URDF and frames of reference

edit

Hi everyone.

I sincerely apologize if my question sounds stupid or has been answered before (I searched the forum and have read the tutorials but didn't find my answer). I'm currently building a URDF file for our robot using xacro. What I've noticed is that the origin of the frame of reference of every link is in the center of that link. For example when modeling hands, the tf will point to the center of the hand. I wanted to change this so tf frames will represent joints of hands and other parts of the robot, and not the middle of link. How is that possible to do?

Btw the current robot model is:

<?xml version="1.0"?>
<robot xmlns:xacro="----KARMA INSUFFICIENT" name="my_robot">
<!-- base cylinder properties -->
<xacro:property name="cyl_radius"                           value=".5"/>
<xacro:property name="cyl_height"                           value=".3"/>
<xacro:property name="cyl_buttom"                           value=".2"/> <!-- NOTE: not used currently -->
<!-- wheel properties -->
<xacro:property name="wheel_height"                         value=".2"/>
<xacro:property name="wheel_dist_to_center"                 value=".2"/>
<!-- laserscanner properties -->
<xacro:property name="laserscanner_dist_to_center"          value=".2"/>
<xacro:property name="laserscanner_height"                  value=".2"/>
<!-- body properties -->
<xacro:property name="body_dist_to_center"                  value=".35"/>
<xacro:property name="body_height"                          value="1.5"/>
<xacro:property name="body_width"                           value="0.07"/>
<!-- shoulder cylinder properties -->
<xacro:property name="shoulder_to_body_distance_x"          value=".15"/>
<xacro:property name="shoulder_rec_size"                    value=".07"/>
<xacro:property name="shoulder_length"                      value=".5"/>
<xacro:property name="shoulder_min_height"                  value=".8"/>
<xacro:property name="shoulder_max_height"                  value=".2"/>
<xacro:property name="max_shoulder_leveler_velocity"        value=".2"/>
<xacro:property name="max_shoulder_leveler_effort"          value=".2"/>
<!-- hands properties -->
<xacro:property name="upper_arm_length"                     value=".5"/>
<xacro:property name="forearm_length"                       value=".2"/>
<xacro:property name="upper_arm_min_angle"                  value=".2"/>
<xacro:property name="upper_arm_max_angle"                  value=".2"/>
<xacro:property name="forearm_min_angle"                    value="-4"/>
<xacro:property name="forearm_max_angle"                    value="4"/>
<xacro:property name="arm_max_effort"                       value=".2"/>
<xacro:property name="arm_max_velocity"                     value=".2"/>
<!-- Starting the robot definition -->
<link name="base_link">
    <visual>
        <geometry>
            <cylinder radius="${cyl_radius}" length="${cyl_height}"/>
        </geometry>
        <material name="blue">
            <color rgba="0 0 .8 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
            <cylinder radius="${cyl_radius}" length="${cyl_height}"/>
        </geometry>
        <origin xyz="0 0 0" rpy="0 0 0"/>
    </collision>
</link>

<joint name="base_to_body_joint" type="fixed">
    <parent link="base_link"/>
    <child link="body"/>
    <origin xyz="${body_dist_to_center} 0 ${(cyl_height + body_height)/2}"/>
</joint>

<link name="body">
    <visual>
        <geometry>
            <box size="${body_width} ${body_width} ${body_height}"/>
        </geometry>
        <material name="red">
            <color rgba="1 0 0 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
            <box size="${body_width} ${body_width} ${body_height}"/>
        </geometry>
    </collision>
</link>

<joint name="shoulder_leveler" type="prismatic">
    <parent link="body"/>
    <child link="shoulder"/>
    <origin xyz="${shoulder_to_body_distance_x} 0 ${(shoulder_min_height + shoulder_max_height - body_height)/2}" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="${shoulder_min_height}" upper="${shoulder_max_height}" 
           effort="${max_shoulder_leveler_effort}" velocity="${max_shoulder_leveler_velocity}" />
</joint>

<link name="shoulder">
    <visual>
        <geometry>
            <box size="${shoulder_rec_size} ${shoulder_length} ${shoulder_rec_size}"/>
        </geometry>
        <material name="red_1">
            <color rgba="0.5 0 0 1"/>
        </material>

    </visual>
    <collision>
        <geometry>
            <box size="${shoulder_rec_size} ${shoulder_length} ${shoulder_rec_size}"/>
        </geometry>
    </collision>
</link>


<xacro:macro name="hand" params="prefix reflect">
    <joint name="${prefix}_shoulder_joint" type="revolute">
        <parent link="shoulder"/>
        <child link="${prefix}_upper_arm"/>
        <axis xyz="0 1 0"/>
        <origin xyz="0 ${reflect * shoulder_length/2} ${-upper_arm_length/2}"/>
        <limit lower ...