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Re: 8.0 Control Systems ToDiscreteTimeModel[]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114585] Re: 8.0 Control Systems ToDiscreteTimeModel[]
  • From: bakshee at wolfram.com
  • Date: Thu, 9 Dec 2010 06:01:16 -0500 (EST)
  • References: <idnqs7$q7d$1@smc.vnet.net>

On Dec 8, 5:40 am, roby <roby.no... at gmail.com> wrote:
> Dear all !
>
> ToDiscreteTimeModel[TransferFunctionModel[1/s, s], Ts, z] // InputForm
>
> behaves just like expected and returns:
>
> TransferFunctionModel[{{{Ts*(1 + z)}}, {{2*(-1 + z)}}}, z,
> SamplingPeriod -> Ts]
>
> But:
>
> ToDiscreteTimeModel[TransferFunctionModel[s, s], Ts, z] // InputForm
>
> With the default Option Method->"BilinearTransform" I would expect
> something like:
>
> TransferFunctionModel[{{{2*(-1 + z)}}, {{Ts*(1 + z)}}}, z,
> SamplingPeriod -> Ts]
>
> but what is returned actualy seems to be some bug/crash:
>
> TransferFunctionModel[{{{2*(-1 + z)}},
> {{Control`ConversionsDump`sdmult$927}}}, z,
>  SamplingPeriod -> Ts]
>
> Any hints, am I missing something ?
>
> Regards Robert


It's a bug, which will be fixed shortly. In the meantime, you
can work it around by entering the denominator as a matrix:

In[2]:= ToDiscreteTimeModel[TransferFunctionModel[{{{s}}, {{1}}}, s],
Ts, z]//InputForm

Out[2]//InputForm=
TransferFunctionModel[{{{2*(-1 + z)}}, {{Ts*(1 + z)}}}, z,
SamplingPeriod -> Ts]

Igor Bakshee
Wolfram Research


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