clarify causality of analysis_point and add test for it

Author: baggepinnen

src/Blocks/analysis_points.jl

@@ -85,15 +85,24 @@ function Base.show(io::IO, ::MIME"text/plain", ap::AnalysisPoint)
 end
 
 """
-    connect(in, ap::AnalysisPoint, out)
-    connect(in, ap_name::Symbol, out)
+    connect(output_connector, ap_name::Symbol, input_connector)
+    connect(output_connector, ap::AnalysisPoint, input_connector)
 
-Connect `in` and `out` with an [`AnalysisPoint`](@ref) inbetween.
-The incoming connection `in` is expected to be of type [`RealOutput`](@ref), and vice versa.
+Connect `output_connector` and `input_connector` with an [`AnalysisPoint`](@ref) inbetween.
+The incoming connection `output_connector` is expected to be of type [`RealOutput`](@ref), and vice versa.
+NOTE: The connection is assumed to be *causal*, meaning that
+```
+connect(C.output, :plant_input, P.input)
+```
+is correct, whereas
+```
+connect(P.input, :plant_input, C.output)
+```
+typically is not (unless the model is an inverse model).
 
 # Arguments:
-- `in`: A connector of type [`RealOutput`](@ref)
-- `out`: A connector of type [`RealInput`](@ref)
+- `output_connector`: A connector of type [`RealOutput`](@ref)
+- `input_connector`: A connector of type [`RealInput`](@ref)
 - `ap`: An explicitly created [`AnalysisPoint`](@ref)
 - `ap_name`: If a name is given, an [`AnalysisPoint`](@ref) with the given name will be created automatically.
 """

test/Blocks/test_analysis_points.jl

@@ -186,12 +186,12 @@ model = SystemModel()
 @named er = Add(k2=-1)
 
 connections = [
-    connect(r.output, :r, filt.input)
-    connect(filt.output, er.input1)
-    connect(pid.ctr_output, model.torque.tau)
-    connect(model.inertia2.flange_b, sensor.flange)
-    connect(er.input2, :y, sensor.phi)
-    connect(er.output, :e, pid.err_input)
+       connect(r.output, :r, filt.input)
+       connect(filt.output, er.input1)
+       connect(pid.ctr_output, :u, model.torque.tau)
+       connect(model.inertia2.flange_b, sensor.flange)
+       connect(sensor.phi, :y, er.input2)
+       connect(er.output, :e, pid.err_input)
 ]
 
 closed_loop = ODESystem(connections, t, systems = [model, pid, filt, sensor, r, er], name = :closed_loop)
@@ -213,4 +213,12 @@ stepres = ControlSystemsBase.step(c2d(lsys, 0.001), 4)
 @test stepres.y[:] ≈ sol(0:0.001:4, idxs=model.inertia2.phi) rtol=1e-4
 
 # plot(stepres, plotx=true, ploty=true, size=(800, 1200), leftmargin=5Plots.mm)
-# plot!(sol, vars = [model.inertia2.phi], sp=1, l=:dash)
+# plot!(sol, vars = [model.inertia2.phi], sp=1, l=:dash)
+
+matrices, ssys = get_sensitivity(closed_loop, :y)
+So = ss(matrices...)
+
+matrices, ssys = get_sensitivity(closed_loop, :u)
+Si = ss(matrices...)
+
+@test tf(So) ≈ tf(Si)