Linear approximation of Simulink model or subsystem
specifies the order of the states in the linearized model for any of the
previous syntaxes.linsys
= linearize(___,'StateOrder',stateorder
)
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
Specify a linearization input at the output of the PID Controller block, which is the input signal for the Water-Tank System block.
io(1) = linio('watertank/PID Controller',1,'input');
Specify a linearization output point at the output of the Water-Tank System block. Specifying the output point as open-loop removes the effects of the feedback signal on the linearization without changing the model operating point.
io(2) = linio('watertank/Water-Tank System',1,'openoutput');
Linearize the model using the specified I/O set.
linsys = linearize(mdl,io);
linsys
is the linear approximation of the plant at the model operating point.
Open the Simulink model.
mdl = 'magball';
open_system(mdl)
Find a steady-state operating point at which the ball height is 0.05
. Create a default operating point specification, and set the height state to a known value.
opspec = operspec(mdl); opspec.States(5).Known = 1; opspec.States(5).x = 0.05;
Trim the model to find the operating point.
options = findopOptions('DisplayReport','off'); op = findop(mdl,opspec,options);
Specify linearization input and output signals to compute the closed-loop transfer function.
io(1) = linio('magball/Desired Height',1,'input'); io(2) = linio('magball/Magnetic Ball Plant',1,'output');
Linearize the model at the specified operating point using the specified I/O set.
linsys = linearize(mdl,io,op);
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
To compute the closed-loop transfer function, first specify the linearization input and output signals.
io(1) = linio('watertank/PID Controller',1,'input'); io(2) = linio('watertank/Water-Tank System',1,'output');
Simulate sys
for 10
seconds and linearize the model.
linsys = linearize(mdl,io,10);
Open the Simulink model.
mdl = 'scdcascade';
open_system(mdl)
Specify parameter variations for the outer-loop controller gains, Kp1
and Ki1
. Create parameter grids for each gain value.
Kp1_range = linspace(Kp1*0.8,Kp1*1.2,6); Ki1_range = linspace(Ki1*0.8,Ki1*1.2,4); [Kp1_grid,Ki1_grid] = ndgrid(Kp1_range,Ki1_range);
Create a parameter value structure with fields Name
and Value
.
params(1).Name = 'Kp1'; params(1).Value = Kp1_grid; params(2).Name = 'Ki1'; params(2).Value = Ki1_grid;
params
is a 6-by-4 parameter value grid, where each grid point corresponds to a unique combination of Kp1
and Ki1
values.
Define linearization input and output points for computing the closed-loop response of the system.
io(1) = linio('scdcascade/setpoint',1,'input'); io(2) = linio('scdcascade/Sum',1,'output');
Linearize the model at the model operating point using the specified parameter values.
linsys = linearize(mdl,io,params);
Open the Simulink model.
mdl = 'scdpwm';
open_system(mdl)
Extract linearization input and output from the model.
io = getlinio(mdl);
Linearize the model at the model operating point.
linsys = linearize(mdl,io)
linsys = D = Step Plant Model 0 Static gain.
The discontinuities in the Voltage to PWM block cause the model to linearize to zero. To treat this block as a unit gain during linearization, specify a substitute linearization for this block.
blocksub.Name = 'scdpwm/Voltage to PWM';
blocksub.Value = 1;
Linearize the model using the specified block substitution.
linsys = linearize(mdl,blocksub,io)
linsys = A = State Space( State Space( State Space( 0.9999 -0.0001 State Space( 0.0001 1 B = Step State Space( 0.0001 State Space( 5e-09 C = State Space( State Space( Plant Model 0 1 D = Step Plant Model 0 Sample time: 0.0001 seconds Discrete-time state-space model.
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
To linearize the Water-Tank System block, specify a linearization input and output.
io(1) = linio('watertank/PID Controller',1,'input'); io(2) = linio('watertank/Water-Tank System',1,'openoutput');
Create a linearization option set, and specify the sample time for the linearized model.
options = linearizeOptions('SampleTime',0.1);
Linearize the plant using the specified options.
linsys = linearize(mdl,io,options)
linsys = A = H H 0.995 B = PID Controll H 0.02494 C = H Water-Tank S 1 D = PID Controll Water-Tank S 0 Sample time: 0.1 seconds Discrete-time state-space model.
The linearized plant is a discrete-time state-space model with a sample time of 0.1
.
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
Specify the full block path for the block you want to linearize.
blockpath = 'watertank/Water-Tank System';
Linearize the specified block at the model operating point.
linsys = linearize(mdl,blockpath);
Open Simulink model.
mdl = 'magball';
open_system(mdl)
Find a steady-state operating point at which the ball height is 0.05
. Create a default operating point specification, and set the height state to a known value.
opspec = operspec(mdl); opspec.States(5).Known = 1; opspec.States(5).x = 0.05;
options = findopOptions('DisplayReport','off'); op = findop(mdl,opspec,options);
Specify the block path for the block you want to linearize.
blockpath = 'magball/Magnetic Ball Plant';
Linearize the specified block at the specified operating point.
linsys = linearize(mdl,blockpath,op);
Open the Simulink model.
mdl = 'magball';
open_system(mdl)
Linearize the plant at the model operating point.
blockpath = 'magball/Magnetic Ball Plant';
linsys = linearize(mdl,blockpath);
View the default state order for the linearized plant.
linsys.StateName
ans = 3x1 cell array {'height' } {'Current'} {'dhdt' }
Linearize the plant and reorder the states in the linearized model. Set the rate of change of the height as the second state.
stateorder = {'magball/Magnetic Ball Plant/height';... 'magball/Magnetic Ball Plant/dhdt';... 'magball/Magnetic Ball Plant/Current'}; linsys = linearize(mdl,blockpath,'StateOrder',stateorder);
View the new state order.
linsys.StateName
ans = 3x1 cell array {'height' } {'dhdt' } {'Current'}
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
To compute the closed-loop transfer function, first specify the linearization input and output signals.
io(1) = linio('watertank/PID Controller',1,'input'); io(2) = linio('watertank/Water-Tank System',1,'output');
Simulate sys
and linearize the model at 0
and 10
seconds. Return the operating points that correspond to these snapshot times; that is, the operating points at which the model was linearized.
[linsys,linop] = linearize(mdl,io,[0,10]);
Open the Simulink model.
mdl = 'watertank';
open_system(mdl)
Vary parameters A
and b
within 10% of their nominal values.
[A_grid,b_grid] = ndgrid(linspace(0.9*A,1.1*A,3),...
linspace(0.9*b,1.1*b,4));
Create a parameter structure array, specifying the name and grid points for each parameter.
params(1).Name = 'A'; params(1).Value = A_grid; params(2).Name = 'b'; params(2).Value = b_grid;
Create a default operating point specification for the model.
opspec = operspec(mdl);
Trim the model using the specified operating point specification, parameter grid. Suppress the display of the operating point search report.
opt = findopOptions('DisplayReport','off'); [op,opreport] = findop(mdl,opspec,params,opt);
op
is a 3-by-4 array of operating point objects that correspond to the specified parameter grid points.
Specify the block path for the plant model.
blockpath = 'watertank/Desired Water Level';
To store offsets during linearization, create a linearization option set and set StoreOffsets
to true
.
options = linearizeOptions('StoreOffsets',true);
Batch linearize the plant at the trimmed operating points, using the specified I/O points and parameter variations.
[linsys,linop,info] = linearize(mdl,blockpath,op,params,options);
You can use the offsets in info.Offsets
when configuring an LPV System block.
info.Offsets
ans = 3x4 struct array with fields: x dx u y StateName InputName OutputName Ts
mdl
— Simulink model nameSimulink model name, specified as a character vector or string. The model must be in the current working folder or on the MATLAB path.
io
— Analysis point setAnalysis point set that contains inputs, outputs, and openings,
specified as a linearization I/O object or a vector of linearization
I/O objects. To create io
:
Each linearization I/O object in io
must correspond to the Simulink model mdl
or some normal mode model
reference in the model hierarchy.
If you omit io
, then linearize
uses
the root level inports and outports of the model as analysis points.
For more information on specifying linearization inputs, outputs, and openings, see Specify Portion of Model to Linearize.
op
— Operating pointOperating point for linearization, specified as one of the following:
Operating point object, created using:
Array of operating point objects, specifying multiple operating points. To create an array of operating point objects, you can:
Extract operating points at multiple snapshot times
using findop
.
Batch trim your model using multiple operating point specifications. For more information, see Batch Compute Steady-State Operating Points for Multiple Specifications.
Batch trim your model using parameter variations. For more information, see Batch Compute Steady-State Operating Points for Parameter Variation.
Vector of positive scalars representing one or more simulation
snapshot times. The software simulates sys
and
linearizes the model at the specified snapshot times.
If you also specify parameter variations using
param
, the software simulates the model for
each snapshot time and parameter grid point combination. This
operation can be computationally expensive.
If you specify parameter variations using param
,
and the parameters:
Affect the model operating point, then specify op
as
an array of operating points with the same dimensions as the parameter
value grid. To obtain the operating points that correspond to the
parameter value combinations, batch trim your model using param
before
linearization. For more information, see Batch Linearize Model at Multiple Operating Points Derived from Parameter Variations.
Do not affect the model operating point, then specify op
as
a single operating point.
blockpath
— Block or subsystemBlock or subsystem to linearize, specified as a character vector or string that contains its full block path.
The software treats the inports and outports of the specified block as open-loop inputs and outputs, which isolates the block from the rest of the model before linearization.
blocksub
— Substitute linearizations for blocks and subsystemsSubstitute linearizations for blocks and subsystems, specified
as a structure or an n-by-1 structure array, where n is
the number of blocks for which you want to specify a linearization.
Use blocksub
to specify a custom linearization
for a block or subsystem. For example, you can specify linearizations
for blocks that do not have analytic linearizations, such as blocks
with discontinuities or triggered subsystems.
To study the effects of varying the linearization of a block on the model dynamics, you can batch linearize your model by specifying multiple substitute linearizations for a block.
Each substitute linearization structure has the following fields:
Name
— Block pathBlock path of the block for which you want to specify the linearization, specified as a character vector or string.
Value
— Substitute linearizationSubstitute linearization for the block, specified as one of the following:
Double — Specify the linearization of a SISO block as a gain.
Array of doubles — Specify the linearization of a MIMO block as an nu-by-ny array of gain values, where nu is the number of inputs and ny is the number of outputs.
LTI model, uncertain state-space model, or uncertain real object — The I/O configuration of
the specified model must match the configuration of the block
specified by Name
. Using an uncertain model
requires Robust Control Toolbox™ software.
Array of LTI models, uncertain state-space models, or uncertain real objects — Batch linearize the model using multiple block substitutions. The I/O configuration of each model in the array must match the configuration of the block for which you are specifying a custom linearization. If you:
Vary model parameters using param
and
specify Value
as a model array, the dimensions
of Value
must match the parameter grid size.
Specify op
as an array of operating
points and Value
as a model array, the dimensions
of Value
must match the size of op
.
Define block substitutions for multiple blocks, and
specify Value
as an array of LTI models for one
or more of these blocks, the dimensions of the arrays must match.
Structure with the following fields:
Field | Description |
---|---|
Specification | Block linearization, specified as a character vector that contains one of the following:
The specified expression or function must return one of the following:
The I/O configuration of the returned model must
match the configuration of the block specified by |
Type | Specification type, specified as one of the following:
|
ParameterNames | Linearization function parameter names, specified as
a cell array of character vectors. Specify You must also specify
the corresponding |
ParameterValues | Linearization function parameter values, specified as
a vector of doubles. The order of parameter values must correspond
to the order of parameter names in |
param
— Parameter samplesParameter samples for linearization, specified as one of the following:
Structure — Vary the value of a single parameter by specifying param
as a
structure with the following fields:
Name
— Parameter name, specified as a character vector or string. You can
specify any model parameter that is a variable in
the model workspace, the MATLAB workspace, or a data dictionary. If
the variable used by the model is not a scalar
variable, specify the parameter name as an
expression that resolves to a numeric scalar
value. For example, to use the first element of
vector V
as a parameter,
use:
param.Name = 'V(1)';
Value
— Parameter sample values, specified as a double array.
For example, vary the value of parameter A
in
the 10% range:
param.Name = 'A';
param.Value = linspace(0.9*A,1.1*A,3);
Structure array — Vary the value of multiple parameters. For example, vary the values of
parameters A
and
b
in the 10% range:
[A_grid,b_grid] = ndgrid(linspace(0.9*A,1.1*A,3),... linspace(0.9*b,1.1*b,3)); params(1).Name = 'A'; params(1).Value = A_grid; params(2).Name = 'b'; params(2).Value = b_grid;
For more information, see Specify Parameter Samples for Batch Linearization.
If param
specifies tunable parameters only,
the software batch linearizes the model using a single model compilation.
To compute the offsets required by the LPV
System block, specify param
, and set options.StoreOffsets
to true
.
You can then return additional linearization information in info
,
and extract the offsets using getOffsetsForLPV
.
stateorder
— State order in linearization resultsState order in linearization results, specified as a cell array of block paths or state names.
The order of the block paths and states in stateorder
indicates the order of the states in linsys
.
You can specify block paths for any blocks in mdl
that have states, or
any named states in mdl
.
You do not have to specify every block and state from mdl
in
stateorder
. The states you specify appear first in
linsys
, followed by the remaining states in their
default order.
options
— Linearization algorithm optionslinearizeOptions
option setLinearization algorithm options, specified as a linearizeOptions
option set.
linsys
— Linearization resultLinearization result, returned as a state-space model or an array of
state-space models. The dimensions of linsys
depend on
the specified parameter variations and block substitutions, and the
operating points at which you linearize the model.
Note
If you specify more than one of op
,
param
, or blocksub.Value
as
an array, then their dimensions must match.
Parameter Variation | Block Substitution | Linearize At... | Resulting linsys
Dimensions |
---|---|---|---|
No parameter variation | No block substitution | Model operating point | Single state-space model |
Single operating point, specified as an operating point
object or snapshot time using
op | |||
N1-by-... -by-Nm
array of operating point objects, specified by
op | N1-by-... -by-Nm | ||
Ns snapshots,
specified as a vector of snapshot times using
op | Column vector of length Ns | ||
N1-by-... -by-Nm
model array for at least one block, specified by
blocksub.Value | Model operating point | N1-by-... -by-Nm | |
Single operating point, specified as an operating point
object or snapshot time using
op | |||
N1-by-... -by-Nm
array of operating points, specified as an array of
operating point objects using
op | |||
Ns snapshots,
specified as a vector of snapshot times using
op | Ns-by-N1-by-... -by-Nm | ||
N1-by-... -by-Nm
parameter grid, specified by
param | Either no block substitution or an
N1-by-... -by-Nm
model array for at least one block, specified by
blocksub.Value | Model operating point | N1-by-... -by-Nm |
Single operating point, specified as an operating point
object or snapshot time using
op | |||
N1-by-... -by-Nm
array of operating point objects, specified by
op | |||
Ns snapshots,
specified as a vector of snapshot times using
op | Ns-by-N1-by-... -by-Nm |
For example, suppose:
op
is a 4-by-3 array of operating point
objects and you do not specify parameter variations or block
substitutions. In this case, linsys
is a 4-by-3
model array.
op
is a single operating point object and
param
specifies a 3-by-4-by-2 parameter
grid. In this case, linsys
is a 3-by-4-by-2
model array.
op
is a row vector of positive scalars with
two elements and you do not specify param
. In
this case, linsys
is a column vector with two
elements.
op
is a column vector of positive scalars
with three elements and param
specifies a
5-by-6 parameter grid. In this case, linsys
is
a 3-by-5-by-6 model array.
op
is a single operating point object, you do
not specify parameter variations, and
blocksub.Value
is a 2-by-3 model array for
one block in the model. In this case, linsys
is
a 2-by-3 model array.
op
is a column vector of positive scalars
with four elements, you do not specify parameter variations, and
blocksub.Value
is a 1-by-2 model array for
one block in the model. In this case, linsys
is
a 4-by-1-by-2 model array.
For more information on model arrays, see Model Arrays.
linop
— Operating pointOperating point at which the model was linearized, returned as an operating point object or an
array of operating point objects with the same dimensions as
linsys
. Each element of linop
is the operating point at which the corresponding
linsys
model was obtained.
If you specify op
as a single operating point object or an array of
operating point objects, then linop
is a copy of
op
. If you specify op
as a
single operating point object and also specify parameter variations using
param
, then linop
is an array
with the same dimensions as the parameter grid. In this case, the elements
of linop
are scalar expanded copies of
op
.
To determine whether the model was linearized at a reasonable
operating point, view the states and inputs in linop
.
info
— Linearization informationLinearization information, returned as a structure with the following fields:
Offsets
— Linearization offsets[]
(default) | structure | structure arrayLinearization offsets that correspond to the operating point
at which the model was linearized, returned as []
if options.StoreOffsets
is false
.
Otherwise, Offsets
is returned as one of the following:
If linsys
is a single state-space model, then Offsets
is a structure.
If linsys
is an array of state-space models, then
Offsets
is a structure array with
the same dimensions as
linsys
.
Each offset structure has the following fields:
Field | Description |
---|---|
x | State offsets used for linearization, returned as a column vector of length
nx, where
nx is the number of states in
linsys . |
y | Output offsets used for linearization, returned as a column vector of length
ny, where
ny is the number of outputs in
linsys . |
u | Input offsets used for linearization, returned as a column vector of length
nu, where
nu is the number of inputs in
linsys . |
dx | Derivative offsets for continuous time systems or updated state values for discrete-time systems, returned as a column vector of length nx. |
StateName | State names, returned as a cell array that contains
nx elements that match the names
in linsys.StateName . |
InputName | Input names, returned as a cell array that contains
nu elements that match the names
in linsys.InputName . |
OutputName | Output names, returned as a cell array that contains
ny elements that match the names
in linsys.OutputName . |
Ts | Sample time of the linearized system, returned as a scalar that matches the sample time in
linsys.Ts . For continuous-time systems,
Ts is 0 . |
If Offsets
is a structure array, you can
configure an LPV System block using
the offsets. To do so, first convert them to the required format using getOffsetsForLPV
. For an example, see Approximating Nonlinear Behavior Using an Array of LTI Systems.
Advisor
— Linearization diagnostic information[]
(default) | LinearizationAdvisor
object | array of LinearizationAdvisor
objectsLinearization diagnostic information, returned as
[]
if
options.StoreAdvisor
is
false
. Otherwise,
Advisor
is returned as one of the
following:
If linsys
is a single state-space
model, Advisor
is a LinearizationAdvisor
object.
If linsys
is an array of
state-space models, Advisor
is an
array of LinearizationAdvisor
objects with the same
dimensions as linsys
.
LinearizationAdvisor
objects store
linearization diagnostic information for individual linearized
blocks. For an example of troubleshooting linearization results
using a LinearizationAdvisor
object, see Troubleshoot Linearization Results at Command Line.
You can specify a substitute linearization for a block or subsystem in your Simulink model using a custom function on the MATLAB path.
Your custom linearization function must have one BlockData
input
argument, which is a structure that the software creates and passes
to the function. BlockData
has the following fields:
Field | Description | ||||||||
---|---|---|---|---|---|---|---|---|---|
BlockName | Name of the block for which you are specifying a custom linearization. | ||||||||
Parameters | Block parameter values, specified as a structure array with Name and Value fields. Parameters contains
the names and values of the parameters you specify in the blocksub.Value.ParameterNames and blocksub.Value.ParameterValues fields. | ||||||||
Inputs |
Input signals to the block for which you are defining a linearization,
specified as a structure array with one structure for each block input.
Each structure in
| ||||||||
ny | Number of output channels of the block linearization. | ||||||||
nu | Number of input channels of the block linearization. | ||||||||
BlockLinearization | Current default linearization of the block, specified as a
state-space model. You can specify a block linearization that depends
on the default linearization using BlockLinearization . |
Your custom function must return a model with nu
inputs
and ny
outputs. This model must be one of the following:
Linear model in the form of a D-matrix
Control System Toolbox LTI model object
Uncertain state-space model or uncertain real object (requires Robust Control Toolbox software)
For example, the following function multiplies the current default block linearization, by a
delay of Td = 0.5
seconds. The delay is represented by a Thiran filter
with sample time Ts = 0.1
. The delay and sample time are parameters
stored in BlockData
.
function sys = myCustomFunction(BlockData) Td = BlockData.Parameters(1).Value; Ts = BlockData.Parameters(2).Value; sys = BlockData.BlockLinearization*Thiran(Td,Ts); end
Save this function to a location on the MATLAB path.
To use this function as a custom linearization for a block or subsystem, specify the
blocksub.Value.Specification
and
blocksub.Value.Type
fields.
blocksub.Value.Specification = 'myCustomFunction'; blocksub.Value.Type = 'Function';
To set the delay and sample time parameter values, specify the blocksub.Value.ParameterNames
and blocksub.Value.ParameterValues
fields.
blocksub.Value.ParameterNames = {'Td','Ts'}; blocksub.Value.ParameterValues = [0.5 0.1];
By default, linearize
automatically sets
the following Simulink model properties:
BufferReuse = 'off'
RTWInlineParameters = 'on'
BlockReductionOpt = 'off'
SaveFormat = 'StructureWithTime'
After linearization, Simulink restores the original model properties.
Simulink Control Design™ software linearizes models using a block-by-block approach. The software individually linearizes each block in your Simulink model and produces the linearization of the overall system by combining the individual block linearizations.
The software determines the input and state levels for each block from the operating point, and obtains the Jacobian for each block at these levels.
For some blocks, the software cannot compute an analytical linearization in this manner. For example:
Some nonlinearities do not have a defined Jacobian.
Some discrete blocks, such as state charts and triggered subsystems, tend to linearize to zero.
Some blocks do not implement a Jacobian.
Custom blocks, such as S-Function blocks and MATLAB Function blocks, do not have analytical Jacobians.
You can specify a custom linearization for any such blocks for which you know the expected linearization. If you do not specify a custom linearization, the software linearizes the model by perturbing the block inputs and states and measuring the response to these perturbations. For each input and state, the default perturbation level is:
for double-precision values.
for single-precision values.
Here, x is the value of the corresponding input or state at the operating point. For information on how to change perturbation levels for individual blocks, see Change Perturbation Level of Blocks Perturbed During Linearization.
For more information, see Linearize Nonlinear Models and Exact Linearization Algorithm
You can linearize your system using full-model numerical perturbation, where the software
computes the linearization of the full model by perturbing the values of root-level inputs
and states. To do so, create a linearizeOptions
object and set the
LinearizationAlgorithm
property to one of the following:
'numericalpert'
— Perturb the inputs and states using forward
differences; that is, by adding perturbations to the input and state values. This
perturbation method is typically faster than the 'numericalpert2'
method.
'numericalpert2'
— Perturb the inputs and states using central
differences; that is, by perturbing the input and state values in both positive and
negative directions. This perturbation method is typically more accurate than the
'numericalpert'
method.
For each input and state, the software perturbs the model and computes a linear model based on
the model response to these perturbations. You can configure the state and input
perturbation levels using the NumericalPertRel
linearization
options.
Block-by-block linearization has several advantages over full-model numerical perturbation:
Most Simulink blocks have a preprogrammed linearization that provides an exact linearization of the block.
You can use linear analysis points to specify a portion of the model to linearize.
You can configure blocks to use custom linearizations without affecting your model simulation.
Structurally nonminimal states are automatically removed.
You can specify linearizations that include uncertainty (requires Robust Control Toolbox software).
You can obtain detailed diagnostic information.
When linearizing multirate models, you can use different rate conversion methods. Full-model numerical perturbation can only use zero-order-hold rate conversion.
For more information, see Linearize Nonlinear Models and Exact Linearization Algorithm.
As an alternative to the linearize
function,
you can linearize models using one of the following methods:
To interactively linearize models, use the Model Linearizer app. For an example, see Linearize Simulink Model at Model Operating Point.
To obtain multiple transfer functions without modifying
the model or creating an analysis point set for each transfer function,
use an slLinearizer
interface.
For an example, see Vary Parameter Values and Obtain Multiple Transfer Functions.
Although both Simulink
Control Design software and the Simulink linmod
function
perform block-by-block linearization, Simulink
Control Design linearization
functionality has a more flexible user interface and uses Control System Toolbox numerical
algorithms. For more information, see Linearization Using Simulink Control Design Versus Simulink.
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