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# Velocity control of PR2 arms and head

Hi all,

I'm trying to implement a visual servoing on PR2. I want to send joint velocities at some rate to the PR2 arms and the head.

After reading this tutorial Moving the arm using the joint trajectory action I computed an complete trajectory for joints of arms with around 800 waypoints (and provide velocity) with a period of 100 Hz, and send them to the robot in Gazebo simulation, it works almost perfectly [see bad video] ( https://www.youtube.com/watch?v=z8Ltc... ).

But when I compute periodically joints velocities and send them to the robot using the joint trajectory action with 1,2 or 3 way points, it seems to me that the arms follows the complete trajectory and stop at the last waypoint, whatever the given velocity.(and the visual servoing task is completed after 8 minutes instead of 13 seconds).

And also the head fall down to the front, while the next way point is computed to raise the head. I got all these results in Gazebo, with ROS Groovy on Ubuntu 12.04. ( I cannot upgrade the ROS neither the OS).

Is someone can point me the mistakes I possibly (probably) made, and suggest me a better way to implement the control, it would be really helpful.

I can provide a more recent video if need, and please excuse the mistakes made by a french speaker.

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I solved one part of the problem.

The main part is the periodicity of the control loop. I had to be sure that the control was sent to the robot in a suitable rate.

The computation time of the block for compute and send the command was evolving between 0.13 to 0.62 secs, but the time length of sent trajectory was really smaller around 0.05 secs

I optimized my code, mainly the most expensive functions, remove some weirds waiting calls. and the computation time drop down under 0.002 sec in ROS time on my computer. Then I could send easily control at 40 Hz.

But there still one problem, the head always fall down.

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The tilt controller in the PR2 head cannot raise the head once it's pointed straight down. You should set a limit in your algorithm so that you don't send head tilt positions below a certain threshold. (I don't remember the exact threshold; you'll have to test and find out).

( 2015-05-26 15:32:14 -0600 )edit

I will check that, but its the same for the controller on the robot and in gazebo simulator ? I had the problems on simulation, I did not checked yet in the real robot.

( 2015-05-27 08:33:25 -0600 )edit

I checked on the real robot, the head falls down. I will look for the threshold. But I'm surprised to not be able to find other peoples with my problem.

( 2015-06-16 09:50:59 -0600 )edit