rm abs on phase margin

Author: baggepinnen

lib/ControlSystemsBase/src/analysis.jl

@@ -506,7 +506,7 @@ function sisomargin(sys::LTISystem, w::AbstractVector{<:Real}; full=false, allMa
     if !allMargins #Only output the smallest margins
         gm, idx = findmin([gm;Inf])
         wgm = [wgm;NaN][idx]
-        pm, idx = findmin([abs.(pm);Inf])
+        pm, idx = findmin([pm;Inf])
         fi = [fi;NaN][idx]
         wpm = [wpm;NaN][idx]
         if full

lib/ControlSystemsBase/test/test_analysis.jl

@@ -245,6 +245,10 @@ Lw = freqresp(L, m[1][])[]
 @test imag(Lw) ≈ 0 atol = 1e-6 # Test definition of gain margin
 @test inv(-real(Lw)) ≈ m[2][] atol = 1e-6 # Test definition of gain margin
 
+# https://github.com/JuliaControl/ControlSystems.jl/issues/961
+P = tf(1,[5, 10.25, 6.25, 1])
+w_180, gm, w_c, pm = margin(50P)
+@test pm[] ≈ -35.1 rtol=1e-2
 
 # RGA
 a = 10