[Octave-bug-tracker] [bug #61061] [octave forge] (control) margin() and step() giving wrong results

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?61061>

                 Summary: [octave forge] (control) margin() and step() giving
wrong results
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Thu 19 Aug 2021 01:33:04 PM UTC
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 5.2.0
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

I am using the package “control” provided by Octave Forge to investigate
the performance of a PLL transfer function.

The transfer function is derived from loop filter components R1 to R4 and C1
to C4. For lower order PLL transfer functions, some components e.g. R3/R4 and
C3/C4 are not used - this means I can either set them to “0” or to
“eps” (the smallest possible value).

I prefer to set them to “eps” to avoid any division by zero in future
calculations.
The resulting transfer function with components set to “eps” looks like:


pll0.TF_cl_fbdiv_out
Transfer function ‘ans’ from input ‘u1’ to output …
6.027e-84 s^6 + 3.668e-52 s^5 + 2.48e-21 s^4 + 9.427e-14 s^3 + 1.085e-07 s^2
y1:
-----------------------------------------------------------------------------------------------------------------------------------------------
7.384e-161 s^10 + 8.987e-129 s^9 + 3.342e-97 s^8 + 3.697e-66 s^7 + 1.25e-35
s^6 + 9.206e-28 s^5 + 1.943e-20 s^4 + 9.427e-14 s^3 + 1.085e-07 s^2


The functions margin(pll0.TF_cl_fbdiv_out) gets me wrong numbers and the
step(pll0.TF_cl_fbdiv_out) function also does not shows any reasonable
transient results.

But when setting the unused components to “0”, the transfer function
results in:

>>pll0.TF_cl_fbdiv_out
Transfer function ‘ans’ from input ‘u1’ to output …
2.479e-21 s^4 + 9.427e-14 s^3 + 1.085e-07 s^2
y1:
----------------------------------------------------------------------------
1.25e-35 s^6 + 9.205e-28 s^5 + 1.943e-20 s^4 + 9.427e-14 s^3 + 1.085e-07 s^2


And the margin(pll0.TF_cl_fbdiv_out) gets me the expected numbers
(PM=135deg@4.21rad/s) and the step(pll0.TF_cl_fbdiv_out) function also shows
reasonable transient results.

Questions:
Is this a bug in the control package that it cannot handle those small values
(e.g. 8.9e-129) in the “eps” transfer function ?
Is there a way to “cleanup” the transfer function parameter values to
avoid such wrong results ?

Why can the margin() and step() functions not properly handle the transfer
function ?





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