.. currentmodule:: control

.. _version-0.9.2:

Version 0.9.2 Release Notes
----------------------------

* Released: 28 May 2022
* `GitHub release page
  <https://github.com/python-control/python-control/releases/tag/0.9.2>`_

This is a minor release that includes I/O system enhancements, optimal
control enhancements, new functionality for stochastic systems,
updated system class functionality, bug fixes and improvements to
Nyquist plots and Nichols charts, and L-infinity norm for linear
systems.

New features
............

* I/O system enhancements:

    - Modify the `ss`, `rss`, and `drss` functions to return
      `LinearIOSystem` objects (instead of `StateSpace` objects).
      This makes it easier to create LTI state space systems that can
      be combined with other I/O systems without having to add a
      conversation step.  Since `LinearIOSystem` objects are also
      `StateSpace` objects, no functionality is lost.  (This change is
      implemented through the introduction of a internal
      `NamedIOSystem` class, to avoid import cycles.)

    - Added a new function `create_statefbk_iosystem` that creates an
      I/O system for implementing a linear state feedback controller
      of the form u = ud - Kp(x - xd).  The function returns an I/O
      system that takes xd, ud, and x as inputs and generates u as an
      output.  The `integral_action` keyword can be used to define a
      set of outputs y = C x for which integral feedback is also
      included: u = ud - Kp(x - xd) - Ki(C x - C xd).

    - The `lqr` and `dlqr` commands now accept an `integral_action`
      keyword that allows outputs to be specified for implementing
      integral action.  The resulting gain matrix has the form K =
      [Kp, Ki].  (This is useful for combining with the
      `integral_action` functionality in `create_statefbk_iosystem`).

* Optimal control enhancements:

    - Allow `t_eval` keyword in `input_output_response` to allow a
      different set of time points to be used for the input vector and
      the computed output.

    - The final cost is now saved in optimal control result.

* Stochastic systems additions:

    - Added two new functions supporting random signals:
      `white_noise`, which creates a white noise vector in continuous
      or discrete time, and `correlation`, which calculates the
      correlation function (or [cross-] correlation matrix), R(tau).

    - Added a new function `create_estimator_iosystem` that matches
      the style of `create_statefbk_iosystem` (#710) and creates an
      I/O system implementing an estimator (including covariance
      update).

    - Added the ability to specify initial conditions for
      `input_output_response` as a list of values, so that for
      estimators that keep track of covariance you can set the initial
      conditions as `[X0, P0]`.  In addition, if you specify a fewer
      number of initial conditions than the number of states, the
      remaining states will be initialized to zero (with a warning if
      the last initial condition is not zero).  This allows the
      initial conditions to be given as `[X0, 0]`.

    - Added the ability to specify inputs for `input_output_response`
      as a list of variables.  Each element in the list will be
      treated as a portion of the input and broadcast (if necessary)
      to match the time vector.  This allows input for a system with
      noise as `[U, V]` and inputs for a system with zero noise as
      `[U, np.zero(n)]` (where U is an input signal and `np.zero(n)`
      gets broadcast to match the time vector).

    - Added new Jupyter notebooks demonstrate the use of these
      functions: `stochresp.ipynb`, `pvtol-outputfbk.ipynb`,
      `kincar-fusion.ipynb`.

* Updated system class functionality:

    - Changed the `LTI` class to use `poles` and `zeros` for
      retrieving poles and zeros, with `pole` and `zero` generating a
      `PendingDeprecationWarning` (which is ignored by default in
      Python).  (The MATLAB compatibility module still uses `pole` and
      `zero`.)

    - The `TimeResponseData` and `FrequencyResponseData` objects now
      implement a `to_pandas` method that creates a simple pandas
      dataframe.

    - The `FrequencyResponseData` class is now used as the output for
      frequency response produced by `freqresp` and a new function
      `frequency_response` has been defined, to be consistent with the
      `input_output_response` function.  A `FrequencyResponseData`
      object can be assigned to a tuple to provide magnitude, phase,
      and frequency arrays, mirroring `TimeResponseData` functionality.

    - The `drss`, `rss`, `ss2tf`, `tf2ss`, `tf2io`, and `ss2io`
      functions now all accept system and signal name arguments (via
      `_process_namedio_keywords`.

    - The `ss` function can now accept function names as arguments, in
      which case it creates a `NonlinearIOSystem` (I'm not sure how
      useful this is, but `ss` is a sort of wrapper function that
      calls the appropriate class constructor, so it was easy enough
      to implement.)

* Added `linform` to compute linear system L-infinity norm.


Improvements, bug fixes
.......................

* Round to nearest integer decade for default omega vector.
* Interpret str-type args to `interconnect` as non-sequence.
* Fixes to various optimization-based control functions.
* Bug fix and improvements to Nyquist plots.
* Improvements to Nichols chart plotting.