PLCs and Fuzzy Logic (3)


Figure 2 illustrates the operation of a fuzzy logic control system. The input to the fuzzy system is the output of the process, which is entered into the system via input interfaces. For example, in a temperature control application, the input data would be entered into the fuzzy controller using an analog input module. This input information would then go through the fuzzy logic process, where the processor would compare the input data to a database to obtain an output. Fuzzy logic processing involves the execution of IF…THEN rules, which are based on the input conditions. An input’s grade specifies how well it fits into a particular graphic input data set. Input data may be represented in a variety of forms, including count value and percentage of error deviation. Thus, if a fuzzy logic system uses an analog input that has a count range from 0 to 4095, the graphs representing the input sets will cover a span from 0 to 4095 counts.

The output of a fuzzy controller is also defined by grades, with the grade determining the appropriate output value for the control element. For example, the output of a fuzzy system could control an air conditioner, which runs faster or slower according to the output’s grade on the output chart.

Figure 3 illustrates the fuzzy logic output chart for this cooling system. The chart has both input and output grades, with the horizontal axis being the input condition (temperature) and the vertical axis being the output (air conditioner motor speed). In this chart, a single input can trigger more than one output condition. For example, if the input temperature is 137.5°F, then the temperature is part of two input sets—it is 50% too cool and 50% normal. Consequently, the input will trigger two outputs—the too cool input condition will trigger a less speed output, while the normal input will trigger a normal speed output. Since the fuzzy logic controller can have only one output, it must complete a process called defuzzification to determine the actual final output value.

The implementation of a fuzzy logic control system is similar to the implementation of PID control using intelligent interfaces, where the module reads the input, processes the information, and provides an output. However, fuzzy controllers are usually independent interfaces. They typically plug into the PLC rack and use the PLC’s I/O system to communicate with the process that is being controlled by fuzzy logic.

to be continued………………………

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