Main Content

Uncertain frequency responses (`ufrd`

) arise naturally when computing the frequency response of an uncertain state-space model (`uss`

). They also arise when frequency response data in an `frd`

model object is combined with an uncertain matrix (`umat`

) such as by adding, multiplying, or concatenating.

To take the frequency response of an uncertain state-space model, use the `ufrd`

command. Construct an uncertain state-space model.

p1 = ureal('p1',10,'pe',50); p2 = ureal('p2',3,'plusm',[-.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]; sys = ss(A,B,C,D)

sys = 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, "get(sys)" to see all properties, and "sys.Uncertainty" to interact with the uncertain elements.

Compute the uncertain frequency response of the uncertain system. Use `ufrd`

command with a frequency grid of 100 points. The result is an uncertain frequency response model object, a `ufrd`

model.

sysg = ufrd(sys,logspace(-2,2,100))

sysg = Uncertain continuous-time FRD model with 2 outputs, 1 inputs, 100 frequency points. 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 "sysg.NominalValue" to see the nominal value, "get(sysg)" to see all properties, and "sysg.Uncertainty" to interact with the uncertain elements.

`ufrd`

Model ObjectsView the properties of the model object.

get(sysg)

Frequency: [100x1 double] FrequencyUnit: 'rad/TimeUnit' ResponseData: [2x1x100x1 umat] NominalValue: [2x1 frd] Uncertainty: [1x1 struct] InputDelay: 0 OutputDelay: [2x1 double] Ts: 0 TimeUnit: 'seconds' InputName: {''} InputUnit: {''} InputGroup: [1x1 struct] OutputName: {2x1 cell} OutputUnit: {2x1 cell} OutputGroup: [1x1 struct] Notes: [0x1 string] UserData: [] Name: '' SamplingGrid: [1x1 struct]

The properties `ResponseData`

and `Frequency`

behave the same as the corresponding properties in Control System Toolbox™ `frd`

objects, except that `ResponseData`

is an uncertain matrix (`umat`

). The properties `InputName`

, `OutputName`

, `InputGroup`

, and `OutputGroup`

behave in exactly the same manner as for all of the Control System Toolbox model objects such as `ss`

, `zpk`

, `tf`

, and `frd`

.

The `NominalValue`

property is an `frd`

object. Hence all functions you can use to analyze `frd`

objects can also analyze `ufrd`

objects. are available. When you use analysis commands such as `bode`

or `step`

with an uncertain model, the command plots random samples of the response to give you a sense of the variation. For instance, plot sampled Bode responses of the system along with the nominal response, using a dot marker so that you can see the individual frequency points.

bode(sysg,'r.',sysg.NominalValue,'b.')

Just as with `umat`

uncertain matrices and `uss`

uncertain models, the `Uncertainty`

property of the `ufrd`

model is a structure containing the uncertain elements. In the model `sysg`

, all uncertain elements are `ureal`

parameters. Change the nominal value of the uncertain element `p1`

within `sysg`

to 14, and plot the Bode response of the (new) nominal system.

sysg.Uncertainty.p1.NominalValue = 14

sysg = Uncertain continuous-time FRD model with 2 outputs, 1 inputs, 100 frequency points. p1: Uncertain real, nominal = 14, 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 "sysg.NominalValue" to see the nominal value, "get(sysg)" to see all properties, and "sysg.Uncertainty" to interact with the uncertain elements.

bode(sysg.NominalValue)

`frd`

model to a `ufrd`

modelA non-uncertain frequency response model is equivalent to an uncertain frequency response model with no uncertain elements. Use the `ufrd`

command to “lift” an `frd`

model to the `ufrd`

class.

sys = rss(3,2,1); sysg = frd(sys,logspace(-2,2,100)); usysg = ufrd(sysg)

usysg = Uncertain continuous-time FRD model with 2 outputs, 1 inputs, 100 frequency points, and no uncertain blocks. Type "usysg.NominalValue" to see the nominal value, "get(usysg)" to see all properties, and "usysg.Uncertainty" to interact with the uncertain elements.

You can also lift arrays of `frd`

objects. See Array Management for Uncertain Objects for more information about how arrays of uncertain objects are handled.