fix for discrete systems in baltrunc

Author: baggepinnen

src/matrix_comps.jl

@@ -540,16 +540,17 @@ function baltrunc(sys::ST; atol = sqrt(eps()), rtol = 1e-3, n = nothing, residua
         A11 = A[i1, i1]
         A12 = A[i1, i2]
         A21 = A[i2, i1]
-        A22 = A[i2, i2]
+        A22 = -A[i2, i2]
+        isdiscrete(sys) && (A22 += I)
         B1 = B[i1, :]
         B2 = B[i2, :]
         C1 = C[:, i1]
         C2 = C[:, i2]
         A2221 = A22\A21
-        A = A11 - A12*(A2221)
-        B = B1 - (A12/A22)*B2
-        C = C1 - C2*A2221
-        D = D - (C2/A22)*B2
+        A = A11 + A12*(A2221)
+        B = B1 + (A12/A22)*B2
+        C = C1 + C2*A2221
+        D = D + (C2/A22)*B2
     else 
         A = sysbal.A[i1,i1]
         B = sysbal.B[i1,:]

test/test_simplification.jl

@@ -68,5 +68,9 @@ y2,x2 = step(sysmin,t)[[1,3]]
     @test sysr.nx == 3
     @test dcgain(sysr)[] ≈ dcgain(sys)[] rtol=1e-10
 
+    sys = c2d(sys, 0.01)
+    sysr,Σ = baltrunc(sys, n=3, residual=true)
+    @test sysr.nx == 3
+    @test dcgain(sysr)[] ≈ dcgain(sys)[] rtol=1e-10
 
 end