Default dt is 0

control/config.py

@@ -15,7 +15,7 @@
 
 # Package level default values
 _control_defaults = {
-    # No package level defaults (yet)
+    'control.default_dt':0
 }
 defaults = dict(_control_defaults)
 
@@ -59,6 +59,9 @@ def reset_defaults():
     from .statesp import _statesp_defaults
     defaults.update(_statesp_defaults)
 
+    from .iosys import _iosys_defaults
+    defaults.update(_iosys_defaults)
+
 
 def _get_param(module, param, argval=None, defval=None, pop=False):
     """Return the default value for a configuration option.
@@ -170,5 +173,6 @@ def use_legacy_defaults(version):
     """
     if version == '0.8.3': 
         use_numpy_matrix(True) # alternatively: set_defaults('statesp', use_numpy_matrix=True)
+        set_defaults('control', default_dt=None)
     else:
         raise ValueError('''version number not recognized. Possible values are: ['0.8.3']''') 

control/dtime.py

@@ -53,21 +53,19 @@
 
 # Sample a continuous time system
 def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None):
-    """Convert a continuous time system to discrete time
-
-    Creates a discrete time system from a continuous time system by
-    sampling.  Multiple methods of conversion are supported.
+    """
+    Convert a continuous time system to discrete time by sampling
 
     Parameters
     ----------
-    sysc : linsys
+    sysc : LTI (StateSpace or TransferFunction)
         Continuous time system to be converted
-    Ts : real
+    Ts : real > 0
         Sampling period
     method : string
-        Method to use for conversion: 'matched', 'tustin', 'zoh' (default)
+        Method to use for conversion, e.g. 'bilinear', 'zoh' (default)
 
-    prewarp_frequency : float within [0, infinity)
+    prewarp_frequency : real within [0, infinity)
         The frequency [rad/s] at which to match with the input continuous-
         time system's magnitude and phase
 
@@ -78,13 +76,13 @@ def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None):
 
     Notes
     -----
-    See `TransferFunction.sample` and `StateSpace.sample` for
+    See :meth:`StateSpace.sample` or :meth:`TransferFunction.sample`` for
     further details.
 
     Examples
     --------
     >>> sysc = TransferFunction([1], [1, 2, 1])
-    >>> sysd = sample_system(sysc, 1, method='matched')
+    >>> sysd = sample_system(sysc, 1, method='bilinear')
     """
 
     # Make sure we have a continuous time system
@@ -95,35 +93,39 @@ def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None):
 
 
 def c2d(sysc, Ts, method='zoh', prewarp_frequency=None):
-    '''
-    Return a discrete-time system
+    """
+    Convert a continuous time system to discrete time by sampling
 
     Parameters
     ----------
-    sysc: LTI (StateSpace or TransferFunction), continuous
-        System to be converted
+    sysc : LTI (StateSpace or TransferFunction)
+        Continuous time system to be converted
+    Ts : real > 0
+        Sampling period
+    method : string
+        Method to use for conversion, e.g. 'bilinear', 'zoh' (default)
 
-    Ts: number
-        Sample time for the conversion
+    prewarp_frequency : real within [0, infinity)
+        The frequency [rad/s] at which to match with the input continuous-
+        time system's magnitude and phase
 
-    method: string, optional
-        Method to be applied,
-        'zoh'        Zero-order hold on the inputs (default)
-        'foh'        First-order hold, currently not implemented
-        'impulse'    Impulse-invariant discretization, currently not implemented
-        'tustin'     Bilinear (Tustin) approximation, only SISO
-        'matched'    Matched pole-zero method, only SISO
+    Returns
+    -------
+    sysd : linsys
+        Discrete time system, with sampling rate Ts
+
+    Notes
+    -----
+    See :meth:`StateSpace.sample` or :meth:`TransferFunction.sample`` for
+    further details.
 
-    prewarp_frequency : float within [0, infinity)
-            The frequency [rad/s] at which to match with the input continuous-
-            time system's magnitude and phase
+    Examples
+    --------
+    >>> sysc = TransferFunction([1], [1, 2, 1])
+    >>> sysd = sample_system(sysc, 1, method='bilinear')
+    """
 
-    '''
     #  Call the sample_system() function to do the work
     sysd = sample_system(sysc, Ts, method, prewarp_frequency)
 
-    # TODO: is this check needed?  If sysc is  StateSpace, sysd is too?
-    if isinstance(sysc, StateSpace) and not isinstance(sysd, StateSpace):
-        return _convertToStateSpace(sysd)       # pragma: no cover
-
     return sysd

control/iosys.py

@@ -38,12 +38,15 @@
 
 from .statesp import StateSpace, tf2ss
 from .timeresp import _check_convert_array
-from .lti import isctime, isdtime, _find_timebase
+from .lti import isctime, isdtime, common_timebase
+from . import config
 
 __all__ = ['InputOutputSystem', 'LinearIOSystem', 'NonlinearIOSystem',
            'InterconnectedSystem', 'input_output_response', 'find_eqpt',
            'linearize', 'ss2io', 'tf2io']
 
+# Define module default parameter values
+_iosys_defaults = {}
 
 class InputOutputSystem(object):
     """A class for representing input/output systems.
@@ -69,9 +72,11 @@ class for a set of subclasses that are used to implement specific
     states : int, list of str, or None
         Description of the system states.  Same format as `inputs`.
     dt : None, True or float, optional
-        System timebase.  None (default) indicates continuous time, True
-        indicates discrete time with undefined sampling time, positive number
-        is discrete time with specified sampling time.
+        System timebase. 0 (default) indicates continuous
+        time, True indicates discrete time with unspecified sampling
+        time, positive number is discrete time with specified
+        sampling time, None indicates unspecified timebase (either  
+        continuous or discrete time).
     params : dict, optional
         Parameter values for the systems.  Passed to the evaluation functions
         for the system as default values, overriding internal defaults.
@@ -86,9 +91,11 @@ class for a set of subclasses that are used to implement specific
         Dictionary of signal names for the inputs, outputs and states and the
         index of the corresponding array
     dt : None, True or float
-        System timebase.  None (default) indicates continuous time, True
-        indicates discrete time with undefined sampling time, positive number
-        is discrete time with specified sampling time.
+        System timebase. 0 (default) indicates continuous
+        time, True indicates discrete time with unspecified sampling
+        time, positive number is discrete time with specified
+        sampling time, None indicates unspecified timebase (either  
+        continuous or discrete time).
     params : dict, optional
         Parameter values for the systems.  Passed to the evaluation functions
         for the system as default values, overriding internal defaults.
@@ -109,7 +116,7 @@ class for a set of subclasses that are used to implement specific
 
     """
     def __init__(self, inputs=None, outputs=None, states=None, params={},
-                 dt=None, name=None):
+                 name=None, **kwargs):
         """Create an input/output system.
 
         The InputOutputSystem contructor is used to create an input/output
@@ -134,10 +141,11 @@ def __init__(self, inputs=None, outputs=None, states=None, params={},
         states : int, list of str, or None
             Description of the system states.  Same format as `inputs`.
         dt : None, True or float, optional
-            System timebase.  None (default) indicates continuous
-            time, True indicates discrete time with undefined sampling
+            System timebase. 0 (default) indicates continuous
+            time, True indicates discrete time with unspecified sampling
             time, positive number is discrete time with specified
-            sampling time.
+            sampling time, None indicates unspecified timebase (either  
+            continuous or discrete time).
         params : dict, optional
             Parameter values for the systems.  Passed to the evaluation
             functions for the system as default values, overriding internal
@@ -153,7 +161,7 @@ def __init__(self, inputs=None, outputs=None, states=None, params={},
         """
         # Store the input arguments
         self.params = params.copy()     # default parameters
-        self.dt = dt                    # timebase
+        self.dt = kwargs.get('dt', config.defaults['control.default_dt'])                    # timebase
         self.name = name                # system name
 
         # Parse and store the number of inputs, outputs, and states
@@ -200,10 +208,8 @@ def __mul__(sys2, sys1):
                              "inputs and outputs")
 
         # Make sure timebase are compatible
-        dt = _find_timebase(sys1, sys2)
-        if dt is False:
-            raise ValueError("System timebases are not compabile")
-
+        dt = common_timebase(sys1.dt, sys2.dt)
+
         # Return the series interconnection between the systems
         newsys = InterconnectedSystem((sys1, sys2))
 
@@ -478,10 +484,8 @@ def feedback(self, other=1, sign=-1, params={}):
                              "inputs and outputs")
 
         # Make sure timebases are compatible
-        dt = _find_timebase(self, other)
-        if dt is False:
-            raise ValueError("System timebases are not compabile")
-
+        dt = common_timebase(self.dt, other.dt)
+
         # Return the series interconnection between the systems
         newsys = InterconnectedSystem((self, other), params=params, dt=dt)
 
@@ -601,10 +605,11 @@ def __init__(self, linsys, inputs=None, outputs=None, states=None,
         states : int, list of str, or None, optional
             Description of the system states.  Same format as `inputs`.
         dt : None, True or float, optional
-            System timebase.  None (default) indicates continuous
-            time, True indicates discrete time with undefined sampling
+            System timebase. 0 (default) indicates continuous
+            time, True indicates discrete time with unspecified sampling
             time, positive number is discrete time with specified
-            sampling time.
+            sampling time, None indicates unspecified timebase (either  
+            continuous or discrete time).
         params : dict, optional
             Parameter values for the systems.  Passed to the evaluation
             functions for the system as default values, overriding internal
@@ -670,7 +675,8 @@ class NonlinearIOSystem(InputOutputSystem):
 
     """
     def __init__(self, updfcn, outfcn=None, inputs=None, outputs=None,
-                 states=None, params={}, dt=None, name=None):
+                 states=None, params={},  
+                 name=None, **kwargs):
         """Create a nonlinear I/O system given update and output functions.
 
         Creates an `InputOutputSystem` for a nonlinear system by specifying a
@@ -722,10 +728,10 @@ def __init__(self, updfcn, outfcn=None, inputs=None, outputs=None,
             operating in continuous or discrete time.  It can have the
             following values:
 
-            * dt = None       No timebase specified
-            * dt = 0          Continuous time system
-            * dt > 0          Discrete time system with sampling time dt
-            * dt = True       Discrete time with unspecified sampling time
+            * dt = 0: continuous time system (default)
+            * dt > 0: discrete time system with sampling period 'dt'
+            * dt = True: discrete time with unspecified sampling period
+            * dt = None: no timebase specified 
 
         name : string, optional
             System name (used for specifying signals).
@@ -741,6 +747,7 @@ def __init__(self, updfcn, outfcn=None, inputs=None, outputs=None,
         self.outfcn = outfcn
 
         # Initialize the rest of the structure
+        dt = kwargs.get('dt', config.defaults['control.default_dt'])
         super(NonlinearIOSystem, self).__init__(
             inputs=inputs, outputs=outputs, states=states,
             params=params, dt=dt, name=name
@@ -865,10 +872,10 @@ def __init__(self, syslist, connections=[], inplist=[], outlist=[],
             operating in continuous or discrete time.  It can have the
             following values:
 
-            * dt = None       No timebase specified
-            * dt = 0          Continuous time system
-            * dt > 0          Discrete time system with sampling time dt
-            * dt = True       Discrete time with unspecified sampling time
+            * dt = 0: continuous time system (default)
+            * dt > 0: discrete time system with sampling period 'dt'
+            * dt = True: discrete time with unspecified sampling period
+            * dt = None: no timebase specified 
 
         name : string, optional
             System name (used for specifying signals).
@@ -881,19 +888,14 @@ def __init__(self, syslist, connections=[], inplist=[], outlist=[],
         # Check to make sure all systems are consistent
         self.syslist = syslist
         self.syslist_index = {}
-        dt = None
         nstates = 0; self.state_offset = []
         ninputs = 0; self.input_offset = []
         noutputs = 0; self.output_offset = []
         system_count = 0
+
         for sys in syslist:
             # Make sure time bases are consistent
-            # TODO: Use lti._find_timebase() instead?
-            if dt is None and sys.dt is not None:
-                # Timebase was not specified; set to match this system
-                dt = sys.dt
-            elif dt != sys.dt:
-                raise TypeError("System timebases are not compatible")
+            dt = common_timebase(dt, sys.dt)
 
             # Make sure number of inputs, outputs, states is given
             if sys.ninputs is None or sys.noutputs is None or \