Getting Electric to work with F/T sensors

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Update to pr2_mechanism 1.6.1 for ROS Fuerte

NOTE: version 1.6.1 of pr2_mechanism has the changes needed. Check CMakesLists.txt:

• roscd pr2_mechanism
  cat CMakeLists.txt | grep rosbuild

You should see:

• rosbuild_make_distribution(1.6.1)

If using pr2_mechanism 1.6.0 or earlier, update packages.

• sudo apt-get update
  sudo apt-get dist-upgrade

Patch URDF

F/T adapters add extra length and angular offset to gripper connection. The extra length can be accounted for by adding an extra link in RDF. However, the anglar displacment causes problems for PR2 inverse kinematics, so an offset of 85 degrees is used with the wrist transmission to cancel it out.

A new linke is also added to the URDF for the Force/Torque sensor. The new frame does not use the axis convention used by the PR2 (X axis forward), but instead used the vendor specified reference frame. In this reference frame, the Z-axis points backwards towards wrist, and X and Y axis are aligned to markings ("+X,+Y) on side of F/T sensor. The data on the force torque topic is aligned to force torque reference frame.

Make a copy of URDF and patch it to add a force/torque link for and angular offset for the wrist transmission. NOTE: At this time, robot_uncalibrated_1.8.2.xml is the newest URDF availble, and may change for a future release.

• cd /etc/ros/fuerte/urdf
  cp robot_uncalibrated_1.8.2.xml robot_uncalibrated_1.8.2_ft.xml

The patch for adding a left F/T sensor can be downloaded here: robot_urdf_left_ft.patch.

The patch for adding a right F/T sensor can be downloaded here: robot_urdf_right_ft.patch.

Patch the URDF with left F/T sensor:

• patch -p1 robot_uncalibrated_1.8.2_ft.xml < robot_urdf_left_ft.patch

Change robot.xml to point to patched URDF:

• rm robot.xml
  ln -s robot_uncalibrated_1.8.2_ft.xml robot.xml

Change robot.launch to enbable F/T sensor and load calibration parameters

Each Force/Torque sensor is calibrated at the factory to create a parameter matrix that maps between sensor readings and force/torque values. ROS parameters must be loaded before pr2_etherCAT is run, so the parameters are available to the driver at start-up.

We've created repository with calibration parameters for all F/T sensors. Checkout this repository with svn:

• svn co https://code.ros.org/svn/wg-ros-pkg/pr2_mods/pr2_force_torque_calibration

The calibration parameters are different for each Force/Torque sensor. The parameter file is named after force/torque serial number. For force/torque sensor with serial number FT11214 the parameter file with be named ft11214_params.yaml. Also, there is common set of parameters used for the F/T amplifier board that is also needed wg035_revF_params.yaml.

Create directory for storing force/torque calibration parameters

• mkdir /etc/ros/sensors/ft 

Copy parameters files in /etc/ros/sensors/ft directory. Copy wg035_revF_params.yaml, and ftXXXXX_params.yaml where XXXXX is serial number of force/torque sensor:

• cp wg035_revF_params.yaml /etc/ros/sensors/ft
  cp ftXXXXX_params.yaml /etc/ros/sensors/ft

Change /etc/ros/fuerte/robot.launch to add something like following. Change ftXXXXX_params.yaml to correct filename for F/T sensor parameters. The following

•     <!-- Left Force/Torque -->
      <group ns="/realtime_loop/l_gripper_motor/">
       <param name="enable_ft_sensor" value="True" type="bool"/>    
       <param name="enable_pressure_sensor" value="True" type="bool"/>
       <rosparam command="load" file="/etc/ros/sensors/ft/ftXXXXX_params.yaml"/>
       <rosparam command="load" file="/etc/ros/sensors/ft/wg035_revF_params.yaml"/>
     </group>

     <!-- Right Force/Torque -->
     <group ns="/realtime_loop/r_gripper_motor/">
       <param name="enable_ft_sensor" value="True" type="bool"/>    
       <param name="enable_pressure_sensor" value="True" type="bool"/>
       <rosparam command="load" file="/etc/ros/sensors/ft/ftXXXXX_params.yaml"/>
       <rosparam command="load" file="/etc/ros/sensors/ft/wg035_revF_params.yaml"/>
     </group>

This is what robot.launch might look like after being modified:

• <launch>
  
      <!-- Robot Description --> <param name="robot_description" textfile="/etc/ros/fuerte/urdf/robot.xml" />
  
      <!-- Robot Analyzer --> <rosparam command="load" file="$(find pr2_bringup)/config/pr2_analyzers.yaml" ns="diag_agg" />
  
      <!-- Robot bringup --> <include file="$(find pr2_bringup)/pr2.launch" />
  
      <!-- Web ui --> <include file="$(find webui)/webui.launch" />
  
      <!-- Left Force/Torque -->
      <group ns="/realtime_loop/l_gripper_motor/">
        <param name="enable_ft_sensor" value="True" type="bool"/>    
        <param name="enable_pressure_sensor" value="True" type="bool"/>
        <rosparam command="load" file="/etc/ros/sensors/ft/ft12345_params.yaml"/>
        <rosparam command="load" file="/etc/ros/sensors/ft/wg035_revF_params.yaml"/>
      </group>
  
      <rosparam file="/etc/ros/robot.yaml"/>
  
  </launch> 

Notice the parameter file uses a flag that will enable and disable the F/T sensor functionality:

• <param name="enable_ft_sensor" value="True" type="bool"/> 

If not specified this flag will default to False and the F/T sensor will not be used.

The parameter file also has a parameter to enable and disable the pressure sensors:

• <param name="enable_pressure_sensor" value="False" type="bool"/>

If not specified this flag will default to True.

If the pressure sensors are not installed (or needed) they should be disabled with the ROS parameter. The pressure sensors are slow and share the SPI bus with F/T amplifier. When the pressure sensor data is read (every 40ms), it will cause a small glitch in the sampling rate for the F/T sensor.