PID 1769-L32E tuning

jmwhitney

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• Sep 19, 2010

• #1

I am tuning a heating PID loop on a CompactLogix PLC and could use a little help. My current settings keep the temperature within 5 degrees Celsius of the set point, but as the temperature fluctuates up and down, the CV does not start to increase until its output until the temperature reaches its set point when the actual temperature is moving down. When the actual temperature is on the way back up towards the setpoint it starts bringing the CV down from 100% so that the over run is only 1 degree above the set point, but on the way down because the CV does not begin increasing the CV percentage before it is less than or equal to the set point, the under run is 4 degrees.
What setting can I adjust to make the CV start opening sooner when the actual temperature is falling?

Thanks in advance,
Jim

 

OkiePC

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• Sep 19, 2010

• #2

It would be a good idea to post all of the settings of the PID.

It sounds like the system is intenitonally tuned to avoid overshoot at the high end.

What are the ramifications of overshoot? Death and destruction? Or no big deal?

How long does it take to cool back down?

What kind of heater are we talking about (gas valve, electric heat strip)?

How fast does the heat command cause a change in the feedback?

 

Ron Beaufort

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• Sep 19, 2010

• #3

let's do this the easy way ...

welcome to the forum ...

(1) please post your complete .ACD program file ... (you'll have to ZIP it first) ...

(2) please set up a trend and graph the loop's response ... post that too ...

 

sthompson

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• Sep 19, 2010

• #4

As Ron requested, the full ACD is the most helpful, because we can see everything that affects the PID. If you can't post the PLC Program for some reason, at the very least Screen Shots of all the PID Settings. Also, a good Trend Graph of the PID is a must for PID Tuning. Trust me, I have had to do it on PLCs that didn't have a good Trending Package, and it sucks. One thing I will have to say for AB, is all their Programming Software has decent Trending Packages.

Stu.....

 

jmwhitney

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• Sep 20, 2010

• #5

The loop is controlling a 2.5 million BTU natural gas tube type burner in a 410 gallon tank of water in a washer. Over shoot of 5 degrees C is no big deal, but undershoot of greater than 3 degrees C is an issue as the temperature is used to sanitize the items. I have attached screen shots in a word doc of the PID set up screens.

Jim

 

jmwhitney

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• Sep 20, 2010

• #6

Attachment

 

sthompson

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• Sep 20, 2010

• #7

jmwhitney said:

Attachment

Click to expand...

We will also need to see the Configuration for the Analog Input and Output for the PID. The first thing I see, is the Periodic Task Time and PID Loop Update Time do not match. These must be the same. Also, I have never seen a Logix PID with those kind of Huge Kp, Ki, and Kd values in them. This tells me there is something wrong with your setup.

Stu.....

 

jmwhitney

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• Sep 20, 2010

• #8

Here is the configuration for the Analog Input and Output for the PID.