Discrete Events Behaving Unexpectedly

[timknab]

There seem to be a couple of weird behaviors when using discrete_events in MTK9. For example, when setting a period of 24 hours for a periodic input into a state, the last event seems to double the input amount.

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
using DifferentialEquations
using Plots
function MTK_func(;name)
    pars = @parameters begin
        input = 10.0
    end

    D = Differential(t)


    vars = @variables begin
        A(t) = 0.0
    end

        eqs = [D(A) ~ -A]
    ODESystem(eqs, t, vars, pars; discrete_events = [24.0 => [A ~ A + input]], name = name)
end

@mtkbuild mtkmod = MTK_func()

tspan = (0.0, 130.0);  # [hr], 28 days

prob = ODEProblem(mtkmod, Pair[], tspan);
sol = solve(prob);

Plots.plot(sol)

And if you use a vector of event times and happen to include 0.0, even though it should have no effect, it skips the first event...

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
using DifferentialEquations
using Plots
function MTK_func(input_times;name)
    pars = @parameters begin
        input = 10.0
    end

    D = Differential(t)


    vars = @variables begin
        A(t) = 0.0
    end

        eqs = [D(A) ~ -A]
    ODESystem(eqs, t, vars, pars; discrete_events = [input_times => [A ~ A + input]], name = name)
end

@mtkbuild mtk1 = PK_func([0.0, 24.0, 48.0, 96.0])
@mtkbuild mtk2 = PK_func([24.0, 48.0, 96.0])

tspan = (0.0, 130.0);  # [hr], 28 days

prob1 = ODEProblem(mtk1, Pair[], tspan);
prob2 = ODEProblem(mtk2, Pair[], tspan);
sol1 = solve(prob1, saveat=0.1);
sol2 = solve(prob2, saveat=0.1);

Plots.plot(sol1)
Plots.plot!(sol2)

Pkg.status()
[0c46a032] DifferentialEquations v7.13.0
[961ee093] ModelingToolkit v9.4.0
[91a5bcdd] Plots v1.40.1

[ChrisRackauckas]

The missed event is fixed by SciML/DiffEqCallbacks.jl#210

But there's still the first issue that it seems a double event in the first.

[ChrisRackauckas]

A fix for that behavior was added to that PR.