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Thursday, May 15 2014

Video from Robogames 2013

The guys at Trossen Robotics released some more highlight videos about people who attended Robogames 2013, inlcuding this one (I'm always horrified when I see or hear myself)...

Enjoy :)

Xachikoma at Robogames 2013

Monday, April 14 2014


I recently posted about the new XL-320 servos by Robotis. I got 4 of these, and thought I should do a quick demo with them before the long and grueling groundwork that will be needed to integrate them in my new robot which will mix AX and XL servos.

So here is GlitchBot, a little walker robot. It uses 3 XL-320 servos, the OpenCM9.04 and OLLO parts exclusively.



GlitchBot walking

I'll have to make steps smoother... here it goes forward thanks to the silicon foam it's walking on, but on my desk it just shuffles in place :p

The design is based on Tictac6 / Twitch / TwitchMX robots. It's basically a 3-servos hexapod that is in fact more of a biped, depending on how you look.

The principle is simple enough: one servo to rotate each leg, and one to swap which trio of feet is on the ground, with a deformable parallelogram configuration to keep the legs parallel to each other.
Each leg is made primarily with one of the batteries, put as close as possible. Flex in the material is taken into account to keep the bot level when it is standing on one trio of leg.


One leg up... there's enough clearance, it's all good!

There is a little glitch sometimes where a command is not executed (which results in this video in the turn, near the end)... I guess the bot is aptly named ^^' I'll have to investigate.

Here is the sketch for OpenCM9.0: GlitchBot.ino

More technical discussions in this thread on Trossen Robotics Forum.

Dynamixel XL-320

I received recently some Dynamixel XL-320 servos for beta-tesing. These are the new mini servos of Robotis' Dynamixel family. They are intended to be used for small robots that do not need a lot of torque, like the cute Darwin-Mini. Yet, what I would really love to do with them is to put them on a bigger robot, with AX and MX servos altogether. I envision a bot where joints could have the proper size of actuator, all the way from the MX-106 for knees and load baring joints to the XL-320 for fingers, sensors, or cosmetic movements (let's make the ears of Darwin-OP wiggle!).

Interfacing with AX / MX servos

Yet, there are 4 major differences that makes these small actuators not so simple to use with their bigger brothers:

  • they are intended to run at 7.4V instead if 11.1V, meaning you can't just put them on the same power supply rail,
  • they have smaller, 2mm pitch Micro-Latch Molex connectors that are incompatible with the Molex Spox 2.50mm of the AX and MX,
  • they use the newer Dynamixel Protocol v2.0, same as the much much bigger Dynamixel Pro. (This was not the case for some earlier beta testers, but since the support website page has been changed, I believe it's going to stay like that from now on).
  • they do not have regular mounting holes for M2 screws, they are intended to fit with OLLO plastic parts and their plastic rivets, meaning they have 4mm holes every 6mm (where AX have 2mm holes every 8mm, and MX have a variety of (non-compatible >_<) hole patterns).

When it comes to the first and second problems, a solution can be found with an adapter board. Aaron Perkins did just that, and I intend to make one too at some point - a smaller and less expensive one though.

The third one is not that bad, since commands in both protocols can be sent in turn on the same line, and there is no risk of misunderstanding. Only difficulty is that the master has to be able to speak fluently in both protocols.

When it comes to interfacing the servos mechanically with other stuff, there are multiple ways to go at it. The most obvious would be to use OLLO parts and rivets, just as intended, but this is not compatible with AX/MX servos, and there is a lot of give in every assembly. Rivets are great for small fun robots or prototyping but n match for screws when you want the thing to stay like that and not wobble around.
In the Bioloid Premium kit, they use a few OLLO-compatible parts, for example in the head to fit the ZIG-110A radio module. This is achieved with what look like M2.5 thermoplastic screws. They have a head with a diameter of 4.3mm, which is enough not to go through the 4mm holes of the OLLO parts.
M2 screws have flat heads around 3.6mm of diameter, but with the help of a small M2 washer (diam 5mm), they can be used to bolt the OLLO parts to regular AX parts. 5mm happens to be exactly the size of the small recess in the parts, in which the shoulder of the rivet usually fits.


Still, the difference in hole pitch (one every 8mm on AX servos, one every 6mm on the XL-320) will necessitate more work to use them together. The smallest simple pattern to fit OLLO parts on AX parts would be to put 5 OLLO holes (4x6mm = 24mm between the two further apart) aligned with 4 AX holes (3x8mm = 24mm), but this arrangement is not very practical. Also, 3 Ollo holes align well enough with 2 AX holes in diagonal to attach (like in the picture), yet I can't see it as a real solution for most cases...


I guess I'll work on some 3D printed mechanical adapter parts.

Other thoughts

The gearbox is 100% plastic, even the motor gear. However, there is a nice feature on the output gear: a torque limiter (the sort of double lobe thingy inside the biggest gear in the photo). This should make breaking the gears less likely in case of shock (like having a robot fall from a table).


The build quality is really good, the architecture is no-fuss, with one PCB on which the motor, STM8 microcontroller and potentiometer are soldered directly.
The sensor is an old-school potentiometer, like the good-old AX servos. There is enough room to put a magnetic sensor like in the MX servos, but the price would have certainly been much higher - such sensors cost maybe $4 more than the potentiometer, and I'm not even counting the magnet and increase of assembly and testing cost.

Some more references

Here are the references of the connectors for the XL-320 (that can be found also here).

  • Male header, 3 ways: 53253-0370 (vertical), 53254-0370 (right angle)
  • Female housing, 3 ways : 51065-0300
  • Contact (24-30AWG): 50212-8000 (reel), 50212-8100 (bag)

More details on the XL-320 page on Robotis support website