IKFast Kinematics Solver¶

Parameters¶

• MYROBOT_NAME - name of robot as in your URDF
• PLANNING_GROUP - name of the planning group you would like to use this solver for, as referenced in your SRDF and kinematics.yaml
• MOVEIT_IK_PLUGIN_PKG - name of the new package you just created
• IKFAST_OUTPUT_PATH - file path to the location of your generated IKFast output.cpp file

To make using this tutorial copy/paste friendly, set a MYROBOT_NAME environment variable with the name of your robot:

export MYROBOT_NAME="panda_arm"

First you will need robot description file that is in Collada or OpenRAVE robot format.

If your robot is not in this format we recommend you create a ROS URDF file.

If your robot is in xacro format you can convert it to urdf using the following command:

rosrun xacro xacro --inorder -o "$MYROBOT_NAME".urdf "$MYROBOT_NAME".urdf.xacro

Once you have your robot in URDF format, you can convert it to Collada (.dae) file using the following command:

rosrun collada_urdf urdf_to_collada "$MYROBOT_NAME".urdf "$MYROBOT_NAME".dae

Often floating point issues arise in converting a URDF file to Collada file, so a script has been created to round all the numbers down to x decimal places in your .dae file. Its probably best if you skip this step initially and see if IKFast can generate a solution with your default values, but if the generator takes longer than, say, an hour, try the following:

export IKFAST_PRECISION="5"
cp "$MYROBOT_NAME".dae "$MYROBOT_NAME".backup.dae  # create a backup of your full precision dae.
rosrun moveit_kinematics round_collada_numbers.py "$MYROBOT_NAME".dae "$MYROBOT_NAME".dae "$IKFAST_PRECISION"

From experience we recommend 5 decimal places, but if the OpenRAVE IKFast generator takes to long to find a solution, lowering the number of decimal places should help.

To see the links in your newly generated Collada file

You may need to install package libsoqt4-dev to have the display working:

openrave-robot.py "$MYROBOT_NAME".dae --info links

This is useful if you have a 7-dof arm and you need to fill in a –freeindex parameter, discussed later.

To test your newly generated Collada file in OpenRAVE:

openrave "$MYROBOT_NAME".dae

You should see your robot.

Select IK Type¶

You need to choose which sort of IK you want. See this page for more info. The most common IK type is transform6d.

Choose Planning Group¶

If your robot has more than one arm or “planning group” that you want to generate an IKFast solution for, choose one to generate first. The following instructions will assume you have chosen one <planning_group_name> that you will create a plugin for. Once you have verified that the plugin works, repeat the following instructions for any other planning groups you have. For example, you might have 2 planning groups:

<planning_group_name> = "left_arm"
<planning_group_name> = "right_arm"

To make it easy to use copy/paste for the rest of this tutorial. Set a PLANNING_GROUP environment variable. eg:

export PLANNING_GROUP="panda_arm"

Identify Link Numbers¶

You also need the link index numbers for the base_link and end_link between which the IK will be calculated. You can count the number of links by viewing a list of links in your model:

openrave-robot.py "$MYROBOT_NAME".dae --info links

A typical 6-DOF manipulator should have 6 arm links + a dummy base_link as required by ROS specifications. If no extra links are present in the model, this gives: baselink=0 and eelink=6. Often, an additional tool_link will be provided to position the grasp/tool frame, giving eelink=7.

The manipulator below also has another dummy mounting_link, giving baselink=1 and eelink=8.

Set the base link and EEF link to the desired index:

export BASE_LINK="0"
export EEF_LINK="8"

If you have a 7 DOF arm you will need to specify a free link:

export FREE_INDEX="1"

Generate IK Solver¶

To generate the IK solution between the manipulator’s base and tool frames for a 6DOF arm, use the following command format. We recommend you name the output ikfast61_”$PLANNING_GROUP”.cpp:

export IKFAST_OUTPUT_PATH=`pwd`/ikfast61_"$PLANNING_GROUP".cpp

For a 6DOF arm:

python `openrave-config --python-dir`/openravepy/_openravepy_/ikfast.py --robot="$MYROBOT_NAME".dae --iktype=transform6d --baselink="$BASE_LINK" --eelink="$EEF_LINK" --savefile="$IKFAST_OUTPUT_PATH"

For a 7 dof arm, you will need to specify a free link:

python `openrave-config --python-dir`/openravepy/_openravepy_/ikfast.py --robot="$MYROBOT_NAME".dae --iktype=transform6d --baselink="$BASE_LINK" --eelink="$EEF_LINK" --freeindex="$FREE_INDEX" --savefile="$IKFAST_OUTPUT_PATH"

The speed and success of this process will depend on the complexity of your robot. A typical 6 DOF manipulator with 3 intersecting axis at the base or wrist will take only a few minutes to generate the IK.

Known issue –freeindex argument is known to have a bug that it cannot handle tree index correctly. Say –baselink=2 –eelink=16 and links index from 3 to 9 is not related to current planning group chain. In that case –freeindex will expect index 2 as link 2, but index 3 as link 10 ... and index 9 as link 16.

You should consult the OpenRAVE mailing list and ROS Answers for information about 5 and 7 DOF manipulators.

Test the Plugin¶

Use the MoveIt RViz Motion Planning Plugin and use the interactive markers to see if correct IK Solutions are found.