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

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vova292 opened this issue May 10, 2018 · 1 comment
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Multiplication of z-domain TF with s-domain TF is wrong #203

vova292 opened this issue May 10, 2018 · 1 comment
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0.8.0

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@vova292
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vova292 commented May 10, 2018

It just multiplies the two as rational polynomials without any conversion of sampling times
@murrayrm
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murrayrm commented Jul 7, 2018

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

@murrayrm murrayrm added this to the 0.8.0 milestone Jul 7, 2018
@murrayrm murrayrm mentioned this issue Jul 7, 2018
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@murrayrm murrayrm closed this as completed Jul 7, 2018
murrayrm added a commit to murrayrm/python-control that referenced this issue Jul 14, 2018
@murrayrm
DOC: update timebase explanations (python-control#203)
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