Installation requirements

You need to install the pr2_cockpit stack which contains the impedance controller from Willow Garage. I recommend to use the Debian packages:

sudo apt-get install ros-electric-pr2-cockpit

Get the articulation stack from Freiburg:

cd ~/ros
svn co https://alufr-ros-pkg.googlecode.com/svn/trunk alufr-ros-pkg
svn co https://alufr-ros-pkg.googlecode.com/svn/branches/articulation-experimental

Compile:

rosmake articulation_pr2

Running the demo

roslaunch /etc/ros/robot.launch
roslaunch pr2_teleop teleop_joystick.launch
roslaunch articulation_pr2 articulation_service.launch

Before you call the articulation service, make sure that the PR2 has firmly grasped the handle.

rosrun articulation_pr2 articulate_object_client

Parameters

All relevant parameters are in the articulation_service.launch file.

1. sigma_position (in meters), default: 0.03, good choices: 0.01 -- 0.05. Assumed noise in the translational components of the observed trajectories (in meters). When sigma_position is small, the robot will always select the most complex model (in our case, the rotational model). In contrast, when sigma_position is large, the simpler models will do better, so the prismatic model (or even the rigid model) become more likely.

2. sigma_orientation (in radians), default: 1.00, good choices: 0.1 -- 1.00. Similar to sigma_position, but for the orientations: This parameter defines the assumed noise in the rotational components of the observed trajectories (in radians). In our case, a large sigma_orientation is reasonable because we grasp thin handles, so the observation of the orientation of the handle during opening is not very accurate.

3. force_gain (scalar), default: 4, good choices: 1 -- 10. This parameter determines how the pulling direction scaled to the next set point (larger values mean that the set point will be placed further away; the actual force is still limited by the saturation values of the impedance controller, see below the rosparam section. Note that this value works in combination with the desired_velocity setting in the service call: <code>next_set_point = current_point + normalized_pulling_direction * desired_velocity * force_gain</code>

4. vel_saturation_trans, vel_saturation_rot (motor current?), default: 6.0, good choices: 1 -- 10. These parameters determine the saturation of the impedance controller (larger values mean that the PR2 will pull stronger; when the values are too small, the PR2 might not have enough force to pull heavier objects open, if too large, it might loose the grasp).

5. min_finger_distance, default: 0.00, good choices: 0.00 - 0.05. The PR2 checks the distance between its finger tips. If the distance drops below min_finger_distance, it will abort the operation.

Typical problems and solutions

• Motion stops too early, PR2 seems to be too weak. Solutions:
  • Increase force_gain.
  • Increase vel_saturation_trans, vel_saturation_rot
• PR2 pulls drawers along an arc instead of a line, the estimated model is rotational (in the console output), or the estimated model looks rotational in RVIZ.
  • increase the sigma_position and/or sigma_orientation.
• PR2 pulls doors along a line instead of an arc, the estimated model is prismatic (in the console output), or the estimated model is a line in RVIZ.
  • decrease the sigma_position and/or sigma_orientation.
• PR2 looses grip on the handle and does not seem to notice.
  • Set min_finger_distance to a suitable value, maybe 0.01 m.