DARwin AX

Dedicated to the new Dynamic Anthropomorphic Robot with Intelligence - Open Platform By Robotis and related projects
20 postsPage 2 of 21, 2
20 postsPage 2 of 21, 2

Post by Bullit » Wed Mar 23, 2011 6:40 pm

Post by Bullit
Wed Mar 23, 2011 6:40 pm

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.
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.
Last edited by Bullit on Thu Mar 24, 2011 9:43 am, edited 1 time in total.
Bullit offline
Savvy Roboteer
Savvy Roboteer
User avatar
Posts: 291
Joined: Wed May 31, 2006 1:00 am
Location: Near robot

Post by Gort » Thu Mar 24, 2011 3:09 am

Post by Gort
Thu Mar 24, 2011 3:09 am

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!
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!
Gort offline
Savvy Roboteer
Savvy Roboteer
User avatar
Posts: 555
Joined: Wed May 31, 2006 1:00 am
Location: KC, MO, USA

Post by i-Bot » Wed Apr 13, 2011 11:48 am

Post by i-Bot
Wed Apr 13, 2011 11:48 am

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 ?
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 ?
i-Bot offline
Savvy Roboteer
Savvy Roboteer
User avatar
Posts: 1142
Joined: Wed May 17, 2006 1:00 am

Post by Bullit » Wed Apr 13, 2011 12:28 pm

Post by Bullit
Wed Apr 13, 2011 12:28 pm

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.
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.
Bullit offline
Savvy Roboteer
Savvy Roboteer
User avatar
Posts: 291
Joined: Wed May 31, 2006 1:00 am
Location: Near robot

Post by Bullit » Mon Apr 25, 2011 12:55 pm

Post by Bullit
Mon Apr 25, 2011 12:55 pm

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.
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.
Bullit offline
Savvy Roboteer
Savvy Roboteer
User avatar
Posts: 291
Joined: Wed May 31, 2006 1:00 am
Location: Near robot

Previous
Previous
20 postsPage 2 of 21, 2
20 postsPage 2 of 21, 2
cron