Allow passing kwargs to connect

[ValentinKaisermayer]

ModelingToolkit.jl/src/systems/abstractsystem.jl

Line 771 in fc6926e

function connect(syss...)

Since the connect method is essentially user defined it would be nice to allow for additional keyword arguments.

function connect(syss...; kwargs...)
    connect(promote_connect_type(map(get_connection_type, syss)...), syss...; kwargs...)
end

The reason is that if I compose a new component e.g. a RealCapacitor out of already existing components (Resistor, Capacitor and Pin) I need the connection to respect that the flow (i.e. current) from the pin of the RealCapacitor and the flow from internal components have different sign conventions. This can be also found in the Modelica specification here and here.

The way I solved this is to use a simple flag to indicate if the connector is inside or outside.

function connect(::Type{Pin}, connectors...; is_inside=fill(true, length(connectors))) # Which connector are inside connectors?
    eqs = [ 
        0 ~ sum([(i_ ? 1 : -1) * c.i for (c, i_) in zip(connectors, is_inside)]) # Flow balance
    ]
    for c in connectors[2:end] 
        push!(eqs, connectors[1].v ~ c.v) # Effort equality
    end
    return eqs
end

and in the definition of the RealCapacitor this can be used as follows:

function RealCapacitor(; name, C=1.0, R_esr=0.1, v0=0.0)
    C_val, R_esr_val = C, R_esr

    @named p = Pin()
    @named n = Pin()
    @named R_esr = Resistor(; R=R_esr_val)
    @named C = Capacitor(; C=C_val, v0=v0)
    connections = [
        connect(p, R_esr.p; is_inside=[false, true])
        connect(R_esr.n, C.p; is_inside=[true, true])
        connect(C.n, n; is_inside=[false, true])
    ]
    return ODESystem(connections, t, [], []; systems=[p, n, R_esr, C], name)
end

It is not really nice and a user is likely to forget about this but the results will be wrong otherwise.

[ValentinKaisermayer]

From the Modelica spec:

...the pin includes the current, which is considered to be positive if it is flowing at the pin into the device.