Task-Based Model Order Reduction Workflow

To perform model order reduction of dense and sparse linear time-invariant (LTI) models using custom reduction criteria, follow these steps:

Create Specification

Create a model order reduction specification object using the reducespec function. The specification object type depends on the model type and the selected algorithm. Currently, the software supports model order reduction for LTI (see Numeric Linear Time Invariant (LTI) Models) and sparse LTI (see sparss and mechss) models.

This table shows the objects you can create.

Model Type	Algorithm	Syntax	Object	Object Functions
Dense LTI	Balanced truncation	R = reducespec(sys,"balanced")	`BalancedTruncation`	`process` `view (balanced)` `getrom (balanced)`
	Balanced truncation of normalized coprime factors (NCF)	R = reducespec(sys,"ncf")	`NCFBalancedTruncation`	`process` `view (ncf)` `getrom (ncf)`
	Modal truncation	R = reducespec(sys,"modal")	`ModalTruncation`	`process` `view (modal)` `getrom (modal)`
Sparse LTI	Balanced truncation	R = reducespec(sysSparse,"balanced")	`SparseBalancedTruncation`	`process` `view (balanced)` `getrom (balanced)`
Sparse LTI	Modal truncation	R = reducespec(sysSparse,"modal")	`SparseModalTruncation`	`process` `view (modal)` `getrom (modal)`

Additionally:

Each object has its own set of properties and an Options property.
All properties of R except Options are read only (derived data). Use dot notation to configure options for R.
```
R = reducespec(sys,"balanced");
R.Options.Goal = "relative";
```
The reducespec function creates only the model order reduction specification object and does not perform any computation. Therefore, all the derived properties are set to [] in this step.

This step allows you to properly configure options before you perform the next steps, which can be computationally expensive, especially in the case of sparse models.

Run Model Reduction Algorithm

Run the model order reduction algorithm using the process function.

This step analyzes the model and computes the derived information you require to generate reduced-order models (ROM), such as the Hankel singular values (balanced truncation) or modal components (modal truncation). Use the following syntax for this function.

R = process(R)

This command populates the properties of the specification object R created in the previous step.

Using this function is optional in the workflow, and is only necessary when you want to:

Compute or update the derived data in R to see the values.
Avoid recomputing the data multiple times. This is critical for sparse models because the algorithm can be computationally expensive. For example, if your application calls view once and getrom twice, then the computation happens three times in the workflow. To avoid this, use process to compute the data once, and then call view and getrom.

Visualize Contributions

To help you decide the order of the reduced model, plot the graphical information using the view function.

For example, you can:

View the Hankel singular value plot and discard the states with negligible contributions (balanced truncation).
Discard the modes based on their locations or DC contributions (modal truncation).

This table lists all the plot types you can visualize based on the model order reduction task.

MOR Algorithm	Plot Types (`type`)	Plot Description
Balanced truncation Balanced truncation of normalized coprime factors	`"sigma"` (default)	Bar chart of Hankel singular values and associated error bound
	`"energy"`	Bar chart of normalized energies
Modal truncation	`"mode"` (default)	Mode locations
	`"damp"`	Mode damping and natural frequencies
	`"contrib"`	Bar chart of normalized DC contributions

Additionally, you can use the command view(R,'-help') to show the help specific to the model order specification object R at the command line. The returned help shows plot types and syntaxes applicable to R.

Obtain Reduced-Order Model

Obtain the reduced-order model using the getrom function. This is based on either an explicit order selection or a combination of criteria you have identified based on the visual analysis in the previous step.

For balanced truncation methods, you can specify custom criteria as one of these.
- Explicitly specify model order using the Order input argument. For example, this command returns a model of order 10.
  rsys = getrom(R,Order=10);
- Retain states based on maximum error or minimum energy using MaxError and MinEnergy input arguments.
  rsys1 = getrom(R,MaxError=0.5); rsys2 = getrom(R,MinEnergy=0.1);
  The first command returns the lowest-order model rsys1 for which the approximation error does not exceed 0.5. The second command discards all states with normalized energy below 0.1. You can also choose between the two state reduction methods.
For modal truncation methods, you can specify custom criteria as one of these.
- Select the desired modes using a logical or index vector. The function selects the modes corresponding to logical ones or indices from the mode vector R.Mode.
  rsys = getrom(R,Selection=[1 5 8 10 20 31]);
- Use one or more mode selection criteria based on frequency range, damping range, or minimum DC contribution of the modes.
  rsys = getrom(R,Damping=[0 0.5]); rsys = getrom(R,Frequency=[0 10],MinDC=0.1);
  For example, the last command includes all modal components with natural frequencies in [0,10] rad/s and a DC contribution of at least 10%.

You can also choose between the two state reduction methods.

The "matchDC" method preserves the DC gain (steady-state value of step response) and tends to match the time response better.
The "truncate" method tends to match the frequency response better. Use this method when you want to control the relative error.

rsys = getrom(R,MaxError=0.5,Method="truncate");

Additionally, you can use the command getrom(R,'-help') to show the help specific to the model order specification object R at the command line. The returned help shows the name-value arguments and syntaxes applicable to R.