ETS.ik_LM

ETS.ik_LM(Tep, q0=None, ilimit=30, slimit=100, tol=1e-06, mask=None, joint_limits=True, k=1.0, method='chan')[source]

Fast Levenberg-Marquardt numerical inverse kinematics solver

Parameters:

Tep (ndarray | SE3) – the desired end-effector pose
q0 (ndarray | None) – the initial joint coordinate vector
ilimit (int) – maximum iterations allowed per search
slimit (int) – maximum search attempts before failure
tol (float) – maximum allowed residual error E
mask (ndarray | None) – a 6-vector weighting Cartesian DoF error priority
joint_limits (bool) – reject solutions with joint limit violations
k (float) – gain value for the damping matrix Wn
method (Literal['chan', 'wampler', 'sugihara']) – one of "chan" (default), "sugihara" or "wampler"

Returns:

an IKSolution containing joint coordinates q, success flag, iterations, searches and residual error value (reason is always empty – this fast C++ solver doesn’t produce a granular failure reason string, unlike ikine_LM())

Return type:

IKSolution

Warning

This method requires the compiled C++ extension. It raises RuntimeError if that extension is unavailable, e.g. in a pure-Python build/wheel or under Pyodide/JupyterLite. Use ikine_LM() instead in those environments.

A method which provides functionality to perform numerical inverse kinematics (IK) using the Levenberg-Marquardt method. This is a fast solver implemented in C++.

See the Inverse Kinematics Docs Page for more details and for a tutorial on numerical IK, see here.

The operation is defined by the choice of the method kwarg.

The step is defined as

\[\begin{split}\vec{q}_{k+1} &= \vec{q}_k + \left( \mat{A}_k \right)^{-1} \bf{g}_k \\ % \mat{A}_k &= {\mat{J}(\vec{q}_k)}^\top \mat{W}_e \ {\mat{J}(\vec{q}_k)} + \mat{W}_n\end{split}\]

where \(\mat{W}_n = \text{diag}(\vec{w_n})(\vec{w_n} \in \mathbb{R}^n_{>0})\) is a diagonal damping matrix. The damping matrix ensures that \(\mat{A}_k\) is non-singular and positive definite. The performance of the LM method largely depends on the choice of \(\mat{W}_n\).

Chan’s Method

Chan proposed

\[\mat{W}_n = λ E_k \mat{1}_n\]

where λ is a constant which reportedly does not have much influence on performance. Use the kwarg k to adjust the weighting term λ.

Sugihara’s Method

Sugihara proposed

\[\mat{W}_n = E_k \mat{1}_n + \text{diag}(\hat{\vec{w}}_n)\]

where \(\hat{\vec{w}}_n \in \mathbb{R}^n\), \(\hat{w}_{n_i} = l^2 \sim 0.01 l^2\), and \(l\) is the length of a typical link within the manipulator. We provide the variable k as a kwarg to adjust the value of \(w_n\).

Wampler’s Method

Wampler proposed \(\vec{w_n}\) to be a constant. This is set through the k kwarg.

Examples

The following example gets the ets of a panda robot object, makes a goal pose Tep, and then solves for the joint coordinates which result in the pose Tep using the ikine_LM method.

>>> import roboticstoolbox as rtb
>>> panda = rtb.models.Panda().ets()
>>> Tep = panda.fkine([0, -0.3, 0, -2.2, 0, 2, 0.7854])
>>> panda.ikine_LM(Tep)
IKSolution: q=[-1.449, -0.8913, 1.068, -2.137, 0.8083, 1.673, 0.1653], success=True, iterations=7, searches=1, residual=1.73e-10

Notes

The value for the k kwarg will depend on the method chosen and the arm you are using. Use the following as a rough guide chan, k = 1.0 - 0.01, wampler, k = 0.01 - 0.0001, and sugihara, k = 0.1 - 0.0001

When using this method, the initial joint coordinates \(q_0\), should correspond to a non-singular manipulator pose, since it uses the manipulator Jacobian.

This class supports null-space motion to assist with maximising manipulability and avoiding joint limits. These are enabled by setting kq and km to non-zero values.

References

J. Haviland, and P. Corke. “Manipulator Differential Kinematics Part I: Kinematics, Velocity, and Applications.” arXiv preprint arXiv:2207.01796 (2022).
J. Haviland, and P. Corke. “Manipulator Differential Kinematics Part II: Acceleration and Advanced Applications.” arXiv preprint arXiv:2207.01794 (2022).