ureal

Uncertain real parameter

Description

Use the ureal uncertain element to represent real numbers whose values are uncertain when modeling dynamic systems with uncertainty. An uncertain real parameter has a nominal value, stored in the NominalValue property, and an uncertainty, which is the potential deviation from the nominal value. ureal stores this deviation equivalently in three different properties:

• PlusMinus — The additive relative deviation from NominalValue

• Range — The absolute range of values, expressed as an interval containing NominalValue

• Percentage — The deviation, expressed as a percentage of NominalValue

When you create an uncertain real parameter, you can specify the uncertainty in any of these three ways. The ureal object automatically calculates the appropriate values for the other two properties.

You can combine ureal uncertain parameters with numeric parameters to create uncertain matrices (umat objects) which you can then use to create uncertain state-space models. Or, you can use them as coefficients in transfer functions. When you use uncertain real parameters to build uncertain dynamic systems, the result is an uncertain model such as a uss or genss model.

Creation

Syntax

p = ureal(name,nominalvalue)

p = ureal(name,nominalvalue,'PlusMinus',plusminus)

p = ureal(name,nominalvalue,'Range',range)

p = ureal(name,nominalvalue,'Percentage',percentage)

p = ureal(name,nominalvalue,___,Name,Value)

Description

p = ureal(name,nominalvalue) creates an uncertain real parameter with the specified nominal value and an uncertainty of ±1. This syntax sets the Name and NominalValue properties of the resulting ureal object.

p = ureal(name,nominalvalue,'PlusMinus',plusminus) sets the uncertainty to the specified deviations from the nominal value. plusminus is a two-element vector of the form [-DL,DR]. The uncertain parameter takes values in the range [nominalvalue-DL,nominalvalue+DR]. If the range is symmetric around the nominal value such that DL = DR, you can use plusminus = DR.

Using this syntax also sets the Mode property of the resulting ureal object to 'PlusMinus'.

p = ureal(name,nominalvalue,'Range',range) sets the uncertainty to the specified absolute range. range is a two-element vector of the form [LOW,HIGH], and the nominal value must fall in this range.

Using this syntax also sets the Mode property of the resulting ureal object to 'Range'.

p = ureal(name,nominalvalue,'Percentage',percentage) sets the uncertainty in terms of percentage deviations from the nominal value. percentage is a two-element vector of the form [-PL,PR]. This syntax sets the lower and upper limits of the uncertainty range such that PL = 100*|1-LOW/nominalvalue| and PR = 100*|1-HIGH/nominalvalue|.

Using this syntax also sets the Mode property of the resulting ureal object to 'Percentage'.

p = ureal(name,nominalvalue,___,Name,Value) sets additional properties using name-value pairs. You can specify multiple name-value pairs. Enclose each property name in single quotes.

Properties

NominalValue — Nominal value

scalar

Nominal value of the uncertain parameter, specified as a real scalar.

Mode — Independent quantification of uncertainty

'PlusMinus' | 'Range' | 'Percentage'

Independent quantification of uncertainty, specified as 'PlusMinus', 'Range', or 'Percentage'. The ureal object stores the uncertainty as a relative deviation from nominal, an absolute range of possible values, and a percentage deviation from nominal. This property specifies which of these three ways of expressing the uncertainty is independent of the nominal value. For instance, if p.Mode = 'Range', then changing the nominal value has no effect on p.Range, but does change the value of both p.PlusMinus and p.Percentage.

The initial value of this property depends on how you create the ureal object. For instance, the following code creates pl with p1.Mode = 'PlusMinus' and p2 with p2.Mode = 'Percentage'.

p1 = ureal('p1',2);
p2 = ureal('p2',2,'Percentage',[-10 20]);

Range — Range of variation

two-element vector

Range of variation of the uncertain parameter, specified as a two-element vector of the form [LOW,HIGH], where LOW and HIGH are real scalars. The uncertain parameter can take any value within this range. The nominal value must fall within this range.

PlusMinus — Deviation from nominal value

[-1 1] (default) | two-element vector

Deviation from nominal value, specified as a two-element vector of the form [-DL,DR], where DL and DR are real positive scalars. The uncertain parameter can take any value in the range [NominalValue-DL,NominalValue+DR]. If you do not specify the uncertainty in any form when you create the ureal parameter, then the default uncertainty is PlusMinus = [-1,1].

Percentage — Percentage deviation from nominal value

two-element vector

Percentage deviation from nominal value, specified as a two-element vector of the form [-PL,PR], where PL and PR are real positive scalars. These values set the range of uncertainty such that PL = 100*|1-LOW/NominalValue| and PR = 100*|1-HIGH/NominalValue|, where [LOW,HIGH] is the value of the Range property.

AutoSimplify — Block simplification level

'basic' (default) | 'full' | 'off'

Block simplification level, specified as 'basic', 'full', or 'off'. In general, when you combine uncertain elements to create uncertain state-space models, the software automatically applies techniques to eliminate redundant copies of the uncertain elements. (See simplify.) Use this property to specify the simplification to apply when you use model arithmetic or interconnection techniques with the uncertain block.

• 'basic' — Apply the elementary simplification method after each arithmetical or interconnection operation.

• 'full' — Apply techniques similar to model reduction.

• 'off' — Perform no simplification.

Name — Name of uncertain element

character vector

Name of the uncertain element, specified as a character vector. When you create an uncertain model or uncertain matrix using uncertain control design blocks, the software tracks the blocks using the name you specify in this property, not the variable name in the MATLAB^® workspace. For example, use the following code to create a ureal parameter and an uncertain dynamic system model.

p1 = ureal('w0',10);
sys = tf(p1,[1 p1])

sys =

  Uncertain continuous-time state-space model with 1 outputs, 1 inputs, 1 states.
  The model uncertainty consists of the following blocks:
    w0: Uncertain real, nominal = 10, variability = [-1,1], 1 occurrences

The Blocks property of the resulting uss model lists the uncertain control design block using w0, which is the Name property of the uncertain parameter used to create sys.

Object Functions

You can use ureal parameters with functions for creating dynamic systems such as tf and ss. You can also combine them with existing dynamic systems models using model arithmetic or commands such as feedback. Doing so creates an uncertain state-space model. You can also combine ureal parameters using common arithmetic operations, which generally results in an uncertain matrix (umat object). Use functions such as actual2normalized and uscale to transform or scale the amount of uncertainty in a ureal parameter. You can also use commands such as usample or usubs to replace real parameters with fixed values. The gridureal command evaluates a ureal parameter over its range and returns a grid of sampled values.

The following list contains a representative subset of the functions you can use with ureal parameter objects.

actual2normalized	Transform actual values to normalized values
append	Group models by appending their inputs and outputs
feedback	Feedback connection of multiple models
get	Access model property values
getLimits	Validity range for uncertain real (ureal) parameters
getNominal	Nominal value of uncertain model
gridureal	Grid ureal parameters uniformly over their range
isuncertain	Check whether argument is uncertain class type
normalized2actual	Convert value for atom in normalized coordinates to corresponding actual value
replaceBlock	Replace or update control design blocks in generalized model
rsampleBlock	Randomly sample Control Design blocks in generalized model
sampleBlock	Sample Control Design blocks in generalized model
ss	State-space model
tf	Transfer function model
umat	Create uncertain matrix
usample	Generate random samples of uncertain model or element
uscale	Scale uncertainty of block or system
usubs	Substitute given values for uncertain elements of uncertain objects

Examples

Uncertain Real Parameter with Specified Deviation from Nominal

Create an uncertain real parameter with a nominal value of 10, and an uncertainty range of ±2. Because this uncertainty is symmetric, you can specify it by setting PlusMinus to 2, instead of explicitly setting it to [-2,2].

p1 = ureal('p1',10,'PlusMinus',2)

Uncertain real parameter "p1" with nominal value 10 and variability [-2,2].

Create another parameter with a nominal value of 10, this time with an asymmetric uncertainty such that the value can decrease by 2 from the nominal but can increase by 5.

p2 = ureal('p2',10,'PlusMinus',[-2 5])

Uncertain real parameter "p2" with nominal value 10 and variability [-2,5].

Examine the properties of the parameter. The Range and Percentage properties are automatically set to values corresponding to this variability.

get(p2)

    NominalValue: 10
            Mode: 'PlusMinus'
           Range: [8 15]
       PlusMinus: [-2 5]
      Percentage: [-20 50]
    AutoSimplify: 'basic'
            Name: 'p2'

Because you specified PlusMinus to create the parameter, the Mode property initializes to PlusMinus. In this mode, when you change the nominal value, PlusMinus remains fixed, while Percentage and Range change to reflect the new range of values the parameter can take. See Change Nominal Value or Uncertainty of Existing Parameter.

Uncertain Real Parameter with Specified Range of Values

Create an uncertain real parameter whose value can vary from 14 to 19, with a nominal value of 15.5. To do so, set the Range property to the lowest and highest values the parameter can take.

p1 = ureal('p1',15.5,'Range',[14,19])

Uncertain real parameter "p1" with nominal value 15.5 and range [14,19].

Examine the properties of the parameter. The PlusMinus and Percentage properties are automatically set to the corresponding values. The Mode property is set to 'Range'.

get(p1)

    NominalValue: 15.5000
            Mode: 'Range'
           Range: [14 19]
       PlusMinus: [-1.5000 3.5000]
      Percentage: [-9.6774 22.5806]
    AutoSimplify: 'basic'
            Name: 'p1'

Uncertain Real Parameter with Specified Percentage Variation

Create an uncertain real parameter with a nominal value of 24, whose value can increase or decrease by 15%. Because this uncertainty is symmetric, you can specify it by setting Percentage to 15, instead of explicitly setting it to [-15,15].

p1 = ureal('p1',24,'Percentage',15)

Uncertain real parameter "p1" with nominal value 24 and variability [-15,15]%.

Create another parameter with a nominal value of 24, this time with an asymmetric uncertainty such that the value can decrease by 20% from the nominal but can increase by 15%.

p2 = ureal('p2',24,'Percentage',[-20,15])

Uncertain real parameter "p2" with nominal value 24 and variability [-20,15]%.

Examine the properties to see the deviation from nominal (PlusMinus) and the range of values (Range) represented by these percentage variations.

get(p2)

    NominalValue: 24
            Mode: 'Percentage'
           Range: [19.2000 27.6000]
       PlusMinus: [-4.8000 3.6000]
      Percentage: [-20 15]
    AutoSimplify: 'basic'
            Name: 'p2'

Change Nominal Value or Uncertainty of Existing Parameter

A ureal parameter stores the uncertainty as a relative deviation from nominal (PlusMinus), an absolute range of possible values (Range), and a percentage deviation from nominal (Percentage). The Mode property specifies which of these three does not change when you change the nominal value of the parameter. For instance, create a parameter with a nominal value of 10 and a relative deviation of ±2.

p1 = ureal('p1',10,'PlusMinus',[-2,2])

Uncertain real parameter "p1" with nominal value 10 and variability [-2,2].

Examine the values of the other properties.

get(p1)

    NominalValue: 10
            Mode: 'PlusMinus'
           Range: [8 12]
       PlusMinus: [-2 2]
      Percentage: [-20 20]
    AutoSimplify: 'basic'
            Name: 'p1'

In PlusMinus mode, when you change the nominal value, the PlusMinus property remains fixed, and the values of the other two ways of expressing the uncertainty are updated to reflect the new values. For instance, change the nominal value to 20.

p1.NominalValue = 20;
get(p1)

    NominalValue: 20
            Mode: 'PlusMinus'
           Range: [18 22]
       PlusMinus: [-2 2]
      Percentage: [-10 10]
    AutoSimplify: 'basic'
            Name: 'p1'

The new uncertain parameter has the same PlusMinus value, but the range and percentage are adjusted to the new values that correspond to 20±2.

If you change the PlusMinus value, the Range and Percentage values are updated to reflect the new uncertainties. The nominal value is unchanged.

p1.PlusMinus = [-4 4];
get(p1)

    NominalValue: 20
            Mode: 'PlusMinus'
           Range: [16 24]
       PlusMinus: [-4 4]
      Percentage: [-20 20]
    AutoSimplify: 'basic'
            Name: 'p1'

Next, change the parameter to Range mode. In this mode, when you change the nominal value, Range remains fixed at [16 24], while Percentage and PlusMinus are updated.

p1.Mode = 'Range';
p1.NominalValue = 22;
get(p1)

    NominalValue: 22
            Mode: 'Range'
           Range: [16 24]
       PlusMinus: [-6 2]
      Percentage: [-27.2727 9.0909]
    AutoSimplify: 'basic'
            Name: 'p1'

Second-Order System with Uncertain Frequency and Damping

Create a model of a second-order system with natural frequency of ${\omega }_0$ = 10±3 rad/s and a damping ratio that can vary from 0.5 to 0.8, with a nominal value of ζ = 0.6.

First, represent the natural frequency and damping ratio values as uncertain real parameters.

w0 = ureal('w0',10,'PlusMinus',[-3 3]);
zeta = ureal('zeta',0.6,'Range',[0.5 0.8]);

Next, use the parameters to specify the coefficients of a transfer function.

sys = tf(1,[1/w0^2 2*zeta/w0 1])

Uncertain continuous-time state-space model with 1 outputs, 1 inputs, 2 states.
The model uncertainty consists of the following blocks:
  w0: Uncertain real, nominal = 10, variability = [-3,3], 3 occurrences
  zeta: Uncertain real, nominal = 0.6, range = [0.5,0.8], 1 occurrences

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

sys is an uncertain state-space (uss) model that depends on the uncertain parameters w0 and zeta. The model sys uses the Name property of the parameters to refer to them and track them.

Examine the step response of the system to get a sense of the responses that the uncertainty represents. The step command automatically takes a number of random samples of an uncertain system.

step(sys,sys.NominalValue)

State-Space Model with Uncertain Entries in State-Space Matrices

You can use ureal parameters to specify uncertain elements in state-space matrices. For instance, create three uncertain real parameters and build state-spaces matrices from them.

p1 = ureal('p1',10,'Percentage',50); 
p2 = ureal('p2',3,'PlusMinus',[-.5 1.2]); 
p3 = ureal('p3',0); 

A = [-p1 p2; 0 -p1]; 
B = [-p2; p2+p3]; 
C = [1 0; 1 1-p3]; 
D = [0; 0];

The matrices constructed with uncertain parameters, A, B, and C, are uncertain matrix (umat) objects. Using them as inputs to ss results in a 2-output, 1-input, 2-state uncertain system.

sys = ss(A,B,C,D)

Uncertain continuous-time state-space model with 2 outputs, 1 inputs, 2 states.
The model uncertainty consists of the following blocks:
  p1: Uncertain real, nominal = 10, variability = [-50,50]%, 2 occurrences
  p2: Uncertain real, nominal = 3, variability = [-0.5,1.2], 2 occurrences
  p3: Uncertain real, nominal = 0, variability = [-1,1], 2 occurrences

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

The display shows that the system includes the three uncertain parameters, referenced by the Name properties of the ureal objects you used to create the system.

Tips

• ureal objects support uncertainty that is skewed, or asymmetric around the nominal value. However, highly skewed ranges can lead to poor numeric conditioning and poor results. Therefore, for meaningful results, avoid highly skewed ranges where the nominal value is orders of magnitude closer to one end of the range than to the other.

  When the uncertainty range of a ureal parameter is not centered at its nominal value, the parameter can take only a restricted range of values. For robust stability analysis, which sometimes requires assigning a parameter values outside the specified range, these restrictions mean that the smallest destabilizing perturbation of the parameter might be outside the actual range of values that the parameter can take. Use getLimits to find the restricted range of values that a skewed ureal parameter can take. For more information, see getLimits.

Version History

Introduced before R2006a

R2020a: Sampling of ureal elements is uniform in actual values

Beginning in R2020a, usample uniformly samples the actual uncertainty range of ureal objects. Previously, usample first normalized the uncertain element, and then sampled uniformly in the normalized range. As a result of this change, you might obtain different results when you use usample to sample ureal elements or uss models that contain them, even if you use the same random seed.

The new implementation yields more uniform sampling for ureal parameters with skewed ranges (nominal value closer to one end of the range than the other).