MATLAB: Does selecting “Ignore Discrete States” option make no difference in the resulting linearized model

simulink

I have a model with both discrete and continuous blocks and I am linearizing a part of this model. Selecting the linearization option "Ignore Discrete States" or unselecting it makes no difference on the resulting linearization. The documentation on this option indicates that the resulting linearized model should a continuous-time model if this option is checked. However, the linearization operation returns a discrete-time model even though I select this option.

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This bug has been fixed in Release 2007b (R2007b). For previous product releases, read below for any possible workarounds:

Documentation on the "Ignore Discrete States" option is missing the information that the linearization sample time should be set to be continuous for the option to be effective.

If the linearization sample time is specified to be '-1' which makes the linearization inherit the sample time from the model, the resulting linearized model will be at the slowest common multiple of sample times in the linearization path. Therefore, a single block in the linearization path which does not have a continuous sample time makes the linearization result in a discrete-time model. The "Ignore Discrete States" option only ignores the discrete states when the linearization sample time is specified to be '0' which refers to a continuous sample time.

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MATLAB: Does the model containing SimDriveline blocks produce algebraic loop warnings in SimDriveline 1.0.1 (R14SP1)

This enhancement has been incorporated in Release 2008a (R2008a). For previous product releases, read below for any possible workarounds:

The algebraic loop is a result of the mode iteration needed to solve for the clutch states of the driveline clutches. In a driveline system, a change in a single clutch state has the potential to lock or unlock other clutches in the system. The process could repeat.

To resolve the clutch states, SimDriveline performs a mode iteration in order to reach the steady-state clutch configuration. In order to do this, the simulation time must be halted while this self-consistent clutch state is found through an algebraic loop. Because simulation time is halted, this mode iteration is not available in generated code versions of models subject to strict real-time requirements. Without mode iteration, the solution to the clutch state is found instead by searching over multiple time steps.

This requirement has the potential to produce in generated code results different from Simulink simulations. You can get a closer match between simulations and generated code by specifying a fixed-step solver in Simulink and disabling mode iteration. Note that you should disable this mode loop only if each clutch is controlled by external signals.

You can disable the algebraic loop by selecting the "Disable mode iteration for clutch locking" option in the Driveline Environment block.

For more information regarding this, you can reference an example in the "Running the Model Without Clutch Mode Iteration" section of the documentation. Access this section by copy/pasting the following command at the MATLAB Command Prompt:

<http://www.mathworks.com/help/releases/R14/toolbox/physmod/drive/a1068663651.html>

MATLAB: How does the linearization option “Ignore Discrete States” work for a system with an integrator in series connection with a discrete time-transfer function

The reason for the issue is how the "Ignore Discrete States" option works.

The "Ignore Discrete States" option operates on discrete-time blocks by converting them to the corresponding state-space representations and use the matrix 'D' for each block as the result of the linearization for the block. Therefore, if there exists a discrete-time block which has a state-space representation where 'D' matrix is zero and in series with the rest of linearization path, then the whole linearization operation returning a static gain of zero is the expected behavior when the "Ignore Discrete States" option is checked.

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Related Solutions

MATLAB: Does the model containing SimDriveline blocks produce algebraic loop warnings in SimDriveline 1.0.1 (R14SP1)

MATLAB: How does the linearization option “Ignore Discrete States” work for a system with an integrator in series connection with a discrete time-transfer function

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