Add the timeevol property to FRD and accept keyword arguments in freqresp(::FRD)

ControlSystemsBase 1.22 calls `freqresp(sys, w; balance)` from `nyquist`. Since `freqresp(::FRD, w)` accepted no keyword arguments, the call was dispatched to the generic `freqresp(::LTISystem, w; balance)`, which failed in `timeevol(::FRD)` and, after that, in `numeric_type(::FRD)`. `FRD` now has the property `timeevol = Continuous()`, consistent with its supertype `LTISystem{Continuous}`, and `freqresp(::FRD, w; kwargs...)` ignores the keyword arguments.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: baggepinnen

src/frd.jl

@@ -91,11 +91,12 @@ end
 Base.lastindex(f::FRD) = length(f)
 function Base.getproperty(f::FRD, s::Symbol)
     s === :Ts && return π / maximum(f.w)
+    s === :timeevol && return Continuous()
     s === :nu && return ninputs(f)
     s === :ny && return noutputs(f)
     getfield(f, s)
 end
-Base.propertynames(f::FRD, private::Bool = false) = (fieldnames(typeof(f))..., :Ts)
+Base.propertynames(f::FRD, private::Bool = false) = (fieldnames(typeof(f))..., :Ts, :timeevol, :nu, :ny)
 
 function Base.show(io::IO, frd::FRD)
     write(io, "Frequency (rad/s)\n")
@@ -120,7 +121,7 @@ function ControlSystemsBase.bode(f::FRD, w::AbstractVector = f.w; unwrap=true)
     reshape(abs.(f.r), f.ny, f.nu, :), angles, f.w
 end
 
-function ControlSystemsBase.freqresp(f::FRD, w::AbstractVector{W} = f.w) where W <: Real
+function ControlSystemsBase.freqresp(f::FRD, w::AbstractVector{W} = f.w; kwargs...) where W <: Real # kwargs such as balance do not apply to frequency-response data
     w == f.w || error("Frequency vector must match the one stored in the FRD")
     reshape(f.r, f.ny, f.nu, :)
 end

test/test_frd.jl

@@ -176,6 +176,8 @@ frd = FRD(w, G);                        # Build a frequency-response data object
 
 @test bode(frd) == bode(G, w)
 @test nyquist(frd) == nyquist(G, w)
+@test ControlSystemsBase.iscontinuous(frd)
+@test freqresp(frd, w; balance = false) == freqresp(frd)
 
 @test size([frd frd]) == (ny, 2nu)
 @test size([frd; frd]) == (2ny, nu)