MATLAB: Does MODRED effect the order of states in the state space Control System Toolbox 8.3 (R2009a)

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I am using MODRED to reduce the states in my state space model. Before the reduction I am reordering the states of the model. The particular order of states is important for me to identify some components (e.g. integrators) later on. Therefore I want to know if MODRED might be reordering the states under certain conditions.

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MODRED will not modify the order of the remaining states. It just finds another system with equivalent dynamics, so it cannot change the order of the states. But since it deletes some existing states, the final transfer function may look very different from the original one.

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MATLAB: Could not find version 8.3 of the MCR.

If your MCR 8.3 has accidentally been removed, you can download it from http://www.mathworks.com/products/compiler/mcr/ . MCR 7.17 and later are available there.

MATLAB: Is there a way to specify the accuracy for the BANDWIDTH function in Control System Toolbox 8.3 (R2009a)

The ability to specify an arbitrary accuracy is not available in Control System Toolbox 8.3 (R2009a). Currently, for transfer function (TF) and zero pole gain (ZPK) models, a fast search algorithm is used to determine the crossover frequency, which has a 1e-3 relative accuracy target. This means that the crossover frequency is determined with only 0.1% accuracy, which in turn means that the gain at this frequency won't be exactly -3dB. For example, for the same fifth order system 'sysd' (defined in the Summary section above), the relative accuracy can be computed as follows:

rel_acc = abs(1-A/A1);

The relative accuracy is equal to 8.0727e-004, which is less than 0.1%.

As a workaround, if higher accuracy is desired, you can modify the "rtol" parameter in the BANDWIDTH function as follows.

NOTE: Making the modification mentioned in the workaround below may affect the behavior of the BANDWIDTH function itself, and any built-in functions or custom functions that make use of it. Please make sure that you retain a copy of the file being modified before you modify it so that you may be able to revert to it when necessary.

1. Execute the following command to open the BANDWIDTH function for LTI models:

edit(fullfile(matlabroot, '\toolbox\control\control\+ltipack\@ltidata\bandwidth.m'))

where 'matlabroot' is to be replaced by the path returned by executing the command 'matlabroot' at the MATLAB prompt.

2. Navigate to line 8 in the file, and set the parameter "rtol" to a value such as 1e-6, and then save it.

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