Error: ArgumentError: `xxx` is neither an observed nor an unknown variable.

[bradcarman]

MWE (note this requires the Hydraulic branch for the ModelingToolkitStandardLibrary): The following code produces the error "ERROR: ArgumentError: mass₊s(t) is neither an observed nor an unknown variable."

using ModelingToolkitStandardLibrary.Hydraulic.IsothermalCompressible: FixedPressure,
                                                                       Orifice, Volume,
                                                                       HydraulicFluid, HydraulicPort, liquid_density
using ModelingToolkitStandardLibrary.Blocks: Ramp, Constant
using ModelingToolkitStandardLibrary.Mechanical.Translational: Force, MechanicalPort, Mass

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
using OrdinaryDiffEq

@mtkmodel System begin
    @components begin
        src = FixedPressure(p = 101325)
        res = Orifice()
        act = Volume(direction = -1, area = 0.1)
        mass = Mass(m=1e-5)
        fluid = HydraulicFluid(density = 1000, bulk_modulus = 2.0e9)
    end
    @equations begin
        connect(src.port, res.port₁)
        connect(res.port₂, act.port)
        connect(act.flange, mass.flange)

        connect(fluid, src.port)
    end
end

@mtkbuild sys = System()

initialization_eqs = [
    sys.act.x ~ 1
    sys.act.dx ~ 0
]

initsys = ModelingToolkit.generate_initializesystem(sys; initialization_eqs)
initsys = structural_simplify(initsys)
initprob = NonlinearProblem(initsys, [t=>0], [])
initsol = solve(initprob; abstol=1e-6)
initsol[sys.mass.s]

The full stack trace is:

Stacktrace:
  [1] build_explicit_observed_function(sys::NonlinearSystem, ts::Num; inputs::Nothing, expression::Bool, eval_expression::Bool, eval_module::Module, output_type::Type, checkbounds::Bool, drop_expr::Function, ps::Vector{…}, return_inplace::Bool, param_only::Bool, op::Type, throw::Bool)
    @ ModelingToolkit C:\Users\bradl\.julia\packages\ModelingToolkit\OYoU3\src\systems\diffeqs\odesystem.jl:501
  [2] #observed#269
    @ C:\Users\bradl\.julia\packages\ModelingToolkit\OYoU3\src\systems\abstractsystem.jl:843 [inlined]
  [3] observed
    @ C:\Users\bradl\.julia\packages\ModelingToolkit\OYoU3\src\systems\abstractsystem.jl:820 [inlined]
  [4] #347
    @ C:\Users\bradl\.julia\packages\ModelingToolkit\OYoU3\src\systems\abstractsystem.jl:1684 [inlined]
  [5] get!(default::ModelingToolkit.var"#347#348"{…}, h::Dict{…}, key::SymbolicUtils.BasicSymbolic{…})
    @ Base .\dict.jl:479
  [6] (::ModelingToolkit.ObservedFunctionCache{NonlinearSystem})(::Num)
    @ ModelingToolkit C:\Users\bradl\.julia\packages\ModelingToolkit\OYoU3\src\systems\abstractsystem.jl:1683
  [7] observed
    @ C:\Users\bradl\.julia\packages\SciMLBase\tEuIM\src\scimlfunctions.jl:4525 [inlined]
  [8] observed
    @ C:\Users\bradl\.julia\packages\SymbolicIndexingInterface\cwAFH\src\index_provider_interface.jl:212 [inlined]
  [9] _getu(sys::SciMLBase.NonlinearSolution{…}, ::SymbolicIndexingInterface.ScalarSymbolic, ::SymbolicIndexingInterface.ScalarSymbolic, sym::Num)
    @ SymbolicIndexingInterface C:\Users\bradl\.julia\packages\SymbolicIndexingInterface\cwAFH\src\state_indexing.jl:155
 [10] getu
    @ C:\Users\bradl\.julia\packages\SymbolicIndexingInterface\cwAFH\src\state_indexing.jl:31 [inlined]
 [11] getindex(A::SciMLBase.NonlinearSolution{…}, sym::Num)
    @ SciMLBase C:\Users\bradl\.julia\packages\SciMLBase\tEuIM\src\solutions\solution_interface.jl:70

The current environment is:

Status `C:\Work\Packages\ModelingToolkitStandardLibrary.jl\Project.toml`
  [d360d2e6] ChainRulesCore v1.25.0
  [2b5f629d] DiffEqBase v6.158.1
  [615f187c] IfElse v0.1.1
⌃ [961ee093] ModelingToolkit v9.45.0
⌃ [0c5d862f] Symbolics v6.13.1
  [37e2e46d] LinearAlgebra

[AayushSabharwal]

I can't reproduce your error. When running the example, I get:

julia> @mtkbuild sys = System()
ERROR: UndefVarError: `Aₒ` not defined in `ModelingToolkitStandardLibrary.Hydraulic.IsothermalCompressible`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
 [1]
   @ ModelingToolkitStandardLibrary.Hydraulic.IsothermalCompressible ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:908
 [2] __Orifice__
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:138 [inlined]
 [3] #_#433
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:25 [inlined]
 [4] macro expansion
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/abstractsystem.jl:2051 [inlined]
 [5]
   @ Main ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:138
 [6] __System__
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:138 [inlined]
 [7] #_#433
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/model_parsing.jl:25 [inlined]
 [8] top-level scope
   @ ~/Julia/SciML/ModelingToolkit.jl/src/systems/abstractsystem.jl:2051
Some type information was truncated. Use `show(err)` to see complete types.

[mgithubk]

@AayushSabharwal I think that comment is related to the port names in your Orifice model. Can you share the code for your orifice model?

[AayushSabharwal]

I'm using whatever is on ModelingToolkitStandardLibrary#Hydraulic

[mgithubk]

I made some changes to it that I haven't pushed yet. Also, I don't have the permissions to push them anymore and need to get them reinstated. I'll let you know when they are.

[mgithubk]

Okay, I created a fork
https://github.com/mgithubk/ModelingToolkitStandardLibrary.jl
It contains updated library components.

There are some changes that need to be made to the model toot though.

#MWE Model Showing Initialization issue with a mass
@mtkmodel System begin
@components begin
src = FixedPressure(p = 101325)
res = Orifice()
act = Volume(direction = -1, area = 0.1)
mass = Mass(m=1e-5)
fluid = HydraulicFluid(density = 1000, bulk_modulus = 2.0e9)
end
@equations begin
connect(src.port, res.port_a)
connect(res.port_b, act.port)
connect(act.flange, mass.flange)
connect(fluid, src.port)
end
end

@mtkbuild sys = System()

initialization_eqs = [
sys.act.x ~ 1
sys.act.dx ~ 0
]

initsys = ModelingToolkit.generate_initializesystem(sys; initialization_eqs)
initsys = structural_simplify(initsys)
initprob = NonlinearProblem(initsys, [t=>0], [])
initsol = solve(initprob; abstol=1e-6)
u0 = unknowns(sys) .=> initsol[unknowns(sys)] # ERROR: ArgumentError: mass₊s(t) is neither an observed nor an unknown variable.

prob = ODEProblem(sys, u0, (0, 1), [])
sol = solve(prob, Rodas5P())
plot(sol; idxs = sys.act.x)

[bradcarman]

I updated the Hydraulic branch of ModelingToolkitStandardLibrary to include these changes needed. As @mgithubk the script to produce the error is now...

using ModelingToolkitStandardLibrary.Hydraulic.IsothermalCompressible: FixedPressure,
                                                                       Orifice, Volume,
                                                                       HydraulicFluid, HydraulicPort, liquid_density
using ModelingToolkitStandardLibrary.Blocks: Ramp, Constant
using ModelingToolkitStandardLibrary.Mechanical.Translational: Force, MechanicalPort, Mass

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
using OrdinaryDiffEq

@mtkmodel System begin
    @components begin
        src = FixedPressure(p = 101325)
        res = Orifice()
        act = Volume(direction = -1, area = 0.1)
        mass = Mass(m=1e-5)
        fluid = HydraulicFluid(density = 1000, bulk_modulus = 2.0e9)
    end
    @equations begin
        connect(src.port, res.port_a)
        connect(res.port_b, act.port)
        connect(act.flange, mass.flange)

        connect(fluid, src.port)
    end
end

@mtkbuild sys = System()

initialization_eqs = [
    sys.act.x ~ 1
    sys.act.dx ~ 0
]

initsys = ModelingToolkit.generate_initializesystem(sys; initialization_eqs)
initsys = structural_simplify(initsys)
initprob = NonlinearProblem(initsys, [t=>0], [])
initsol = solve(initprob; abstol=1e-6)
initsol[sys.mass.s]

[AayushSabharwal]

Apparently this happens because sys.mass.s doesn't appear in any of the equations in initsys. It's not in the algebraic equations, it doesn't have a default, and isn't in initialization_eqs so it never shows up. As a result, structural_simplify completely ignores it.

[bradcarman]

And yet it's an unknown of sys...

julia> unknowns(sys)
6-element Vector{SymbolicUtils.BasicSymbolic{Real}}:
 act₊x(t)
 act₊rho(t)
 mass₊s(t)
 mass₊v(t)
 act₊p(t)
 act₊drho(t)

How is this possible?

[AayushSabharwal]

I don't understand why it isn't possible 😅 mass.s is an unknown of sys because of the equation D(mass.s) ~ mass.v.

Let me ask this question: how is it meant to solve for the initial value of mass.s, given that the only equation it's involved in is D(mass.s) ~ mass.v and it doesn't have a default? Equivalently, consider this system:

@variables x(t) y(t)
@mtkbuild sys = ODESystem([D(x) ~ y, y ~ 3t], t)

If I now do ODEProblem(sys, [], (0.0, 1.0)) what determines the initial value of x?

[AayushSabharwal]

The initialization system is formed from:

1. Algebraic equations
2. Observed equations
3. Defaults
4. Initialization equations
5. variable => value mappings passed to ODEProblem

mass.s appears in none of these. So initialization can't solve for it.

[mgithubk]

@AayushSabharwal Would mass.s be in the defaults mentioned above If we set a guess for s?

@component function Mass(; name, m, g = 0)
pars = @parameters begin
m = m
g = g
end
@nAmed flange = MechanicalPort()

vars = @variables begin
    s(t), _[guess = 0]_
    v(t)
    f(t)
end

eqs = [flange.v ~ v
       flange.f ~ f
       D(s) ~ v
       D(v) ~ f / m + g]

return compose(ODESystem(eqs, t, vars, pars; name = name),
    flange)

end