Target loop shape for control system tuning
Use the TuningGoal.LoopShape object to specify a target gain profile (gain as a function of frequency) of an open-loop response. The TuningGoal.LoopShape requirement constrains the open-loop, point-to-point response (L) at a specified location in your control system. Use this requirement for control system tuning with tuning commands, such as systune or looptune.
When you tune a control system, the target open-loop gain profile is converted into constraints on the inverse sensitivity function inv(S) = (I + L) and the complementary sensitivity function T = 1–S. These constraints are illustrated for a representative tuned system in the following figure.
Where L is much greater than 1, a minimum gain constraint on inv(S) (green shaded region) is equivalent to a minimum gain constraint on L. Similarly, where L is much smaller than 1, a maximum gain constraint on T (red shaded region) is equivalent to a maximum gain constraint on L. The gap between these two constraints is twice the CrossTol parameter, which specifies the frequency band where the loop gain can cross 0 dB.
For multi-input, multi-output (MIMO) control systems, values in the gain profile greater than 1 are interpreted as minimum performance requirements. Such values are lower bounds on the smallest singular value of the open-loop response. Gain profile values less than one are interpreted as minimum roll-off requirements, which are upper bounds on the largest singular value of the open-loop response. For more information about singular values, see sigma.
Use TuningGoal.LoopShape when the loop shape near crossover is simple or well understood (such as integral action). To specify only high gain or low gain constraints in certain frequency bands, use TuningGoal.MinLoopGain and TuningGoal.MaxLoopGain. When you do so, the software determines the best loop shape near crossover.
Req = TuningGoal.LoopShape(location,loopgain) creates a tuning requirement for shaping the open-loop response measured at the specified location. The magnitude of the single-input, single-output (SISO) transfer function loopgain specifies the target open-loop gain profile. You can specify the target gain profile (maximum gain across the I/O pair) as a smooth transfer function or sketch a piecewise error profile using an frd model.
Req = TuningGoal.LoopShape(location,loopgain,crosstol) specifies a tolerance on the location of the crossover frequency. crosstol expresses the tolerance in decades. For example, crosstol = 0.5 allows gain crossovers within half a decade on either side of the target crossover frequency specified by loopgain. When you omit crosstol, the tuning requirement uses a default value of 0.1 decades. You can increase crosstol when tuning MIMO control systems. Doing so allows more widely varying crossover frequencies for different loops in the system.
Req = TuningGoal.LoopShape(location,wcrange) specifies a range for the target gain crossover frequency. The range is a vector of the form wcrange = [wc1,wc2]. This syntax is equivalent to using the geometric mean sqrt(wc1*wc2) as wc and setting crosstol to the half-width of wcrange in decades. Using a range instead of a single wc value increases the ability of the tuning algorithm to enforce the target loop shape for all loops in a MIMO control system.
Location where the open-loop response shape to be constrained is measured, specified as a string or cell array of strings that 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:
The loop shape requirement applies to the point-to-point open-loop transfer function at the specified loop-opening location. That transfer function is the open-loop response obtained by injecting signals at the loop-opening location and measuring the return signals at the same point.
If location is a cell array of loop-opening locations, then the loop shape requirement applies to the MIMO open-loop transfer function.
Target open-loop gain profile 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. When you do so, the software automatically maps your specified gain profile to a zpk model whose magnitude approximates the desired gain profile. Use viewSpec(Req) to plot the magnitude of that zpk model.
For multi-input, multi-output (MIMO) control systems, values in the gain profile greater than 1 are interpreted as minimum performance requirements. These values are lower bounds on the smallest singular value of L. Gain profile values less than one are interpreted as minimum roll-off requirements, which are upper bounds on the largest singular value of L. For more information about singular values, see sigma.
Tolerance in the location of crossover frequency, in decades. specified as a scalar value. For example, crosstol = 0.5 allows gain crossovers within half a decade on either side of the target crossover frequency specified by loopgain. Increasing crosstol increases the ability of the tuning algorithm to enforce the target loop shape for all loops in a MIMO control system.
Target crossover frequency, specified as a positive scalar value. Express wc in units of rad/TimeUnit, where TimeUnit is the TimeUnit property of the control system model you are tuning.
Range for target crossover frequency, specified as a vector of the form [wc1,wc2]. Express wc in units of rad/TimeUnit, where TimeUnit is the TimeUnit property of the control system model you are tuning.
Location at which the open-loop response shape to be constrained is measured, specified as a string or cell array of strings that 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.LoopShape requirement.
Target loop shape 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. Use viewSpec(Req) to plot the magnitude of the zpk model LoopGain.
Tolerance on gain crossover frequency, in decades.
The initial value of CrossTol is set by the crosstol input when you create the requirement object.
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.
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)
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.
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.
Feedback loops to open when evaluating the requirement, specified as a cell array of strings that identify loop-opening locations. The available loop-opening locations depend on what kind of system you are tuning:
All feedback loops are closed by default, except where there is a permanent loop-opening defined in an slTuner interface.
Name of the requirement object, specified as a string.
For example, if Req is a requirement:
Req.Name = 'LoopReq';
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), 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 requirement is a hard constraint.
For the TuningGoal.LoopShape requirement, f(x) is given by:
S = D–1[I – L(s,x)]–1D is the scaled sensitivity function.
L(s,x) is the open-loop response being shaped.
D is an automatically-computed loop scaling factor. (If the LoopScaling property is set to 'off', then D = I.)
T = S – I is the complementary sensitivity function.
WS and WT are weighting functions derived from the specified loop shape.
Create a target gain profile requirement for the following control system. Specify integral action, gain crossover at 1, and a roll-off requirement of 40 dB/decade.
The requirement should apply to the open-loop response measured at the loopswitch block X. Specify a crossover tolerance of 0.5 decades.
LS = frd([100 1 0.0001],[0.01 1 100]); Req = TuningGoal.LoopShape('X',LS,0.5);
The software converts LS into a smooth function of frequency that approximates the piecewise-specified requirement. Display the requirement using viewSpec.
The green and red regions indicate the bounds for the inverse sensitivity, inv(S) = 1-G*C, and the and the complementary sensitivity, T = 1-S, respectively. The gap between these regions at 0 dB gain reflects the specified crossover tolerance, which is half a decade to either side of the target loop crossover.
When you use viewSpec(Req,CL) to validate a tuned closed-loop model of this control system, CL, the tuned values of S and T are also plotted.
Create separate loop shape requirements for the inner and outer loops of the following control system.
For the inner loop, specify a loop shape with integral action, gain crossover at 1, and a roll-off requirement of 40 dB/decade. Additionally, specify that this loop shape requirement should be enforced with the outer loop open.
LS2 = frd([100 1 0.0001],[0.01 1 100]); Req2 = TuningGoal.LoopShape('X2',LS2); Req2.Openings = 'X1';
Specifying 'X2' for the location indicates that Req2 applies to the point-to point, open-loop transfer function at the loop opening location X2. Setting Req2.Openings indicates that the loop switch at X1 is open when Req2 is enforced.
By default, Req2 imposes a stability requirement on the inner loop as well as the loop shape requirement. In some control systems, however, inner-loop stability might not be required, or might be impossible to achieve. In that case, remove the stability requirement from Req2 as follows.
Req2.Stabilize = false;
For the outer loop, specify a loop shape with integral action, gain crossover at 0.1, and a roll-off requirement of 20 dB/decade.
LS1 = frd([10 1 0.01],[0.01 0.1 10]); Req1 = TuningGoal.LoopShape('X1',LS1);
Specifying 'X1' for the location indicates that Req1 applies to the point-to point, open-loop transfer function at the loop opening location X1. You do not have to set Req1.Openings because this loop shape is enforced with the inner loop closed.
You may need to tune the control system with both loop shaping requirements Req1 and Req2. To do so, use both requirements as inputs to the tuning command. For example, suppose CL0 is a tunable genss model of the closed-loop control system. In that case, the following command tunes the control system to both requirements.
[CL,fSoft] = systune(CL0,[Req1,Req2]);
Create a loop-shape requirement for the feedback loop on 'q' in the following control system, which is the Simulink model rct_airframe2. Specify that the loop-shape requirement is enforced with the 'az' loop open.
Open the model.
Create a loop shape requirement that enforces integral action with a crossover a 2 rad/s for the 'q' loop. This loop shape corresponds to a loop shape of 2/s.
s = tf('s'); shape = 2/s; Req = TuningGoal.LoopShape('q',shape);
Specify the location at which to open an additional loop when enforcing the requirement.
Req.Openings = 'az';
To use this requirement to tune the Simulink model, create an slTuner interface to the model. Identify the block to tune in the interface.
ST0 = slTuner('rct_airframe2','MIMO Controller');
Designate both az and q as analysis points in the slTuner interface.
This command makes q available as an analysis location. It also allows the tuning requirement to be enforced with the loop open at az.
You can now tune the model using Req and any other tuning requirements. For example:
[ST,fSoft] = systune(ST0,Req);
Create a tuning requirement specifying that the open-loop response of loop identified by 'X' cross unity gain between 50 and 100 rad/s.
Req = TuningGoal.LoopShape('X',[50,100]);
Examine the resulting requirement to see the target loop shape.
The plot shows that the requirement specifies an integral loop shape, with crossover around 70 rad/s, the geometrical mean of the range [50,100]. The gap at 0 dB between the minimum low-frequency gain (green region) and the maximum high-frequency gain (red region) reflects the allowed crossover range [50,100].