O.K. Here is what i have and i have done..
I have a tank about 24" tall 2' round that gets filled with a liquid, The liquid is pumped in from a fitting on the bottom of the tank (Bottom filling prevents foaming) there is a pressure sensor on he side of he tank about 3" from the bottom the pressure sensor is 0-1psi (About 30WC), There is a spring return valve on the inlet to regulate the product going in (0-60psi..60 psi is 100% open) The tank should maintain a level set by the operator,for argument sake lets say 50%, The tank feeds 4 piston pumps that pump product into containers, anywhere from 125ml per pump to 1 liter per pump(Again selectable by the operator)..Now for the hardware part..
Control logix processor
0-60psi IP transducer
10 HP motor pushing the liquid.
What i have done so far..
First off i completely ignored all advise to read up about PID's..I went to site hooked up the pressure sensor and the ip transducer and started to "play" with the P I and D settings.. I got the PID loop working sort of but ran into another problem that i will post at the end..
Now that i got the play part out of my system i went home and just spent the last 4 or 5 hours reading Ron's P I D tutorial also read why P only control wont work and the thread Ron, Peter and Alan had back in 2003 about "How often should i trigger the PID"....Of course now i see why my test didn't work out the way i wanted, But the reading brings up some questions..
First i have read that the PID loop should be scheduled..why?? Is this just to save scan time?..the reading says it should be the same as the loop update time (More questions to come on that one!)..my question is why? What happens if it runs continuously?
Second "update time" and |"steady state"..At the begging of the run the operator plugs in the value he wants the tank to be at, Now i don't care if it takes 10 seconds or 1 minute for the liquid to reach that point..what matters is once its selected it stays close.. the machine fills 1200 cartons an hour, so worst case i am pulling 20 liters of liquid out of this tank per min.. how should i go about selecting an update time?
Third, After reading all this info (BTW Thanks a ton Ron Beaufort..an excellent tutorial, I got the ones from your web site)..(I also must admit while i read P and I word for word i kinda skim read D ) I can see why my test didn't work but i think i also have another problem..I stated out with the tank empty and my P at 1 started the pump and pushed water into the tank.. It was a classic example of why Ron says P alone wont work..It wouldn't hit the set point..it was close but not quite..It would work for this application tho..next i did an "Overflow" (The valve is opened 100% and the tank fills to capacity and the pushes the liquid out the overflow tube..(used for CIP at the end of a shift)..Everything worked o.k... Now when i put in a smaller set point..say back to 50% the valve closed..However i GUESS that caused a slight change in pressure in the tank..The level sensor read -14 so the valve went to full open then the tank read overflow and shut the valve..back to -14 back to valve open 100%..it kept oscillating..i walked away for 3 or 4 Min's to see if it would even itself out but it wouldnt..If i set the set point back to overflow it would take about 1 min for the pressure in the tank to simmer down and i would get the proper reading..
To try and overcome this i took the pressure reading every .5 seconds and MOV it to the PV of the PID block..this didnt do to much, i then tried 1 and 2 seconds..all it did was slow the oscillations down..I THINK again i say THINK that the valve is closing and opening to quickly..I know the PID should be slowed down but still dont quite grasp what loop update time should be or how to calculate..or even if this will help..
My scale on the input card is 4ma=0 and 20ma=28 (For 28WC)
and the ouput card is 4ma=0 20ma=60 (For 60psi)
any help OR nudges would be appreciated
Also as a side note..how does the auto tune feature for PIDE work?? Is it close or more headache? (in relation to to PIDE)
Thanks
Darren
I have a tank about 24" tall 2' round that gets filled with a liquid, The liquid is pumped in from a fitting on the bottom of the tank (Bottom filling prevents foaming) there is a pressure sensor on he side of he tank about 3" from the bottom the pressure sensor is 0-1psi (About 30WC), There is a spring return valve on the inlet to regulate the product going in (0-60psi..60 psi is 100% open) The tank should maintain a level set by the operator,for argument sake lets say 50%, The tank feeds 4 piston pumps that pump product into containers, anywhere from 125ml per pump to 1 liter per pump(Again selectable by the operator)..Now for the hardware part..
Control logix processor
0-60psi IP transducer
10 HP motor pushing the liquid.
What i have done so far..
First off i completely ignored all advise to read up about PID's..I went to site hooked up the pressure sensor and the ip transducer and started to "play" with the P I and D settings.. I got the PID loop working sort of but ran into another problem that i will post at the end..
Now that i got the play part out of my system i went home and just spent the last 4 or 5 hours reading Ron's P I D tutorial also read why P only control wont work and the thread Ron, Peter and Alan had back in 2003 about "How often should i trigger the PID"....Of course now i see why my test didn't work out the way i wanted, But the reading brings up some questions..
First i have read that the PID loop should be scheduled..why?? Is this just to save scan time?..the reading says it should be the same as the loop update time (More questions to come on that one!)..my question is why? What happens if it runs continuously?
Second "update time" and |"steady state"..At the begging of the run the operator plugs in the value he wants the tank to be at, Now i don't care if it takes 10 seconds or 1 minute for the liquid to reach that point..what matters is once its selected it stays close.. the machine fills 1200 cartons an hour, so worst case i am pulling 20 liters of liquid out of this tank per min.. how should i go about selecting an update time?
Third, After reading all this info (BTW Thanks a ton Ron Beaufort..an excellent tutorial, I got the ones from your web site)..(I also must admit while i read P and I word for word i kinda skim read D ) I can see why my test didn't work but i think i also have another problem..I stated out with the tank empty and my P at 1 started the pump and pushed water into the tank.. It was a classic example of why Ron says P alone wont work..It wouldn't hit the set point..it was close but not quite..It would work for this application tho..next i did an "Overflow" (The valve is opened 100% and the tank fills to capacity and the pushes the liquid out the overflow tube..(used for CIP at the end of a shift)..Everything worked o.k... Now when i put in a smaller set point..say back to 50% the valve closed..However i GUESS that caused a slight change in pressure in the tank..The level sensor read -14 so the valve went to full open then the tank read overflow and shut the valve..back to -14 back to valve open 100%..it kept oscillating..i walked away for 3 or 4 Min's to see if it would even itself out but it wouldnt..If i set the set point back to overflow it would take about 1 min for the pressure in the tank to simmer down and i would get the proper reading..
To try and overcome this i took the pressure reading every .5 seconds and MOV it to the PV of the PID block..this didnt do to much, i then tried 1 and 2 seconds..all it did was slow the oscillations down..I THINK again i say THINK that the valve is closing and opening to quickly..I know the PID should be slowed down but still dont quite grasp what loop update time should be or how to calculate..or even if this will help..
My scale on the input card is 4ma=0 and 20ma=28 (For 28WC)
and the ouput card is 4ma=0 20ma=60 (For 60psi)
any help OR nudges would be appreciated
Also as a side note..how does the auto tune feature for PIDE work?? Is it close or more headache? (in relation to to PIDE)
Thanks
Darren