Fix prismatic joint motion_subspace returning 12 elements instead of 6

[Copilot]

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
• Tests: Added motion_subspace dimension validation for all joint types (prismatic, revolute, fixed)

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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