Fix negative phase margin

lib/ControlSystemsBase/src/ControlSystemsBase.jl

@@ -244,7 +244,7 @@ function __init__()
             print(io, " for automatic discretization (applicable to systems without delays or nonlinearities only).")
         end
         plots_id = Base.PkgId(UUID("91a5bcdd-55d7-5caf-9e0b-520d859cae80"), "Plots")
-        if exc.f !== nothing && nameof(exc.f) === :plot && parentmodule(argtypes[1]) == @__MODULE__() && !haskey(Base.loaded_modules, plots_id)
+        if exc.f isa Function && nameof(exc.f) === :plot && parentmodule(argtypes[1]) == @__MODULE__() && !haskey(Base.loaded_modules, plots_id)
             printstyled(io, "\nPlotting is not available unless Plots.jl is loaded manually. Call `using Plots` before plotting.", color=:green, bold=true)
         elseif (exc.f == /) && argtypes[2] <: DelayLtiSystem
             print(io, "A delayed system can not be inverted. Consider use of the function `feedback`.")

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/src/connections.jl

@@ -140,12 +140,12 @@ Base.typed_hcat(::Type{S}, X::Union{AbstractArray{<:Number,1}, AbstractArray{<:N
 
 ## Mixed-type array creation
 # When creating an array of systems, an error may be thrown if the default Base.vect is called that tries to promote all systems to a common type. E.g., when using non-proper transfer functions and statespace systems. We thus opt out of the conversion with the method below
-function Base.vect(X::LTISystem...)
-    LTISystem[X...]
+function Base.vect(X0::LTISystem, X::LTISystem...)
+    LTISystem[X0, X...]
 end
 
-function Base.vect(X::T...) where T <: LTISystem
-    T[X...]
+function Base.vect(X0::T, X::T...) where T <: LTISystem
+    T[X0, X...]
 end
 
 """

lib/ControlSystemsBase/test/runtests.jl

@@ -4,13 +4,13 @@ import Base.isapprox        # In framework and test_synthesis
 import SparseArrays: sparse # In test_matrix_comps
 import Polynomials: conv            # In test_conversion and test_synthesis
 using Aqua
-# @testset "Aqua" begin
-#     Aqua.test_all(ControlSystemsBase;
-#         ambiguities = false, # causes 100s of hits in all dependencies
-#         stale_deps = true,  # Aqua complains about itself https://github.com/JuliaTesting/Aqua.jl/issues/78 
-#         project_toml_formatting = false, # https://github.com/JuliaTesting/Aqua.jl/issues/105
-#     )
-# end
+@testset "Aqua" begin
+    Aqua.test_all(ControlSystemsBase;
+        ambiguities = false, # causes 100s of hits in all dependencies
+        stale_deps = true,  # Aqua complains about itself https://github.com/JuliaTesting/Aqua.jl/issues/78 
+        project_toml_formatting = false, # https://github.com/JuliaTesting/Aqua.jl/issues/105
+    )
+end
 
 
 include("framework.jl")

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