Uncertain Matrices

Uncertain matrices (class umat) are built from doubles and uncertain elements, using traditional MATLAB^® matrix building syntax. Uncertain matrices can be added, subtracted, multiplied, inverted, transposed, etc., resulting in uncertain matrices. The rows and columns of an uncertain matrix are referenced in the same manner that MATLAB references rows and columns of an array, using parenthesis, and integer indices. The NominalValue of a uncertain matrix is the result obtained when all uncertain elements are replaced with their own NominalValue. The uncertain elements making up a umat are accessible through the Uncertainty gateway, and the properties of each element within a umat can be changed directly. The properties are:

Properties	Meaning	Class
NominalValue	Nominal value of element	double
Uncertainty	Uncertain blocks in the matrix, stored as a structure whose fields are named after the uncertain blocks, and contain the uncertain elements, such as ureal.	struct
SamplingGrid	Sampling grid, for umat arrays, stored as a structure whose fields are named after the sampling variables, and contain the sample values associated with the corresponding model in the array.	struct
Name	umat name. When you convert a static control design block such as ureal to an uncertain matrix using umat(blk), the Name property of the block is preserved.	char

Using usubs, specific values may be substituted for any of the uncertain elements within a umat. The command usample generates a random sample of the uncertain matrix, substituting random samples (within their ranges) for each of the uncertain elements.

The command wcnorm computes tight bounds on the worst-case (maximum over the uncertain elements' ranges) norm of the uncertain matrix.

Standard MATLAB numerical matrices (i.e., double) naturally can be viewed as uncertain matrices without any uncertainty.

Create and Manipulate Uncertain Matrices

You create uncertain matrices (umat objects) by creating uncertain parameters and using them to build matrices. You can then use uncertain matrices to build uncertain state-space models. This example shows how to create an uncertain matrix, access and change its uncertain parameters, extract elements, and perform matrix arithmetic.

For example, create two uncertain real parameters, and use them to create a 3-by-2 uncertain matrix.

a = ureal('a',3); 
b = ureal('b',10,'Percentage',20); 
M = [-a, 1/b; b, a+1/b; 1, 3]

Uncertain matrix with 3 rows and 2 columns.
The uncertainty consists of the following blocks:
  a: Uncertain real, nominal = 3, variability = [-1,1], 2 occurrences
  b: Uncertain real, nominal = 10, variability = [-20,20]%, 3 occurrences

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

Examine and Modify umat Properties

M is a umat object. Examine its properties using get.

get(M)

    NominalValue: [3x2 double]
     Uncertainty: [1x1 struct]
    SamplingGrid: [1x1 struct]
            Name: ''

The nominal value of M is the matrix obtained by replacing all the uncertain elements with their nominal values.

M.NominalValue

ans = 3×2

   -3.0000    0.1000
   10.0000    3.1000
    1.0000    3.0000

The Uncertainty property is a structure containing the uncertain elements (the Control Design Blocks) of M.

M.Uncertainty

ans = struct with fields:
    a: [1x1 ureal]
    b: [1x1 ureal]

M.Uncertainty.a

Uncertain real parameter "a" with nominal value 3 and variability [-1,1].

Use the Uncertainty property for direct access to the uncertain elements. For example, check the Range of the uncertain element a within M.

M.Uncertainty.a.Range

ans = 1×2

     2     4

The range is [2,4] because you created the ureal parameter a with a nominal value 3 and the default uncertainty of +/- 1. Change the range to [2.5,5].

M.Uncertainty.a.Range = [2.5,5]

Uncertain matrix with 3 rows and 2 columns.
The uncertainty consists of the following blocks:
  a: Uncertain real, nominal = 3, variability = [-0.5,2], 2 occurrences
  b: Uncertain real, nominal = 10, variability = [-20,20]%, 3 occurrences

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

This change to a only takes place within M. Verify that the variable a in the MATLAB workspace still has the original range.

a.Range

ans = 1×2

     2     4

You cannot combine elements that have a common internal name, but different properties. So, for example, entering M.Uncertainty.a - a would generate an error, because the realp parameter a in the workspace has different properties from the element a in M.

Row and Column Referencing

You can use standard row-column referencing to extract elements from a umat. For example, extract a 2-by-2 selection from M consisting of its second and third rows.

Msub = M(2:3,:)

Uncertain matrix with 2 rows and 2 columns.
The uncertainty consists of the following blocks:
  a: Uncertain real, nominal = 3, variability = [-0.5,2], 1 occurrences
  b: Uncertain real, nominal = 10, variability = [-20,20]%, 2 occurrences

Type "Msub.NominalValue" to see the nominal value and "Msub.Uncertainty" to interact with the uncertain elements.

You can use single indexing only if the umat is a single column or row. Make a single-column selection from M and use single-index references to access elements of it.

Msing = M([2 1 2 3],2);
Msing(2)

Uncertain matrix with 1 rows and 1 columns.
The uncertainty consists of the following blocks:
  b: Uncertain real, nominal = 10, variability = [-20,20]%, 1 occurrences

Type "ans.NominalValue" to see the nominal value and "ans.Uncertainty" to interact with the uncertain elements.

You can use indexing to change the value of any element of a umat. For example, set the (3,2) entry of M to an uncertain parameter c.

c = ureal('c',3,'Percentage',40);
M(3,2) = c

Uncertain matrix with 3 rows and 2 columns.
The uncertainty consists of the following blocks:
  a: Uncertain real, nominal = 3, variability = [-0.5,2], 2 occurrences
  b: Uncertain real, nominal = 10, variability = [-20,20]%, 2 occurrences
  c: Uncertain real, nominal = 3, variability = [-40,40]%, 1 occurrences

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

M now has three uncertain blocks.

Matrix Operations on umat Objects

You can perform many matrix operations on a umat object, such as matrix-multiply, transpose, and inverse. You can also combine uncertain matrices with numeric matrices that do not have uncertainty.

For example, premultiply M by a 1-by-3 numeric matrix, resulting in a 1-by-2 umat.

M1 = [2 3 1]*M;

Verify that the first entry of M1 is as expected, -2*a + 3*b + 1.

d = M1(1) - (-2*M.Uncertainty.a + 3*M.Uncertainty.b + 1)

Uncertain matrix with 1 rows, 1 columns, and no uncertain blocks.

Type "d.NominalValue" to see the nominal value and "d.Uncertainty" to interact with the uncertain elements.

Transpose M, form a product, and invert it. As expected, the product of a matrix and its inverse is the identity matrix. You can verify this by sampling the result.

H = M.'*M; 
K = inv(H); 
usample(K*H,3)

ans = 
ans(:,:,1) =

    1.0000    0.0000
   -0.0000    1.0000


ans(:,:,2) =

    1.0000    0.0000
   -0.0000    1.0000


ans(:,:,3) =

    1.0000    0.0000
    0.0000    1.0000

Lifting a Double Matrix to umat

You can convert a numeric matrix to a umat object with no uncertain elements. Use the umat command to lift a double matrix to the umat class. For example:

Md = [1 2 3;4 5 6]; 
M = umat(Md)

Uncertain matrix with 2 rows, 3 columns, and no uncertain blocks.

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

You can also convert higher-dimension numeric matrices to umat. When you do so, the software interprets the third dimension and beyond as array dimensions. For example, convert a random three-dimensional numeric array to umat.

Md = randn(4,5,6); 
M = umat(Md)

6x1 array of uncertain matrices with 4 rows, 5 columns, and no uncertain blocks.

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

The result is a one-dimensional array of uncertain matrices, rather than a three-dimensional uncertain array. Similarly, a four-dimensional numeric array converts to a two-dimensional array of umat objects.

Md = randn(4,5,6,7); 
M = umat(Md)

6x7 array of uncertain matrices with 4 rows, 5 columns, and no uncertain blocks.

Type "M.NominalValue" to see the nominal value and "M.Uncertainty" to interact with the uncertain elements.

See Array Management for Uncertain Objects for more information about multidimensional arrays of uncertain objects.