# Time Prop. PID Temp. Loop

## 11 posts in this topic

I have taken on a Programing Project using an Fx2n controler in which I will need to control 4 electric heaters. I have never programed a Mitsubishi before this one. Could some one send me an example of setting up a time Prop. Temp. loop. Thanks RHime.

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What is "time Prop. Temp."?

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I believe that he wants the control to be via a relay output and not 4-20mA.. That is, according to the MV calculated by the PID algorithm, the output will turn on for a period of time. (Similar to a Temperature Controller with a Relay Output) Hope that helps... Sorry i don;t have any Mitsubishi Examples.... :*-(

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I would suggest reading the FX Series Programming Manual II, in section 10 there are example programs for PID control and all the equations that go with it. Go to www.meau.com and look for manual number JY992D88101D

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I would consider SMC ( sliding mode control ). This is much more robust than PID and much simpler.

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Pardon my ignorance Peter, but I am unfamilar with the above SMC, never alone heard of it....Could you please educate me ... Thanks

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SMC is basically a on-off type of control. Because of this there is no need to convert analog values of a PID to PWM on-off control. Sliding mode control uses the error and and rate of error to compute a result. If this result is negative then the output is turned of. If the result is positve the output is turned on. e = SP - PV e' is the rate of change of E. This is similar to a differentiator. s = T*e' + e if s < 0 then Off else On When starting the heater control the SP will be much higher than the PV and the error will be positive. However the error rate will be negative because the error rate will decrease as the system heats up. If you think about it the error rate must always have the opposite sign as the error if the system is to converge. In otherwords e*e' < 0 when the system is converging. The time constant T just controls how fast the PV converges on the SP. If T is made too short, the PV will overshoot the SP. If T is too large, the PV will approach the SP slowly. However in anycase the PV will reach and hold the SP and even large changes in the load will not affect the control. This simple form of SMC has only the time constant to adjust. The main difference between SMC and PID is that PID is more concerned with having the right output for a given error and SMC is more concerned with the error being reduced at the desired rate. SMC tries to bring s in the equation above to 0. When s has reached 0, the system is said to have reached the sliding mode or plane. For a detailed example see: Sliding Mode Control I have example programs for the MicroLogix and AD DL250. Someday I should do one for the FX2N. Also search "For the PID Challenged" on the plcs.net forum.

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Thanks, Peter.....a heap.. and thank you for the layman terms in the explanation.... BTW Thought of registeing ?

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Peter thanks for the link also.... I hope that this isn't asking to much but could you print the MicroLogix sample code to a .pdf format ? If you can't create a pdf then try this little utility... Adobe Acrobat file generator Also you might consider submitting this as an article to chakorules ?? Just a thought Edited by Sleepy Wombat

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Peter, followed your info on SMC and generated an Excel spread sheet as attached...pretty cool stuff... Now to translate this to Omron LL.... Sliding_Mode_Control.zip

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Excellent!!!!! You understand! Good, now there is another to spread the word. Notice that the X and Y are two first order differential equations. Y is the measured valued but X is the actual value. Most people only consider what Y is doing without any concern for how their bread is burning because the don't know what is really happening inside the oven ( X ). Ignorance is bliss but is also ignorance. Have you tried playing around with the set points and time constants? More good info. Nasa Space Station Furnace Save this .pdf. It is one of the best on SMC on the net.