pid algorithm

[ggchavhan]

Hi

i want to make PID code by using MLX1000, I dont want to use PID instruction and real type data format. is possible, if yes please help me.

GAJANAN

[widelto]

If you do not want to use the PID instruction, it´s going to be difficult, but try to use comparison instructions to turn on and off your ouput variable, be careful to avoid instability, try to use a hysteresis.

hope this will help you

[SNK]

There was a project some time ago that does just what you want

See post http://www.plctalk.net/qanda/showthr...hlight=hot+rod

(I used a portion of the code for a vacuum system, achieving a SP very acurately based on a contactor and feedback,

[TConnolly]

Originally Posted by ggchavhan

Hi
...I dont want to use PID instruction and real type data format...

Thats a good thing because the PID instructions is NOT available on the ML1000 and the ML1000 does not uspport real numbers. You must step up to the ML1200 to get these features.

Fortunately, there is an alternatve that has been discussed at length here - the sliding mode controller.

Here are some of the threads that cover it: http://www.plctalk.net/qanda/search.php?searchid=813844

[KP_EENG]

If you have to stick with the Micrologix 1000, you may want to look into a separate PID controller such as the CN77000 Series controller from Omega Engineering.

[Tom Jenkins]

Why are you locking in on PID control? I realize that this is the most common algorithm for analog control, but it may not be the best for your application. In many cases a simple deadband controller (like your home thermostat) can do just as good or even better job of control in a much simpler manner. The SMC mentioned above is another alternate technique.

I'm not saying PID isn't the best choice, just that you should consider alternatives.

[KP_EENG]

I agree with Tom, there are alternative methods of control. I was going to mention Fuzzy Logic, which can easily be implemented in the Micrologix, but every time I mention Fuzzy Logic, a lot of people get all bent out of shape. If you are interested, take a look at the GE Forum on www.mrplc.com for a fuzzy logic algorithm implemented in the Micrologix world. You can possibly alter it to work with the ML1000.

Best regards

[Peter Nachtwey]

Sliding mode control is the easiest to implement by far but there could be problems if there is excess dead time. I like the idea of an external PID controller. One can pack some very sophisticated algorithms into a little 8 bit micro controller because the update time is so slow.

KP_EENG, you do have a nice but simple fuzzy logic algorithm but for SISO applications fuzzy logic is over kill and still a lot more code than what is necessary to get the job done.. SMC is perfect for temperature control systems with little or no dead time. SMC really is nothing more than simple on-off control with a derivative term(s) that is used to dynamically change the at temperature band.

[ggchavhan]

Thanks Every Body
Actually my client is expects low cost solution using MLX only. I tried my best but could not suceed as problem of math calculation overflow. If I could get certain idea to go further by using integer data I can proceed and I will be thankful.

Other solution suggested for fuzzy logic I don't have any Idea of fuzzy logic, if anybody having sample code for it I can assure you I will complete the project.

Actually this is boiler drum level control loop, I know, the code sould be very safe as per as boiler operation is concern, but stull client is forcing me to make the logic on MLX only because he is having spare MLX and he don't want to go for new one.

Please help me

GAJANAN

[chud]

I just did a boiler plc upgrade. Converted the program and installed new soft and hardware. As soon as I started I saw no PID control, basically just used comparators. Would have personally used pid myself but then again this was done yonks ago.
Is it a full boiler control you have to do (feed to boiler, water level,steam and furnace pressure,furnace temperature)