PLCs and Fuzzy Logic (8)

Membership Functions and Rule Creation
To provide enhanced resolution and accuracy, this system uses five–membership function (fivelabel) fuzzy sets for the two inputs and a seven–membership function fuzzy set for the output.

The offset input is named X (deviation between part and box) and the offset rate of change input is named DeltaX (rate of change of deviation). The fuzzy set for the output is named S (speed), which corresponds to the motor speed of conveyor B. The range of each fuzzy input and output variable is from 0 to 4095 counts. This corresponds to a range of ±24 inches for the deviation between the part and box positions, a range of ±10 inches/second for the rate of change of the offset, and a range of ±10 inches/second for the speed of the box conveyor.
The fuzzy logic database for this system contains 25 rules. Figure 11 shows a matrix of the rules, describing the desired output according to the deviation between the part and the box and the rate of change of deviation. This matrix includes a description of the rule inputs and outputs, as well as their respective membership function labels. Once the fuzzy controller receives the inputs, it will determine the final output value based on a logical addition of the selected outcomes.

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