Robot slides on plane with no actuation. Unexpected continuous motion

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Hello,
I’m simulating a 4-leg robot (spherical foot at each tip) with a CMG in the center in Simscape Multibody. I run the model without any external forces or actuation and with small damping/stiffness in revolute joints. Each spherical foot uses a Spatial Contact Force block with static friction = 0.6 and dynamic = 0.5. Gravity is normal (plane is horizontal, parallel to XY plane).
Problem: when I run the simulation from rest (no actuation), the whole robot begins to glide on the plane and accelerates — continuous sliding that should not occur. No external actuators are enabled and gravity should not produce horizontal motion.
Where to look and what to test. Any known causes or fixes?
Thanks!

Answers (1)

Satyam
Satyam 2 minutes ago
Edited: Satyam 1 minute ago
There could be a few possible causes for Sliding in Simscape Multibody Model. You could try out troubleshooting through these steps and check if it solves your problem.
Contact Force Settings:
  • Double-check the Smoothing parameter in the Spatial Contact Force block. Too much smoothing can cause unrealistic sliding.
  • Try reducing the Transition Region Width for friction.
Solver Settings:
  • Use a smaller fixed-step size or increase the accuracy if using a variable-step solver. Numerical errors can accumulate and cause drift.
Initial Conditions:
  • Ensure all initial velocities and joint positions are set to zero, especially for floating base and legs.
Model Symmetry:
  • Imbalances or slight misalignments in the robot or foot geometry can cause unintended motion.
Contact Penetration:
  • Excessive penetration between foot and ground can cause large contact forces. Adjust Contact Stiffness and Damping.
Friction Model:
  • Check if “Velocity threshold” for friction is set too high or too low. This affects when static friction switches to dynamic.
Mass and Inertia:
  • Ensure all bodies have realistic mass and inertia values.

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