Documentation Center

  • Trial Software
  • Product Updates

usubs

Substitute given values for uncertain elements of uncertain objects

Syntax

B = usubs(M,ElementName1,value1,ElementName2,value2,...)
B = usubs(M,S)
B = usubs(M,...,'-once')
B = usubs(M,...,'-batch')

Description

Use usubs to substitute a specific value for an uncertain element of an uncertain model object. The value can itself be uncertain. It needs to be the correct size, but otherwise can be of any class, and can be an array. Hence, the result can be of any class. In this manner, uncertain elements act as symbolic placeholders, for which specific values (which can also contain other placeholders too) can be substituted.

B = usubs(M,ElementName1,value1,ElementName2,value2,...) sets the elements in M, identified by ElementName1, ElementName2, etc., to the values in value1, value2, etc. respectively.

Any value can also be the string 'NominalValue' or 'Random' (or only partially specified) in which case the nominal value, or a random instance of the atom is used.

B = usubs(M,S) instantiates the uncertain elements of M to the values specified in the structure S. The field names of S are the names of the uncertain elements to replace. The values are the corresponding replacement values. To provide several replacement values, make S a struct array, where each struct contains one set of replacement values. A structure such as S typically comes from robustness analysis commands such as robuststab, usample, or wcgain.

B = usubs(M,...,'-once') performs vectorized substitution in the uncertain model array M. Each uncertain element is replaced by a single value, but this value may change across the model array. To specify different substitute values for each model in the array M, use:

  • A cell array for each valueN that causes the uncertain element ElementNameN in M(:,:,k) to be replaced by valueN(k). For example, if M is a 2-by-3 array, then a 2-by-3 cell array value1 replaces ElementName1 of the model M(:,:,k) with the corresponding value1(k).

  • A struct array S that specifies one set of substitute values S(k) for each model M(:,:,k).

Numeric array formats are also accepted for value1,value2,.... For example, value1 can be a 2-by-3 array of LTI models, a numeric array of size [size(name1) 2 3], or a 2-by-3 matrix when the uncertain element name1 is scalar-valued. The array sizes of M, S, value1,value2,... must agree along non-singleton dimensions. Scalar expansion takes place along singleton dimensions.

Vectorized substitution ('-once') is the default for model arrays when no substitution method is specified.

B = usubs(M,...,'-batch') performs batch substitution in the uncertain model array M. Each uncertain element is replaced by an array of values, and the same values are used for all models in M. In batch substitution, B is a model array of size [size(M) VS], where VS is the size of the array of substitute values.

Examples

expand all

Evaluate Uncertain Matrix for Multiple Values of Uncertain Parameters

Evaluate an uncertain matrix at several different values of the uncertain parameters of the matrix.

Create an uncertain matrix with two uncertain parameters.

a = ureal('a',5);
b = ureal('b',-3);
M = [a b];

Evaluate the matrix at four different combinations of values for the uncertain parameters a and b.

B = usubs(M,'a',[1;2;3;4],'b',[10;11;12;13]);

This command evaluates M for the four different (a,b) combinations (1,10), (2,11), and so on. Therefore, B is a 1-by-2-by-4 array of numeric values containing the four evaluated values of M.

Evaluate Uncertain Matrix over Grid of Uncertain Parameters

Evaluate an uncertain matrix over a 3-by-4 grid of values of the uncertain parameters of the matrix.

Create a 2-by-2 uncertain matrix with two uncertain parameters.

a = ureal('a',5);
b = ureal('b',-3);
M = [a b;0 a*b];

Build arrays of values for the uncertain parameters.

aval = [1;2;3;4];
bval = [10;20;30];
[as,bs] = ndgrid(aval,bval);

This command builds two 4-by-3 grids of values.

Evaluate M over the parameter grids A and B.

B = usubs(M,'a',as,'b',bs);

This command evaluates M for each four different combination of values (A(k),B(k)) B is a 2-by-2-by-4-by-3 array of numeric values, which is a 4-by-3 array of values of M, i.e., M evaluated over the parameter grids.

Instantiate Uncertain Parameter by Batch Substitution of Parameter for Array of Values

Evaluate an array of uncertain models, substituting an array of values for an uncertain parameter.

Create a 1-by-2 uncertain matrix with two uncertain parameters.

a = ureal('a',5);
b = ureal('b',-3);
M = [a b];

Replace a by each of the values 1, 2, 3, and 4.

Ma = usubs(M,'a',[1;2;3;4]);

This command returns a 4-by-1 array of 1-by-2 uncertain matrices that contain one uncertain parameter b.

For each model in the array Ma, evaluate b at 10, 20, and 30.

B = usubs(Ma,'b',[10;20;30],'-batch');

The '-batch' flag causes usubs to evaluate each model in the array at all three values of b. Thus B is a 4-by-3 array of M values.

The '-batch' syntax here yields the same result as the parameter grid approach used in the previous example:

aval = [1;2;3;4];
bval = [10;20;30];
[as,bs] = ndgrid(aval,bval);
B = usubs(M,'a',as,'b',bs);

Instantiate Uncertain Parameter Using Different Value for Each Entry in Array

Evaluate an array of uncertain models, substituting a different value for the uncertain parameter in each entry in the array.

Create a 1-by-2 uncertain matrix with two uncertain parameters.

a = ureal('a',5);
b = ureal('b',-3);
M = [a b];

Replace a by each of the values 1, 2, 3, and 4.

Ma = usubs(M,'a',[1;2;3;4]);

This command returns a 4-by-1 array of 1-by-2 uncertain matrices that contain one uncertain parameter b.

For each model in the array Ma, evaluate b. Use b = 10 for the first entry in the array, b = 20 for the second entry, and so on.

B = usubs(Ma,'b',{10;20;30;40},'-once');

The '-once' flag causes usubs to evaluate the first model in the array using the first specified value for b, the second model for the second specified value, etc.

Replace Uncertain Parameters with Values Returned by usample

Replace the uncertain parameters in an uncertain models by values specified in struct array form, as returned by usample.

This is useful, for example, when you have multiple uncertain models that use the same set of parameters, and you want to evaluate all models at the same parameter values.

Create two uncertain matrices that have the same uncertain parameters, a and b.

a = ureal('a',5);
b = ureal('b',-3);
M1 = [a b];
M2 = [a b;0 a*b];

Generate some random samples of M1.

[M1rand,samples] = usample(M1,5);

M1rand is an array of five values of M1, evaluated at randomly generated values of a and b. These a and b values are returned in the struct array samples.

Examine the struct array samples.

samples
samples = 

5x1 struct array with fields:
    a
    b

The field names of samples correspond to the uncertain parameters of M1. The values are the parameter values used to generate M1rand. Because M2 has the same parameters, you can use this structure to evaluate M2 at the same set of values.

M2rand = usubs(M2,samples);

This command returns a 1-by-5 array of instantiations of M2.

See Also

| |

Was this topic helpful?