handle 1/(1 + L) for delay systems

JuliaControl · Dec 4, 2023 · 28c3712 · 28c3712
1 parent 9d401b8
commit 28c3712
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diff --git a/lib/ControlSystemsBase/src/types/DelayLtiSystem.jl b/lib/ControlSystemsBase/src/types/DelayLtiSystem.jl
@@ -95,7 +95,11 @@ end
 
 
 function /(anything, sys::DelayLtiSystem)
-    all(iszero, sys.Tau) || error("A delayed system can not be inverted. Consider use of the function `feedback`.")
+    if !all(iszero, sys.Tau)
+        ny,nu = size(sys)
+        ny == nu || error("The denominator system must be square")
+        return anything * feedback(I(nu), sys - I(nu))
+    end
     /(anything, sys.P.P) # If all delays are zero, invert the inner system
 end
 

diff --git a/lib/ControlSystemsBase/test/test_delayed_systems.jl b/lib/ControlSystemsBase/test/test_delayed_systems.jl
@@ -56,6 +56,11 @@ P2_fr = (im*ω .+ 1) ./ (im*ω .+ 2)
 @test freqresp(P2 * delay(1), ω)[:] ≈ P2_fr .* exp.(-im*ω) rtol=1e-15
 @test freqresp(delay(1) * P2, ω)[:] ≈ P2_fr .* exp.(-im*ω) rtol=1e-15
 
+
+# Division / feedback
+@test freqresp(1/(1+P1), ω) ≈ freqresp(feedback(I(size(P1, 1)), P1), ω) rtol=1e-15
+
+
 ## append
 P12 = append(P1, P2)
 G12 = [P1 tf(0); tf(0) P2]