mes.titech.ac.jp/ro...rwalker_e.html
For some reason, it triggered a kind of revelation : "it can be done". I spent the next year working on my own skating robot (maybe I will talk more about it in an other thread) with the pretext of entering it in the French qualifications of Eurobot.
In the meantime, I discussed at length with my blacksmith friend what would be needed to start building a tachikoma.
Lately, I came across Zenta's hexapods. Their smooth and lifelike motions stunned me. It was exactly the kind of smooth motion a tachikoma would need! The fact that the code was freely available encouraged me to start the project (thank's Xan, Zenta, Kurte and other contributors!).
We settled on doing this project progressively :
First, a prototype of one leg with small and inexpensive servos, then the first version of the body + 4 legs, but without any load (external power source, external brain ...), and then the "real" version, bigger, with better servos, and everything inside the body. A last step would be to add foot pressure sensors, IMU, camera, you name it... and to add some AI.
We may or may not go all the way with this project, depending on the success of previous milestones, time and available disposable income, but at least it will be fun to go as far as possible.
Here is the kinematic model of the leg:
H = hip
K = knee
F = foot
The axes of the first 3 joints (starting from the body) all cross in one point, making the system nearly equivalent to a spherical joint between the body and the femur, and a hinge joint between the femur and tibia.
Alan KM6VV worked on a similar but slightly different design but if you know of some other similar projects, I would be delighted to hear about it.
To build this, one of the cool things is that we can make a module with two servos with orthogonal axes and use two of these modules (+ other brackets) to make a leg. I considered printing some using a 3D printing service, but it ended up being too costly compared to the rest of the prototype, so I built it from 1mm aluminum sheets.
First leg Prototype
Here are some pictures of the very first leg prototype. The servos are the cheapest available, so if I damage some during the software development phase, it's no big deal. The "real" tachikoma will have better servos. The wooden tibia is just for show.
The second leg prototype has some slight modifications : the 3rd and 4th servo are permuted. This does not change anything in term of kinematics, it just shortens the femur. The second servo has been replaced by a stronger (but equally cheap) one.
The body the leg is mounted on is a tough (and heavy) test body. The wheel is fixed and currently just for style. The sort of decoration on the tibia was just to test the possibility of making a shell around the leg to make it look like the original tachikoma... but this is not going to happen anytime soon :/
I designed a lighter and cuter body to be used when the bulk of the testing is over. It is made of 2mm styrene sheets, with spacers cut from 5mm styrene. In the pictures, all the M2 screws are metal, but I switched most of them to nylon to save weight (went from 36g to 30g). it is not very rigid by itself, but when the legs are in place, it looks sufficient. It it proved to be too flexible, I will rebuild it with aluminum.
Detail of the rotation axis:
That's all for today!
Soon, some info on the (epic) quest to find an IK solution for this beast ^^