Simulate Fuzzy Inference Systems in Simulink

You can simulate a fuzzy inference system (FIS) in Simulink^® using either the Fuzzy Logic Controller or Fuzzy Logic Controller with Ruleviewer blocks. Alternatively, you can evaluate fuzzy systems at the command line using evalfis.

Using the Fuzzy Logic Controller, you can simulate traditional type-1 fuzzy inference systems (mamfis and sugfis) and type-2 fuzzy inference systems (mamfistype2 and sugfistype2). The Fuzzy Logic Controller with Ruleviewer block supports only type-1 systems.

For more information on creating fuzzy inference systems, see Build Fuzzy Systems Using Fuzzy Logic Designer and Build Fuzzy Systems at the Command Line.

Simulate Fuzzy Inference System

Once you have implemented a fuzzy inference system using Fuzzy Logic Designer, using Neuro-Fuzzy Designer, or at the command line, you can simulate the system in Simulink.

For this example, you control the level of water in a tank using a fuzzy inference system implemented using a Fuzzy Logic Controller block. Open the sltank model.

open_system('sltank')

For this system, you control the water that flows into the tank using a valve. The outflow rate depends on the diameter of the output pipe, which is constant, and the pressure in the tank, which varies with water level. Therefore, the system has nonlinear characteristics.

The two inputs to the fuzzy system are the water level error, level, and the rate of change of the water level, rate. The output of the fuzzy system is the rate at which the control valve is opening or closing, valve.

To implement a fuzzy inference system, specify the FIS name parameter of the Fuzzy Logic Controller block as the name of a FIS object in the MATLAB® workspace. In this example, the block uses the mamfis object tank.

For more information on this system, see Water Level Control in a Tank.

As a first attempt to control the water level, set the following rules in the FIS. These rules adjust the valve based on only the water level error.

• If the water level is okay, then do not adjust the valve.

• If the water level is low, then open the valve quickly.

• If the water level is high, then close the valve quickly.

Specify the rules by creating a vector of fisrule objects and assigning it to the Rules property of the tank FIS object.

rule1 = "If level is okay then valve is no_change";
rule2 = "If level is low then valve is open_fast";
rule3 = "If level is high then valve is close_fast";
rules = [rule1 rule2 rule3];
tank.Rules = fisrule(rules);

Simulate the model, and view the water level.

open_system('sltank/Comparison')
sim('sltank',100)

These rules are insufficient for controlling the system, since the water level oscillates around the setpoint.

To reduce the oscillations, add two more rules to the system. These rules adjust the valve based on the rate of change of the water level when the water level is near the setpoint.

• If the water level is okay and increasing, then close the valve slowly.

• If the water level is okay and decreasing, then open the valve slowly.

To add these rules, use the addRule function.

rule4 = "If level is okay and rate is positive then valve is close_slow";
rule5 = "If level is okay and rate is negative then valve is open_slow";
newRules = [rule4 rule5];
tank = addRule(tank,newRules);

Simulate the model.

sim('sltank',100)

The water level now tracks the setpoint without oscillating.

You can also simulate fuzzy systems using the Fuzzy Logic Controller with Ruleviewer block. The sltankrule model is the same as the sltank model, except that it uses the Fuzzy Logic Controller with Ruleviewer block.

open_system('sltankrule')

During simulation, this block displays the Rule Viewer from the Fuzzy Logic Designer app.

sim('sltankrule',100)

If you pause the simulation, you can examine the FIS behavior by manually adjusting the input variable values in the Rule Viewer, and observing the inference process and output.

You can also access the Fuzzy Logic Designer editors from the Rule Viewer. From the Rule Viewer, you can then adjust the parameters of your fuzzy system using these editors, and export the updated system to the MATLAB workspace. To simulate the updated FIS, restart the simulation. For more information on using these editors, see Build Fuzzy Systems Using Fuzzy Logic Designer.

Access Intermediate Fuzzy Inference Results

You can access intermediate fuzzy inference results using the Fuzzy Logic Controller block. You can use this data to visualize the fuzzy inference process or troubleshoot the performance of your FIS. To access this data, enable the corresponding parameters in the block, and connect signals to the corresponding output ports.

Block Parameter	Description	Output Port
Fuzzified Inputs	Fuzzified input values, obtained by evaluating the input membership functions of each rule at the current input values.	fi
Rule firing strengths	Rule firing strengths, obtained by evaluating the antecedent of each rule.	rfs
Rule outputs	Rule outputs, obtained by evaluating the consequent of each rule.	ro
Aggregated outputs	Aggregate output for each output variable, obtained by combining the corresponding outputs from all the rules.	ao

For more information, see Fuzzy Logic Controller.

Simulation Modes

The Fuzzy Logic Controller block has the following two simulation modes:

• Interpreted execution — Simulate fuzzy systems using precompiled MEX files. Using this option reduces the initial compilation time of the model.

• Code generation — Simulate fuzzy system without precompiled MEX files. Use this option when simulating fuzzy systems for code generation applications. Doing so simulates your system using the same code path used for generated code.

To select a simulation mode, set the Simulate using parameter of the block. By default, the block uses Interpreted execution mode for simulation.

Map Command-Line Functionality to Fuzzy Logic Controller Block

The parameters and ports of the Fuzzy Logic Controller block map to the input and output arguments of evalfis or the properties of evalfisOptions. The following table shows the block parameters and ports that map to evalfis arguments.

evalfis Argument	Description	Block Parameter or Port
fis	Fuzzy inference system	FIS name
input, when a single row	Input variable values	in
output, when a single row	Output variable values	out
fuzzifiedIn	Fuzzified inputs	fi
ruleOut	Rule outputs	ro
aggregateOut	Aggregated outputs	ao
ruleFiring	Rule firing strengths	rfs

The following table shows the block parameters that map to evalfisOptions properties.

evalfisOptions Property	Description	Block Parameter or Port
NumSamplePoints	Number of points in output fuzzy sets	Number of samples for output discretization
OutOfRangeInputValueMessage	Diagnostic message behavior when an input is out of range	Out of range input value
NoRuleFiredMessage	Diagnostic message behavior when no rules fire	No rule fired
EmptyOutputFuzzySetMessage	Diagnostic message behavior when an output fuzzy set is empty	Empty output fuzzy set

The remaining parameters of the Fuzzy Logic Controller block do not map to arguments of evalfis. Also, unlike the Fuzzy Logic Controller block, evalfis does not support fixed-point data for simulation or code generation.