Tuning Fuzzy Inference Systems

Designing a complex fuzzy inference system (FIS) with large number of inputs and membership functions (MFs) is a challenging problem due to large number of MF parameters and increasing number of rules. A promising solution to this problem is to design FIS trees, which include hierarchically connected modular FISs having small number of inputs. Designing a FIS tree with appropriate MF parameters and rules, however, is a difficult task when adequate expert knowledge is not available for a complex system. Hence, the data-driven approach of tuning and optimizing MF and rule parameters offers an elegant solution to this problem.

You can tune a fuzzy system using the tunefis command. The following figure shows how a fuzzy system is tuned with input/output training data. The tuning process uses an optimization method on an optimization problem created from a fuzzy system.

In each iteration, the optimization method generates multiple sets of solutions, which are values for the selected parameters of the fuzzy system. The fuzzy system is updated with each solution and then evaluated using the input training data. The evaluated output is compared with the output training data to generate costs of the solutions. This process continues for multiple iterations until the stop condition is met, and then it returns the minimum cost solution with the optimized fuzzy system parameters. For an example that uses this approach, see Tune Mamdani Fuzzy Inference System.

If input/output training data is not available, you can use a custom model in place of the training data to evaluate a fuzzy system for cost measurement. This approach is shown in the following figure.

In this case, the custom model uses the fuzzy system to minimize the cost of achieving specific performance goals. The parameter solution that produces the best performance of the custom model is returned as the optimization result. For example, in a robot navigation model, the performance goal is to minimize the travelled distances to target positions without colliding with any of the obstacles. Hence, the navigation model uses the fuzzy system to control the robot’s heading direction to achieve the goal. For an example that uses this approach, see Tune Fuzzy Systems using Custom Cost Function.

You can select individual MF and rule parameters of a fuzzy system for optimization.

Tune Membership Function Parameters

This example shows how to specify parameter settings for each input and output MF of a FIS and tune it.

Create a FIS.

fis = mamfis;
fis = addInput(fis,[0 10],'NumMFs',3);
fis = addOutput(fis,[0 1],'NumMFs',3);
fis = addRule(fis,[1 1 1 1;1 1 1 1;1 1 1 1]);

Extract input and output parameter settings from the FIS.

[in,out] = getTunableSettings(fis)
in = 
  VariableSettings with properties:

                   Type: "input"
           VariableName: "input1"
    MembershipFunctions: [1×3 fuzzy.tuning.MembershipFunctionSettings]
                FISName: "fis"

out = 
  VariableSettings with properties:

                   Type: "output"
           VariableName: "output1"
    MembershipFunctions: [1×3 fuzzy.tuning.MembershipFunctionSettings]
                FISName: "fis"

The parameter settings are represented by VariableSettings objects that include the FIS name, variable type, variable name, and MF parameter settings. Examine the parameter settings of MF 1 of input 1.

in(1).MembershipFunctions(1).Parameters
ans = 
  NumericParameters with properties:

    Minimum: [-Inf -Inf -Inf]
    Maximum: [Inf Inf Inf]
       Free: [1 1 1]

For each parameter value of an input/output MF, you can specify whether it is available for tuning and its minimum and maximum values. By default, all MF parameters are free for tuning and their ranges are set to [-Inf,Inf]. Make MF 1 of input 1 nontunable.

in(1).MembershipFunctions(1) = setTunable(in(1).MembershipFunctions(1),false);

Similarly, make the first parameter of MF 2 of input 1 nontunable.

in(1).MembershipFunctions(2).Parameters.Free(1) = false;

Set minimum ranges for second and third parameters of MF 3 of input 1 to 0.

in(1).MembershipFunctions(3).Parameters.Minimum(2:3) = 0;

Set maximum ranges for second and third parameters of MF 3 of input 1 to 15.

in(1).MembershipFunctions(3).Parameters.Maximum(2:3) = 15;

Note that the default minimum and maximum range values of tunable MF parameters are set to corresponding input/output ranges in the tuning process.

Finally, make the output nontunable.

out = setTunable(out,false);

Now that you have configured the parameters, specify input and output training data. Generate some data for this example.

x = (0:0.1:10)';
y = abs(sin(2*x)./exp(x/5));

Specify options for tunefis. Use genetic algorithm for optimization.

options = tunefisOptions("Method","ga");

Specify maximum 5 generations for optimization.

options.MethodOptions.MaxGenerations = 5;

If you have Parallel Computing Toolbox™ software, you can improve the speed of the tuning process by setting options.UseParallel to true. If you do not have Parallel Computing Toolbox software, set options.UseParallel to false.

By default, tunefis uses root mean squared error (RMSE) for cost calculation. You can change the cost function to norm1 or norm2 by setting options.DistanceMetric.

options.DistanceMetric = "norm1";

Tune fis using the parameter settings, training data, and tuning options.

rng('default')  % for reproducibility
[fisout,optimout] = tunefis(fis,[in;out],x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              100           32.84           32.84        0
    2              147           32.84           32.84        1
    3              194           32.84           32.84        2
    4              241           32.84           32.84        3
    5              288           32.84           32.84        4
Optimization terminated: maximum number of generations exceeded.

fisout includes the updated parameter values. optimout provides additional outputs of the optimization method and any error message that are returned during the update process of the input fuzzy system using the optimized parameter values.

optimout
optimout = struct with fields:
    tuningOutputs: [1×1 struct]
     errorMessage: []

optimout.tuningOutputs
ans = struct with fields:
             x: [5 9.1667 5.8333 10 14.1667]
          fval: 32.8363
      exitflag: 0
        output: [1×1 struct]
    population: [50×5 double]
        scores: [50×1 double]

You can optionally tune fis using either input or output parameter settings. In this example, output parameter settings are set to nontunable. Therefore, tuning fis with only input parameter settings produces the same results.

rng('default')
tunefis(fis,in,x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              100           32.84           32.84        0
    2              147           32.84           32.84        1
    3              194           32.84           32.84        2
    4              241           32.84           32.84        3
    5              288           32.84           32.84        4
Optimization terminated: maximum number of generations exceeded.

Note that the best optimization costs shown in column 3 of both tuning results are the same, which indicates that the optimization results are the same in both cases.

Tune Fuzzy Rules

You can specify only rule parameters of a fuzzy system for tuning.

Use getTunableSettings to get rule parameter settings from a fuzzy system. Rule parameter settings are returned as the third output argument.

[~,~,rule] = getTunableSettings(fis)
rule=3×1 object
  3×1 RuleSettings array with properties:

    Index
    Antecedent
    Consequent
    FISName

Each rule parameter settings include FIS name, index of the rule in the FIS, and parameter settings for the rule antecedent and consequent (the rule clause).

The parameter settings for a rule clause include three options:

  • Whether the input/output MF indices are available for tuning. By default, clause parameters are free for tuning.

  • Whether the clause allows use of NOT logic, in other words, whether it allows negative MF indices. By default, rules do not allow NOT logic.

  • Whether the clause allows absence of input/output variables, in other words, if it allows zero MF indices. By default, absence of a variable is allowed.

rule(1).Antecedent(1)
ans = 
  ClauseParameters with properties:

      AllowNot: 0
    AllowEmpty: 1
          Free: 1

Allow NOT logic in the antecedent of rule 1.

rule(1).Antecedent.AllowNot = true;

Make the consequent of rule 1 not available for tuning.

rule(1).Consequent.Free = 0;

Do not allow absence of a variable in the consequent of rule 2.

rule(2).Consequent.AllowEmpty = false;

Set rule 3 nontunable.

rule(3) = setTunable(rule(3),false);

Set options.DistanceMetric to norm2.

options.DistanceMetric = "norm2";

Tune fis with the rule parameter settings.

rng('default')  % for reproducibility
fisout = tunefis(fis,rule,x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              100           1.648           2.575        0
    2              147           1.648           2.448        1
    3              194           1.648           2.212        2
    4              241           1.648           2.052        3
    5              288           1.648           1.874        4
Optimization terminated: maximum number of generations exceeded.

Because you made rule 3 nontunable, you can exclude rule 3 when you tune fis.

rng('default')  % for reproducibility
tunefis(fis,rule(1:2),x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              100           1.648           2.575        0
    2              147           1.648           2.448        1
    3              194           1.648           2.212        2
    4              241           1.648           2.052        3
    5              288           1.648           1.874        4
Optimization terminated: maximum number of generations exceeded.

Note that the best optimization costs shown in column 3 of both tuning results are the same, which indicates the optimization results are the same in both cases.

Learn Fuzzy Rules

You can tune a fuzzy system without any rules in tunefis. In this case, set the OptimizationType option of tunefisOptionsto learning so that tunefis learns rules for the FIS.

fisin = fis;
fisin.Rules = [];
options.OptimizationType = 'learning';

Specify maximum size of the rule base to 3. This value specifies the maximum number of rules in the tuned FIS.

options.NumMaxRules = 3;

Note that the size of the tuned rule base may be less than NumMaxRules, because tunefis removes duplicate rules from the tuned FIS. If you do not specify NumMaxRules, then tunefis adds the maximum number of rules determined by the combinations of input MFs. The default input MF combinations include zero MF indices, which allow absence of variables. The default combinations exclude negative MF indices, so that NOT logic is not allowed.

Set options.DistanceMetric to rmse and tune the FIS.

options.DistanceMetric = "rmse";
rng('default')  % for reproducibility
fisout = tunefis(fisin,[],x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              400           0.165          0.2973        0
    2              590           0.165          0.2891        1
    3              780           0.165          0.2685        2
    4              970           0.165          0.2548        3
    5             1160           0.165          0.2378        4
Optimization terminated: maximum number of generations exceeded.

During the tuning process, the FIS automatically learns rules after cost optimization with the training data. Examine the tuned rules.

fisout.Rules
ans = 
  1×3 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf3 => output1=mf1 (1)"
    2    "input1==mf1 => output1=mf2 (1)"
    3    "input1==mf2 => output1=mf1 (1)"

You can remove some of the existing rules and learn additional rules.

fisout.Rules(2:end) = [];
rng('default')  % for reproducibility
fisout = tunefis(fisin,[],x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              400           0.165          0.2973        0
    2              590           0.165          0.2891        1
    3              780           0.165          0.2685        2
    4              970           0.165          0.2548        3
    5             1160           0.165          0.2378        4
Optimization terminated: maximum number of generations exceeded.
fisout.Rules
ans = 
  1×3 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf3 => output1=mf1 (1)"
    2    "input1==mf1 => output1=mf2 (1)"
    3    "input1==mf2 => output1=mf1 (1)"

You can also tune existing rules and learn new rules.

fisout.Rules(2:end) = [];
fisout.Rules(1).Antecedent = 1;
fisout.Rules(1).Consequent = 1;
[~,~,rule] = getTunableSettings(fisout);
rng('default')
fisout = tunefis(fisin,rule,x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              400           0.165          0.3075        0
    2              590           0.165          0.2738        1
    3              780           0.165          0.2545        2
    4              970           0.165          0.2271        3
    5             1160           0.165          0.2083        4
Optimization terminated: maximum number of generations exceeded.
fisout.Rules
ans = 
  1×3 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf1 => output1=mf2 (1)"
    2    "input1==mf2 => output1=mf1 (1)"
    3    "input1==mf3 => output1=mf1 (1)"

Tune MF and Rule Parameters

You can tune all FIS parameters together.

[in,out,rule] = getTunableSettings(fis);
options = tunefisOptions('Method','ga');
options.MethodOptions.MaxGenerations = 5;
rng('default')  % for reproducibility
fisout = tunefis(fis,[in;out;rule],x,y,options);
                                  Best           Mean      Stall
Generation      Func-count        f(x)           f(x)    Generations
    1              400          0.1624          0.2997        0
    2              590          0.1624          0.2776        1
    3              780          0.1624          0.2653        2
    4              970          0.1592          0.2486        0
    5             1160          0.1592          0.2342        1
Optimization terminated: maximum number of generations exceeded.

For a large fuzzy system, tuning all FIS parameters in the same tuning process may take several iterations to obtain the expected results. Hence, you can tune parameters in two steps:

  1. Tune or learn rule parameters only.

  2. Tune both MF and rule parameters.

The first step is less computationally expensive due to the small number of rule parameters. It quickly converges to a fuzzy rule base during training. In the second step, using the rule base from the first step as an initial condition provides fast convergence of the parameter tuning process.

Tune FIS Tree Parameters

You can tune the parameters of a FIS tree using a similar two-step process to the one described above for tuning a FIS.

Create a FIS tree to model sin(x)+cos(x)exp(x) as shown in the following figure.

Create fis1 as a Sugeno type FIS, which results in faster tuning process due to computationally efficient defuzzification method. Add two inputs with range [0 10] having three MFs each. Use a smooth differentiable MF, such as gaussmf, to match characteristics of the data type you are modeling.

fis1 = sugfis('Name','fis1');
fis1 = addInput(fis1,[0 10],'NumMFs',3,'MFType','gaussmf');
fis1 = addInput(fis1,[0 10],'NumMFs',3,'MFType','gaussmf');

Add an output with range [–1.5 1.5] having nine MFs to provide maximum granularity corresponding to each combination of the input MFs. The output range is set according to the values of sin(x)+cos(x).

fis1 = addOutput(fis1,[-1.5 1.5],'NumMFs',9);

Create fis2 as a Sugeno type FIS as well. Add two inputs. Use [-1.5 1.5] as the range of the first input, which is the output of fis1. The second input is the same as the inputs of fis1, so it also uses range [0 10]. Add three MFs for each of the inputs.

fis2 = sugfis('Name','fis2');
fis2 = addInput(fis2,[-1.5 1.5],'NumMFs',3,'MFType','gaussmf');
fis2 = addInput(fis2,[0 10],'NumMFs',3,'MFType','gaussmf');

Add an output with range [0 1] having nine MFs to provide maximum granularity corresponding to each combination of the input MFs. The output range is set according to the values of sin(x)+cos(x)exp(x).

fis2 = addOutput(fis2,[0 1],'NumMFs',9);

Connect the inputs and the outputs as shown in the diagram. Output 1 of fis1 connects to input 1 of fis2, inputs 1 and 2 of fis1 connect to each other, and input 2 of fis1 connects to input 2 of fis2.

con1 = ["fis1/output1" "fis2/input1"];
con2 = ["fis1/input1" "fis1/input2"];
con3 = ["fis1/input2" "fis2/input2"];

Finally, create a FIS tree using fis1, fis2, con1, con2, and con3.

fisT = fistree([fis1 fis2],[con1;con2;con3]);

Specify an additional output to the FIS tree to access the output of fis1.

fisT.Outputs = ["fis1/output1";fisT.Outputs];

Generate input and output training data.

x = (0:0.1:10)';
y1 = sin(x)+cos(x);
y2 = y1./exp(x);
y = [y1 y2];

Tune the FIS tree parameters in two steps. First, use a global optimization method such as particle swarm or genetic algorithm to learn the rules of the FIS tree. Create tunefis options for learning with the particleswarm method.

options = tunefisOptions('Method','particleswarm','OptimizationType','learning');

This tuning step uses small number of iterations to learn a rule base without overfitting the training data. The rule base provides an educated initial condition for the second step to optimize all the FIS tree parameters together. Set the maximum iteration number to 5, and learn the rule base.

options.MethodOptions.MaxIterations = 5;
rng('default')  % for reproducibility
fisTout1 = tunefis(fisT,[],x,y,options);
                                 Best            Mean     Stall
Iteration     f-count            f(x)            f(x)    Iterations
    0             100          0.6682          0.9395        0
    1             200          0.6682           1.023        0
    2             300          0.6652          0.9308        0
    3             400          0.6259           0.958        0
    4             500          0.6259           0.918        1
    5             600          0.5969          0.9179        0
Optimization ended: number of iterations exceeded OPTIONS.MaxIterations.

To tune all the FIS tree parameters, the second step uses a local optimization method such as pattern search or simulation annealing method. Local optimization is generally faster than global optimization and can produce better results when the input fuzzy system parameters are already consistent with the training data.

Use the patternsearch method for optimization. Set the number of iterations to 25.

options.Method = 'patternsearch';
options.MethodOptions.MaxIterations = 25;

Use getTunableSettings to get input, output, and rule parameter settings from fisTout1.

[in,out,rule] = getTunableSettings(fisTout1);

Tune all FIS tree parameters.

rng('default') % for reproducibility
fisTout2 = tunefis(fisTout1,[in;out;rule],x,y,options);
Iter     Func-count       f(x)      MeshSize     Method
    0           1       0.596926             1      
    1           3       0.551284             2     Successful Poll
    2          13       0.548551             4     Successful Poll
    3          20       0.546331             8     Successful Poll
    4          33       0.527482            16     Successful Poll
    5          33       0.527482             8     Refine Mesh
    6          61       0.511532            16     Successful Poll
    7          61       0.511532             8     Refine Mesh
    8          92       0.505355            16     Successful Poll
    9          92       0.505355             8     Refine Mesh
   10         128       0.505355             4     Refine Mesh
   11         175       0.487734             8     Successful Poll
   12         212       0.487734             4     Refine Mesh
   13         265       0.487734             2     Refine Mesh
   14         275       0.486926             4     Successful Poll
   15         328       0.486926             2     Refine Mesh
   16         339       0.483683             4     Successful Poll
   17         391       0.483683             2     Refine Mesh
   18         410       0.442624             4     Successful Poll
   19         462       0.442624             2     Refine Mesh
   20         469        0.44051             4     Successful Poll
   21         521        0.44051             2     Refine Mesh
   22         542       0.435381             4     Successful Poll
   23         594       0.435381             2     Refine Mesh
   24         614       0.398872             4     Successful Poll
   25         662       0.398385             8     Successful Poll
   26         698       0.398385             4     Refine Mesh
Maximum number of iterations exceeded: increase options.MaxIterations.

The optimization cost reduces from 0.59 to 0.39 in the second step.

Tune FIS Tree with Selected Fuzzy Systems

You can tune specific fuzzy systems in a FIS tree. To get parameter settings of the specific fuzzy systems, use getTunableSettings. For example, after learning the rule base of the previous FIS tree, separately tune fis1 and fis2 parameters. First, get the parameter settings for fis1.

[in,out,rule] = getTunableSettings(fisTout1,"FIS","fis1");

Tune fis1 parameters of the tree.

rng('default')
fisTout2 = tunefis(fisTout1,[in;out;rule],x,y,options);
Iter     Func-count       f(x)      MeshSize     Method
    0           1       0.596926             1      
    1           3       0.551284             2     Successful Poll
    2          18       0.510362             4     Successful Poll
    3          28       0.494804             8     Successful Poll
    4          56       0.494804             4     Refine Mesh
    5          84       0.493422             8     Successful Poll
    6         107       0.492883            16     Successful Poll
    7         107       0.492883             8     Refine Mesh
    8         136       0.492883             4     Refine Mesh
    9         171       0.492883             2     Refine Mesh
   10         178       0.491534             4     Successful Poll
   11         213       0.491534             2     Refine Mesh
   12         229       0.482682             4     Successful Poll
   13         264       0.482682             2     Refine Mesh
   14         279       0.446645             4     Successful Poll
   15         313       0.446645             2     Refine Mesh
   16         330        0.44657             4     Successful Poll
   17         364        0.44657             2     Refine Mesh
   18         384       0.446495             4     Successful Poll
   19         418       0.446495             2     Refine Mesh
   20         461       0.445938             4     Successful Poll
   21         495       0.445938             2     Refine Mesh
   22         560       0.422421             4     Successful Poll
   23         594       0.422421             2     Refine Mesh
   24         597       0.397265             4     Successful Poll
   25         630       0.397265             2     Refine Mesh
   26         701       0.390338             4     Successful Poll
Maximum number of iterations exceeded: increase options.MaxIterations.

In this case, the optimization cost is improved by tuning only fis1 parameter values.

Next, get the parameter settings for fis2 and tune the fis2 parameters.

[in,out,rule] = getTunableSettings(fisTout2,"FIS","fis2");
rng('default') 
fisTout3 = tunefis(fisTout2,[in;out;rule],x,y,options);
Iter     Func-count       f(x)      MeshSize     Method
    0           1       0.390338             1      
    1           2       0.374103             2     Successful Poll
    2           5       0.373855             4     Successful Poll
    3          10       0.356619             8     Successful Poll
    4          33       0.356619             4     Refine Mesh
    5          43       0.350715             8     Successful Poll
    6          65       0.349417            16     Successful Poll
    7          65       0.349417             8     Refine Mesh
    8          87       0.349417             4     Refine Mesh
    9          91       0.349356             8     Successful Poll
   10         112       0.349356             4     Refine Mesh
   11         138       0.346102             8     Successful Poll
   12         159       0.346102             4     Refine Mesh
   13         172       0.345938             8     Successful Poll
   14         193       0.345938             4     Refine Mesh
   15         222       0.342721             8     Successful Poll
   16         244       0.342721             4     Refine Mesh
   17         275       0.342721             2     Refine Mesh
   18         283       0.340727             4     Successful Poll
   19         312       0.340554             8     Successful Poll
   20         335       0.340554             4     Refine Mesh
   21         366       0.340554             2     Refine Mesh
   22         427       0.337873             4     Successful Poll
   23         457       0.337873             2     Refine Mesh
   24         521        0.33706             4     Successful Poll
   25         551        0.33706             2     Refine Mesh
   26         624       0.333193             4     Successful Poll
Maximum number of iterations exceeded: increase options.MaxIterations.

The optimization cost is further reduced by tuning fis2 parameter values. To avoid overfitting of individual FIS parameter values, you can further tune both fis1 and fis2 parameters together.

[in,out,rule] = getTunableSettings(fisTout3);
rng('default') 
fisTout4 = tunefis(fisTout3,[in;out;rule],x,y,options);
Iter     Func-count       f(x)      MeshSize     Method
    0           1       0.333193             1      
    1           8       0.326804             2     Successful Poll
    2          91       0.326432             4     Successful Poll
    3         116       0.326261             8     Successful Poll
    4         154       0.326261             4     Refine Mesh
    5         205       0.326261             2     Refine Mesh
    6         302       0.326092             4     Successful Poll
    7         352       0.326092             2     Refine Mesh
    8         391       0.325964             4     Successful Poll
    9         441       0.325964             2     Refine Mesh
   10         478        0.32578             4     Successful Poll
   11         528        0.32578             2     Refine Mesh
   12         562       0.325691             4     Successful Poll
   13         612       0.325691             2     Refine Mesh
   14         713       0.229273             4     Successful Poll
   15         763       0.229273             2     Refine Mesh
   16         867        0.22891             4     Successful Poll
   17         917        0.22891             2     Refine Mesh
   18        1036       0.228688             4     Successful Poll
   19        1086       0.228688             2     Refine Mesh
   20        1212       0.228688             1     Refine Mesh
   21        1266       0.228445             2     Successful Poll
   22        1369       0.228441             4     Successful Poll
   23        1381       0.227645             8     Successful Poll
   24        1407       0.226125            16     Successful Poll
   25        1407       0.226125             8     Refine Mesh
   26        1447       0.226125             4     Refine Mesh
Maximum number of iterations exceeded: increase options.MaxIterations.

Overall, the optimization cost reduces from 0.59 to 0.22 in three steps.

Tune with Custom Cost Function

Suppose you want to modify the previous FIS tree as shown in the following diagram.

Create the FIS tree.

fis1 = sugfis('Name','fis1');
fis1 = addInput(fis1,[0 10],'NumMFs',3,'MFType','gaussmf');
fis1 = addOutput(fis1,[-1 1],'NumMFs',3);

fis2 = sugfis('Name','fis2');
fis2 = addInput(fis2,[0 10],'NumMFs',3,'MFType','gaussmf');
fis2 = addOutput(fis2,[-1 1],'NumMFs',3);

fis3 = sugfis('Name','fis3');
fis3 = addInput(fis3,[0 10],'NumMFs',3,'MFType','gaussmf');
fis3 = addOutput(fis3,[0 1],'NumMFs',3);

con = ["fis1/input1" "fis2/input1";"fis2/input1" "fis3/input1"];

fisT = fistree([fis1 fis2 fis3],con);

To implement the addition and multiplication operations, use a custom cost function. For this example, use the function customcostfcn, included at the end of the example. Learn a rule base with this cost function.

options.Method = 'particleswarm';
options.MethodOptions.MaxIterations = 5;
rng('default')
fisTout1 = tunefis(fisT,[],@(fis)customcostfcn(fis,x,y),options);
                                 Best            Mean     Stall
Iteration     f-count            f(x)            f(x)    Iterations
    0             100           0.746           1.318        0
    1             200          0.5787           1.236        0
    2             300          0.5787           1.104        1
    3             400          0.5787           1.097        0
    4             500          0.5171           1.155        0
    5             600          0.5171           1.067        1
Optimization ended: number of iterations exceeded OPTIONS.MaxIterations.

Tune all parameters of the FIS tree.

options.Method = 'patternsearch';
options.MethodOptions.MaxIterations = 25;
[in,out,rule] = getTunableSettings(fisTout1);
rng('default')
fisTout2 = tunefis(fisTout1,[in;out;rule],@(fis)customcostfcn(fis,x,y),options);
Iter     Func-count       f(x)      MeshSize     Method
    0           1        0.51705             1      
    1          11       0.514884             2     Successful Poll
    2          21       0.512873             4     Successful Poll
    3          43       0.512873             8     Successful Poll
    4          56       0.512873             4     Refine Mesh
    5          79       0.512873             2     Refine Mesh
    6         106       0.512869             4     Successful Poll
    7         129       0.512869             2     Refine Mesh
    8         174       0.512869             1     Refine Mesh
    9         197       0.512862             2     Successful Poll
   10         242       0.512862             1     Refine Mesh
   11         314       0.512862           0.5     Refine Mesh
   12         388       0.512862          0.25     Refine Mesh
   13         422       0.510163           0.5     Successful Poll
   14         429       0.509153             1     Successful Poll
   15         439       0.509034             2     Successful Poll
   16         460       0.509034             4     Successful Poll
   17         483       0.507555             8     Successful Poll
   18         495       0.507555             4     Refine Mesh
   19         519       0.507555             2     Refine Mesh
   20         565       0.507555             1     Refine Mesh
   21         636       0.507555             2     Successful Poll
   22         682       0.507555             1     Refine Mesh
   23         755       0.507555           0.5     Refine Mesh
   24         799       0.507554             1     Successful Poll
   25         872       0.507554           0.5     Refine Mesh
   26         947       0.507554          0.25     Refine Mesh
Maximum number of iterations exceeded: increase options.MaxIterations.

You can add more input/output MFs and specify additional FIS tree outputs to improve the tuning performance. Using additional MF parameters and more training data for additional FIS tree outputs can further fine tune the outputs of fis1, fis2, and fis3.

Custom Optimization Method

You can implement your own FIS parameter optimization method using getTunableSettings, getTunableValues, and setTunableValues. This example uses these functions to tune a rule base of a fuzzy system.

Create a FIS to approximate sin(θ), where θ varies from 0 to 2π.

fisin = mamfis;

Add an input with range [0 2π] having five MFs of Gaussian type, and an output with range [–1 1] having five MFs of Gaussian type.

fisin = addInput(fisin,[0 2*pi],'NumMFs',5,'MFType','gaussmf');
fisin = addOutput(fisin,[-1 1],'NumMFs',5,'MFType','gaussmf');

Add 5 rules.

fisin = addRule(fisin,[1 1 1 1;2 2 1 1;3 3 1 1;4 4 1 1;5 5 1 1]);
fisin.Rules
ans = 
  1×5 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf1 => output1=mf1 (1)"
    2    "input1==mf2 => output1=mf2 (1)"
    3    "input1==mf3 => output1=mf3 (1)"
    4    "input1==mf4 => output1=mf4 (1)"
    5    "input1==mf5 => output1=mf5 (1)"

Set DisableStructuralChecks to true for faster FIS update.

fisin.DisableStructuralChecks = true;

Get the rule parameter settings.

[~,~,rule] = getTunableSettings(fisin);

Make the antecedent nontunable. In the consequent, do not allow NOT logic (negative MF indices) or empty variables (zero MF indices) in the rules.

for i = 1:numel(rule)
    rule(i).Antecedent.Free = false;
    rule(i).Consequent.AllowNot = false;
    rule(i).Consequent.AllowEmpty = false;
end

Generate data for tuning.

x = (0:0.1:2*pi)';
y = sin(x);

To tune the rule parameters, use the customtunefis function defined at the end of this example. Set the number of iterations to 2, and do not allow invalid parameter values when updating the FIS using setTunableValues.

numite = 2;
ignoreinvp = false;
fisout = customtunefis(fisin,rule,x,y,numite,ignoreinvp);
Initial cost = 1.170519
Iteration 1: Cost = 0.241121
Iteration 2: Cost = 0.241121

Display tuned rules.

fisout.Rules
ans = 
  1×5 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf1 => output1=mf4 (1)"
    2    "input1==mf2 => output1=mf5 (1)"
    3    "input1==mf3 => output1=mf3 (1)"
    4    "input1==mf4 => output1=mf1 (1)"
    5    "input1==mf5 => output1=mf2 (1)"

Allow NOT logic in the rules, and optimize the FIS again.

for i = 1:numel(rule)
    rule(i).Consequent.AllowNot = true;
end
fisout = customtunefis(fisin,rule,x,y,numite,ignoreinvp);
Initial cost = 1.170519
Iteration 1: Cost = 0.357052
Iteration 2: Cost = 0.241121
fisout.Rules
ans = 
  1×5 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf1 => output1=mf4 (1)"
    2    "input1==mf2 => output1=mf5 (1)"
    3    "input1==mf3 => output1=mf3 (1)"
    4    "input1==mf4 => output1=mf1 (1)"
    5    "input1==mf5 => output1=mf2 (1)"

With NOT logic, there are more combinations of rule parameters, and it generally takes more iterations to tune a FIS.

Next, reset AllowNot to false and set AllowEmpty to true, in other words, allow absence of variables (zero output MF indices) in the consequent. Tune the FIS with the updated rule parameter settings.

for i = 1:numel(rule)
    rule(i).Consequent.AllowNot = false;
    rule(i).Consequent.AllowEmpty = true;
end

try
    fisout = customtunefis(fisin,rule,x,y,numite,ignoreinvp);
catch me
    disp("Error: "+me.message)
end
Initial cost = 1.170519
Error: Rule consequent must have at least one nonzero membership function index.

The tuning process fails since the FIS only contains one output, which must be nonzero (nonempty) in the rule consequent. To ignore invalid parameter values, specify IgnoreInvalidParameters with setTunableValues.

Set ignoreinvp to true, which specifies IgnoreInvalidParameters value in the call to setTunableValues used in customtunefis.

ignoreinvp = true;
fisout = customtunefis(fisin,rule,x,y,numite,ignoreinvp);
Initial cost = 1.170519
Iteration 1: Cost = 0.241121
Iteration 2: Cost = 0.241121
fisout.Rules
ans = 
  1×5 fisrule array with properties:

    Description
    Antecedent
    Consequent
    Weight
    Connection

  Details:
                   Description           
         ________________________________

    1    "input1==mf1 => output1=mf4 (1)"
    2    "input1==mf2 => output1=mf5 (1)"
    3    "input1==mf3 => output1=mf3 (1)"
    4    "input1==mf4 => output1=mf1 (1)"
    5    "input1==mf5 => output1=mf2 (1)"

In this case, the tuning process bypasses the invalid values and uses only valid parameter values for optimization.

By default, tunefis ignores invalid values when updating fuzzy system parameters. You can change this behavior by setting tunefisOptions.IgnoreInvalidParameters to false.

Generate FIS from Data and Tune

You can generate a FIS from the training data using genfis and then optimize the FIS with tunefis. In this approach, the tuning process can employ a local optimization method because the rule base is derived from the training data.

This example describes the tuning steps to approximate the function

sin(2x)exp(x5),

where the input x varies from 0 to 10.

Generate training data.

x = (0:0.1:10)';
y = sin(2*x)./exp(x/5);

Create options for genfis that specify five MFs, a Gaussian MF for the input, and a constant MF for the output.

goptions = genfisOptions('GridPartition','NumMembershipFunctions',5, ...
    'InputMembershipFunctionType','gaussmf', ...
    'OutputMembershipFunctionType','constant');

Generate the initial FIS, and get its parameter settings.

fisin = genfis(x,y,goptions);
[in,out,rule] = getTunableSettings(fisin);

Use pattern search method for optimization, setting the maximum number of iterations to 25, and tune the FIS.

toptions = tunefisOptions('Method','patternsearch');
toptions.MethodOptions.MaxIterations = 25;
rng('default')
fisout = tunefis(fisin,[in;out],x,y,toptions);
Iter     Func-count       f(x)      MeshSize     Method
    0           1       0.346649             1      
    1          19       0.346649           0.5     Refine Mesh
    2          28       0.295219             1     Successful Poll
    3          34       0.295069             2     Successful Poll
    4          48       0.295069             1     Refine Mesh
    5          56       0.295064             2     Successful Poll
    6          71       0.294986             4     Successful Poll
    7          82       0.294986             2     Refine Mesh
    8          98       0.294986             1     Refine Mesh
    9         112       0.293922             2     Successful Poll
   10         128       0.293922             1     Refine Mesh
   11         131        0.29151             2     Successful Poll
   12         144       0.290141             4     Successful Poll
   13         156       0.290141             2     Refine Mesh
   14         171       0.290006             4     Successful Poll
   15         184       0.290006             2     Refine Mesh
   16         200       0.290006             1     Refine Mesh
   17         207       0.289743             2     Successful Poll
   18         223       0.289743             1     Refine Mesh
   19         243       0.289743           0.5     Refine Mesh
   20         257       0.286935             1     Successful Poll
   21         260       0.282278             2     Successful Poll
   22         263       0.281878             4     Successful Poll
   23         267       0.280144             8     Successful Poll
   24         272       0.280144             4     Refine Mesh
   25         278       0.275167             8     Successful Poll
   26         284       0.275167             4     Refine Mesh
Maximum number of iterations exceeded: increase options.MaxIterations.

You can increase the number of iterations to further optimize the cost.

Validate Training Results

To avoid overfitting of training data sets, check the performance of a trained fuzzy system with validation data. One common approach is to divide available data into training and validation data sets so that both data sets include similar characteristics.

If training and validation performances differ significantly, you can:

  • Change the maximum number of tuning iterations. For example, if the training performance is better than validation performance, you can reduce the number of tuning iterations.

  • Resample the training and validation data sets to maintain homogeneity.

To see how the validation performance can vary based on nonhomogeneous sampling, create a single-input-single-output fuzzy inference system to model the function

sin(2x)exp(x5),

where the input varies from 0 to 10 and the output range is [0 1]. Add five default MFs to the input and output. Get the input and output parameter settings.

fisin = sugfis;
fisin = addInput(fisin,[0 10],'NumMFs',5,'MFType','gaussmf');
fisin = addOutput(fisin,[0 1],'NumMFs',5);
[in,out] = getTunableSettings(fisin);

Create tunefis options for learning with the particleswarm optimization method, and set the maximum number of iterations to 5.

options = tunefisOptions('Method','particleswarm','OptimizationType','learning');
options.MethodOptions.MaxIterations = 5;

Divide the available data into two sets without maintaining homogeneity in the data sets.

x = (0:0.1:10)';
n = numel(x);
midn = floor(n/2);
trnX = x(1:midn);
vldX = x(midn+1:end);
f = @(x)(sin(2*x)./exp(x/5));
trnY = f(trnX);
vldY = f(vldX);

Tune the FIS parameters.

rng('default')
fisout = tunefis(fisin,[in;out],trnX,trnY,options);
                                 Best            Mean     Stall
Iteration     f-count            f(x)            f(x)    Iterations
    0             100          0.4279          0.5932        0
    1             200          0.3846          0.6183        0
    2             300          0.3751          0.5675        0
    3             400          0.3606           0.568        0
    4             500          0.3606          0.5596        1
    5             600          0.3598          0.5307        0
Optimization ended: number of iterations exceeded OPTIONS.MaxIterations.

Find the optimization cost for validation data using the findcost function defined at the end of this example.

vldCost = findcost(fisout,vldX,vldY)
vldCost = 0.1780

Note that the validation cost differs from the best training cost shown in column 3 of the tuning result.

Next, resample the available data, and create two homogeneous data sets for training and validation.

trnX = x(1:2:end);
vldX = x(2:2:end);
f = @(x)(sin(2*x)./exp(x/5));
trnY = f(trnX);
vldY = f(vldX);

Tune the FIS with the new training data.

rng('default')
fisout = tunefis(fisin,[in;out],trnX,trnY,options);
                                 Best            Mean     Stall
Iteration     f-count            f(x)            f(x)    Iterations
    0             100          0.3445          0.5852        0
    1             200          0.2996          0.5616        0
    2             300          0.2907          0.5381        0
    3             400          0.2878          0.5334        0
    4             500          0.2878          0.5624        1
    5             600          0.2877             Inf        0
Optimization ended: number of iterations exceeded OPTIONS.MaxIterations.

Find the optimization cost for the new validation data.

vldCost = findcost(fisout,vldX,vldY)
vldCost = 0.2803

In this case, homogeneous sampling reduces the difference between the training and validation costs.

Ways to Improve Tuning Results

You can improve the training error of a tuned fuzzy systems by following these guidelines.

  • Use multiple steps in a tuning process. For example, first learn the rules of a fuzzy system, and then tune input/output MF parameters using the learned rule base. You can also separately tune individual FIS parameters in a FIS tree and then tune all the fuzzy systems together to generalize the parameter values.

  • Increase the number of iterations in both the rule-learning and parameter-tuning phases. Doing so increases the duration of the optimization process. It can also increase validation error due to overtuned system parameters with the training data.

  • Use global optimization methods, such as ga and particleswarm, in both rule-learning and parameter-tuning phases. Global optimizers ga and particleswarm perform better for large parameter tuning ranges than local optimizers. The local optimizers patternsearch and simulannealbnd perform better for small parameter ranges. If rules are already added to a FIS tree using training data, then patternsearch and simulannealbnd can produce faster convergence compared to ga and particleswarm. For more information on these optimization methods and their options, see ga, particleswarm, patternsearch, and simulannealbnd.

  • Change the clustering technique used by genfis. Depending on the clustering technique, the generated rules can differ in their representation of the training data. Hence, the use of different clustering techniques can affect the performance of tunefis.

  • Change FIS properties. Try changing properties such as the type of FIS, number of inputs, number of input/output MFs, MF types, and number of rules. A Sugeno system has fewer output MF parameters (assuming constant MFs) and faster defuzzification. Therefore, for fuzzy systems with a large number of inputs, a Sugeno FIS generally converges faster than a Mamdani FIS. Small numbers of MFs and rules reduce the number of parameters to tune, producing a faster tuning process. Furthermore, a large number of rules might overfit the training data.

  • Modify tunable parameter settings for MFs and rules. For example, you can tune the support of a triangular MF without changing its peak location. Doing so reduces the number of tunable parameters and can produce a faster tuning process for specific applications. For rules, you can exclude zero MF indices by setting the AllowEmpty tunable setting to false, which reduces the overall number of rules during the learning phase.

  • Change FIS tree properties, such as the number of fuzzy systems and the connections between the fuzzy systems.

  • Use different ranking and grouping of the inputs to the FIS tree. For more information about creating FIS trees, see Fuzzy Trees.

Local functions

function cost = customcostfcn(fis,x,y)

tY = evalfis(fis,x);
sincosx = tY(:,1)+tY(:,2);
sincosexpx = sincosx.*tY(:,3);
actY = [sincosx;sincosexpx];
d = y(:)-actY;
cost = sqrt(mean(d.*d));

end

function fis = customtunefis(fis,rule,x,y,n,ignore)

% Show initial cost.
cost = findcost(fis,x,y);
fprintf('Initial cost = %f\n',cost);

% Optimize rule parameters.
numMFs = numel(fis.Outputs.MembershipFunctions);
for ite = 1:n
    for i = 1:numel(rule)
        % Get consequent value.
        pval = getTunableValues(fis,rule(i));
        % Loop through output MF indices to minimize the cost.
        % Use output indices according to AllowNot and AllowEmpty.
        allowNot = rule(i).Consequent.AllowNot;
        allowEmpty = rule(i).Consequent.AllowEmpty;
        if allowNot && allowEmpty
            mfID = -numMFs:numMFs;
        elseif allowNot && ~allowEmpty
            mfID = [-numMFs:-1 1:numMFs];
        elseif ~allowNot && allowEmpty
            mfID = 0:numMFs;
        else
            mfID = 1:numMFs;
        end
        cost = 1000;
        minCostFIS = fis;
        for j = 1:length(mfID)
            % Update consequent value.
            pval(1) = mfID(j);
            % Set updated consequent value to the FIS.
            fis = setTunableValues(fis,rule(i),pval,'IgnoreInvalidParameters',ignore);
            % Evaluate cost.
            rmse = findcost(fis,x,y);
            % Update FIS with the minimum cost.
            if rmse<cost
                cost = rmse;
                minCostFIS = fis;
            end
        end
        fis = minCostFIS;
    end
    fprintf('Iteration %d: Cost = %f\n',ite,cost);
end

end

function cost = findcost(fis,x,y)

actY = evalfis(fis,x);
d = y - actY;
cost = sqrt(mean(d.*d));

end

See Also

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