Fix prismatic joint motion_subspace returning 12 elements instead of 6 #151

Copilot · 2026-01-23T16:23:07Z

Prismatic joints were generating 12-element motion subspace vectors instead of the required 6-element spatial vectors due to concatenating zeros(3) three times.

Changes

std_joint.py: Changed prismatic joint motion_subspace from zeros(3) to zeros(1) to match revolute joint pattern
parametric_joint.py: Applied same fix to parametric joints

Technical Details

Before:

elif self.type in ["prismatic"]:
    axis = self.axis
    zero = self.math.zeros(3)  # Creates [0, 0, 0]
    return self.math.vertcat(axis[0], axis[1], axis[2], zero, zero, zero)  # 12 elements

After:

elif self.type in ["prismatic"]:
    axis = self.axis
    z = self.math.zeros(1)  # Creates [0]
    return self.math.vertcat(axis[0], axis[1], axis[2], z, z, z)  # 6 elements

Prismatic joints now correctly return spatial vectors with axis values in positions 0-2 and zeros in positions 3-5, inverse of revolute joints.

Original prompt

This section details on the original issue you should resolve

<issue_title>[Question] Using Prismatic Joint</issue_title>
<issue_description>Hi Dev,

Trying to use the library with robot that has prismatic joint. Is there any reason why the dimension for the motion_subspace seems to be different.
if self.type == "fixed":
            return self.math.zeros(6, 1)
        elif self.type in ["revolute", "continuous"]:
            axis = self.axis
            z = self.math.zeros(1)
            return self.math.vertcat(z, z, z, axis[0], axis[1], axis[2])
        elif self.type in ["prismatic"]:
            axis = self.axis
            zero = self.math.zeros(
                3,
            )
            return self.math.vertcat(axis[0], axis[1], axis[2], zero, zero, zero)
and is there a good way to translate from a normal axis definition to the one use in adam. </issue_description>

<agent_instructions>Please fix the motion subspace dimension for the prismatic joint
</agent_instructions>

Comments on the Issue (you are @copilot in this section)
@Giulero Hi @podshara ! Thanks for opening the issue. This is clearly a bug! The prismatic joint subspace should be as well 6D vector. Opening a PR to fix this!

Fixes [Question] Using Prismatic Joint #150

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📚 Documentation preview 📚: https://adam-docs--151.org.readthedocs.build/en/151/

Co-authored-by: Giulero <[email protected]>

Giulero · 2026-01-27T08:47:20Z

@copilot please remove the tests added in this pr.

Co-authored-by: Giulero <[email protected]>

Copilot · 2026-01-27T08:48:59Z

@copilot please remove the tests added in this pr.

Done. Removed both test files in commit 193ae77.

Giulero · 2026-01-23T16:34:10Z

tests/parametric/test_parametric_joint_motion_subspace.py

+"""Test parametric joint motion subspace dimensions."""
+
+import pytest
+import numpy as np
+
+from adam.parametric.model.parametric_factories.parametric_joint import ParametricJoint
+from adam.numpy.numpy_like import SpatialMath
+from adam.model import Pose
+
+
+class SimpleLink:
+    """Minimal link object for testing."""
+
+    def __init__(self):
+        self.name = "test_link"
+        self.link_offset = 0.0
+        self.inertial = type("obj", (object,), {"origin": Pose.zero(SpatialMath())})()
+
+    def compute_joint_offset(self, joint, parent_offset):
+        return 0.0
+
+    def get_principal_length_parametric(self):
+        return 1.0
+
+
+class SimpleJoint:
+    """Minimal joint object for testing."""
+
+    def __init__(self, joint_type, axis):
+        self.name = f"test_{joint_type}"
+        self.parent = "link1"
+        self.child = "link2"
+        self.joint_type = joint_type
+        self.axis = axis
+        self.origin = type("obj", (object,), {"xyz": [0, 0, 0], "rpy": [0, 0, 0]})()
+        self.limit = None
+
+
+@pytest.fixture
+def spatial_math():
+    """Create a SpatialMath instance."""
+    return SpatialMath()
+
+
+@pytest.fixture
+def simple_link(spatial_math):
+    """Create a SimpleLink instance."""
+    return SimpleLink()
+
+
+def test_parametric_prismatic_joint_motion_subspace_dimension(spatial_math, simple_link):
+    """Test that parametric prismatic joint motion subspace has correct dimension (6x1)."""
+    joint = SimpleJoint("prismatic", [1, 0, 0])
+    parametric_joint = ParametricJoint(joint, spatial_math, simple_link)
+    motion_sub = parametric_joint.motion_subspace()
+
+    # Check shape is (6, 1)
+    assert motion_sub.shape == (6, 1), f"Expected shape (6, 1), got {motion_sub.shape}"
+
+    # Check total size is 6
+    assert motion_sub.array.size == 6, f"Expected 6 elements, got {motion_sub.array.size}"
+
+
+def test_parametric_prismatic_joint_motion_subspace_values(spatial_math, simple_link):
+    """Test that parametric prismatic joint motion subspace has correct values."""
+    # Test X-axis prismatic joint
+    joint = SimpleJoint("prismatic", [1, 0, 0])
+    parametric_joint = ParametricJoint(joint, spatial_math, simple_link)
+    motion_sub = parametric_joint.motion_subspace()
+
+    expected = np.array([[1.0], [0.0], [0.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Y-axis prismatic joint
+    joint = SimpleJoint("prismatic", [0, 1, 0])
+    parametric_joint = ParametricJoint(joint, spatial_math, simple_link)
+    motion_sub = parametric_joint.motion_subspace()
+
+    expected = np.array([[0.0], [1.0], [0.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Z-axis prismatic joint
+    joint = SimpleJoint("prismatic", [0, 0, 1])
+    parametric_joint = ParametricJoint(joint, spatial_math, simple_link)
+    motion_sub = parametric_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [1.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+
+def test_parametric_revolute_joint_motion_subspace_values(spatial_math, simple_link):
+    """Test that parametric revolute joint motion subspace has correct values."""
+    # Test X-axis revolute joint
+    joint = SimpleJoint("revolute", [1, 0, 0])
+    parametric_joint = ParametricJoint(joint, spatial_math, simple_link)
+    motion_sub = parametric_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [0.0], [1.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)


We do not need this test. please remove it

Giulero · 2026-01-23T16:34:21Z

tests/test_joint_motion_subspace.py

+"""Test joint motion subspace dimensions."""
+
+import pytest
+import numpy as np
+
+from adam.model.std_factories.std_joint import StdJoint
+from adam.numpy.numpy_like import SpatialMath
+
+
+class SimpleJoint:
+    """Minimal joint object for testing."""
+
+    def __init__(self, joint_type, axis):
+        self.name = f"test_{joint_type}"
+        self.parent = "link1"
+        self.child = "link2"
+        self.joint_type = joint_type
+        self.axis = axis
+        self.origin = None
+        self.limit = None
+
+
+@pytest.fixture
+def spatial_math():
+    """Create a SpatialMath instance."""
+    return SpatialMath()
+
+
+def test_prismatic_joint_motion_subspace_dimension(spatial_math):
+    """Test that prismatic joint motion subspace has correct dimension (6x1)."""
+    joint = SimpleJoint("prismatic", [1, 0, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    # Check shape is (6, 1)
+    assert motion_sub.shape == (6, 1), f"Expected shape (6, 1), got {motion_sub.shape}"
+
+    # Check total size is 6
+    assert motion_sub.array.size == 6, f"Expected 6 elements, got {motion_sub.array.size}"
+
+
+def test_prismatic_joint_motion_subspace_values(spatial_math):
+    """Test that prismatic joint motion subspace has correct values."""
+    # Test X-axis prismatic joint
+    joint = SimpleJoint("prismatic", [1, 0, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[1.0], [0.0], [0.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Y-axis prismatic joint
+    joint = SimpleJoint("prismatic", [0, 1, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[0.0], [1.0], [0.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Z-axis prismatic joint
+    joint = SimpleJoint("prismatic", [0, 0, 1])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [1.0], [0.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+
+def test_revolute_joint_motion_subspace_dimension(spatial_math):
+    """Test that revolute joint motion subspace has correct dimension (6x1)."""
+    joint = SimpleJoint("revolute", [1, 0, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    # Check shape is (6, 1)
+    assert motion_sub.shape == (6, 1), f"Expected shape (6, 1), got {motion_sub.shape}"
+
+    # Check total size is 6
+    assert motion_sub.array.size == 6, f"Expected 6 elements, got {motion_sub.array.size}"
+
+
+def test_revolute_joint_motion_subspace_values(spatial_math):
+    """Test that revolute joint motion subspace has correct values."""
+    # Test X-axis revolute joint
+    joint = SimpleJoint("revolute", [1, 0, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [0.0], [1.0], [0.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Y-axis revolute joint
+    joint = SimpleJoint("revolute", [0, 1, 0])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [0.0], [0.0], [1.0], [0.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+    # Test Z-axis revolute joint
+    joint = SimpleJoint("revolute", [0, 0, 1])
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    expected = np.array([[0.0], [0.0], [0.0], [0.0], [0.0], [1.0]])
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)
+
+
+def test_fixed_joint_motion_subspace(spatial_math):
+    """Test that fixed joint motion subspace is all zeros."""
+    joint = SimpleJoint("fixed", None)
+    std_joint = StdJoint(joint, spatial_math)
+    motion_sub = std_joint.motion_subspace()
+
+    # Check shape is (6, 1)
+    assert motion_sub.shape == (6, 1), f"Expected shape (6, 1), got {motion_sub.shape}"
+
+    # Check all zeros
+    expected = np.zeros((6, 1))
+    np.testing.assert_array_almost_equal(motion_sub.array, expected)


We do not need this test. please remove it

Initial plan

93a548d

Copilot AI assigned Copilot and Giulero Jan 23, 2026

Copilot started work on behalf of Giulero January 23, 2026 16:23 View session

Copilot AI and others added 4 commits January 23, 2026 16:25

Fix prismatic joint motion_subspace to return 6x1 vector

9cfc572

Co-authored-by: Giulero <[email protected]>

Add tests for joint motion_subspace dimensions

fa0dea1

Co-authored-by: Giulero <[email protected]>

All existing tests pass with the fix

bf02dd6

Co-authored-by: Giulero <[email protected]>

Remove test-generated urdf file and update .gitignore

30194e3

Co-authored-by: Giulero <[email protected]>

Copilot AI changed the title ~~[WIP] Fix motion subspace dimension for prismatic joint~~ Fix prismatic joint motion_subspace returning 12 elements instead of 6 Jan 23, 2026

Copilot AI requested a review from Giulero January 23, 2026 16:32

Copilot finished work on behalf of Giulero January 23, 2026 16:32

Copilot started work on behalf of Giulero January 27, 2026 08:47 View session

Remove added test files as requested

193ae77

Co-authored-by: Giulero <[email protected]>

Copilot finished work on behalf of Giulero January 27, 2026 08:49

Giulero approved these changes Jan 27, 2026

View reviewed changes

Giulero marked this pull request as ready for review January 27, 2026 08:52

Apply black

e67bbdd

Giulero merged commit ab2e8ed into main Jan 27, 2026
12 checks passed

Giulero deleted the copilot/fix-prismatic-joint-dimension branch January 27, 2026 10:31

Giulero mentioned this pull request Jan 27, 2026

[Question] Using Prismatic Joint #150

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Fix prismatic joint motion_subspace returning 12 elements instead of 6 #151

Fix prismatic joint motion_subspace returning 12 elements instead of 6 #151

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Copilot AI commented Jan 23, 2026 •

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Giulero commented Jan 27, 2026

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Fix prismatic joint motion_subspace returning 12 elements instead of 6 #151

Fix prismatic joint motion_subspace returning 12 elements instead of 6 #151

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Copilot AI commented Jan 23, 2026 •

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