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handle MIMO tf feedback and division by converting to ss #1007

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Merged
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Jul 9, 2025
Merged

handle MIMO tf feedback and division by converting to ss #1007

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9 changes: 6 additions & 3 deletions lib/ControlSystemsBase/src/connections.jl
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Expand Up @@ -259,7 +259,8 @@

function feedback(G1::TransferFunction{<:TimeEvolution,<:SisoRational}, G2::TransferFunction{<:TimeEvolution,<:SisoRational}; pos_feedback::Bool = false)
if !issiso(G1) || !issiso(G2)
error("MIMO TransferFunction feedback isn't implemented.")
@warn("MIMO TransferFunction feedback isn't implemented yet, converting to a state-space object and back. Consider converting your transfer functions to state-space form using `ss` as soon as possible.")
return tf(feedback(ss(G1), ss(G2); pos_feedback))
end
G1num = numpoly(G1)[]
G1den = denpoly(G1)[]
Expand All @@ -274,7 +275,8 @@
#Efficient implementations
function feedback(L::TransferFunction{<:TimeEvolution,T}) where T<:SisoRational
if size(L) != (1,1)
error("MIMO TransferFunction feedback isn't implemented, use L/(1+L)")
@warn("MIMO TransferFunction feedback isn't implemented yet, converting to a state-space object and back. Consider converting your transfer functions to state-space form using `ss` as soon as possible.")
return tf(feedback(ss(L)))
end
P = numpoly(L)
Q = denpoly(L)
Expand All @@ -283,7 +285,8 @@

function feedback(L::TransferFunction{TE, T}) where {TE<:TimeEvolution, T<:SisoZpk}
if size(L) != (1,1)
error("MIMO TransferFunction feedback isn't implemented, use L/(1+L)")
@warn("MIMO TransferFunction feedback isn't implemented yet, converting to a state-space object and back. Consider converting your transfer functions to state-space form using `ss` as soon as possible.")
return tf(feedback(ss(L)))

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end
#Extract polynomials and create P/(P+Q)
k = L.matrix[1].k
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14 changes: 12 additions & 2 deletions lib/ControlSystemsBase/src/types/TransferFunction.jl
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Expand Up @@ -227,12 +227,22 @@ function /(n::Number, G::TransferFunction)
matrix = fill(entry, 1, 1)
return TransferFunction(matrix, G.timeevol)
else
error("MIMO TransferFunction inversion isn't implemented yet")
error("MIMO TransferFunction inversion isn't implemented yet, consider converting your transfer functions to state-space form using `ss`")
end
end
/(G::TransferFunction, n::Number) = G*(1/n)
/(G1::TransferFunction, G2::TransferFunction) = G1*(1/G2)
Base.:(/)(sys1::LTISystem, sys2::TransferFunction) = *(promote(sys1, ss(1/sys2))...) # This special case is needed to properly handle improper inverse transfer function (1/s)
function /(f1::TransferFunction, f2::TransferFunction)
if issiso(f2)
T = numeric_type(f2)
One = one(1/one(T)) # This gymatics is to ensure that we get floats out of integer system division. We will always get floats for ss division in the branch below, so this is required for type stability
f1*(One/f2)
else
@warn "MIMO TransferFunction inversion isn't implemented yet, converting to state-space object and back. Consider converting your transfer functions to state-space form using `ss` as soon as possible."
tf(ss(f1) / ss(f2))
end
end



#####################################################################
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18 changes: 18 additions & 0 deletions lib/ControlSystemsBase/test/test_transferfunction.jl
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Expand Up @@ -271,4 +271,22 @@ if VERSION >= v"1.8.0-rc1"
@test @test_logs (:warn, r"deprecated") tf(1).Ts == 0
end

# Test MIMO TransferFunction feedback behavior
@test_logs (:warn, r"MIMO TransferFunction feedback isn't implemented yet") feedback(C_222)
@test_logs (:warn, r"MIMO TransferFunction feedback isn't implemented yet") feedback(C_222, C_222)
# Test that feedback returns state-space for MIMO
fb_result = @test_logs (:warn, r"MIMO TransferFunction feedback isn't implemented yet") feedback(C_222)
@test fb_result isa TransferFunction
fb_result2 = @test_logs (:warn, r"MIMO TransferFunction feedback isn't implemented yet") feedback(C_222, C_222)
@test fb_result2 isa TransferFunction

# Test MIMO TransferFunction division behavior
Ci_222 = tf(ssrand(2,2,2))
# Test that division returns state-space for MIMO
div_result = @test_logs (:warn, r"MIMO TransferFunction inversion isn't implemented yet") C_222 / Ci_222
@test div_result isa TransferFunction

# Test improved error messages in MIMO TransferFunction inversion
@test_throws ErrorException("MIMO TransferFunction inversion isn't implemented yet, consider converting your transfer functions to state-space form using `ss`") 1 / Ci_222

end
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