Multiplication of z-domain TF with s-domain TF is wrong #203

Activity

1. murrayrm commented on Jul 7, 2018

   

   murrayrm

   on Jul 7, 2018

   Member

   More actions

   This behavior is actually correct: all LTI systems (including the TransferFunction class) have a timebase dt. The timebase can any of the following (as described here):

   • dt = None: no timebase specified
   • dt = 0: continuous time system
   • dt > 0: discrete time system with sampling period ‘dt’
   • dt = True: discrete time with unspecified sampling period

   If you create system without a timebase (the default case) then it will automatically be converted to the appropriate timebase when it is operated upon.

   The following code demonstrates the behavior:

   tf1 = control.TransferFunction([1,1],[1,2,3])       # unspecified timebase
   tf2 = control.TransferFunction([1,1],[1,2,3], 0)    # continuous time
   tf3 = control.TransferFunction([1,1],[1,2,3], True) # discrete time, unspecified sampling
   tf4 = control.TransferFunction([1,1],[1,2,3], 1)    # discrete time, with sample time
   
   tf1*tf2             # OK; convert tf1 to continuous time
   tf1*tf3             # OK; convert tf1 to discrete time
   tf1*tf4             # OK; convert tf1 to discrete time time
   tf2*tf3             # Error; incompatible timebases
   tf2*tf4             # Error; incompatible timebases
   tf3*tf4             # OK; convert tf3 to fixed sample time
   
2. murrayrm

   added this to the 0.8.0 milestone on Jul 7, 2018
3. murrayrm

   closed this as completedon Jul 7, 2018
4. murrayrm

   added a commit that references this issue on Jul 14, 2018

   DOC: update timebase explanations (python-control#203)

   1039edd