RoboSavvy Forum

DARwin AX

2011-04-25T12:55:41+01:00

I've now updated my Darwin to the new 4096 firmware. The new firmware changes the working angle of RX-28M's from 300 degrees to 360 degrees so where the number space used to be 1024 steps over 300 degrees it is now 4096 steps over 360 degrees. The other big change in the RX-28M firmware is the change from CW/CCW slope to PID parameters. Thus far the Darwin code only has default P with no I or D terms used. This change seemed to impact a couple of my walking parameters a bit and required some re-tunning of the holonomic gait to get it back where I had it previously.
Overall after re-tuning I think the changes in performance are very subtle. I think the one thing I noticed being different is the range at which the gyro gains where useful. I'm not really sure why this is yet but it seems I can push the gyro gains even higher than before without getting oscillation. For instance with the new firmware / software I have been able to get my Darwin to walk quite well on soft carpet which required significant gyro gain given the surface variability. These settings although they worked well on soft carpet did not work that well on short carpet or a hard surface. Darwin would still walk on the hard surface but the settling time made the walk seem a bit odd as he over stabilized planting each footstep.

Statistics: Posted by Bullit — Mon Apr 25, 2011 12:55 pm

DARwin AX

2011-04-13T12:28:44+01:00

I'll be demonstrating my Darwin at RoboGames.
I have not yet implemented the update to 4096 for the RX-28M. I'll do that after RoboGames as it requires refactoring my pose based moves.

Statistics: Posted by Bullit — Wed Apr 13, 2011 12:28 pm

DARwin AX

2011-04-13T11:48:45+01:00

A couple of intersting updates are on sourceforge.

The actual actual firmware for the CM-730 is now there.

Also Robotis now made the step from 1024 to 4096 resolution on the RX28-M servos. Will be good to here from Bullit if this makes a noticable difference to perfomance.

Are Robotis or Bullit taking the DARwin to Robogames ?

Statistics: Posted by i-Bot — Wed Apr 13, 2011 11:48 am

DARwin AX

2011-03-24T03:09:09+01:00

Sounds like I am going to have to make do with my AX-18Fs, I am never going to be able to buy 20 RX-28Ms! They sound very cool but very expensive!

Statistics: Posted by Gort — Thu Mar 24, 2011 3:09 am

DARwin AX

2011-03-24T09:43:47+01:00

Thanks SJ for those clarifications.

It seems to me that the rx-28m holds its position more finely then the other Robotis servos. When I think about this, it makes sense given the precision of the potentiometer and the fact its read by a 10bit A2D on the ATMega8 in the servo. One can imagine there is no way that the Robotis potentiometer servos achieve true 0.29 degree precision. The rx-28m's on the other hand use a 12bit magnetic encoder and a digital SPI interface to the sensor therefore the output is over-sampled and we can imagine that it achieves the 0.29 degree precision.

The Robotis ex-106+'s have an interesting arrangement of potentiometer on the final output and a magnetic encoder on the motor. The magnetic encoder there is likely only quadrature of the motor and gives some finer resolution between steps from the potentiometer. This solution has some problems in that the motor position can be finely controlled but the output includes gearing backlash. I have found the ex-106+'s have the greatest backlash of the Robotis servos. Robotis also struggled quite a bit getting calibration of the ex's to work well as one can imagine given the arrangement of potentiometer and motor encoder.

If you look at the specs for the Murata potentiometers used by Robotis in the servos its easy to understand why they wear out and why they are noisy.

Statistics: Posted by Bullit — Wed Mar 23, 2011 6:40 pm

DARwin AX

2011-03-23T16:09:02+01:00

The main superiority of RX28M is that we do not have to worry about the potentiometer wear any more, which results in annoying servo whine and dreadful shaking after some while.

And we used to have some controller side code that keeps track of all joint angles of servo and returns them if requested (that's the sync read). The distributed code does not rely on controller side code and lacks that feature. However you can use any controller (cm720, usb2dynamixel, arduino..) instead I think.

Finally the hardness (or max torque) setting comments are about prototype OPs with AX-18 servos. For some reason we could not control the maximum torque of AX servos in the way we control RX and EX servos.

Statistics: Posted by SJ — Wed Mar 23, 2011 4:09 pm

DARwin AX

2011-03-23T12:08:28+01:00

Thanks bullit for the clafication on the two versions.

I started with the UPenn code and agree it is more complete and the best start point if Robocup is the objective. The code hacks I put in the article are not valid for the UPenn code.
To run the UPenn code I had a different approach where I emulated the CM-730 on a Cortex M3 (actually an NXP not STM because I prefer the USB bootloader). This used the on chip USB interface to talk with the main controller unlike the CM730 which uses an FTDI chip. I had to write linux USB device drivers and rewrite the low level c and lua from UPenn. This got DARwin communication manager working. I was looking at adapting the kinematics on UPenn when the Robotis code appeared. The Robotis code seemed to offer a quick start opportunity to get more people involved, so I jumped over to that.

The UPenn code seems to have reference to a SYNC READ command, but that did not appear to make it to the Robotis docs or code. Do you use the same firmware in the CM-730 for both UPenn and Robotis main controller software ?

The superiority of the RX28M is interesting. Since you have experience of a wide range of Robotis servos including some with mag encoders, do you think the improvement is due to mechanics, encoders, or the servo firmware ?

In the UPenn code, I see in the DarwinOPCommManager.lua code, the comment " We can control hardness with RX28" and in the DarwinLCCommManager.lua code the comment " Dynamixel torque limit..... making it useless for hardness control". Can bullit, JS, or anyone explain this and what servos it applies to ?

Statistics: Posted by i-Bot — Wed Mar 23, 2011 12:08 pm

DARwin AX

2011-03-22T18:52:04+01:00

There are actually two sources of code for the Darwin-OP:

The UPenn (mostly lua) source which was put up late last year and hasn't been updated since. It contains a full ZMP and IK walk generator along with ball following and game finite state machines. This code include mass, length, cm for all joints and robot. There is a bunch of clever stuff here but its not as clean as the Robotis implementation.

The Robotis source which is more current contains an IK walk generator which does not use mass, cm or ZMP. It does however use the geometric configuration of the robot. Line the UPenn code it supports Actions or pose playing which is joint angle based and does not use IK or balance. The Robotis code does have an interface to roboplus which is convenient to do pose based moves. This source does expose fully the Robotis pose motion engine and it interpolation methods. This should be a good source for anyone working with dynamixels for pose based motion.

I've been fortunate enough to have a Darwin-OP to play with for the last month or so. I've worked with both code bases. Both have their merits. I've managed to tune the Robotis gait generator quite nicely and its one of the most impressive walking gaits I've worked with. Like i-bot I am looking at porting some of this to my other platforms however I believe one of the reasons it works so well on the Darwin-OP is the rx-28m. The rx-28m's are clearly superior in their ability to hold a tight position without oscillation.

Both these sources have simple gyro balance with no kalman filtering or real PID (above the servos themselves) yet it seems to work quite well in the Darin-OP. I've tried similar implementations with other Robotis servos including the Bioloid only to find great sensitivity to oscillations hence previously I've resorted to kalman filtering and class C PID's.

Statistics: Posted by Bullit — Tue Mar 22, 2011 6:52 pm

DARwin AX

2011-03-22T16:57:59+01:00

Hi i-Bot

I've been reading through your articles and (as usual) it's a very clever approach to adapting it.

I was actually quite interested and surprised to see you managed to track down the "bridge" in software/hardware to the emulation of the CM-730.

I was now wondering, once the CM-730 emulation is complete, how deep are the algorithms calibrated for the DarwinOP weight, center of mass, sensor positioning, etc.

From the DarwinOP main website it still seems to maintain this Motion/Action division where a Motion is triggered by an Action but I trust the approach is not as simplistic as this.
There's certainly a lot more going on with all that processing power under the hood.

Looking forward for further developments

Regards

Statistics: Posted by PedroR — Tue Mar 22, 2011 4:57 pm

DARwin AX

2011-03-22T00:32:39+01:00

Hi Mohit,
It is not my code, I just made some changes to the Robotis code. At this time they only provide code for linux. You might adapt it to Windows if you are good with C++ and .NET. It simply uses the USB2Dynamixel as a serial port.

Statistics: Posted by i-Bot — Tue Mar 22, 2011 12:32 am

DARwin AX

2011-03-21T20:21:45+01:00

Hi I-bot,
Can I use your cm730.zip code on windows and control all 18 Ax12+ with Usb2Dynamixel ?
I have no cm730 board
Please Help
Thanks.

Statistics: Posted by MOHIT JINDAL — Mon Mar 21, 2011 8:21 pm

DARwin AX

2011-03-21T18:36:32+01:00

Excellent,

I'm looking for something just like this to leverage the darwin code into the OpenHRP simulator. It should be a fairly simple matter to feed the servo commands into the simulator as opposed to the USB2Dynamixel.

I'm also working on a java program to replace RoboPlus Motion. It will read, write, and edit motion files and will run under Linux. If I can connect this program to the darwin framework then maybe I can factor the Windows PC out of the configuration all together.

Keep up the good work!

Statistics: Posted by Fritzoid — Mon Mar 21, 2011 6:36 pm

DARwin AX

2011-03-21T16:48:13+01:00

I put up a draft of the second part. I don't use the CM5 or CM510, just the USB2Dynamixel directly.

The second part might be updated as I, or others try the DARwin software with Roboplus. Still I hope there is enough to get others started experimenting.

http://robosavvy.com/site/index.php?opt ... 7&Itemid=2

Statistics: Posted by i-Bot — Mon Mar 21, 2011 4:48 pm

DARwin AX

2011-03-21T12:02:12+01:00

I've been spending a lot of time with the darwin code myself and can see some daunting problems that need to be overcome before it can be used with a CM-5/510. Not the least of which are the transmission speed and command protocol.

So it is with great interest that I await your second article.

Statistics: Posted by Fritzoid — Mon Mar 21, 2011 12:02 pm

DARwin AX

2011-03-20T23:24:56+01:00

What are the differences between the CM-720 and the CM-730?

Statistics: Posted by Gort — Sun Mar 20, 2011 11:24 pm