3. Creating a Rule Base with Pre-Generated Rules

Rules describe, in words, the relationships between input and output linguistic variables based on their linguistic terms. The rule base of a fuzzy system determines the output values of the fuzzy system based on the input values.

The vehicle-position input linguistic variable you created in step 1 has five membership functions, and the vehicle-orientation input linguistic variable has seven. Therefore, you can construct rules to associate 35 possible combinations of the linguistic terms of these input linguistic variables with the linguistic terms of the steering-angle output linguistic variable you created in step 2.

Note��The table in the Complete Rule Bases topic illustrates this rule base.

Complete the following steps to create the complete rule base for the vehicle maneuvering example. You must have created both input linguistic variables and the output linguistic variable before creating this rule base.

1. Click the Rules tab of the Fuzzy System Designer to display the Rules page.
2. Select Operate�Pre-Generate Rules to display the Pre-Generate Rules dialog box.
3. Select AND (Minimum) from the Antecedent connective pull-down menu. This antecedent connective specifies that the smallest degree of membership of the antecedents determines the truth value of the aggregated rule antecedent for each rule.
4. Enter a Degree of support of 1 to specify the weight that you want to apply to each rule. The final rule weight for each rule is equal to the Degree of support multiplied by the truth value of the aggregated rule antecedent.
5. Select Minimum from the Consequent implication pull-down menu to specify that the fuzzy logic controller truncates the output membership functions at the value of the corresponding rule weights before performing defuzzification. Defuzzification is the process of converting the degrees of membership of output linguistic variables into numerical values.
6. Click the OK button to return to the Rules page. The Fuzzy System Designer populates the Rules list with all 35 combinations of linguistic terms of the input variables. Each rule uses the same antecedent connective, degree of support, and implication method that you specified in the Pre-Generate Rules dialog box.

Notice that the consequent of each rule in the Rules list is THEN 'steering-angle' IS 'neg-large'. However, the linguistic term of the steering-angle output variable depends on the combination of linguistic terms of the input variables. Therefore, you must modify the pre-generated rules to use the correct consequents.

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