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03-27-2013 06:21 PM

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If modeling a system using PID is too hard or too difficult, I want to use fuzzy logic, but according to the NI control manual, fuzzy logic is good for a continuous system.

Can it be used on a discrete system? for example, turn on a heater for 2 second and shut off, wait for 10 minutes, and then meausre again and go through fuzzy logic?

Can my idea be fulfilled?

Thanks

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03-28-2013 09:49 AM

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Hi,

Actually PID isn't that difficult to implement compared to Fuzzy logic. Fuzzy logic is a set of artificial intelligence rules that deals with reasoning algorithms used to emulate human thinking and decision making.

I think you can use it for your project but I strongly recommend you using PID control. With PID control you just need to identify your system's transfer function and then define your PID controller in order to make it stable.

Regards,

Miguel Fonseca

Applications Engineer

National Instruments

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03-29-2013 02:31 PM

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The difficult part is deriving the transfer function, Or I should say I dont know how to derive a one for our complex chemical system. But we do know wha's gonna happen so we proposed to use fuzzy logic to define rules as many as we can.

When we are doing fuzzy system, besides the NI control manual, do you have any other good outside resources for further reading? thank you Mike

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03-29-2013 02:56 PM

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Hi,

Unfortunately there is no specific content that we can provide about fuzzy logic rather than the content we have in our website. I strongly recommend you to look for related articles over the internet.

Regards

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03-29-2013 03:52 PM

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@ivy037 wrote:

The difficult part is deriving the transfer function, Or I should say I dont know how to derive a one for our complex chemical system. But we do know wha's gonna happen so we proposed to use fuzzy logic to define rules as many as we can.

You do not need a transfer function in order to use PID. In fact, if you did have a good mathematical model of your system's transfer function, you wouldn't need PID control - you could use that model to create a more accurate controller that precisely reflected your system dynamics.

What are you trying to control? PID works well for systems in which you want to control one measurement using one output that have an approximately linear response.

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03-29-2013 11:33 PM

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It's about neuturalization of wasted water that contains metalic ions like Fe2+, Zn2+, non-mentalic ion like Cl, and so on.

Lots of article I read point out the neuturalizaotion process is quite non-linear, besides, if chemical reagent is added to the wasted water, we need to wait until everything becomes steady and then measure again and decide which reagents should be added. So this model is not continuous and our output is on/off (we want to use proportional valve but too expensive ;( out of our budget...)

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To work with fuzzy logic does not require any transfer function as it is a fully descriptive, the disadvantage is that you have to know very well the operation of the plant and has something to do with trial and error, but when setting the rules correctly, your system is completely stable.

03-30-2013 07:52 PM

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