gangoffour accepts ss (#322)

Author: baggepinnen

src/analysis.jl

@@ -464,7 +464,7 @@ Given a transfer function describing the Plant `P` and a transferfunction descri
 `T = PC/(1+PC)` Complementary sensitivity function
 
 Only supports SISO systems"""
-function gangoffour(P::TransferFunction,C::TransferFunction)
+function gangoffour(P::LTISystem,C::LTISystem)
     if P.nu + P.ny + C.nu + C.ny > 4
         error("gangoffour only supports SISO systems")
     end

test/test_pid_design.jl

@@ -3,6 +3,7 @@
 
 P = tf(1,[1,1])^4
 gangoffourplot(P,tf(1))
+gangoffourplot(ss(P),ss(1))
 ωp = 0.8
 kp,ki,C = loopshapingPI(P,ωp,phasemargin=60, doplot=true)
 @test kp ≈ 0.82274734724854