ERROR: ArgumentError: ... is neither an observed nor an unknown variable

[bradcarman]

The following attempt to get the initial values gives the error: ERROR: ArgumentError: drho_1(t) is neither an observed nor an unknown variable. . The variable drho_1(t) is a part of the system, I should be able to retrieve the initial value.

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


reg_pow(x,a) = sign(x)*abs(x)^a
reg_pow_safe(x, a, delta = 0.1) = ifelse(abs(x/delta) >= 1, sign(x)*abs(x/delta)^a * delta^a, (delta^a*x)/delta)

pars = @parameters begin
    ρ₀ = 1000
    β = 2e9
    A = 0.1
    m = 100
    L = 1

    p_s = 100e5
    p_r = 10e5
    C1 = 1.35
    C2 = 1.35
    c = 1000
    A_p = 0.00094

    transition=0.1
end   
vars = @variables begin
    p_1(t) = p_s
    p_2(t) = p_r
    x(t)=0
    dx(t)=0
    ddx(t), [guess=0]
    rho_1(t), [guess=ρ₀]
    rho_2(t), [guess=ρ₀]
    drho_1(t), [guess=0]
    drho_2(t), [guess=0]
    m_1(t), [guess=0]
    m_2(t), [guess=0]
    u_1(t), [guess=0]
    u_2(t), [guess=0]
end

eqs = [

    D(x) ~ dx
    D(dx) ~ ddx

    D(m_1) ~ u_1*ρ₀*A_p
    D(m_2) ~ u_2*ρ₀*A_p

    # volume & mass
    m_1 ~ rho_1*(L+x)*A
    m_2 ~ rho_2*(L-x)*A

    # density
    rho_1 ~ ρ₀*(1 + p_1/β)
    rho_2 ~ ρ₀*(1 + p_2/β)
    
    # actuator force balance
    m*ddx ~ (p_1 - p_2)*A - c*dx
    
    # orifices
    (p_s - p_1) ~ C1*ρ₀*reg_pow(u_1, 2)  # orifice 1
    (p_r - p_2) ~ C2*ρ₀*reg_pow(u_2, 2)  # orifice 2

]

@mtkbuild sys = ODESystem(eqs, t, vars, pars)

initprob = ModelingToolkit.InitializationProblem(sys, 0.0)
initsol = solve(initprob, FastShortcutNonlinearPolyalg()) # Success
initsol[sys.drho_1] #ERROR: ArgumentError: drho_1(t) is neither an observed nor an unknown variable.

Versions...

  [961ee093] ModelingToolkit v9.61.0
  [8913a72c] NonlinearSolve v4.3.0
  [1dea7af3] OrdinaryDiffEq v6.90.1

[bradcarman]

Ooops, nevermind, drho_1 isn't in the equations.