[S7] PID control between control valve and load cell

Hey,

I'm working on the automation of a reaction vessel and have a basic knowledge of siemens S7.

my problem is the next:

i'm dosing the polyol addition through a control valve. the polyol is captured in a reaction vessel. that vessel is on load cells.

So i have the analog value of the load cell as input and have an analog output of 4-10mA for controlling the control valve.

Now i wanna be able to say:

fill this vessel till it has 50 kg of polyol and then totally close the valve. I know it is possible with a P,PI or PID controller.

but how the hell do I begin on this? the vessel needs to be filled as fast and as accurate possible. But i can't have an overschoot because i cant get the polyol back out of the vessel.

could you please help me.

maybe a short example of a grafcet or program could be a hudge help for me...

tnx
Steven Vandermeulen

How and when your vessel must be emptyed? If loading and unloading of vessel work simultaneously, yes - you can use PID. If you must to fill vessel till 50 kg only, you don't need PID. Only "fast speed" (large opening angle) and "low speed" (small opening angle) can be used.

so i should go for like:

100% opening till 90%
75% opening till 95%
50% opening till 97.5%
25% opening till 99%

something like that?

Yes thats the type of thing, you must also have a value which you would expect to continue after sending the close signal, so don't close on target, you have to close early to talke account of the time the valve reacts, plus the material in the air.

Yes, plus Peter said.