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AI – CS364 Fuzzy Logic Fuzzy Logic 3 03 rd October 2006 Dr Bogdan L. Vrusias
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Contents Fuzzy Inference Fuzzification of the input variables Rule evaluation Aggregation of the rule outputs Defuzzification Mamdani Sugeno
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Fuzzy Inference The most commonly used fuzzy inference technique is the so-called Mamdani method. In 1975, Professor Ebrahim Mamdani of London University built one of the first fuzzy systems to control a steam engine and boiler combination. He applied a set of fuzzy rules supplied by experienced human operators.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Mamdani Fuzzy Inference The Mamdani-style fuzzy inference process is performed in four steps: 1.Fuzzification of the input variables 2.Rule evaluation (inference) 3.Aggregation of the rule outputs (composition) 4.Defuzzification.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Mamdani Fuzzy Inference We examine a simple two-input one-output problem that includes three rules:Rule: 1 IFx is A3IFproject_fundingis adequate ORy is B1ORproject_staffingis small THENz is C1THENriskis lowRule: 2 IFx is A2IFproject_fundingis marginal ANDy is B2ANDproject_staffingis large THENz is C2THENriskis normalRule: 3 IFx is A1IFproject_fundingis inadequate THEN z is C3THENriskis high
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 1: Fuzzification The first step is to take the crisp inputs, x1 and y1 (project funding and project staffing), and determine the degree to which these inputs belong to each of the appropriate fuzzy sets.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation The second step is to take the fuzzified inputs, (x=A1) = 0.5, (x=A2) = 0.2, (y=B1) = 0.1 and (y=B2) = 0.7, and apply them to the antecedents of the fuzzy rules. If a given fuzzy rule has multiple antecedents, the fuzzy operator (AND or OR) is used to obtain a single number that represents the result of the antecedent evaluation. This number (the truth value) is then applied to the consequent membership function.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation RECAL: To evaluate the disjunction of the rule antecedents, we use the OR fuzzy operation. Typically, fuzzy expert systems make use of the classical fuzzy operation union: A B (x) = max [ A (x), B (x)] Similarly, in order to evaluate the conjunction of the rule antecedents, we apply the AND fuzzy operation intersection: A B (x) = min [ A (x), B (x)]
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation Now the result of the antecedent evaluation can be applied to the membership function of the consequent. There are two main methods for doing so: –Clipping –Scaling
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation The most common method of correlating the rule consequent with the truth value of the rule antecedent is to cut the consequent membership function at the level of the antecedent truth. This method is called clipping (alpha-cut). Since the top of the membership function is sliced, the clipped fuzzy set loses some information. However, clipping is still often preferred because it involves less complex and faster mathematics, and generates an aggregated output surface that is easier to defuzzify.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation While clipping is a frequently used method, scaling offers a better approach for preserving the original shape of the fuzzy set. The original membership function of the rule consequent is adjusted by multiplying all its membership degrees by the truth value of the rule antecedent. This method, which generally loses less information, can be very useful in fuzzy expert systems.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 2: Rule Evaluation clipping scaling
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 3: Aggregation of the rule outputs Aggregation is the process of unification of the outputs of all rules. We take the membership functions of all rule consequents previously clipped or scaled and combine them into a single fuzzy set. The input of the aggregation process is the list of clipped or scaled consequent membership functions, and the output is one fuzzy set for each output variable.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 3: Aggregation of the rule outputs
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 4: Defuzzification The last step in the fuzzy inference process is defuzzification. Fuzziness helps us to evaluate the rules, but the final output of a fuzzy system has to be a crisp number. The input for the defuzzification process is the aggregate output fuzzy set and the output is a single number.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 4: Defuzzification There are several defuzzification methods, but probably the most popular one is the centroid technique. It finds the point where a vertical line would slice the aggregate set into two equal masses. Mathematically this centre of gravity (COG) can be expressed as:
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 4: Defuzzification Centroid defuzzification method finds a point representing the centre of gravity of the fuzzy set, A, on the interval, ab. A reasonable estimate can be obtained by calculating it over a sample of points.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Step 4: Defuzzification
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Fuzzy Inference Mamdani-style inference, as we have just seen, requires us to find the centroid of a two-dimensional shape by integrating across a continuously varying function. In general, this process is not computationally efficient. Michio Sugeno suggested to use a single spike, a singleton, as the membership function of the rule consequent. A singleton, or more precisely a fuzzy singleton, is a fuzzy set with a membership function that is unity at a single particular point on the universe of discourse and zero everywhere else.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Fuzzy Inference Sugeno-style fuzzy inference is very similar to the Mamdani method. Sugeno changed only a rule consequent. Instead of a fuzzy set, he used a mathematical function of the input variable. The format of the Sugeno-style fuzzy rule is IFx is A ANDy is B THENz is f(x, y) where x, y and z are linguistic variables; A and B are fuzzy sets on universe of discourses X and Y, respectively; and f(x, y) is a mathematical function.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Fuzzy Inference The most commonly used zero-order Sugeno fuzzy model applies fuzzy rules in the following form: IFx is A ANDy is B THENz is k where k is a constant. In this case, the output of each fuzzy rule is constant. All consequent membership functions are represented by singleton spikes.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Rule Evaluation
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Aggregation of the Rule Outputs
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Sugeno Defuzzification Weighted Average (WA)
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Mamdani or Sugeno? Mamdani method is widely accepted for capturing expert knowledge. It allows us to describe the expertise in more intuitive, more human- like manner. However, Mamdani-type fuzzy inference entails a substantial computational burden. On the other hand, Sugeno method is computationally effective and works well with optimisation and adaptive techniques, which makes it very attractive in control problems, particularly for dynamic nonlinear systems.
AI – CS364 Fuzzy Logic 03 rd October 2006Bogdan L. Vrusias © Closing Questions??? Remarks??? Comments!!! Evaluation!
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