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Connecting Time-Delay Systems

When two time-delay systems are connected in series, the delays can sum in the resulting system:
Wolfram Language code: {tfm, ssm} = {TransferFunctionModel[Unevaluated[{{(E^-5 s/1 + 2 s)}}], s, SamplingPeriod -> None, SystemsModelLabels -> None], StateSpaceModel[{{{0, 1}, {-3, -(2/3)}}, {{0}, {Subscript[, (7/3)]}}, {{1, 0}}, {{0}}}, SamplingPeriod -> None, SystemsModelLabels -> None]};
Wolfram Language code: series = SystemsModelSeriesConnect[tfm, ssm]//Simplify
Wolfram Language code: Plot[Evaluate[Table[OutputResponse[sys, UnitStep[t], {t, 0, 20}], {sys, {tfm, ssm, series}}]], {t, 0, 20}, PlotLegends -> {"tfm", "ssm", "series"}]
Time delays can sum in a feedback connection as well:
Wolfram Language code: feedback = SystemsModelFeedbackConnect[ssm, tfm]//Simplify
Wolfram Language code: Plot[Evaluate[Table[OutputResponse[sys, UnitStep[t], {t, 0, 20}], {sys, {tfm, ssm, feedback}}]], {t, 0, 20}, PlotLegends -> {"tfm", "ssm", "feedback"}]