python-control · murrayrm · Jun 9, 2019 · Apr 28, 2019 · May 26, 2019 · May 27, 2019
diff --git a/control/__init__.py b/control/__init__.py
@@ -68,6 +68,7 @@
 from .robust import *
 from .config import *
 from .sisotool import *
+from .iosys import *
 
 # Exceptions
 from .exception import *

diff --git a/control/bdalg.py b/control/bdalg.py
@@ -224,6 +224,11 @@ def feedback(sys1, sys2=1, sign=-1):
     scalars, then TransferFunction.feedback is used.
 
     """
+    # Allow anything with a feedback function to call that function
+    try:
+        return sys1.feedback(sys2, sign)
+    except AttributeError:
+        pass
 
     # Check for correct input types.
     if not isinstance(sys1, (int, float, complex, np.number,

diff --git a/control/iosys.py b/control/iosys.py
diff --git a/control/lti.py b/control/lti.py
@@ -178,6 +178,28 @@ def timebaseEqual(sys1, sys2):
     else:
         return sys1.dt == sys2.dt
 
+# Find a common timebase between two or more systems
+def _find_timebase(sys1, *sysn):
+    """Find the common timebase between systems, otherwise return False"""
+
+    # Create a list of systems to check
+    syslist = [sys1]
+    syslist.append(*sysn)
+
+    # Look for a common timebase
+    dt = None
+
+    for sys in syslist:
+        # Make sure time bases are consistent
+        if (dt is None and sys.dt is not None) or \
+           (dt is True and isdiscrete(sys)):
+            # Timebase was not specified; set to match this system
+            dt = sys.dt
+        elif dt != sys.dt:
+            return False
+    return dt
+
+
 # Check to see if a system is a discrete time system
 def isdtime(sys, strict=False):
     """
@@ -200,6 +222,15 @@ def isdtime(sys, strict=False):
     if isinstance(sys, LTI):
         return sys.isdtime(strict)
 
+    # Check to see if object has a dt object
+    if hasattr(sys, 'dt'):
+        # If no timebase is given, answer depends on strict flag
+        if sys.dt == None:
+            return True if not strict else False
+
+        # Look for dt > 0 (also works if dt = True)
+        return sys.dt > 0
+
     # Got passed something we don't recognize
     return False
 
@@ -225,6 +256,13 @@ def isctime(sys, strict=False):
     if isinstance(sys, LTI):
         return sys.isctime(strict)
 
+    # Check to see if object has a dt object
+    if hasattr(sys, 'dt'):
+        # If no timebase is given, answer depends on strict flag
+        if sys.dt is None:
+            return True if not strict else False
+        return sys.dt == 0
+
     # Got passed something we don't recognize
     return False
 

diff --git a/control/statesp.py b/control/statesp.py
@@ -115,7 +115,7 @@ class StateSpace(LTI):
     sampling time.
     """
 
-    def __init__(self, *args):
+    def __init__(self, *args, **kw):
         """
         StateSpace(A, B, C, D[, dt])
 
@@ -152,6 +152,9 @@ def __init__(self, *args):
         else:
             raise ValueError("Needs 1 or 4 arguments; received %i." % len(args))
 
+        # Process keyword arguments
+        remove_useless = kw.get('remove_useless', True)
+
         A, B, C, D = [_matrix(M) for M in (A, B, C, D)]
 
         # TODO: use super here?
@@ -183,7 +186,7 @@ def __init__(self, *args):
             raise ValueError("C and D must have the same number of rows.")
 
         # Check for states that don't do anything, and remove them.
-        self._remove_useless_states()
+        if remove_useless: self._remove_useless_states()
 
     def _remove_useless_states(self):
         """Check for states that don't do anything, and remove them.