TuningGoal.Gain

Gain constraint for control system tuning

Description

Use the TuningGoal.Gain object to specify a constraint that limits the gain from a specified input to a specified output. Use this tuning goal for control system tuning with tuning commands such as systune or looptune.

When you use TuningGoal.Gain, the software attempts to tune the system so that the gain from the specified input to the specified output does not exceed the specified value. By default, the constraint is applied with the loop closed. To apply the constraint to an open-loop response, use the Openings property of the TuningGoal.Gain object.

You can use a gain constraint to:

• Enforce a design requirement of disturbance rejection across a particular input/output pair, by constraining the gain to be less than 1

• Enforce a custom roll-off rate in a particular frequency band, by specifying a gain profile in that band

Creation

Syntax

Req = TuningGoal.Gain(inputname,outputname,gainvalue)

Req = TuningGoal.Gain(inputname,outputname,gainprofile)

Description

Req = TuningGoal.Gain(inputname,outputname,gainvalue) creates a tuning goal that constrains the gain from inputname to outputname to remain below the value gainvalue.

You can specify the inputname or outputname as cell arrays (vector-valued signals). If you do so, then the tuning goal constrains the largest singular value of the transfer matrix from inputname to outputname. See sigma for more information about singular values.

Req = TuningGoal.Gain(inputname,outputname,gainprofile) specifies the maximum gain as a function of frequency. You can specify the target gain profile (maximum gain across the I/O pair) as a smooth transfer function. Alternatively, you can sketch a piecewise error profile using an frd model.

Input Arguments

inputname — Input signals for tuning goal

character vector | cell array of character vectors

Input signals for the tuning goal, specified as a character vector or, for multiple-input tuning goals, a cell array of character vectors.

• If you are using the tuning goal to tune a Simulink^® model of a control system, then inputname can include:

  • Any model input.

  • Any linear analysis point marked in the model.

  • Any linear analysis point in an slTuner (Simulink Control Design) interface associated with the Simulink model. Use addPoint (Simulink Control Design) to add analysis points to the slTuner interface. Use getPoints (Simulink Control Design) to get the list of analysis points available in an slTuner interface to your model.

  For example, suppose that the slTuner interface contains analysis points u1 and u2. Use 'u1' to designate that point as an input signal when creating tuning goals. Use {'u1','u2'} to designate a two-channel input.

• If you are using the tuning goal to tune a generalized state-space (genss) model of a control system, then inputname can include:

  • Any input of the genss model

  • Any AnalysisPoint location in the control system model

  For example, if you are tuning a control system model, T, then inputname can be any input name in T.InputName. Also, if T contains an AnalysisPoint block with a location named AP_u, then inputname can include 'AP_u'. Use getPoints to get a list of analysis points available in a genss model.

  If inputname is an AnalysisPoint location of a generalized model, the input signal for the tuning goal is the implied input associated with the AnalysisPoint block:

  

For more information about analysis points in control system models, see Mark Signals of Interest for Control System Analysis and Design.

outputname — Output signals for tuning goal

character vector | cell array of character vectors

Output signals for the tuning goal, specified as a character vector or, for multiple-output tuning goals, a cell array of character vectors.

• If you are using the tuning goal to tune a Simulink model of a control system, then outputname can include:

  • Any model output.

  • Any linear analysis point marked in the model.

  • Any linear analysis point in an slTuner (Simulink Control Design) interface associated with the Simulink model. Use addPoint (Simulink Control Design) to add analysis points to the slTuner interface. Use getPoints (Simulink Control Design) to get the list of analysis points available in an slTuner interface to your model.

  For example, suppose that the slTuner interface contains analysis points y1 and y2. Use 'y1' to designate that point as an output signal when creating tuning goals. Use {'y1','y2'} to designate a two-channel output.

• If you are using the tuning goal to tune a generalized state-space (genss) model of a control system, then outputname can include:

  • Any output of the genss model

  • Any AnalysisPoint location in the control system model

  For example, if you are tuning a control system model, T, then outputname can be any output name in T.OutputName. Also, if T contains an AnalysisPoint block with a location named AP_u, then outputname can include 'AP_u'. Use getPoints to get a list of analysis points available in a genss model.

  If outputname is an AnalysisPoint location of a generalized model, the output signal for the tuning goal is the implied output associated with the AnalysisPoint block:

  

For more information about analysis points in control system models, see Mark Signals of Interest for Control System Analysis and Design.

gainvalue — Maximum linear gain

scalar

Maximum linear gain, specified as a scalar. The gain constraint Req specifies that the gain from inputname to outputname is less than gainvalue.

If the signals inputname or outputname are vector-valued signals, then gainvalue constrains the largest singular value of the transfer matrix from inputname to outputname. See sigma for more information about singular values.

gainprofile — Gain profile as function of frequency

SISO transfer function

Gain profile as a function of frequency, specified as a SISO transfer function. The gain constraint Req specifies that the gain from inputname to outputname at a particular frequency is less than gainprofile. You can specify gainprofile as a smooth transfer function (tf , zpk, or ss model). Alternatively, you can sketch a piecewise gain profile using a frd model or the makeweight (Robust Control Toolbox) function. When you do so, the software automatically maps the gain profile onto a zpk model. The magnitude of this zpk model approximates the desired gain profile. Use viewGoal(Req) to plot the magnitude of the zpk model.

If inputname or outputname are cell arrays, gainprofile applies to all I/O pairs from inputname to outputname

If you are tuning in discrete time (that is, using a genss model or slTuner interface with nonzero Ts), you can specify gainfprofile as a discrete-time model with the same Ts. If you specify gainfprofile in continuous time, the tuning software discretizes it. Specifying the gain profile in discrete time gives you more control over the gain profile near the Nyquist frequency.

Properties

MaxGain — Maximum gain as function of frequency

SISO zpk model object

Maximum gain as a function of frequency, specified as a SISO zpk model.

The software automatically maps the gainvalue or gainprofile input arguments to a zpk model. The magnitude of this zpk model approximates the desired gain profile. The tuning goal derives and is stored in the MaxGain property. Use viewGoal(Req) to plot the magnitude of MaxGain.

Focus — Frequency band in which tuning goal is enforced

[0,Inf] (default) | [0,pi/Ts] | row vector of two elements

Frequency band in which tuning goal is enforced, specified as a row vector of the form [min,max]. For continuous time, the default value is equal to [0,Inf]. For discrete time, the default value is equal to [0,pi/Ts], where Ts is the model sample time.

Set the Focus property to limit enforcement of the tuning goal to a particular frequency band. Express this value in the frequency units of the control system model you are tuning (rad/TimeUnit). For example, suppose Req is a tuning goal that you want to apply only between 1 and 100 rad/s. To restrict the tuning goal to this band, use the following command:

Req.Focus = [1,100];

Stabilize — Stability requirement on closed-loop dynamics

1 (true) (default) | 0 (false)

Stability requirement on closed-loop dynamics, specified as 1 (true) or 0 (false).

By default, TuningGoal.Gain imposes a stability requirement on the closed-loop transfer function from the specified inputs to outputs, in addition to the gain requirement. If stability is not required or cannot be achieved, set Stabilize to false to remove the stability requirement. For example, if the gain constraint applies to an unstable open-loop transfer function, set Stabilize to false.

InputScaling — Input signal scaling

[ ] (default) | vector of positive real elements

Input signal scaling, specified as a vector of positive real elements.

Use this property to specify the relative amplitude of each entry in vector-valued input signals when the choice of units results in a mix of small and large signals. This information is used to scale the closed-loop transfer function from Input to Output when the tuning goal is evaluated.

Suppose T(s) is the closed-loop transfer function from Input to Output. The tuning goal is evaluated for the scaled transfer function D_o^–1T(s)D_i. The diagonal matrices D_o and D_i have the OutputScaling and InputScaling values on the diagonal, respectively.

The default value, [] , means no scaling.

OutputScaling — Output signal scaling

[ ] (default) | vector of positive real elements

Output signal scaling, specified as a vector of positive real values.

Use this property to specify the relative amplitude of each entry in vector-valued output signals when the choice of units results in a mix of small and large signals. This information is used to scale the closed-loop transfer function from Input to Output when the tuning goal is evaluated.

Suppose T(s) is the closed-loop transfer function from Input to Output. The tuning goal is evaluated for the scaled transfer function D_o^–1T(s)D_i. The diagonal matrices D_o and D_i have the OutputScaling and InputScaling values on the diagonal, respectively.

The default value, [] , means no scaling.

Input — Input signal names

cell array of character vectors

Input signal names, specified as a cell array of character vectors that identify the inputs of the transfer function that the tuning goal constrains. The initial value of the Input property is set by the inputname input argument when you construct the tuning goal.

Output — Output signal names

cell array of character vectors

Output signal names, specified as a cell array of character vectors that identify the outputs of the transfer function that the tuning goal constrains. The initial value of the Output property is set by the outputname input argument when you construct the tuning goal.

Models — Models to which tuning goal applies

NaN (default) | vector of scalars

Models to which the tuning goal applies, specified as a vector of indices.

Use the Models property when tuning an array of control system models with systune, to enforce a tuning goal for a subset of models in the array. For example, suppose you want to apply the tuning goal, Req, to the second, third, and fourth models in a model array passed to systune. To restrict enforcement of the tuning goal, use the following command:

Req.Models = 2:4;

When Models = NaN, the tuning goal applies to all models.

Openings — Feedback loops to open when evaluating the tuning goal

{} (default) | cell array of character vectors

Feedback loops to open when evaluating the tuning goal, specified as a cell array of character vectors that identify loop-opening locations. The tuning goal is evaluated against the open-loop configuration created by opening feedback loops at the locations you identify.

If you are using the tuning goal to tune a Simulink model of a control system, then Openings can include any linear analysis point marked in the model, or any linear analysis point in an slTuner (Simulink Control Design) interface associated with the Simulink model. Use addPoint (Simulink Control Design) to add analysis points and loop openings to the slTuner interface. Use getPoints (Simulink Control Design) to get the list of analysis points available in an slTuner interface to your model.

If you are using the tuning goal to tune a generalized state-space (genss) model of a control system, then Openings can include any AnalysisPoint location in the control system model. Use getPoints to get the list of analysis points available in the genss model.

For example, if Openings = {'u1','u2'}, then the tuning goal is evaluated with loops open at analysis points u1 and u2.

Name — Name of tuning goal

[] (default) | character vector

Name of the tuning goal, specified as a character vector.

For example, if Req is a tuning goal:

Req.Name = 'LoopReq';

Examples

Disturbance Rejection Goal

Create a gain constraint that enforces a disturbance rejection requirement from a signal 'du' to a signal 'u'.

Req = TuningGoal.Gain('du','u',1);

This requirement specifies that the maximum gain of the response from 'du' to 'u' not exceed 1 (0 dB).

Custom Roll-off Specification

Create a tuning goal that constrains the response from a signal 'du' to a signal 'u' to roll off at 20 dB/decade at frequencies greater than 1. The tuning goal also specifies disturbance rejection (maximum gain of 1) in the frequency range [0,1].

gmax = frd([1 1 0.01],[0 1 100]);
Req = TuningGoal.Gain('du','u',gmax);

These commands use a frd model to specify the gain profile as a function of frequency. The maximum gain of 1 dB at the frequency 1 rad/s, together with the maximum gain of 0.01 dB at the frequency 100 rad/s, specifies the desired rolloff of 20 dB/decade.

The software converts gmax into a smooth function of frequency that approximates the piecewise specified requirement. Display the gain profile using viewGoal.

viewGoal(Req)

The dashed line shows the gain profile, and the region indicates where the requirement is violated.

Tips

• This tuning goal imposes an implicit stability constraint on the closed-loop transfer function from Input to Output, evaluated with loops opened at the points identified in Openings. The dynamics affected by this implicit constraint are the stabilized dynamics for this tuning goal. The MinDecay and MaxRadius options of systuneOptions control the bounds on these implicitly constrained dynamics. If the optimization fails to meet the default bounds, or if the default bounds conflict with other requirements, use systuneOptions to change these defaults.

Algorithms

When you tune a control system using a TuningGoal object, the software converts the tuning goal into a normalized scalar value f(x), where x is the vector of free (tunable) parameters in the control system. The software then adjusts the parameter values to minimize f(x) or to drive f(x) below 1 if the tuning goal is a hard constraint.

For TuningGoal.Gain, f(x) is given by:

$$f\left(x\right)={\Vert W_F\left(s\right)D_o^{-1}T\left(s,x\right)D_i\Vert }_{\infty },$$

or its discrete-time equivalent, for discrete-time tuning. Here, T(s,x) is the closed-loop transfer function from Input to Output. D_o and D_i are diagonal matrices with the OutputScaling and InputScaling property values on the diagonal, respectively. $${\Vert \cdot \Vert }_{\infty }$$ denotes the H_∞ norm (see getPeakGain).

The frequency weighting function W_F is the regularized gain profile, derived from the maximum gain profile you specify. The gains of W_F and 1/MaxGain roughly match inside the frequency band Focus. W_F is always stable and proper. Because poles of W_F close to s = 0 or s = Inf might lead to poor numeric conditioning of the systune optimization problem, it is not recommended to specify maximum gain profiles with very low-frequency or very high-frequency dynamics.

To obtain W_F, use:

WF = getWeight(Req,Ts)

where Req is the tuning goal, and Ts is the sample time at which you are tuning (Ts = 0 for continuous time). For more information about regularization and its effects, see Visualize Tuning Goals.

Version History

Introduced in R2011b

R2016a: Functionality moved from Robust Control Toolbox

Prior to R2016a, this functionality required a Robust Control Toolbox™ license.

R2012b: TuningGoal.MaxGain renamed to TuningGoal.Gain

Additionally, the GainLimit property of this tuning goal has been remained to MaxGain. Replace instances of TuningGoal.MaxGain in your code with TuningGoal.Gain, and replace references to the GainLimit property with MaxGain.