Merge pull request #2517 from SciML/fb/continuousclock

feat: add `SolverStepClock`

Author: ChrisRackauckas

src/clock.jl

@@ -124,3 +124,26 @@ function Base.:(==)(c1::Clock, c2::Clock)
 end
 
 is_concrete_time_domain(x) = x isa Union{AbstractClock, Continuous}
+
+"""
+    SolverStepClock <: AbstractClock
+    SolverStepClock()
+    SolverStepClock(t)
+
+A clock that ticks at each solver step (sometimes referred to as "continuous sample time"). This clock **does generally not have equidistant tick intervals**, instead, the tick interval depends on the adaptive step-size slection of the continuous solver, as well as any continuous event handling. If adaptivity of the solver is turned off and there are no continuous events, the tick interval will be given by the fixed solver time step `dt`. 
+
+Due to possibly non-equidistant tick intervals, this clock should typically not be used with discrete-time systems that assume a fixed sample time, such as PID controllers and digital filters.
+"""
+struct SolverStepClock <: AbstractClock
+    "Independent variable"
+    t::Union{Nothing, Symbolic}
+    "Period"
+    SolverStepClock(t::Union{Num, Symbolic}) = new(value(t))
+end
+SolverStepClock() = SolverStepClock(nothing)
+
+sampletime(c) = nothing
+Base.hash(c::SolverStepClock, seed::UInt) = seed ⊻ 0x953d7b9a18874b91
+function Base.:(==)(c1::SolverStepClock, c2::SolverStepClock)
+    ((c1.t === nothing || c2.t === nothing) || isequal(c1.t, c2.t))
+end

src/systems/diffeqs/abstractodesystem.jl

@@ -1053,6 +1053,10 @@ function DiffEqBase.ODEProblem{iip, specialize}(sys::AbstractODESystem, u0map =
         discrete_cbs = map(affects, clocks, svs) do affect, clock, sv
             if clock isa Clock
                 PeriodicCallback(DiscreteSaveAffect(affect, sv), clock.dt)
+            elseif clock isa SolverStepClock
+                affect = DiscreteSaveAffect(affect, sv)
+                DiscreteCallback(Returns(true), affect,
+                    initialize = (c, u, t, integrator) -> affect(integrator))
             else
                 error("$clock is not a supported clock type.")
             end

test/clock.jl

@@ -477,3 +477,43 @@ y = res.y[:]
     # y ~ x(k-1) + kp * u
     # Instead. This should be equivalent to the above, but gve me an error when I tried
 end
+
+## Test continuous clock
+
+c = ModelingToolkit.SolverStepClock(t)
+k = ShiftIndex(c)
+
+@mtkmodel CounterSys begin
+    @variables begin
+        count(t) = 0
+        u(t) = 0
+    end
+    @equations begin
+        count(k + 1) ~ Sample(c)(u)
+    end
+end
+
+@mtkmodel FirstOrderSys begin
+    @variables begin
+        x(t) = 0
+    end
+    @equations begin
+        D(x) ~ -x + sin(t)
+    end
+end
+
+@mtkmodel FirstOrderWithStepCounter begin
+    @components begin
+        counter = CounterSys()
+        fo = FirstOrderSys()
+    end
+    @equations begin
+        counter.u ~ fo.x
+    end
+end
+
+@mtkbuild model = FirstOrderWithStepCounter()
+prob = ODEProblem(model, [], (0.0, 10.0))
+sol = solve(prob, Tsit5(), kwargshandle = KeywordArgSilent)
+
+@test sol.prob.kwargs[:disc_saved_values][1].t == sol.t[1:2:end] # Test that the discrete-tiem system executed at every step of the continuous solver. The solver saves each time step twice, one state value before discrete affect and one after.