Fixes details in documentation for Pink IK Null-space posture task

[michaellin6]

Description

Fixes typo in NullSpacePostureTask documentation. Adds additional tests for NullSpacePostureTask. Makes QP solver for Pink a parameter in config file so users can choose other solver options, but still defaults to 'daqp'.

Fixes # (issue)
Fixes issues raised by community user: #4417

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation update

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Overview

Greptile Summary

This PR addresses issue #4417 by fixing documentation and adding configurability to the Pink IK controller. The changes include:

• Documentation Fix (Partial): Corrected the mathematical formula in the null space posture task documentation from (q* - q) to (q - q*) at lines 32 and 75
• QP Solver Configuration: Added qp_solver parameter to PinkIKControllerCfg allowing users to choose between different solvers (defaults to "daqp")
• Enhanced Testing: Added two comprehensive integration tests (test_solve_ik_with_null_space_posture_task and test_solve_ik_with_different_posture_offsets) that verify the IK solver behavior with the null space posture task

Critical Issue Found:
The documentation change creates an inconsistency between the updated formulas (lines 32, 75) and both:

1. The actual implementation at line 200 which uses pin.difference(configuration.model, self.target_q, configuration.q) computing (q* - q)
2. The unchanged docstring at line 176 which still documents the old formula (q* - q)

This inconsistency needs to be resolved by either reverting the documentation changes or updating the implementation to match.

Confidence Score: 2/5

• This PR contains a critical documentation-implementation mismatch that needs resolution before merging
• The QP solver configuration change is solid and well-tested, but the documentation fix introduced an inconsistency between the mathematical formulas in the docs and the actual implementation, which could confuse users and lead to incorrect usage
• Pay close attention to null_space_posture_task.py - the documentation changes conflict with the implementation

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/controllers/pink_ik/null_space_posture_task.py	Updated error formula documentation but introduced inconsistency with implementation
source/isaaclab/isaaclab/controllers/pink_ik/pink_ik.py	Made QP solver configurable via cfg parameter, straightforward change
source/isaaclab/isaaclab/controllers/pink_ik/pink_ik_cfg.py	Added qp_solver configuration parameter with clear documentation
source/isaaclab/test/controllers/test_null_space_posture_task.py	Added comprehensive integration tests for solve_ik and improved error message formatting

Sequence Diagram

sequenceDiagram
    participant User
    participant PinkIKController
    participant Config as PinkIKControllerCfg
    participant Solver as Pink solve_ik
    participant Task as NullSpacePostureTask

    User->>Config: Set qp_solver (e.g., "daqp", "quadprog")
    User->>PinkIKController: Initialize with config
    PinkIKController->>Task: Set up NullSpacePostureTask
    Task->>Task: Initialize with controlled_frames & controlled_joints
    
    User->>PinkIKController: compute(curr_joint_pos, dt)
    PinkIKController->>Task: compute_error(configuration)
    Task->>Task: pin.difference(target_q, current_q)
    Note over Task: Computes (q* - q) [Implementation]<br/>Docs say (q - q*) [Inconsistency]
    Task-->>PinkIKController: Error vector
    
    PinkIKController->>Task: compute_jacobian(configuration)
    Task->>Task: Build null space projector N = I - J+ @ J
    Task-->>PinkIKController: Null space Jacobian
    
    PinkIKController->>Solver: solve_ik(config, tasks, dt, solver=cfg.qp_solver)
    Note over Solver: Uses configurable solver<br/>(previously hardcoded "daqp")
    Solver-->>PinkIKController: Joint velocities
    PinkIKController-->>User: Target joint positions

Loading

[greptile-apps]

_{4 files reviewed, 2 comments}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

The documentation now states the formula is (q - q*) but the actual implementation at line 200 uses pin.difference(configuration.model, self.target_q, configuration.q) which computes (q* - q). Either the documentation fix is incorrect, or the implementation needs to be updated to match.

Same issue at line 75 and in the docstring at line 176.

[greptile-apps]

Additional Comments (1)

source/isaaclab/isaaclab/controllers/pink_ik/null_space_posture_task.py
Docstring still uses old formula (q* - q) but lines 32 and 75 were changed to (q - q*). Need to ensure all three locations are consistent with the actual implementation at line 200.