Oh no!!!! YAFC ( yet another fuzzy controller )
You are not the first to ask about implement fuzzy logic in ladder. I think only on person has made fuzzy logic work ( Ashwin ) and that was on a Control Logix. Even then he complained that the Control Logix was slow.
Search this site for the user named Ashwin and for the topic of fuzzy logic.
Warning! Warning! Warning!
Don't do it because it is a waste of time.
This sounds like a student project.
Ladder is NOT the language to use to implement fuzzy logic.
If you must follow the path of hype then try this
http://ffll.sourceforge.net/downloads.htm
Free software but it is in C and not ladder as it should be.
http://www.aaai.org/AITopics/html/fuzzy.html
More information for your misguided adventure.
If you must do this on a PLC then use structured text.
Better yet, don't use a PLC and get a small microncontroller evaluation kit where you can use C code.
Now answer this. Why bother? Don't say because it is the assignment. I would fail you for accepting the assignment. Even if you could implement the fuzzy logic in ladder that doesn't mean you should. Who would use it? How would one set it up? What about an operator interface for the fuzzy logic?
You have been warned. Give yourself a reason as to why you should continue this project.