BugFix: tf2ss now handles static MIMO gains with no Slycot

roryyorke · roryyorke · commit 02080e051ee9 · 2017-02-11T14:22:00.000+02:00
Check for static-gain (i.e., constant) transfer function matrix, and
handle specially.

Added unit tests for static SISO and MIMO plants.
diff --git a/control/statesp.py b/control/statesp.py
@@ -10,6 +10,7 @@
 
 # Python 3 compatibility (needs to go here)
 from __future__ import print_function
+from __future__ import division # for _convertToStateSpace
 
 """Copyright (c) 2010 by California Institute of Technology
 All rights reserved.
@@ -647,6 +648,7 @@ def _convertToStateSpace(sys, **kw):
     """
 
     from .xferfcn import TransferFunction
+    import itertools
     if isinstance(sys, StateSpace):
         if len(kw):
             raise TypeError("If sys is a StateSpace, _convertToStateSpace \
@@ -679,16 +681,26 @@ def _convertToStateSpace(sys, **kw):
                 ssout[3][:sys.outputs, :states],
                 ssout[4], sys.dt)
         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")
-
-            # TODO: do we want to squeeze first and check dimenations?
-            # I think this will fail if num and den aren't 1-D after
-            # the squeeze
-            lti_sys = lti(squeeze(sys.num), squeeze(sys.den))
-            return StateSpace(lti_sys.A, lti_sys.B, lti_sys.C, lti_sys.D,
-                              sys.dt)
+            # No Slycot.  Scipy tf->ss can't handle MIMO, but static MIMO is an easy special case we can check for here
+            maxn = max(max(len(n) for n in nrow)
+                       for nrow in sys.num)
+            maxd = max(max(len(d) for d in drow)
+                       for drow in sys.den)
+            if 1==maxn and 1==maxd:
+                D = empty((sys.outputs,sys.inputs),dtype=float)
+                for i,j in itertools.product(range(sys.outputs),range(sys.inputs)):
+                    D[i,j] = sys.num[i][j][0] / sys.den[i][j][0]
+                return StateSpace([], [], [], D, sys.dt)
+            else:
+                if (sys.inputs != 1 or sys.outputs != 1):
+                    raise TypeError("No support for MIMO without slycot")
+
+                # TODO: do we want to squeeze first and check dimenations?
+                # I think this will fail if num and den aren't 1-D after
+                # the squeeze
+                lti_sys = lti(squeeze(sys.num), squeeze(sys.den))
+                return StateSpace(lti_sys.A, lti_sys.B, lti_sys.C, lti_sys.D,
+                                  sys.dt)
 
     elif isinstance(sys, (int, float, complex)):
         if "inputs" in kw:
diff --git a/control/tests/convert_test.py b/control/tests/convert_test.py
@@ -184,6 +184,28 @@ def testConvertMIMO(self):
         if (not slycot_check()):
             self.assertRaises(TypeError, control.tf2ss, tfcn)
 
+    def testTf2ssStaticSiso(self):
+        """Regression: tf2ss for SISO static gain"""
+        import control
+        gsiso = control.tf2ss(control.tf(23, 46))
+        self.assertEqual(0, gsiso.states)
+        self.assertEqual(1, gsiso.inputs)
+        self.assertEqual(1, gsiso.outputs)
+        # in all cases ratios are exactly representable, so assert_array_equal is fine
+        np.testing.assert_array_equal([[0.5]], gsiso.D)
+
+    def testTf2ssStaticMimo(self):
+        """Regression: tf2ss for MIMO static gain"""
+        import control
+        # 2x3 TFM
+        gmimo = control.tf2ss(control.tf([[ [23],   [3],  [5] ], [ [-1],  [0.125],  [101.3] ]],
+                                         [[ [46], [0.1], [80] ], [  [2],   [-0.1],      [1] ]]))
+        self.assertEqual(0, gmimo.states)
+        self.assertEqual(3, gmimo.inputs)
+        self.assertEqual(2, gmimo.outputs)
+        d = np.matrix([[0.5, 30, 0.0625], [-0.5, -1.25, 101.3]])
+        np.testing.assert_array_equal(d, gmimo.D)
+
 
 def suite():
    return unittest.TestLoader().loadTestsFromTestCase(TestConvert)
