urdf: add base_footprint

diff --git a/urdf/a4wd3.urdf b/urdf/a4wd3.urdf
index 5148bd86ae3e3e2a4967b895dd6997f6f23fb4eb..16effb6d0c3cd30880e315afe6fe75c3885bbb22 100644 (file)
--- a/urdf/a4wd3.urdf
+++ b/urdf/a4wd3.urdf
@@ -49,6 +49,24 @@
     </collision>
   </link>
 
+  <!-- base_footprint: planar frame at GROUND level (wheel contact), for 2D navigation.
+       Wheels dia 158 mm -> rolling radius 79 mm below the axle plane (= base_link origin).
+       Child of base_link (NOT parent): hw_node publishes /odom with child_frame_id
+       base_link and the EKF broadcasts odom->base_link, so base_link must stay the
+       odometry-attached frame; the footprint hangs 79 mm below it. -->
+  <joint name="base_link_to_base_footprint" type="fixed">
+    <parent link="base_link"/>
+    <child link="base_footprint"/>
+    <origin xyz="0 0 -0.079" rpy="0 0 0"/>
+  </joint>
+  <link name="base_footprint">
+    <visual>
+      <origin xyz="0 0 0" rpy="0 0 0"/>
+      <geometry><box size="0.001 0.001 0.001"/></geometry>
+      <material name="footprint_invisible"><color rgba="0.0 0.0 0.0 0.0"/></material>
+    </visual>
+  </link>
+
   <!-- Lidar mount (2026-10-13, final): 3x 18 mm distance holders stand on the FLAT CHASSIS DECK
        (z=35.0 mm) at the plate's 3 chassis screw holes; the raised central ring (top 43.5 mm)
        is UNDER THE PLATE CENTER and clears it by ~9.5 mm (verified from the RViz mesh: all 3