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	$$\begin{array}{r}E\frac{dx}{dt}=Ax(t)+Bu(t)\\ y(t)=Cx(t)+Du(t)\end{array}$$	sparss
Discrete-time sparse first-order model	$$\begin{array}{r}Ex\left[k+1\right]=Ax\left[k\right]+Bu\left[k\right]\\ y\left[k\right]=Cx\left[k\right]+Du\left[k\right]\end{array}$$	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.

For examples, see the sparss and mechss reference pages.

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 N₁ of substructure S₁ and DOFs N₂ of substructure S₂, these include:

• Displacement matching: q(N₁) = q(N₂)

• Action/reaction principle: g(N₁) + g(N₂) = 0 where g(N₁) are the forces exerted by S₂ on S₁, and g(N₂) the forces exerted by S₁ on S₂.

These constraints can be summarized by the equations:

$$\begin{array}{l}M\ddot{q}+C\dot{q}+Kq=Bu+g,Hq=0,g=-H^T\lambda ,\\ y=Fq+G\dot{q}+Du,\end{array}$$

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 object

• Combining sparse models with any non-FRD model like tf, ss, and zpk yields a sparse model object

• Combining 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.

You can convert between continuous-time and discrete-time, and resample mechss models using c2d, d2c and d2d with the 'tustin' method. The 'tustin' method computes the second-order form of the Tustin discretization. This is equivalent to applying Tustin to the first-order sparss equivalent of the mechss model. This form is more favorable to modal approximation since it preserves symmetry. (since R2024a)

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:

• Low-frequency gain computation using dcgain

• Frequency-response evaluation using evalfr and freqresp

• Inverting models using inv

• Implicit-explicit relation conversions using imp2exp

• Padé approximations using pade

• Input, output and internal delay specification

• I/O selection and indexing

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.