# MTK acausal modeling not working with @discrete\_events

**URL:** <https://discourse.julialang.org/t/mtk-acausal-modeling-not-working-with-discrete-events/116683>\
**Category:** Modelling & Simulations\
**Tags:** modelingtoolkit\
**Created:** [July 5, 2024, 11:22pm UTC](https://discourse.julialang.org/t/mtk-acausal-modeling-not-working-with-discrete-events/116683 "2024-07-05T23:22:56Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![JosephChoi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/josephchoi/32/221042_2.png) [@JosephChoi](https://discourse.julialang.org/u/JosephChoi)\
**Post date:** [July 5, 2024, 11:22pm UTC](https://discourse.julialang.org/t/mtk-acausal-modeling-not-working-with-discrete-events/116683/1 "2024-07-05T23:22:56Z")

</div>

I’m trying to model an unsteady-state CSTR with discrete-event PI controller updates using MTK’s acausal component-based modeling framework, but I’m having trouble with @discrete\_events (or @continuous\_events) not being able to “see” connector variable information from outside components. Am I doing something wrong or are these begin blocks simply not supported with the acausal modeling framework yet?

```julia
@mtkmodel PIcontroller begin
    @components begin
        readingsIn = Stream()
        signalOut = ValveSignal()
    end
    @variables begin
        I_err(t) = 0 # Integral error
    end
    @parameters begin
        c_BSP = 0.32 # Desired concentration (mol/L)
        K_b = 0.0 # Bias (-)
        K_c = 9.0 # Proportional gain (-)
        K_I = 0.18 # Integral gain (-)

        eventStart = 200 # Enable PI Controller at t=200 min
        eventInterval = 30 # Update signal every 30 mins

        s = 0 # No initial signal to open valve
    end
    @equations begin
        D(I_err) ~ c_BSP - readingsIn.c_B
        signalOut.s ~ s
    end
    @discrete_events begin
        ((mod(t-eventStart,eventInterval) == 0) & (t >= eventStart)) => [s ~ K_b + K_c*(c_BSP - readingsIn.c_B) + K_I*I_err]
    end
end

```

I’m able to connect all my components, use `@mtkbuild`, create an `ODEProblem()`, but when I get to `solve()`, I get `ERROR: UndefVarError: picontroller₊readingsIn₊c_B not defined`. I’ve isolated this as the issue because whenever I replace `readingsIn.c_B` in the discrete\_events block with some number, everything runs properly (although I get the wrong solution of course).

I’ve also tried something like this

```julia
@mtkmodel PIcontroller begin
    @components begin
        readingsIn = Stream()
        signalOut = ValveSignal()
    end
    @variables begin
        I_err(t) = 0 # Integral error
        B(t)
    end
    @parameters begin
        c_BSP = 0.32 # Desired concentration (mol/L)
        K_b = 0.0 # Bias (-)
        K_c = 9.0 # Proportional gain (-)
        K_I = 0.18 # Integral gain (-)

        eventStart = 200 # Enable PI Controller at t=200 min
        eventInterval = 30 # Update signal every 30 mins

        s = 0 # No initial signal to open valve
    end
    @equations begin
        D(I_err) ~ c_BSP - B
        B ~ readingsIn.c_B
        signalOut.s ~ s
    end
    @discrete_events begin
        ((mod(t-eventStart,eventInterval) == 0) & (t >= eventStart)) => [s ~ K_b + K_c*(c_BSP - B) + K_I*I_err]
    end
end

```

but `@discrete_events` can’t see `B` either. I’ve found that `@discrete_events` can only see `B` if I do something like `D(B) ~ 0`, but then I can’t do `B ~ readingsIn.c_B` because `@mtkbuild` will tell me I have too many equations. Any insight would be appreciated.

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [July 5, 2024, 11:35pm UTC](https://discourse.julialang.org/t/mtk-acausal-modeling-not-working-with-discrete-events/116683/2 "2024-07-05T23:35:23Z")

</div>

Open an issue. This may be related to another example we’ve been working on.