Xevelabs

Behold Xachikoma's new aluminum body!

Also to try and improve the quality of the pictures, thanks to my friend Landry I have a new big lightbox (still a WIP though)!

A lot more photos - and a more convenient viewer - in the gallery!

This week and the last, I received all the CNC milled and 3D printed parts for the legs!

CNC milled goodness

The main element of the tibia is carved into 20x20x1.5mm square tubes (many thanks to the guys at AIPL!). The smaller part is used to support the wheel axis, and fits inside the tube. Having the two ball bearing fit tightly inside the bloc proved to be a challenge as the first ones I bought were a little too cheap and deformed when inserted. Sourcing parts from a better supplier solved the problem, but I still lost a week on this ><.

Sweet 3D printed stuff

Then we have all the 3D printed stuff, made by the awesome guys at i.materialise. I designed five different parts, three of which are printed in ABS (ivory white) for rigidity and dimensional accuracy, and the other two in polyamide (white) for better details. It's interesting to see that in fact polyamide can be pretty rigid too when used with >4mm thick walls.

This "big" and convoluted part makes the junction between the Dynamixel servos and the custom made tibia. I noticed quite a few design faults (a missing bevel here, a hole too small there...), but nothing unsurmountable. I should probably have made it with bigger walls: it deforms a little too much to my taste when twisted along the longitudinal axis (which will happen in normal operation). Also it's a little too fragile around the hole where the ball bearing is inserted. I actually cracked one when trying to put the bearing the first time... luckily I could repair it by putting a droplet of acetone on the crack. After 5 minutes the two sides were neatly fused together again.

This one holds the servo used to orient the wheel (14g, 1.4kg.cm, analog, metal gears). Its axis is aligned with the contact point between wheel and ground, hence no need for tremendous torque.

I had a little surprise when I drilled the holes... some support material was left inside the part, and when the drill bit hits this material, it makes a breaking sound... Gave me quite a scare, I thought the drill bit or the part had just shattered ^^".

Last ABS printed one. It serves two purposes: providing a second attach point on the servo axis, and holding the motor in place at an adjustable position. While it does perfectly well at the first task, I'm afraid I kind of messed up concerning the second one.

The motor is clamped between the two flexible sides of the part. Two screws and nuts passing through the holes above and below the motor provide the clamping action. From the few test I have made so far, this system is able to block the motor in place, but it pretty quickly start to slip under the action of the tension of the belt. Also, signs of fatigue are visible on the part, as well as cracks around the holes... I think I will need to redesign it to be less adjustable (now I that I have the whole system at hand, I know the desired position of the motor with a very narrower margin) and more robust. I also think will not rely and the material flexibility this time, and/or I'll switch to polyamide.

Last but not least, the two pulleys. I received three of each in polyamide and one of each in ABS, and all of them are perfectly functional. I'll go into more detail about those in a future post.

Final words

I find the overall quality of the assembly extremely good, however I'm not used to playing with such precision tools, so maybe I'm easy to impress. Compared to the hand-made stuff I did until now, it's a big leap in term of quality and repeatability!

I'm pretty satisfied with the 3D printed parts (except the motor support). It's the first time I use this kind of service, and it opens a whole new world of possibilities, at an affordable prince.

Here are at random a few things I learned along the way (take it with a grain of salt): - if you want to have a screw or an axis going through a hole, make it the right diameter from the get go. - if you want to tap a hole, make it smaller than the recommended initial drill diameter, 0.2mm less if it's polyamide, 1mm less if it's ABS. - ABS is easy to drill and easy to tap, polyamide is relatively easy to drill but not that easy to tap. Use the 3rd tap a few times if you have important length of nylon screws to put in... - always check, check and check again your CAO before sending the parts for printing -_-"

Anyway, now I have to work on redesigning the problematic part, then assembling all of these, and do all the electronics and wiring... still a lot of work left.

At first, the Xachikoma project was completely separate from my participation to Eurobot. However, building two complex robots at the same time would have been way too time consuming, and nearly impossible to financially overcome. That's why, as I hinted in previous posts, I chose to merge the two projects into an Eurobot-playing version of the Xachikoma.

Of course, the two projects had different goals: one needed to be able to play by this year's Eurobot rules, whatever its means of achieving it, when the other had to look and act accurately like a Tachikoma... Not exactly a perfect match!

So the idea is this: build a robot which has the possibilities of moving like a Tachikoma (legs with enough degrees of freedom (DOF) and powered wheels), fitted with lots of sensors and equipments to be able to score some points and not be ridiculously unadapted for the competition, all of this without giving too much importance to the cosmetic aspect. This way, I can work on the mechanics, electronics and leg movements this year, and make an accurate-looking Tachikoma later. Also, I don't want to show up at the competition with a robot so unadapted it would have to rely on luck to be accepted past the approval phase. That would be kind of wasting the time of the referees and other participants.

The main structural differences between the original robot and the one I'll make this year are these: the Xachikoma won't have a pod, and probably no arms or eyes either :/ . The body, while still round, will be significantly different from the cute rounded blue shell we are accustomed to. On top of the body will probably be a moving structure destined to maintain the beacon support at exactly the right height and inclination whatever the body movements. On the bottom part of the body, the Neato Piccolo LDS (Laser Distance Sensor, aka the lidar I referred to in previous posts) will be mounted head down, but this still needs some consideration...

The chassis of the robot is heavily based on the Bioloid Premium Kit I talked about earlier. This is the core of the robot: 16 AX-12+ (at least 4 of which will be replaced by AX-18F), plus custom parts for the body itself and the tibia. With the AX-12+ and even more powerful AX-18F, the robot will be able to easily bear the payload (Beagleboard-xM, Neato LDS, PSEye camera, controllers, batteries, dynamically stabilized beacon support, other sensors...).

For size comparison, here is the chassis (I just began the 3D model of the body) next to a tower of playing elements. The perimeter when standing like that is a little below the 120cm mark, and it can easily be shrunk or widened by changing the leg's position.

The leg has two parts: the first one is fully built from the Bioloid Premium Kit elements, and goes from the body to the knee. It is composed of 4 Dynamixel servos, arranged to have kinematics identical to the Xachikoma prototype, and to the Tachikoma. Particularly, the axes of the first three servos are concurrent, and the axes of the last two as well.

The second part of the leg is custom made, and goes from the knee to the wheel. It's composed of (from top to bottom): a servo support (weird shaped white piece, 3D printed), a metal-geared digital micro-servo, a second servo support (second white part, 3D printed), the tibia cut out of a 20x20 aluminum tube, a motor with its bracket (yellow part), two pulleys(also 3D printed) and their timing belt (not shown), and a wheel. Add to this a force sensor to know when the leg is on the ground, a magnetic encoder for motor control, probably one or more contact or proximity sensors, and a Baby Orangutan to manage all of this.

The wheels are nice and have good traction. Only problem could be the color (green!) ^^. The rotation axis of the small servo passes through the contact point of the wheel with the ground, which has the advantage of not affecting too much the kinematics of the rest of the leg.

The servo supports are tricky parts: they must allow for +/- 90° rotation of the lower part of the leg, and yet be able to support all the weight of the robot, even with occasional shock. And on top of that, as it's the outermost part of the leg, it has to be as small as possible to avoid breaking the perimeter limit. It also mustn't get in the way of the tibia movement by reducing the range of the other servos of the leg. All these consideration led to this (arguably efficient) design.

They will be printed in ABS plastic by the cool guys at i.materialise. ABS is a strong and rigid material, with a is very dimensionally stable and accurate printing process (meaning the part has little risk of being deformed while printed). Using Finite Element Analysis, the profile of the part has been refined to remove as much as possible the weak points of the design. Here is what I got on one of these tests: As you can see, the stress is spread across the whole lower part, with two potential small weak spots (in red). The first iterations had just one big crimson area at the bottom of the part, so this is an improvement ^^. Have a look here for an interesting tutorial about FEA.

The pulleys are more of a bet. I wanted a way to drive the wheel without slipping, yet I did not have enough space to fit gears, or commercially available timing belt pulleys... so I will try to print them with white polyamide (called "white, strong and flexible" by Shapeways).

The belt will be a 225 5M HTD (225mm length, 5mm between two teeth, High Torque Drive), but with non conventional width. I will try to reduce it to around 3mm, instead of 9. This would allow for pulleys to be way smaller (7mm width instead of the commercially available 20mm). I redesigned the pulleys from what I could find about the 5M HTD specification (ie. not much), and took this chance to make them just the way I need to mount on my mechanism. For example, I added a small ring on the side of the pulley that will allow perfect centering of the multi-pole magnetic ring that goes with the magnetic encoder (the blue ring). The pulley on the wheel will be mounted with six nylon screws onto the wheel, while the one going on the motor will fit on its shaft with a M3 grub screw.

Quite a few details are still in the design phase (like the body... a big detail!) but this is all I can show right now :)

In the next few weeks I should finalize this design, have the plastic parts printed, the aluminum part CNCed and the other parts shipped or hand-made. To be continued !

Il reste... grosso modo 5 jours, et le robot n'est toujours pas homologuable.

Au chapitre des trucs qui font pas plaisir, la CMUcam a décidé qu'avoir 4 ports servos c'était trop, alors elle en a tué 2. Après arrachages de cheveux et tests, la source de l'avarie n'a pas été formellement identifiée mais le problème a été réglé en utilisant les deux restants.

Sinon, niveau choses qui marchent :

- La balise à mettre sur le support surplombant le conteneur a été réalisée avec une bouteille de Yop, une pile 9v, un régu ajustable et quelques leds blanches. Les couleurs sont faites en scotch d'électricien.

Éteinte:

Allumée :

Pouvoir l'allumer est important pour avoir des couleurs facilement reconnaissables quelles que soient les conditions d'éclairage. Je m'en fais pas trop pour les matchs officiels, où l'éclairage est extrêmement puissant, mais plus pour tout ce qui viens avant : tests et homologations.

La luminosité est réglable pour pouvoir avoir toujours des couleurs bien saturées, mais pas complètement cramées. Ça fera déjà un truc qui ne nécessitera pas de calibration avant les matchs...

L'algorithme de recherche de la balise est codé, et donne des résultats plutôt satisfaisants : il trouve sans problème la balise même a 4m de distance, est plutôt rapide (recherche dans l'image en multi-résolution) et ne risque pas trop de faire de faux-positif.

- la détection de l'adversaire avec les capteurs ultrasons est fonctionnelle, et envoi à la carte principale un angle à fuir ainsi que l'intensité de la répulsion. La balise à poser sur l'adversaire est faite avec un fond de bouteille... ca fait un petit peu cheap mais bon ^^'. Plus de détails après la coupe, ou en regardant le code sur le svn.

- la boussole fait bien son boulot de boussole et permet au robot d'avoir une idée d'où regarder pour trouver la balise.

Voila, maintenant il reste beaucoup de code a faire/finaliser/tester pour la camera, mais la majorité des algorithmes important sont faits, donc ce qu'il reste est plutôt orienté "faire marcher en conditions réelles ce qui marche deja sur des cas de test".

Après une longue attente que je pensais briser en annonçant que le dossier projet était validé, que l'équipe était enfin officiellement inscrite, etc. Hé bien non, on attends encore... donc pour patienter, voila une petite vidéo de la base roulante du robot !