PID Tuning Help Needed Please!!!

[Naval2069]

Hello,
I have a problem with tuning my PID loop in my RS5000 project.
In my program I am using a PID loop to control the KW output of an engine. The Process variable of my PID loop is a level switch in a tank. I need to maintain a 80% full tank while running the highest amount of KW from the engine. The engine is capable of producing a range of 75 to 150 kw. What I am looking for in this loop is once I go below 80% (my set point) I want to start ramping down the engine, and the farther above 80% tank level I want to run the KW full out. My problem is I hit 82% of the tank level then the engine sends full KW and as expected the tank level begins to drop, but as my tank level falls below 80% and the KW decreases my tank level never reaches 80% again until I am running at minimum KW (75). How should I adjust my Kp, Ki, Kd? Is there a good site for understanding PID loops?
Thanks,
Navin

 

[VAN]

OK, How I am understanding this.

Process Variable - Liquid Level Switch at some set point.
Control Variable - Some motor
P - Proportional
I - Integral
D - Derivative

You don't like that the PID keeps the LL just below the set point?
Or
How much are you off by?

 

[Naval2069]

The tank level hits about 78.4% before the KW drops to its lowest output but that's OK, however I don't want the kw to keep swinging from 150 to 75.

 

[daba]

IMHO this is not a suitable control algorithm that a PID can control...

The reasoning... your PV is not a proportional signal that the PID can calculate with, it is either On or Off. So PV and SP are not "related" in the same fashion that an analog PV would be.

This means that the PID calculation has absolutely no knowledge how far away the PV is from the SP. It could be just over/under, or in a different ball-park, so how can the PID calculation know what to set the output.

I believe you will have to "roll-your-own" control algorithm, and forget PID completely.

 

[g.mccormick]

Why not replace the level switch with a continuous level transmitter. The PID loop needs a continuously changing feedback to calculate correctly.

 

[Naval2069]

I am sorry, I miss spoke. The actual input to the PID loop is a level transmitter that does send out an analog output to my PLC.

 

[JHarbin]

I am sorry, I miss spoke. The actual input to the PID loop is a level transmitter that does send out an analog output to my PLC.

Can you control the process manually? In other words, can you put the loop into manual mode and change the output to the engine while watching the tank level and get the results you want?

If so, then you can tune the loop, if not, there isn't any point in discussing this and the "roll your own" option will be better. This said, the exercise above, will help you to know how to "roll your own".

 

[Highland Controls]

It sounds like your proportional band is too large. Set integral and derivative to zero. Decrease the proportional until you have much more gentle swings in output. You will even want to get it to a point where it is pretty slow to respond. At that point, start adding integral until it maintains setpoint. You probably don't need derivative in this application.

 

[Peter Nachtwey]

It sounds to me the real problem is that what ever is feeding the tank can't keep up with the engines fuel demands unless the engine is running close to 75 KW. So, does the PID output go to 100% when the tank level is below your set point at 80%. If so, no amount of fussing with PIDs is going to help.

 

[Highland Controls]

It is hard to interpret from the original post, but I think that his capacities are OK, but the loop is just very unstable.

 

[xzen]

What type of engine are we talking about. If it is a combustion engine, it has a very unlinear output that might cause problems.

 

[g.mccormick]

What type of engine are we talking about. If it is a combustion engine, it has a very unlinear output that might cause problems.

That is going to depend on what type of governor the engine is equipped with.