search

RobotLocomotion / drake

Model-based design and verification for robotics.
https://drake.mit.edu
Other
3.27k stars 1.26k forks source link

Algebraic loop causes confusing "Recursion limit error" #16636

Closed RussTedrake closed 2 years ago

RussTedrake commented 2 years ago

Posted on Stack Overflow: https://stackoverflow.com/questions/70910589/recursion-error-while-evaluating-contact-information with a small notebook reproducing the error.

The diagram was constructed with MBP contact_results looping directly back to the MBP actuation_input_port as seen in this diagram: algebraic_loop Something is defeating our attempts to discover the algebraic loop. One of the systems in the loop is a python leaf system.

As a result, calling diagram.Publish(context) resulted in the confusing error RecursionError: maximum recursion depth exceeded while calling a Python object.

The desired outcome would be to provide the "algebraic loop" error message at the time of constructing the diagram.

jwnimmer-tri commented 2 years ago

Possibly related to #12786? If so, then there is a fail-fast diagnostic already in place, but possibly not when using point contact?

https://github.com/RobotLocomotion/drake/blob/6e0c78de480f9f6fec4d3ad0448d77f20259cf08/multibody/plant/compliant_contact_manager.cc#L443-L455

xuchenhan-tri commented 2 years ago

Hmm, I thought the non-contact force calculation in MbP got the same treatment as that in the contact manager. Apparently not... I'll take a look at this when I get some time.

xuchenhan-tri commented 2 years ago

Like @jwnimmer-tri said above, the reason that this algebraic loop went undetected is due to #12786 where the non-contact forces' calculation can't properly depend on input ports. But the reason that it's not caught by the existing fail-fast diagnostic is not because of point contact but due to the fact that it doesn't go through the compliant contact manager. I believe routing all non-contact force calculations through the diagnostic should resolve this.