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Package: TuningGoal
Tracking requirement for control system tuning
Use the TuningGoal.Tracking object to specify a frequencydomain tracking requirement between specified inputs and outputs. This requirement specifies the maximum relative error (gain from reference input to tracking error) as a function of frequency. Use this requirement for control system tuning with tuning commands such as systune or looptune.
You can specify the maximum error profile directly by providing a transfer function. Alternatively, you can specify a target DC error, peak error, and response time. These parameters are converted to the following transfer function that describes the maximum frequencydomain tracking error:
Here, ω_{c} is 2/(response time). The following plot illustrates these relationships for an example set of values.
Req = TuningGoal.Tracking(inputname,outputname,responsetime,dcerror,peakerror) creates a tuning requirement Req that constrains the tracking performance from inputname to outputname in the frequency domain. This tuning requirement specifies a maximum error profile as a function of frequency given by:
The tracking bandwidth ω_{c} = 2/responsetime. The maximum relative steadystate error is given by dcerror, and peakerror gives the peak relative error across all frequencies.
You can specify a MIMO tracking requirement by specifying signal names or a cell array of multiple signal names for inputname or outputname. For MIMO tracking requirements, use the InputScaling property to help limit crosscoupling. See Properties.
Req = TuningGoal.Tracking(inputname,outputname,maxerror) specifies the maximum relative error as a function of frequency. You can specify the target error profile (maximum gain from reference signal to tracking error signal) as a smooth transfer function. Alternatively, you can sketch a piecewise error profile using an frd model.
inputname 
Input signal for requirement, specified as a string or a cell array of strings for vectorvalued signals. The signals available to designate as input signals for the tuning requirement are as follows.
If inputname is a loopswitch channel of a generalized model, the input signal for the requirement is the implied input associated with the switch:

outputname 
Output signal for requirement, specified as a string or a cell array of strings for vectorvalued signals. The signals available to designate as output signals for the tuning requirement are as follows.
If outputname is a loopswitch channel of a generalized model, the output signal for the requirement is the implied output associated with the switch:

responsetime 
Target response time, specified as a positive scalar value. The tracking bandwidth is given by ω_{c} = 2/responsetime.Express the target response time in the time units of the models to be tuned. For example, when tuning a model T, if T.TimeUnit is 'minutes', then express the target response time in minutes. 
dcerror 
Maximum steadystate fractional tracking error, specified as a positive scalar value. For example, dcerror = 0.01 sets a maximum steadystate error of 1%. If inputname or outputname are vectorvalued, dcerror applies to all I/O pairs from inputname to outputname. Default: 0.001 
peakerror 
Maximum fractional tracking error across all frequencies, specified as a positive scalar value. For example, peakerror = 0.1 sets a maximum steadystate error of 10%. Default: 1 
maxerror 
Target tracking error profile as a function of frequency, specified as a SISO numeric LTI model. maxerror is the maximum gain from reference signal to tracking error signal. You can specify maxerror as a smooth transfer function (tf, zpk, or ss model). Alternatively, you can sketch a piecewise error profile using a frd model. When you do so, the software automatically maps the error profile to a zpk model. The magnitude of the zpk model. approximates the desired error profile. Use show(Req) to plot the magnitude of the zpk model. maxerror must be a SISO LTI model. If inputname or outputname are cell arrays, maxerror applies to all I/O pairs from inputname to outputname. 
Input 
Reference signal names. String or cell array of strings specifying the names of the signals to be tracked, populated by the inputname argument. 
InputScaling 
Reference signal scaling, specified as a vector of positive real values. For a MIMO tuning requirement, when the choice of units results in a mix of small and large signals in different channels of the response, use this property to specify the relative amplitude of each entry in the vectorvalued step input. This information is used to scale the offdiagonal terms in the transfer function from reference to tracking error. This scaling ensures that crosscouplings are measured relative to the amplitude of each reference signal. For example, suppose that Req is a requirement that signals {'y1','y2'} track reference signals {'r1','r2'}. Suppose further that you require the outputs to track the references with less than 10% crosscoupling. If r1 and r2 have comparable amplitudes, then it is sufficient to keep the gains from r1 to y2 and r2 and y1 below 0.1. However, if r1 is 100 times larger than r2, the gain from r1 to y2 must be less than 0.001 to ensure that r1 changes y2 by less than 10% of the r2 target. To ensure this result, set the InputScaling property as follows. Req.InputScaling = [100,1]; This tells the software to take into account that the first reference signal is 100 times greater than the second reference signal. The default value, [] , means no scaling. Default: [] 
Output 
Output signal names. String or cell array of strings specifying the names of the signals that must track the reference signals, populated by the outputname argument. 
MaxError 
Maximum error as a function of frequency, expressed as a SISO zpk model. This property stores the maximum tracking error as a function of frequency (maximum gain from reference signal to tracking error signal). If you use the syntax Req = TuningGoal.Tracking(inputname,outputname,maxerror), then the MaxError property is the zpk equivalent or approximation of the LTI model you supplied as the maxerror input argument. If you use the syntax Req = TuningGoal.Tracking(inputname,outputname,resptime,dcerror,peakerror, then the MaxError is a zpk transfer function given by:
MaxError is a SISO LTI model. If inputname or outputname are cell arrays, MaxError applies to all I/O pairs from inputname to outputname. Use show(Req) to plot the magnitude of MaxError. 
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. 
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 loopopening locations. The available loopopening locations depend on what kind of system you are tuning:
All feedback loops are closed by default, except where there is a permanent loopopening defined in an slTuner interface. Default: {} 
Name 
Name of the requirement object, specified as a string. For example, if Req is a requirement: Req.Name = 'LoopReq'; Default: [] 
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.Tracking requirement, f(x) is given by:
T(s,x) is the closedloop transfer function from Input to Output. denotes the H_{∞} norm (see norm).
Create a tracking requirement specifying that a signal 'theta' track a signal 'theta_ref'. The required response time is 2, in the time units of the control system you are tuning. The maximum steadystate error is 0.1%.
Req = TuningGoal.Tracking('theta_ref','theta',2,0.001);
Since peakerror is unspecified, this requirement uses the default value, 1.
Create a tracking requirement specifying that a signal 'theta' track a signal 'theta_ref'. The maximum relative error is 0.01 (1%) in the frequency range [0,1]. The relative error increases to 1 (100%) at the frequency 100.
Use a frd model model to specify the error profile as a function of frequency.
err = frd([0.01 0.01 1],[0 1 100]); Req = TuningGoal.Tracking('theta_ref','theta',err);
The software converts err into a smooth function of frequency that approximates the piecewise specified requirement. Display the error requirement using viewSpec.
viewSpec(Req)
The yellow region indicates where the requirement is violated.
evalSpec  looptune  looptune (for slTuner)  slTuner  systune  systune (for slTuner)  TuningGoal.Gain  TuningGoal.LoopShape  viewSpec