TuningGoal.MinLoopGain class

Package: TuningGoal

Minimum loop gain constraint for control system tuning

Description

Use the TuningGoal.MinLoopGain object to enforce a minimum loop gain in a particular frequency band. Use this requirement with control system tuning commands such as systune or looptune.

This requirement imposes a minimum gain on the open-loop frequency response (L) at a specified location in your control system. You specify the minimum open-loop gain as a function of frequency (a minimum gain profile). For MIMO feedback loops, the specified gain profile is interpreted as a lower bound on the smallest singular value of L.

When you tune a control system, the minimum gain profile is converted to a minimum gain constraint on the inverse of the sensitivity function, inv(S)  = (I + L).

The following figure shows a typical specified minimum gain profile (dashed line) and a resulting tuned loop gain, L (blue line). The green region represents gain profile values that are forbidden by this requirement. The figure shows that when L is much larger than 1, imposing a minimum gain on inv(S) is a good proxy for a minimum open-loop gain.

TuningGoal.MinLoopGain and TuningGoal.MaxLoopGain specify only low-gain or high-gain constraints in certain frequency bands. When you use these requirements, systune and looptune determine the best loop shape near crossover. When the loop shape near crossover is simple or well understood (such as integral action), you can use TuningGoal.LoopShape to specify that target loop shape.

Construction

Req = TuningGoal.MinLoopGain(location,loopgain) creates a tuning requirement for boosting the gain of a SISO or MIMO feedback loop. The requirement specifies that the open-loop frequency response (L) measured at the specified locations exceeds the minimum gain profile specified by loopgain.

You can specify the minimum gain profile as a smooth transfer function or sketch a piecewise error profile using an frd model or the makeweight command. Only gain values greater than 1 are enforced.

For MIMO feedback loops, the specified gain profile is interpreted as a lower bound on the smallest singular value of L.

Req = TuningGoal.MinLoopGain(location,fmin,gmin) specifies a minimum gain profile of the form loopgain = K/s (integral action). The software chooses K such that the gain value is gmin at the specified frequency, fmin.

Input Arguments

location

Location at which the minimum open-loop gain is constrained, specified as a string or cell array of strings. These strings identify one or more loop-opening locations in the control system to tune. What loop-opening locations are available depends on what kind of system you are tuning:

  • If you are tuning a Simulink® model of a control system, you can use any linear analysis point marked in the model, or any linear analysis point in an slTuner interface associated with the Simulink model. Use addPoint to add analysis points to the slTuner interface. Use getPoints to get the list of analysis points available in an slTuner interface to your model.

  • If you are tuning a generalized state-space (genss) model of a control system, you can use any AnalysisPoint location in the control system model. For example, the following code creates a PI loop with an analysis point at the plant input 'u'.

    AP = AnalysisPoint('u');
    G = tf(1,[1 2]);
    C = ltiblock.pid('C','pi');
    T = feedback(G*AP*C,1);
    

    You can use the string 'u' to refer to the analysis point at the plant input. Use getPoints to get the list of analysis points available in a genss model.

If location is a cell array of loop-opening locations, then the minimum gain requirement applies to the resulting MIMO loop.

loopgain

Minimum open-loop gain as a function of frequency.

You can specify loopgain as a smooth SISO transfer function (tf, zpk, or ss model). Alternatively, you can sketch a piecewise gain profile using a frd model or the makeweight command. For example, the following frd model specifies a minimum gain of 100 (40 dB) below 0.1 rad/s, rolling off at a rate of –20 dB/dec at higher frequencies.

loopgain = frd([100 100 10],[0 1e-1 1]);

When you use an frd model to specify loopgain, the software automatically maps your specified gain profile to a zpk model. The magnitude of this model approximates the desired gain profile. Use viewSpec(Req) to plot the magnitude of that zpk model.

Only gain values larger than 1 are enforced. For multi-input, multi-output (MIMO) feedback loops, the gain profile is interpreted as a lower bound on the smallest singular value of L. For more information about singular values, see sigma.

fmin

Frequency of minimum gain gmin, specified as a scalar value in rad/s.

Use this argument to specify a minimum gain profile of the form loopgain = K/s (integral action). The software chooses K such that the gain value is gmin at the specified frequency, fmin.

gmin

Value of minimum gain occurring at fmin, specified as a scalar absolute value.

Use this argument to specify a minimum gain profile of the form loopgain = K/s (integral action). The software chooses K such that the gain value is gmin at the specified frequency, fmin.

Properties

MinGain

Minimum open-loop gain as a function of frequency, specified as a SISO zpk model.

The software automatically maps the input argument loopgain onto a zpk model. The magnitude of this zpk model approximates the desired gain profile. Alternatively, if you use the fmin and gmin arguments to specify the gain profile, this property is set to K/s. The software chooses K such that the gain value is gmin at the specified frequency, fmin.

Use viewSpec(Req) to plot the magnitude of the open-loop minimum gain profile.

Focus

Frequency band in which tuning requirement is enforced, specified as a row vector of the form [min,max].

Set the Focus property to limit enforcement of the requirement 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 requirement that you want to apply only between 1 and 100 rad/s. To restrict the requirement to this band, use the following command:

Req.Focus = [1,100];

Default: [0,Inf] for continuous time; [0,pi/Ts] for discrete time, where Ts is the model sampling time.

Stabilize

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

When Stabilize is true, this requirement stabilizes the specified feedback loop, as well as imposing gain or loop-shape requirements. Set Stabilize to false if stability for the specified loop is not required or cannot be achieved.

Default: 1 (true)

LoopScaling

Toggle for automatically scaling loop signals, specified as 'on' or 'off'.

In multi-loop or MIMO control systems, the feedback channels are automatically rescaled to equalize the off-diagonal terms in the open-loop transfer function (loop interaction terms). Set LoopScaling to 'off' to disable such scaling and shape the unscaled open-loop response.

Default: 'on'

Location

Location at which minimum loop gain is constrained, specified as a string or cell array of strings. These strings identify one or more loop-opening locations in the control system to tune.

The value of the Location property is set by the location input argument when you create the TuningGoal.Sensitivity requirement.

Models

Models to which the tuning requirement 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 requirement for a subset of models in the array. For example, suppose you want to apply the tuning requirement, Req, to the second, third, and fourth models in a model array passed to systune. To restrict enforcement of the requirement, use the following command:

Req.Models = 2:4;

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

Default: NaN

Openings

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

If you are using the requirement 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 interface associated with the Simulink model. Use addPoint to add analysis points and loop openings to the slTuner interface. Use getPoints to get the list of analysis points available in an slTuner interface to your model.

If you are using the requirement 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.

Default: {}

Name

Name of the requirement object, specified as a string.

For example, if Req is a requirement:

Req.Name = 'LoopReq';

Default: []

Algorithms

When you tune a control system using a TuningGoal object to specify a tuning requirement, the software converts the requirement into a normalized scalar value f(x). Here, 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 requirement is a hard constraint.

For the TuningGoal.MinLoopGain requirement, f(x) is given by:

f(x)=WS(D1SD).

WS is the minimum loop gain profile, MaxGain. D is a diagonal scaling (for MIMO loops). S is the sensitivity function at Location.

Although S is a closed-loop transfer function, driving f(x) < 1 is equivalent to enforcing a lower bound on the open-loop transfer function, L, in a frequency band where the gain of L is greater than 1. To see why, note that S = 1/(1 + L). For SISO loops, when |L| >> 1, |S | ≈ 1/|L|. Therefore, enforcing the open-loop minimum gain requirement, |L| > |WS|, is roughly equivalent to enforcing |WsS| < 1. For MIMO loops, similar reasoning applies, with ||S|| ≈ 1/σmin(L), where σmin is the smallest singular value.

For an example illustrating the constraint on S, see Minimum Loop Gain as Constraint on Sensitivity Function.

Examples

Minimum Loop Gain Requirement

Create a requirement that boosts the open-loop gain of a feedback loop to greater than a specified profile.

Suppose that you are tuning a control system that has a loop-opening location identified by PILoop. Specify that the open-loop gain measured at that location exceed a minimum gain of 10 (20 dB) below 0.1 rad/s, rolling off at a rate of -20 dB/dec at higher frequencies. Use an frd model to sketch this gain profile.

loopgain = frd([10 10 0.1],[0 1e-1 10]);
Req = TuningGoal.MinLoopGain('PILoop',loopgain);

The software converts loopgain into a smooth function of frequency that approximates the piecewise-specified requirement. Display the requirement using viewSpec.

viewSpec(Req)

The green region indicates where the requirement is violated, except that gain values less than 1 are not enforced. Therefore, this requirement only specifies a minimum gain at frequencies below 1 rad/s.

You can use Req as an input to looptune or systune when tuning the control system.

Integral Minimum Gain Specified as Gain Value at Single Frequency

Create a requirement that specifies a minimum loop gain profile of the form L = K / s. The gain profile attains the value of -20 dB (0.01) at 100 rad/s.

Req = TuningGoal.MinLoopGain('X',100,0.01);
viewSpec(Req)

viewSpec confirms that the requirement is correctly specified. You can use this requirement to tune a control system that has a loop-opening location identified as 'X'. Since loop gain values less than 1 are ignored, this requirement specifies minimum gain only below 1 rad/s, with no restriction on loop gain at higher frequency.

Minimum Loop Gain as Constraint on Sensitivity Function

Examine a minimum loop gain requirement against the tuned loop gain. A minimum loop gain requirement is converted to a constraint on the gain of the sensitivity function at the requirement location.

To see this relationship between the requirement and the sensitivity function, tune the following closed-loop system with analysis points at X1 and X2. The control system has tunable PID controllers C1 and C2.

Create a model of the control system.

G2 = zpk([],-2,3);
G1 = zpk([],[-1 -1 -1],10);
C20 = ltiblock.pid('C2','pi');
C10 = ltiblock.pid('C1','pid');
X1 = AnalysisPoint('X1');
X2 = AnalysisPoint('X2');
InnerLoop = feedback(X2*G2*C20,1);
CL0 = feedback(G1*InnerLoop*C10,X1);
CL0.InputName = 'r';
CL0.OutputName = 'y';

Specify some tuning requirements, including a minimum loop gain requirement. Tune the control system to these requirements.

Rtrack = TuningGoal.Tracking('r','y',10,0.01);
Rreject = TuningGoal.Gain('X2','y',0.1);
Rgain = TuningGoal.MinLoopGain('X2',100,10000);
Rgain.Openings = 'X1';

[CL,fSoft] = systune(CL0,[Rtrack,Rreject,Rgain]);
Final: Soft = 1.07, Hard = -Inf, Iterations = 81

Examine the TuningGoal.MinLoopGain requirement against the corresponding tuned response.

viewSpec(Rgain,CL)

The plot shows the achieved loop gain for the loop at X2 (blue line). The plot also shows the inverse of the achieved sensitivity function, S, at the location X2 (green line). The inverse sensitivity function at this location is given by inv(S) = I+L. Here, L is the open-loop point-to-point loop transfer measured at X2.

The minimum loop gain requirement Rgain is constraint on inv(S), represented in the plot by the green shaded region. The constraint on inv(S) can be thought of as a minimum gain constraint on L that applies where the gain of L (or the smallest singular value of L, for MIMO loops) is greater than 1.

Requirement without Stability Constraint on Inner Loop

Create a requirement that specifies a minimum loop gain of 20 dB (100) at 50 rad/s on the inner loop of the following control system.

Req = TuningGoal.MinLoopGain('X2',50,100);

Configure the requirement to apply to the loop gain of the inner loop measured with the outer loop open.

Req.Openings = 'X2';

Setting Req.Openings tells the tuning algorithm to enforce this requirement with loops open at the specified location.

By default, tuning using TuningGoal.MinLoopGain imposes a stability requirement as well as the minimum loop gain requirement. Practically, in some control systems it is not possible to achieve a stable inner loop. When this occurs, remove the stability requirement for the inner loop by setting the Stabilize property to false.

Req.Stabilize = false;

The tuning algorithm still imposes a stability requirement on the overall tuned control system, but not on the inner loop alone.

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