10.31.1 has issues with variable and equation counts #4048
Description
Describe the bug 🐞
I am seeing errors on 10.31.1 (but not on 10.31.0) like ExtraEquationsSystemException: The system is unbalanced. There are 85 highest order derivative variables and 91 equations..
I can't share that code, so instead I ran on the Standard library and am also seeing errors there:
Expected behavior
Standard library's test to pass.
Minimal Reproducible Example 👇
@v1.12) pkg> test ModelingToolkitStandardLibraryError & Stacktrace
Test Summary: | Pass Fail Error Broken Total Time
Core | 524 2 9 1 536 12m11.9s
Blocks: utils | 12 1 13 19.7s
Array Guesses | 2 2 1.9s
Scalarized Guesses | 4 4 0.6s
SISO Check | 1 1 13.7s
Blocks: math | 93 93 1m16.3s
Blocks: nonlinear | 15 15 11.1s
Blocks: continuous | 76 76 1m03.9s
Blocks: sources | 63 1 64 2m44.7s
Blocks: analysis points | 60 60 2m00.6s
Analog Circuits | 99 99 1m48.1s
Digital Circuits | 34 34 0.9s
Chua Circuit Demo | 1 1 5.2s
Thermal Circuits | 12 2 1 15 17.8s
Heat systems | 6 6 4.4s
Heat flow system | 1 1 0.8s
Radiator system | 3 3 2.3s
Thermal Collector | 3 3 1.7s
FixedHeatFlow with alpha=0.0 test | 2 2 8.7s
Thermal Demo | 10 10 18.2s
Magnetic | 3 3 3.8s
Mechanical Rotation | 19 1 20 36.2s
two inertias | 6 6 12.6s
two inertias with driving torque | 6 6 9.3s
first example | 1 1 6.2s
Stick-Slip | 2 2 4.4s
sensors | 1 1 0.2s
Position | 4 4 3.2s
Mechanical Translation | 15 1 16 29.4s
Free | 2 2 2.5s
Spring, Damper, Mass, Fixed | 4 4 14.0s
driven spring damper mass | 4 4 5.9s
sources & sensors | 5 1 6 7.0s
Translational | 3 3 4.4s
TranslationalPosition | 2 1 3 2.6s
PositionSensor & ForceSensor | 2 2 2.3s
AccelerationSensor | 1 1 0.3s
Mechanical Translation Modelica | 7 7 9.1s
Multi-Domain | 2 2 4 4.8s
DC motor | 1 1 0.3s
DC motor with speed sensor | 1 1 0.1s
Electrical Heating Circuit | 2 2 4.4s
Hydraulic IsothermalCompressible | 3 3 6 42.1s
Fluid Domain and Tube | 2 2 34.8s
Valve | 1 1 4.7s
DynamicVolume and minimum_volume feature | 1 1 0.5s
Actuator System | 1 1 1.4s
Prevent Negative Pressure | 1 1 0.3sone of the errors:
Got exception outside of a @test
ExtraVariablesSystemException: The system is unbalanced. There are 104 highest order derivative variables and 103 equations.
More variables than equations, here are the potential extra variable(s):
In particular, the unset input(s) are:
pos₊s₊u(t)
The rest of potentially unset variable(s) are:
piston₊mass₊v(t)
valve₊vSA₊x(t)
valve₊vBR₊x(t)
valve₊mass₊s(t)
valve₊mass₊v(t)
piston₊x(t)
piston₊vol_a₊x(t)
piston₊vol_a₊moving_volume₊x(t)
piston₊vol_a₊moving_volume₊m(t)
piston₊vol_b₊x(t)
piston₊vol_b₊moving_volume₊x(t)
piston₊vol_b₊moving_volume₊m(t)
piston₊mass₊s(t)
pos₊s₊u(t)
pos₊flange₊f(t)
snk₊port₊dm(t)
Note that the process of determining extra variables is a best-effort heuristic. The true extra variables are dependent on the model and may not be in this list.
Stacktrace:
(Interestingly only ExtraVariablesSystemException, not ExtraEquationsSystemException.
Environment (please complete the following information):
- Output of
versioninfo()
Julia Version 1.12.2
Commit ca9b6662be4 (2025-11-20 16:25 UTC)
Build Info:
Official https://julialang.org release
Platform Info:
OS: macOS (arm64-apple-darwin24.0.0)
CPU: 12 × Apple M2 Max
WORD_SIZE: 64
LLVM: libLLVM-18.1.7 (ORCJIT, apple-m2)
GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 8 virtual cores)
Environment:
JULIA_PKG_USE_CLI_GIT = true