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PID STATUS 2

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Hi, I got the PID documentation from another post in this forum, I'm getting the STATUS value of 2 in my PID config Struct, what does it mean? also How do I know if it is performing the autotune procedure?, My coding is quite similar from the video tutorial for PID, but when I try to run the PID autotune it stays there forever (I asumme this because the Autotune done bit never turns 1), and I see no action in my control value, it simply stays in 100% of the output value. From the documentation I think the status for normal operation is 4.

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Hello,

 

The PID status of 2 means your PID autotune process is running, and it is in the second stage of the process.  (there are 3 all together)  If the control value is not changing, this could be due to the set-up of your system, or maybe you are no longer energizing the autotune function block. 

 

What type of output are you running (analog output, PWM output, etc.)?  I believe I have seen the CV not doing anything for a PWM output if the frequency was not set for the output (a value of 0Hz in the frequency register) so it may be something like this that is causing the issue.

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I'm running the PID for an analog output (4-20mA I have them configured as 0 to 1000 in the output range, I tried manually operating the value by storing an int value from 0 to 1000 in the selected output and it works fine, 250 means 25% 500 50% and so on) , I deenergized the autotune block, because I thought it was frozen, should I just reenergize it and wait?

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Hello,

 

Depending on how resposive your system is, it may take a while to perform a PID stage.  You can try to run the autotune process again.  It will be very helpful to know where the SP is set to, and how the CV and PV are reacting.

 

(Assuming this is a heating application...)

For the autotune process, you should see your output be 100% until it reaches the set-point.  At that point, the output will drop to 0% and let the momentum of the system take it back down below the set point.  This will occur four times (once for initialization, and then 3 stages).  Once it has completed these stages, you will see the PID status integer turn to a '0', unless you run the PID loop right after the autotune is complete, in which case it will return a status of '4'.

 

For a cooling process, it is the same, except the output will be at 100% driving the PV down to the SP.

 

Hope this helps

 

Please let me know a little more about your system and what its purpose is for further troubleshooting.

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It is a consistency control of pulp made from recicled paper,  I have a consistency transmitter that trasmit the % of solids in the water(thats my PV, I configured it to have a range of 200 to 600 in the analog In, this is because the range of my transmitter is 2% to 6% of solids in the water) it is reading the signal from the trasmitter perfectly, I control the consistency in this stage using a control valve using an electronic positioner(thats my output, the CV), it is an reverse relationship, if I open the valve the consistency willl drop down, this is because when the valve opens it injects water into the system thus reducing the relationship between paper and water. According to the process engineer, the dead time(not sure if this is the correct translation, I know this as "tiempo muerto") is really low, he states that when you open the valve the consistency drops down really quickly. we have the SP set to 350 and the process value is usually around 320, but when I run the PID it just goes 100% open, I have the reverse action bit set to 0.

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What is your sample time value?  Have you successfully ran PID Autotune, or are you manually entering PID values to run PID?  What are your P, I, &D values?   

 

When you run PID, do you see the process value increasing?  Does it pass your set-point?

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I just succeded in running the PID process, it took more than 15 minutes. The parameters are these, sample time=100, P=500, I=145,D=144. the current Process value is slightly above my set point. Thanks for the help.

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I control a water flow process to GPM through a variable frequency drive on a pump. I also control water level at flow through the use of a control valve. I have found the auto tune results for both of these processes to be unusable. The auto tune values were extremely large for P, I and D similar to yours.

 

I have manually tuned these processes and they perform very well with very different values. The flow process has a sample time=25, P=40, I=.6, D=.1 (note the decimals before 6 and 1). The water level process has a sample time of 100, P=20, I=1, D=0.

 

I want my integral to wind up fast. Once your at setpoint most of the control output is all integral.

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