re: Fuzzy Control

From: james.becker.b@bayer.com
Date: Sun Dec 31 2000 - 04:27:29 MET

  • Next message: Pim van den Broek: "Fuzzy relations vs. Mamdani model (continued)"

    There are a few industrial practitioners who search this newsgroup.

    There are a few people who have done fuzzy logic controllers which
    mimic the response of PID controllers. But one has to question the
    reason for doing this. Why put in a great new V-8 engine and then
    adjust it to respond like a VW Beetle? An old Beetle.

    The best way to implement a fuzzy logic controller is to look at the
    system response to discover which other inputs can effect the
    controller you are creating. Include these inputs in logic and you
    can create a multivariable controller. A typical PID replacement
    fuzzy logic controller uses the deviation from set point (SP-PV) and
    the derivative of the PV as inputs.

    There is so much more an engineer can do to adjust the response
    of a fuzzy logic controller to the specific system. It is possible to make
    the controller respond quickly and have little or no overshoot. This is
    accomplished by adjusting the controller response in individual
    matrix blocks or the membership functions.

    Also, the engineer can create the equivalent of feedforward controls
    or any other type of advanced control done with PID controllers.

    I suggest that you look at the Inform Software website for their product
    called FuzzyTECH, www.fuzzytech.com. You can download the
    software for a trial. I suggest that you look at the boiler simulation that
    comes with the product. You can modify the logic and make the
    controller respond in almost any way you want it to operate. By the way,
    the entire User's Manual is included in .pdf format with the download.

    The company is in Germany, but they have an office in Chicago.

    Jim Becker

    Subject: Fuzzy Control
    From: Gregory L. Hansen (glhansen@steel.ucs.indiana.edu)
    Date: Wed Oct 11 2000 - 01:19:57 MET DST

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    Is this the right place to ask about fuzzy logic in control systems? It's
    the only fuzzy logic newsgroup I could find.

    I'm trying to understand fuzzy logic control systems compared to more
    traditional systems like the PID controllers. The examples go through
    decision matrices and elaborate logic and calculation to do what seems to
    be the moral equivalent of "if sensor moves away from setpoint, adjust
    parameter in a direction that will bring the system back", which seems
    equivalent to the integral term in a PID controller. What does fuzzy gain
    you?

    It's been said that fuzzy logic lets you use simple English rules and
    focus on what you want the system to do rather than trying to model the
    system, and that it easily handles non-linear systems, in contrast to
    control theory. And I suppose that's true to a point. But even a simple
    PID can handle systems that are non-linear to a degree, because all it
    does is produce an output related to the deviation of the system from the
    ideal, which is pretty much focusing on what you want the system to do,
    and there are simple and even automated methods to calibrate the run
    parameters. And in a fuzzy system you still need to define control
    regions -- number, locations, widths, overlaps. You still need to
    defuzzify the output, you need to tweak parameters. It seems like those
    simple English statements are just code words for the modeling and
    mathematics that must eventually be done. And I'd be surprised if fuzzy
    controllers never suffer instabilities.

    I like fuzzy logic. It looks fun and interesting, and probably very
    useful in things like expert systems. But I feel a little cheated in how
    I've seen fuzzy control systems explained. The popular media seems to
    pretend that all non-fuzzy systems are stupid on/off affairs and that
    control theory doesn't exist. They bemoan the fact that the U.S. hasn't
    been putting many resources into fuzzy logic, but don't mention if that
    might be because U.S. industry is satisfied with traditional controllers.
    The more technical articles compare traditional controllers in fuzzy logic
    terms, like the numerous if...then statements a PID controller must have
    rather than just giving a discretized formula that it follows relentlessly
    with no decision element whatsoever; a formula, I might add, that seems
    simpler than determining the membership of an input to various sets,
    cutting out sections of an output set, and finding the centroid. I feel
    like a lot of hype surrounds the field.

    --
    "A good plan executed right now is far better than a perfect plan
    executed next week."
                           -Gen. George S. Patton
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