Simscape Rotational Friction – Instability Issue in Joint Simulation

Hello,

I am trying to simulate joint friction in a robot using the Rotational Friction block in Simscape. I have measured the torque-velocity relationship from an actual robot joint, as shown in the plot below.

Based on this data, I estimated the friction parameters as follows:

• Breakaway friction torque: 20 Nm
• Coulomb friction torque: 10 Nm
• Breakaway friction velocity: 0.1 rad/s
• Viscous friction coefficient: 50 Nm·s/rad

I applied these values in the Rotational Friction block and connected it to each joint as shown in the Simscape model below.

The solver is configured to fixed-step with automatic solver selection, and the step size is set to 0.001 s (1 kHz). However, I am encountering unstable or diverging behavior during simulation.

I would appreciate it if you could help me understand:

• Why this divergence is happening
• Whether there are any best practices or recommended solver settings when using the Rotational Friction block in this context

Thank you in advance for your support.

[Yifeng Tang]

It'll be easier if you could attached the model that exhibit such behavior. it's different to diagnose from a snapshot, without seeing what's in the model, the parameters, and solver settings.

[Umar]

Hi @ donghoo,

I do agree with @Yifeng Tang’s comments, however, the only advice I can give is that the instability is probably related to the discontinuous nature of Coulomb friction. Even without Matlab Simulink access, here are some general approaches that often help:

1. Solver settings: Try ode15s or ode23s (stiff solvers) instead of fixed-step

2. Regularization: Most friction blocks have options to smooth the velocity discontinuity

3. Initial conditions: Make sure your initial joint velocities aren't exactly zero

4. Step size: 0.001s might be too large for the friction dynamics - try reducing it

Please bear in mind that the key is avoiding the mathematical discontinuity at zero velocity that causes numerical issues. Hope this helps.