fix(hangar_sim): use the hull rolling radius for the mecanum wheels

src/hangar_sim/config/control/picknik_ur.ros2_control.yaml

@@ -46,7 +46,10 @@ platform_velocity_controller:
     rear_right_wheel_command_joint_name: "rear_right_wheel"
     rear_left_wheel_command_joint_name: "rear_left_wheel"
 
-    kinematics.wheels_radius: 0.0666
+    # Hull rolling radius (perimeter/2*pi = 75.630 mm), NOT the sphere-peak
+    # radius (75.9002 mm) or the static ride height (75.0889 mm). See the
+    # wheel-geometry comment in hangar_sim/description/hangar_scene.xml.
+    kinematics.wheels_radius: 0.0756
     kinematics.sum_of_robot_center_projection_on_X_Y_axis: 0.59
     wheel_separation_multiplier: 1.0
     wheel_radius_multiplier: 1.0
@@ -98,7 +101,10 @@ platform_velocity_controller_nav2:
     rear_right_wheel_command_joint_name: "rear_right_wheel"
     rear_left_wheel_command_joint_name: "rear_left_wheel"
 
-    kinematics.wheels_radius: 0.0666
+    # Hull rolling radius (perimeter/2*pi = 75.630 mm), NOT the sphere-peak
+    # radius (75.9002 mm) or the static ride height (75.0889 mm). See the
+    # wheel-geometry comment in hangar_sim/description/hangar_scene.xml.
+    kinematics.wheels_radius: 0.0756
     kinematics.sum_of_robot_center_projection_on_X_Y_axis: 0.59
     wheel_separation_multiplier: 1.0
     wheel_radius_multiplier: 1.0

src/hangar_sim/description/hangar_scene.xml

@@ -17,10 +17,9 @@
          a + r            = 75.9002 mm  peak, reached only at each sphere
          a*cos(pi/N) + r  = 75.0889 mm  between spheres; the static ride height,
                                         which ur5e_ridgeback.xacro anchors to
-         hull perimeter/2pi = 75.630 mm rolling radius, for the controllers'
-                                        kinematics.wheels_radius — which still reads
-                                        0.0666 in picknik_ur.ros2_control.yaml and is
-                                        corrected on feat/19667-fuse-odometry
+         hull perimeter/2pi = 75.630 mm rolling radius, which is what the
+                                        controllers use for kinematics.wheels_radius
+                                        (0.0756 in picknik_ur.ros2_control.yaml)
        The 0.811 mm ripple between the first two is inherent to approximating a
        circle with N spheres — a*(1-cos(pi/N)) — and sizing them tangent would not
        remove it. They are not tangent as built: tangency wants r = 10.309 mm, so

src/hangar_sim/test/test_base_geometry.py

@@ -49,11 +49,24 @@
 from pathlib import Path
 
 import pytest
+import yaml
 from ament_index_python.packages import get_package_share_directory
 
 DESCRIPTION = Path(__file__).resolve().parent.parent / "description"
 MJCF = DESCRIPTION / "ur5e_ridgeback.xml"
 XACRO = DESCRIPTION / "ur5e_ridgeback.xacro"
+CONTROL_YAML = (
+    Path(__file__).resolve().parent.parent
+    / "config"
+    / "control"
+    / "picknik_ur.ros2_control.yaml"
+)
+
+# Both mecanum controller instances drive the same four wheels.
+MECANUM_CONTROLLERS = (
+    "platform_velocity_controller",
+    "platform_velocity_controller_nav2",
+)
 RIDGEBACK_XACRO = (
     Path(get_package_share_directory("ridgeback_description"))
     / "urdf"
@@ -172,6 +185,59 @@ def _static_ride_height() -> float:
     return ring * math.cos(math.pi / count) + radius
 
 
+def _rolling_radius() -> float:
+    """Radius of a circle with the roller hull's perimeter — the rolling radius.
+
+    One turn of the wheel lays down the hull's perimeter on the floor, so wheel
+    odometry and the drive-side `1/wheels_radius` both need `perimeter / 2pi`,
+    which is neither of the other two radii the ring produces. The hull of N equal
+    circles of radius `r` centred on a ring of radius `a` is N common external
+    tangents — each as long as a centre-polygon edge, `2*a*sin(pi/N)` — joined by
+    arcs that together sweep one full turn of radius `r`.
+    """
+    ring, radius, count = _roller_ring()
+    perimeter = count * 2 * ring * math.sin(math.pi / count) + 2 * math.pi * radius
+    return perimeter / (2 * math.pi)
+
+
+def _configured_wheels_radius(controller: str) -> float:
+    parameters = yaml.safe_load(CONTROL_YAML.read_text())[controller]["ros__parameters"]
+    radius = parameters.get("kinematics.wheels_radius")
+    assert (
+        radius is not None
+    ), f"{controller} has no kinematics.wheels_radius in {CONTROL_YAML.name}"
+    return float(radius)
+
+
+@pytest.mark.parametrize("controller", MECANUM_CONTROLLERS)
+def test_controller_wheels_radius_is_the_rolling_radius(controller: str) -> None:
+    """The one radius of the three that wheel odometry and the drive IK may use.
+
+    Held to 0.1 mm: tight enough to tell the rolling radius apart from the peak
+    (0.27 mm above it) and from the static ride height (0.54 mm below it), loose
+    enough for the value to stay written to four decimals.
+    """
+    ring, radius, count = _roller_ring()
+    configured = _configured_wheels_radius(controller)
+    assert configured == pytest.approx(_rolling_radius(), abs=1e-4), (
+        f"{controller} uses kinematics.wheels_radius = {configured:.4f} m, but the "
+        f"roller hull rolls at {_rolling_radius():.6f} m. The other two radii the "
+        f"ring produces — the {ring + radius:.6f} m peak and the "
+        f"{ring * math.cos(math.pi / count) + radius:.6f} m static ride height — are "
+        f"not interchangeable with it: odometry and the drive-side 1/wheels_radius "
+        f"both scale directly with this number."
+    )
+
+
+def test_both_mecanum_controllers_agree_on_the_wheels() -> None:
+    """They swap in for each other at runtime, so the base must not change speed."""
+    radii = {c: _configured_wheels_radius(c) for c in MECANUM_CONTROLLERS}
+    assert len(set(radii.values())) == 1, (
+        f"the mecanum controllers disagree on the wheel radius: {radii}. Switching "
+        f"between them would change how fast the base drives."
+    )
+
+
 def _mjcf_wheel_heights() -> dict[str, float]:
     """World z of each wheel body, accumulated down the MJCF body nesting."""
     heights: dict[str, float] = {}