Merge pull request #837 from JuliaControl/generibal

make `balance_statespace` work for more system types

docs/src/man/numerical.md

@@ -114,3 +114,5 @@ Linear systems with zero-order-hold inputs can be exactly simulated in discrete
 
 For discrete-time systems, the function [`lsim!`](@ref) accepts a pre-allocated workspace objects that can be used to avoid allocations for repeated simulations.
 
+### Numerical balancing
+If you are only interested in the simulated outputs, not the state trajectories, you may consider applying balancing to the statespace model using [`balance_statespace`](@ref) before simulating, see the section on [State-space balancing](@ref) above. If the state trajectories are of interest, balancing can still be performed before simulation, and the inverse transformation applied to the state trajectories after simulation.

lib/ControlSystemsBase/src/types/conversion.jl

@@ -187,9 +187,10 @@ end
 #     balance_statespace(A2, B2, C2, perm)
 # end
 
-function balance_statespace(sys::StateSpace, perm::Bool=false)
+function balance_statespace(sys::AbstractStateSpace, perm::Bool=false)
     A, B, C, T = balance_statespace(sys.A,sys.B,sys.C, perm)
-    return ss(A,B,C,sys.D,sys.timeevol), T
+
+    return basetype(sys)(A,B,C,sys.D, ntuple(i->getfield(sys, i+4), fieldcount(typeof(sys))-4)...), T
 end
 
 # Method that might fail for some exotic types, such as TrackedArrays