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The optenv qualifier points to environmental variables as mentioned in the roslaunch documentation. There's no way to set an environmental variable from the include tag AFAIK, but someone can correct me if I'm wrong. In any case, I don't think that would get you the behavior you want. The syntax is $(optenv VARIABLE_NAME default_value), so if the variable is set, it will use that value everywhere. As you're using the lidar macro twice, the variables are used in multiple places and won't have different values.

One option is to leave the file alone and put in some "dummy" joints/links to get the orientation you want, which in your case might look similar to this:

# robot, namespace stuff, etc.
...
# Note: you only have to include the lidar macro file (sick_lms1xx_mount.urdf.xacro) once.
<xacro:include filename="$(find jackal_description)/urdf/accessories/sick_lms1xx_mount.urdf.xacro" />

<link name="base_link" />

<joint name="base_front_joint" type="fixed">
  <origin xyz="0.1 0 0" rpy="0 0 0" />  # Puts it 10 cm forward.
  <parent link="base_link" />
  <child link="front_link" />
</joint>

<link name="front_link" />

<xacro:sick_lms1xx_mount prefix="front"
                         parent_link="front_link"
                         topic="front_lidar_scan"/>

<joint name="base_rear_joint" type="fixed">
  <origin xyz="-0.1 0 0" rpy="0 0 ${-pi}" />  # Puts it 10 cm back and spins it 180 deg.
  <parent link="base_link" />
  <child link="rear_link" />
</joint>

<link name="rear_link" />

<xacro:sick_lms1xx_mount prefix="rear"
                         parent_link="rear_link"
                         topic="rear_lidar_scan"/>
...

You may note that there's repetition there, so you could create your own macro for adding the lidar(s), which includes the pose parameters as arguments:

<xacro:include filename="$(find jackal_description)/urdf/accessories/sick_lms1xx_mount.urdf.xacro" />
<xacro:macro name="sick_lm1xx" params="prefix parent topic xyz rpy" />
  <joint name="base_${prefix}_joint" type="fixed">
    <origin xyz="${xyz}" rpy="${rpy}" />  # Puts it 10 cm forward.
    <parent link="${parent}" />
    <child link="${prefix}_link" />
  </joint>

  <link name="${prefix}_link" />

  <xacro:sick_lms1xx_mount prefix="${prefix}"
                           parent_link="${prefix}_link"
                           topic="${topic}"/>
</xacro:macro>

<link name="base_link" />

<xacro:sick_lm1xx prefix="front"
                  parent="base_link"
                  topic="front_lidar_scan"
                  xyz="0.1 0 0"
                  rpy="0 0 0" />

<xacro:sick_lm1xx prefix="rear"
                  parent="base_link"
                  topic="rear_lidar_scan"
                  xyz="-0.1 0 0"
                  rpy="0 0 ${-pi}" />

At this point you may as well write your own version of the original macro with xyz and rpy parameters and use that instead. The source is in the Jackal GitHub repo.

The optenv qualifier points to environmental variables as mentioned in the roslaunch documentation. There's no way to set an environmental variable from the include tag AFAIK, but someone can correct me if I'm wrong. In any case, I don't think that would get you the behavior you want. The syntax is $(optenv VARIABLE_NAME default_value), so if the variable is set, it will use that value everywhere. As you're using the lidar macro twice, the variables are used in multiple places and won't have different values.

One option is to leave the file sick_lms1xx_mount.urdf.xacro alone and put in some "dummy" joints/links to get the orientation you want, which in your case might look similar to this:

# robot, namespace stuff, etc.
...
# Note: you only have to include the lidar macro file (sick_lms1xx_mount.urdf.xacro) once.
<xacro:include filename="$(find jackal_description)/urdf/accessories/sick_lms1xx_mount.urdf.xacro" />

<link name="base_link" />

<joint name="base_front_joint" type="fixed">
  <origin xyz="0.1 0 0" rpy="0 0 0" />  # Puts it 10 cm forward.
  <parent link="base_link" />
  <child link="front_link" />
</joint>

<link name="front_link" />

<xacro:sick_lms1xx_mount prefix="front"
                         parent_link="front_link"
                         topic="front_lidar_scan"/>

<joint name="base_rear_joint" type="fixed">
  <origin xyz="-0.1 0 0" rpy="0 0 ${-pi}" />  # Puts it 10 cm back and spins it 180 deg.
  <parent link="base_link" />
  <child link="rear_link" />
</joint>

<link name="rear_link" />

<xacro:sick_lms1xx_mount prefix="rear"
                         parent_link="rear_link"
                         topic="rear_lidar_scan"/>
...

You may note that there's repetition there, so you could create your own macro for adding the lidar(s), which includes the pose parameters as arguments:

<xacro:include filename="$(find jackal_description)/urdf/accessories/sick_lms1xx_mount.urdf.xacro" />
<xacro:macro name="sick_lm1xx" params="prefix parent topic xyz rpy" />
  <joint name="base_${prefix}_joint" type="fixed">
    <origin xyz="${xyz}" rpy="${rpy}" />  # Puts it 10 cm forward.
    <parent link="${parent}" />
    <child link="${prefix}_link" />
  </joint>

  <link name="${prefix}_link" />

  <xacro:sick_lms1xx_mount prefix="${prefix}"
                           parent_link="${prefix}_link"
                           topic="${topic}"/>
</xacro:macro>

<link name="base_link" />

<xacro:sick_lm1xx prefix="front"
                  parent="base_link"
                  topic="front_lidar_scan"
                  xyz="0.1 0 0"
                  rpy="0 0 0" />

<xacro:sick_lm1xx prefix="rear"
                  parent="base_link"
                  topic="rear_lidar_scan"
                  xyz="-0.1 0 0"
                  rpy="0 0 ${-pi}" />

At this point you may as well write your own version of the original macro with xyz and rpy parameters and use that instead. The source is in the Jackal GitHub repo.