How to solve a degenerate mass distribution on its follower side.

How to solve a degenerate mass distribution in simmechanics?

Answer 1

[Samar]

Hello @sanjeev kumar,

When Simscape/SimMechanics reports a "degenerate mass distribution", it means there is a degree of freedom with no physical inertia, which leads to nonsensical (infinite) acceleration. Here are the common causes and corresponding fixes:

1. Joint-to-joint without mass: You cannot connect two joints directly without a solid or inertia block in between. Fix: Insert a small “dummy” Solid/Inertia block or use a composite joint (like 6DOF) if appropriate.
2. Unconnected or floating bodies: A body is not properly grounded—especially with imports from CAD. Fix: Ensure main parts are fixed via a weld or 6DOF joint to the world frame. Refer the discussion given here: www.mathworks.com/matlabcentral/answers/484741-sharing-solution-about-degenerate-mass-on-follower-side-issues-when-evaluate-simscape-mechanic-mod
3. Frame alignment issues: Overlapping coordinate frames with mismatched orientations can cause numerical issues. Fix: Verify base and follower frames are aligned correctly in orientation and position.
4. Ill-conditioned inertia: Inertia tensor may be singular or degenerate (zero, Inf, NaN, etc.). Fix: Confirm your solids have valid geometry-based inertia or defined positive-definite inertia matrices.

Simulation Solver Tips

Even with structural corrections, if face persists, follow these steps:

• Use a smaller fixed-step solver (e.g., ODE5 with step ~1e4 s) to improve numeric stability. A similar discussion is given here: www.mathworks.com/matlabcentral/answers/178218-block-error-solver-configuration-an-implicit-6-dof-joint-is-attached-to-a-degenerate-mass-distribut
• Incorporate local (implicit) solvers like backward Euler in the Solver Configuration block to handle stiff dynamics. Refer this discussion for more info: www.mathworks.com/matlabcentral/answers/865440-degenerate-mass-distribution-after-reducing-step-size-and-using-state-space-model

By ensuring every scheduled motion has real inertia and using appropriate solver settings, you will eliminate degenerate mass errors and simulate reliably.