## Sparse Model Basics

### Model Objects

Use the `sparss`

and `mechss`

objects to represent sparse first-order and second-order
systems, respectively. Such sparse models arise from finite element analysis (FEA)
and are useful in fields like structural analysis, fluid flow, heat transfer and
electromagnetics. FEA involves analyzing a problem using finite element method (FEM)
where a large system is subdivided into numerous, smaller components or finite
elements (FE) which are then analyzed separately. These numerous components when
combined together result in large sparse models which are computationally expensive
and inefficient to be represented by traditional dense model objects like `ss`

.

Sparse matrices provide efficient storage of double or logical data that has a large percentage of zeros. While full (or dense) matrices store every single element in memory regardless of value, sparse matrices store only the nonzero elements and their row indices. For this reason, using sparse matrices can significantly reduce the amount of memory required for data storage. For more information, see Computational Advantages of Sparse Matrices.

The following table illustrates the types of sparse models that can be represented:

Model Type | Mathematical Representation | Model Object |
---|---|---|

Continuous-time sparse first-order model |
$$\begin{array}{r}E\frac{dx}{dt}=A\text{}x(t)+B\text{}u(t)\\ y(t)=C\text{}x(t)+D\text{}u(t)\end{array}$$ | `sparss` |

Discrete-time sparse first-order model |
$$\begin{array}{r}E\text{}x\left[k+1\right]=A\text{}x\left[k\right]+B\text{}u\left[k\right]\\ y\left[k\right]=C\text{}x\left[k\right]+D\text{}u\left[k\right]\end{array}$$ | `sparss` |

Continuous-time sparse second-order model |
$$\begin{array}{r}\text{M}\ddot{q}(t)+\text{C}\dot{q}(t)+\text{K}q(t)\text{=B}u(t)\\ y(t)\text{=F}q(t)+\text{G}\dot{q}(t)+\text{D}u(t)\end{array}$$ | `mechss` |

Discrete-time sparse second-order model |
$$\begin{array}{r}\text{M}q\left[k+2\right]+\text{C}q\left[k+1\right]+\text{K}q\left[k\right]\text{=B}u\left[k\right]\\ y\left[k\right]\text{=F}q\left[k\right]+\text{G}q\left[k+1\right]+\text{D}u\left[k\right]\end{array}$$ | `mechss` |

You can use `sparssdata`

and `mechssdata`

to access the model matrices in sparse form. You can also
use the `spy`

command to visualize the sparsity of both first-order and second-order model
matrices.

You can also convert any non-FRD model into a `sparss`

or a `mechss`

object respectively. Conversely, you can use the `full`

command to convert sparse models to dense storage `ss`

. Converting to dense storage is
not recommended for large sparse models as it result in high memory usage and poor
performance.

### Combining Sparse Models

#### Signal-Based Connections

All standard signal-based connections listed under Model Interconnection are supported for sparse model objects. Interconnecting models using signals allows you to construct models for control systems. You can conceptualize your control system as a block diagram containing multiple interconnected components, such as a plant and a controller connected in a feedback configuration. Using model arithmetic or interconnection commands, you combine models of each of these components into a single model representing the entire block diagram.

In series and feedback connections of `sparss`

models, eliminating the internal signals can lead to
undesirable fill-in in the `A`

matrix. To prevent this, the
software eliminates only those signals that do not affect the sparsity of
`A`

and adds the remaining signals to the state vector. In
general, this produces a differential algebraic equation (DAE) model of the
interconnection where the `A`

matrix has a block arrow
structure as depicted in the following figure:

Here, each diagonal block is a sub-component of the sparse model. The last row
and column combines the `Interface`

and
`Signal`

groups to capture all couplings and connections
between components.

The same rules apply to second-order `mechss`

model objects. You can use `showStateInfo`

to print a summary of the state vector partition
into components, interfaces, and signals

Use the `xsort`

command to sort states based on state partition. In the
sorted model, all components appear first, followed by the interfaces, and then
followed by a single group of all internal signals.

#### Physical Coupling

You can use the `interface`

command to specify physical couplings between the
components of a `mechss`

model. `interface`

uses *dual assembly* formulation
where the global set of degrees of freedom (DoFs) is retained and an assembly is
made by describing coupling constraints at the interface. For rigid couplings
between DOFs *N _{1}* of substructure

*S*and DOFs

_{1}*N*of substructure

_{2}*S*, these include:

_{2}Displacement matching:

*q(N*_{1}) = q(N_{2})Action/reaction principle:

*g(N*where_{1}) + g(N_{2}) = 0*g(N*are the forces exerted by_{1})*S*on_{2}*S*, and_{1}*g(N*the forces exerted by_{2})*S*on_{1}*S*._{2}

These constraints can be summarized by the equations:

$$\begin{array}{l}M\text{}\ddot{q}+C\text{}\dot{q}+K\text{}q=B\text{}u+g,\text{}H\text{}q=0,\text{}g=-{H}^{T}\lambda ,\\ y=Fq+G\dot{q}+Du,\end{array}$$

where *g* is the vector of the interface
forces and *H* is a suitable DOF-selecting matrix.

You can also specify non-rigid couplings between the DOFs using the `interface`

command.

For more information, see `interface`

and Rigid Assembly of Model Components.

#### Combining Models of Different Types

The following precedence rules apply when combining models of different types:

Combining sparse models with FRD models yields an

`frd`

model objectCombining sparse models with any non-FRD model like

`tf`

,`ss`

, and`zpk`

yields a sparse model objectCombining

`sparss`

and`mechss`

models yields a`mechss`

model object.Currently, sparse models cannot be combined with generalized or uncertain models,

`genss`

and`uss`

(Robust Control Toolbox).

For examples, see the `mechss`

reference page.

### Time-Domain Analysis

All standard time-domain analysis functions listed under Time and Frequency Domain Analysis are supported for `sparss`

and `mechss`

model objects.

You must specify a final time or time vector when using time-domain response functions for sparse models. For example:

tf = 10; step(sys,tf) t = 0:0.001:1; initial(sys,x0,t)

The time response functions rely on custom fixed-step DAE solvers that have been
developed especially for large-scale sparse models. You can configure the DAE solver
type and enable parallel computing by using the `SolverOptions`

property of the `sparss`

and `mechss`

model objects. Parallel computing can be used to accelerate
`step`

or `impulse`

response simulation for
multi-input models as the response for each input channel is simulated in parallel.
Enabling parallel computing requires a Parallel Computing Toolbox™ license.

The available solver options are outlined in the table below:

DAE Solver | Description | Usage |
---|---|---|

`'trbdf2'` [2] | Fixed step solver with accuracy of `o(h^2)` ,
where `h` is the step size.
`'trbdf2'` is 50% more computationally
expensive than `'hht'` . This is the default DAE
solver option for both `sparss` and `mechss` models. | Available for both `sparss` and `mechss` models. |

`'trbdf3'` | Fixed step solver with accuracy of `o(h^3)` .
`'trbdf3'` is 50% more computationally
expensive than `'trbdf2'` . | Available for both `sparss` and `mechss` models. |

`'hht'` [1] | Fixed step solver with accuracy of `o(h^2)` ,
where `h` is the step size.
`'hht'` is the fastest but can run into
difficulties with high initial acceleration like the impulse
response of a system. | Available for `mechss` models only. |

To enable parallel computing and solver selection, use the following syntax:

sys.SolverOptions.UseParallel = true; sys.SolverOptions.DAESolver = 'trbdf3';

For an example, see Linear Analysis of Cantilever Beam.

### Frequency-Domain Analysis

All standard frequency-domain analysis functions listed under Time and Frequency Domain Analysis are supported for `sparss`

and `mechss`

model objects. For frequency response computations, enabling
parallel computing speeds the response computation by distributing the set of
frequencies across available workers. Enabling parallel computing requires a
Parallel Computing Toolbox license.

You must specify a frequency vector when using frequency response functions for sparse models. For example:

w = logspace(0,8,1000); bode(sys,w) sigma(sys,w)

For an example, see Linear Analysis of Cantilever Beam.

### Continuous and Discrete Conversions

You can convert between continuous-time and discrete-time, and resample `sparss`

models using `c2d`

, `d2c`

and `d2d`

. The following table outlines
the available methods for `sparss`

models:

Method | Description | Usage |
---|---|---|

`'tustin'` | Bilinear Tustin approximation | Available with `c2d` , `d2c` and
`d2d`
functions. |

`'damped'` | Damped Tustin approximation based on
TRBDF2[2] formula. This method provides damping at infinity in contrast
to the `'tustin'` method, that is, the high
frequency dynamics are filtered out and do not contribute to an
accumulation of modes near `z = -1` in the
discrete model (a source of numerical instability). | Available with `c2d`
only. |

**Note**

Currently, `mechss`

models are not supported for continuous and discrete
conversions. Convert to `sparss`

model form for discretization.

### Sparse Linearization

#### Linearize Simulink Model

You can obtain a sparse linearized model from a Simulink^{®} model when a Descriptor
State-Space (Simulink) or Sparse
Second Order block is present.

The Sparse Second Order block is configured to always linearize to a

`mechss`

model. As a result, the overall linearized model is a second-order sparse model when this block is present.Check the

**Linearize to sparse model**option in the Descriptor State-Space (Simulink) block to linearize to a`sparss`

model. You can also achieve this by setting the`LinearizerToSparse`

parameter to`'on'`

in the Descriptor State-Space (Simulink). By default, the**Linearize to sparse model**box is unchecked and the`LinearizerToSparse`

is`'off'`

. The block-level`LinearizeToSparse`

setting is ignored when you specify a replacement for the block, either with the`SCDBlockLinearizationSpecification`

block parameter, or the`blocksub`

input to`linearize`

(Simulink Control Design).

This linearization workflow requires Simulink Control Design™ software.

For an example, see Linearize Simulink Model to a Sparse Second-Order Model Object.

#### Linearize Structural or Thermal PDE Model

You can obtain a sparse linearized model from a structural or a thermal PDE
model by using the `linearize`

(Partial Differential Equation Toolbox) function.

For a structural analysis model,

`linearize`

extracts a`mechss`

model.For a thermal analysis model,

`linearize`

extracts a`sparss`

model.

Use `linearizeInput`

(Partial Differential Equation Toolbox) to specify the inputs of the linearized model in
terms of boundary conditions, loads, or internal heat sources in the PDE model.
Use `linearizeOutput`

(Partial Differential Equation Toolbox) to specify the outputs of the linearized model
in terms of regions of the geometry, such as cells (for 3-D geometries only),
faces, edges, or vertices.

This linearization workflow requires Partial Differential Equation Toolbox™ software.

For examples, see Linear Analysis of Cantilever Beam and Linear Analysis of Tuning Fork.

### Model Order Reduction

*Since R2023b*

To compute low-order approximations of sparse state-space models, use the
`reducespec`

function. `reducespec`

is the entry
point for model order reduction workflows in Control System Toolbox™ software. For the full workflow, see Task-Based Model Order Reduction Workflow.

The software supports sparse model order reduction using these methods.

Balanced truncation — Obtain low-order approximation by discarding states with low contribution.

Modal truncation — Obtain low-order approximation by discarding undesired modes.

For examples, see `SparseBalancedTruncation`

, `SparseModalTruncation`

, and Sparse Modal Truncation of Linearized Structural Beam Model.

### Other Supported Functionality

Additionally, the following functionality is currently supported for sparse models:

### Limitations

The following operations are currently not supported for sparse models:

Pole/zero and stability margin computation

Compensator design and tuning

You can use the `full`

command to convert small sparse models to dense storage `ss`

to perform the above
operations. Converting to dense storage is not recommended for large sparse models
as it may saturate available memory and cause severe performance degradation.

The following limitations exist for sparse linearization:

When you set the

`linearize`

(Simulink Control Design) option`BlockReduction`

to`'off'`

, the model is always linearized to a dense`ss`

model. This is because block substitution is disabled in this case.Sparse linearization is incompatible with block substitutions involving tunable or uncertain models. The

**Linearize to sparse model**option should be unchecked when trying to linearize to`genss`

or`uss`

(Robust Control Toolbox) models.Snapshot linearization may not work when the Descriptor State-Space (Simulink) or Sparse Second Order block is present. Since snapshot linearization requires simulation, the simulation capabilities are currently limited for the Descriptor State-Space (Simulink) and Sparse Second Order blocks.

## References

[1] H. Hilber, T. Hughes &
R. Taylor. " Improved numerical dissipation for time integration algorithms in
structural dynamics." *Earthquake Engineering and Structural
Dynamics*, vol. 5, no. 3, pp. 283-292, 1977.

[2] M. Hosea and L. Shampine.
"Analysis and implementation of TR-BDF2." *Applied Numerical
Mathematics*, vol. 20, no. 1-2, pp. 21-37, 1996.

## See Also

`sparss`

| `mechss`

| `showStateInfo`

| `getx0`

| `full`

| `sparssdata`

| `mechssdata`

| `xsort`

| `interface`

| `spy`

| Descriptor
State-Space (Simulink) | Sparse Second
Order