diff --git a/control/tests/convert_test.py b/control/tests/convert_test.py
index b395996e0..e95e03bcf 100644
--- a/control/tests/convert_test.py
+++ b/control/tests/convert_test.py
@@ -206,6 +206,29 @@ def testTf2ssStaticMimo(self):
d = np.matrix([[0.5, 30, 0.0625], [-0.5, -1.25, 101.3]])
np.testing.assert_array_equal(d, gmimo.D)
+ def testSs2tfStaticSiso(self):
+ """Regression: ss2tf for SISO static gain"""
+ import control
+ gsiso = control.ss2tf(control.ss([], [], [], 0.5))
+ np.testing.assert_array_equal([[[0.5]]], gsiso.num)
+ np.testing.assert_array_equal([[[1.]]], gsiso.den)
+
+ def testSs2tfStaticMimo(self):
+ """Regression: ss2tf for MIMO static gain"""
+ import control
+ # 2x3 TFM
+ a = []
+ b = []
+ c = []
+ d = np.matrix([[0.5, 30, 0.0625], [-0.5, -1.25, 101.3]])
+ gtf = control.ss2tf(control.ss(a,b,c,d))
+
+ # we need a 3x2x1 array to compare with gtf.num
+ # np.testing.assert_array_equal doesn't seem to like a matrices
+ # with an extra dimension, so convert to ndarray
+ numref = np.asarray(d)[...,np.newaxis]
+ np.testing.assert_array_equal(numref, np.array(gtf.num) / np.array(gtf.den))
+
def suite():
return unittest.TestLoader().loadTestsFromTestCase(TestConvert)
diff --git a/control/xferfcn.py b/control/xferfcn.py
index 1559fa047..be21f8509 100644
--- a/control/xferfcn.py
+++ b/control/xferfcn.py
@@ -1098,40 +1098,48 @@ def _convertToTransferFunction(sys, **kw):
return sys
elif isinstance(sys, StateSpace):
- try:
- from slycot import tb04ad
- if len(kw):
- raise TypeError(
- "If sys is a StateSpace, " +
- "_convertToTransferFunction cannot take keywords.")
-
- # Use Slycot to make the transformation
- # Make sure to convert system matrices to numpy arrays
- tfout = tb04ad(sys.states, sys.inputs, sys.outputs, array(sys.A),
- array(sys.B), array(sys.C), array(sys.D), tol1=0.0)
-
- # Preallocate outputs.
- num = [[[] for j in range(sys.inputs)] for i in range(sys.outputs)]
- den = [[[] for j in range(sys.inputs)] for i in range(sys.outputs)]
-
- for i in range(sys.outputs):
- for j in range(sys.inputs):
- num[i][j] = list(tfout[6][i, j, :])
- # Each transfer function matrix row
- # has a common denominator.
- den[i][j] = list(tfout[5][i, :])
- # print(num)
- # print(den)
- except ImportError:
- # If slycot is not available, use signal.lti (SISO only)
- if (sys.inputs != 1 or sys.outputs != 1):
- raise TypeError("No support for MIMO without slycot")
-
- lti_sys = lti(sys.A, sys.B, sys.C, sys.D)
- num = squeeze(lti_sys.num)
- den = squeeze(lti_sys.den)
- # print(num)
- # print(den)
+
+ if 0==sys.states:
+ # Slycot doesn't like static SS->TF conversion, so handle
+ # it first. Can't join this with the no-Slycot branch,
+ # since that doesn't handle general MIMO systems
+ num = [[[sys.D[i,j]] for j in range(sys.inputs)] for i in range(sys.outputs)]
+ den = [[[1.] for j in range(sys.inputs)] for i in range(sys.outputs)]
+ else:
+ try:
+ from slycot import tb04ad
+ if len(kw):
+ raise TypeError(
+ "If sys is a StateSpace, " +
+ "_convertToTransferFunction cannot take keywords.")
+
+ # Use Slycot to make the transformation
+ # Make sure to convert system matrices to numpy arrays
+ tfout = tb04ad(sys.states, sys.inputs, sys.outputs, array(sys.A),
+ array(sys.B), array(sys.C), array(sys.D), tol1=0.0)
+
+ # Preallocate outputs.
+ num = [[[] for j in range(sys.inputs)] for i in range(sys.outputs)]
+ den = [[[] for j in range(sys.inputs)] for i in range(sys.outputs)]
+
+ for i in range(sys.outputs):
+ for j in range(sys.inputs):
+ num[i][j] = list(tfout[6][i, j, :])
+ # Each transfer function matrix row
+ # has a common denominator.
+ den[i][j] = list(tfout[5][i, :])
+ # print(num)
+ # print(den)
+ except ImportError:
+ # If slycot is not available, use signal.lti (SISO only)
+ if (sys.inputs != 1 or sys.outputs != 1):
+ raise TypeError("No support for MIMO without slycot")
+
+ lti_sys = lti(sys.A, sys.B, sys.C, sys.D)
+ num = squeeze(lti_sys.num)
+ den = squeeze(lti_sys.den)
+ # print(num)
+ # print(den)
return TransferFunction(num, den, sys.dt)
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