loopsyn

Loop-shaping controller design with tradeoff between performance and robustness

Syntax

[K,CL,gamma,info] = loopsyn(G,Gd)

[K,CL,gamma,info] = loopsyn(G,Gd,alpha)

[K,CL,gamma,info] = loopsyn(G,Gd,alpha,ord)

Description

loopsyn balances performance and robustness by blending two loop-shaping methods.

• Mixed-sensitivity design (mixsyn), which tends to optimize performance and decoupling at the expense of robustness

• The Glover-McFarlane method (ncfsyn), which maximizes robustness to plant uncertainty

You can adjust the tradeoff between performance and robustness to obtain satisfactory time-domain responses while avoiding fragile designs with plant inversion or flexible mode cancellation.

[K,CL,gamma,info] = loopsyn(G,Gd) computes a stabilizing controller K that shapes the open-loop response G*K to approximately match the specified loop shape Gd. The mixed-sensitivity performance gamma indicates the closeness of the match. loopsyn tries to minimize gamma, subject to the constraint that the robustness with K (as measured by ncfmargin) is no worse than half the maximum achievable robustness. The function also returns the closed-loop transfer function CL and a structure info containing further information about the controller synthesis.

[K,CL,gamma,info] = loopsyn(G,Gd,alpha) explicitly specifies the tradeoff between performance and robustness with parameter alpha in the interval [0,1]. Within this interval, smaller alpha favors performance (mixsyn design) and larger alpha favors robustness (ncfsyn design). When you specify alpha, loopsyn tries to minimize gamma, subject to the constraint that the robustness is no worse than alpha times the maximum achievable robustness.

[K,CL,gamma,info] = loopsyn(G,Gd,alpha,ord) specifies the order of the controller K. To use this syntax, you must specify alpha such that 0 < alpha < 1.

Examples

Design Loop-Shaping Controller

Consider the following plant.

s = zpk('s');
G = (s-10)/(s+100);

Design a controller that yields a closed-loop step response with a rise time of about 4 s. A simple target loop shape for this requirement is Gd = wc/s, where the target crossover frequency wc is related to the desired rise time by t = 2/wc.

wc = 0.5; 
Gd = wc/s;

Obtain the controller using loopsyn.

[K,CL,gamma] = loopsyn(G,Gd);
gamma

gamma = 
1.1744

This value of gamma is close to 1, indicating a fairly good match between the achieved loop shape and the target loop shape. Compare the achieved open-loop response G*K with the desired response Gd.

sigma(G*K,"b",Gd,"r--",{0.01,10})
grid on
legend("Actual","Target")

Examine the step response of the closed-loop system.

step(CL)

Specify Tradeoff Between Performance and Robustness

You can use the input argument alpha to specify how much loopsyn favors either performance (mixsyn design) or robustness (ncfsyn design). By default, loopsyn computes a balanced design, alpha = 0.5. To change the balance, change alpha. Consider the following plant and target loop shape.

G = tf(25,[1 10 10 10]); 
Gd = tf(0.5,[1 0]);

Design a loop-shaping controller that maximizes performance (minimizes gamma) subject to the constraint that the robustness (as determined by ncfmargin) is no worse than 75% of the maximum achievable robustness. To do so, set alpha to 0.75.

alpha = 0.75;
[K,CL,gamma,info] = loopsyn(G,Gd,alpha);

The maximum achievable robustness margin is returned in the info structure. Compare that value to the margin achieved by this controller. For ncfmargin, use the shaped plant and corresponding controller, also returned in info.

info.emax

ans = 
0.6474

ncfmargin(info.Gs,info.Ks)

ans = 
0.4857

These values confirm that the achieved robustness is 75% of the maximum robustness achievable by setting alpha = 1 for the pure ncfsyn design. For this plant, the alpha = 0.75 design yields a good match to the loop shape without sacrificing much robustness. Examine the performance and loop shape with this controller.

gamma

gamma = 
1.2143

sigma(G*K,"b",Gd,"r--",{0.01,10})
grid on
legend("Actual","Target")

For details on how to choose a good value of alpha for your application, see Loop-Shaping Controller Design.

Specify Different Shapes for Each Loop in MIMO System

When designing a controller for a MIMO system, if you specify a scalar Gd, then loopsyn applies the same target loop shape to all feedback channels. You can specify a different shape for each loop using a diagonal Gd of size Ny-by-Ny, where Ny is the number of feedback loops, or the number of outputs of G. Consider the following control system with a two-output, two-input plant.

s = tf('s');
G = [(1+0.1*s)/(1+s) 0.1/(s+2) ; 0 (s+2)/(s^2+s+3)];

Design a controller for this plant such as the first feedback channel has a crossover frequency of 1 rad/s and the second has a crossover frequency of 5 rad/s. To do so, create the loop shapes wc/s for each loop and use the append command to create the diagonal Gd.

wc = [1 5]; 
Gd = append(wc(1)/s,wc(2)/s);

Design the controller.

[K,CL] = loopsyn(G,Gd);

Compare the achieved loop shapes with the target loop shape.

bodemag(G*K,Gd,{0.1,100})
grid on
legend("Actual","Target")

Specify Order of Tuned Controller

You can limit the order of the controller that loopsyn designs using the order argument. To specify a controller order, you must use a value of the parameter alpha such that 0 < alpha < 1.

Load a two-input, two-output plant.

load plant_loopsynOrderExample.mat G
size(G)

State-space model with 2 outputs, 2 inputs, and 7 states.

Design a controller for this plant with loop shape Gd = 0.5/s. Use alpha = 0.5 and let loopsyn select the controller order.

alpha = 0.5;
Gd = tf(0.5,[1 0]);
[K0,CL0,gamma0] = loopsyn(G,Gd,alpha); 
order(K0)

ans = 
5

loopsyn returns a controller with five states. Use loopsyn again to design a three-state controller.

ord = 3;
[K,CL,gamma,info] = loopsyn(G,Gd,alpha,ord); 
order(K)

ans = 
3

For this plant, reducing the controller order from five to three yields a small decrease in performance.

gamma0

gamma0 = 
1.0334

gamma

gamma = 
1.1831

Use a model-reduction command such as balred to identify suitable target orders to try for ord.

Two-Degree-of-Freedom Loop-Shaping Controller

loopsyn can also provide a two-input, one output controller suitable for implementing the two-degree-of-freedom (2-DOF) architecture of the following illustration.

This architecture can be useful for mitigating the derivative kick that can occur when the reference signal changes. To obtain a controller suitable for this implementation, specify a plant and target loop shape, and call loopsyn.

G = tf(8625,[1 2.389 -5606]);
Gd = tf(80,[1 0])*tf(240,[1 240]);

[K,CL,gamma,info] = loopsyn(G,Gd);

The K2dof field of the info output contains the 2-DOF controller. For a plant with Nu inputs and Ny outputs, K2dof has Nu outputs and 2*Ny inputs. In this example, because G is SISO, K2dof has one output and two inputs.

K2dof = info.K2dof;
size(K2dof)

State-space model with 1 outputs, 2 inputs, and 4 states.

To use the controller, create a closed-loop system with the architecture shown previously.

L2dof = G*K2dof;
L2dof.InputName = {'r','y'};
L2dof.OutputName = 'y';
CL2dof = connect(L2dof,'r','y');

Compare the closed-loop step response with the two architectures. For this system, the 2-dof architecture substantially reduces the overshoot in the response.

step(CL,CL2dof)
legend("1-dof","2-dof")

Input Arguments

G — Plant

dynamic system model

Plant, specified as a dynamic system model such as a state-space (ss) model. If G is a generalized state-space model with uncertain or tunable control design blocks, then loopsyn uses the nominal or current value of those elements. G can be SISO or MIMO, and can be a continuous-time or discrete-time model. G must have at least as many inputs as outputs. G cannot have time delays. Use pade to approximate delays.

Gd — Target loop shape

dynamic system model

Target loop shape, specified as a dynamic system model such as a ss, tf, or zpk model. You can also provide Gd as a frequency-response data (frd) model specifying the desired gain at specific frequencies. Gd cannot have time delays. Use pade to approximate delays.

For a MIMO plant G with Ny outputs:

• Specify SISO Gd to use the same desired loop shape for all loops.

• Specify Ny-by-Ny diagonal Gd to use a different shape for each feedback loop. One way to construct such a diagonal Gd is to specify a Gdi for each channel, and then use Gd = append(Gd1,...,GdNy).

In general, use a target loop shape that has high gain at low frequencies for reference tracking and disturbance rejection, and low gain at high frequencies for robustness against plant uncertainty. For more information about how to choose your target loop shape, see Loop Shaping for Performance and Robustness.

alpha — Balance between performance and robustness

0.5 (default) | scalar in [0,1]

Balance between performance and robustness, specified as a scalar value in the range [0,1]. Use alpha to adjust the balance between performance and robustness as follows:

• alpha = 0 gives the mixsyn design.

• alpha = 1 gives the ncfsyn design.

loopsyn maximizes performance (minimizes gamma) subject to the constraint that the robustness (as measured by ncfmargin) is no worse than alpha*emax, where emax is the maximum robustness achievable by the ncfsyn design. The default value alpha = 0.5 yields a balanced design. You can adjust alpha between 0 and 1 to find the right tradeoff for your application. For an example that shows the effect of varying alpha, see Loop-Shaping Controller Design.

ord — Controller order

positive integer

Controller order, specified as a positive integer. To use this option, you must specify alpha such that 0 < alpha < 1.

Output Arguments

K — Loop-shaping controller

ss model

Loop-shaping controller, returned as a state-space (ss) model. K shapes the open-loop response G*K to approximately match the specified loop shape Gd. The controller minimizes the performance gamma subject to the constraint that the stability margin as computed by ncfmargin does not exceed alpha*emax, where emax is the maximum margin achievable by ncfsyn.

CL — Closed-loop system

ss model

Closed-loop system, returned as a state-space (ss) model. The closed-loop system is given by feedback(G*K,eye(ny)), where ny is the number of outputs of G.

gamma — Controller performance

nonnegative scalar | Inf

Controller performance, returned as a nonnegative scalar value or Inf. A value near or below 1 indicates that G*K is close to Gd. Values much greater than one indicate a poor match between the achieved and desired loop shapes. If loopsyn cannot find a stabilizing controller, gamma is Inf.

gamma is the mixed-sensitivity performance, the cost function minimized by mixsyn, and is given by

$$\gamma ={\Vert \left[\begin{array}{c}{W}_{1}S\\ W_{3}T\end{array}\right]\Vert }_{\infty },S={\left(I+GK\right)}^{-1},T=I-S,$$

where W₁ and W₃ are the mixsyn weights. loopsyn derives these weights from Gd to enforce the desired loop shape.

info — Additional information about controller synthesis

structure

Additional information about the controller synthesis, returned as a structure containing the following fields.

Field	Description
W	Shaping prefilter, returned as a state-space (ss) model. The value of W is such that the shaped plant Gs = G*W has the desired loop shape Gd.
Gs	Shaped plant Gs = G*W, returned as an ss model.
Ks	H∞ controller for the shaped plant Gs (see ncfsyn), returned as an ss model. For computing the robustness margin with ncfmargin, use Gs and Ks.
emax	Maximum achievable robustness margin, returned as a scalar. This value is the robustness achieved by the pure ncfsyn design. For information on interpreting this value, see ncfmargin.
W1,W3	Weighting functions for the mixsyn formulation of the loop-shaping goal, returned as ss models. loopsyn derives these weights from Gd to enforce the desired loop shape.
K2dof	Two-degree-of-freedom controller, returned as a ss model with 2*ny inputs and nu outputs, where ny and nu are the numbers of outputs and inputs of G, respectively. This controller is suitable for a two-degree-of-freedom implementation that separates the reference signal from the feedback signal, as in the architecture of the following diagram. For an example that shows how to use K2dof, see Two-Degree-of-Freedom Loop-Shaping Controller.

Version History

Introduced before R2006a

R2021b: loopsyn ignores frequency range

Starting in R2021b, loopsyn automatically generates a design that balances performance and robustness. As a result of this change, loopsyn might return different results from previous releases. Additionally, you can no longer specify a frequency range for loop shaping using the syntax loopsyn(G,Gd,[wmin,wmax]). The loopsyn command ignores any [wmin,wmax] input. Further, the info output structure no longer contains a Range field.