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Old December 12th, 2005, 10:46 PM   #31
darrenj
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Quote:
Originally Posted by Peter Nachtwey
Darrenj, yur pump still looks awfully big even if it supplies other tanks. Do you know the formula that relates power to flow and pressure?

Air activated valves will also add a lot of lag to the process. Have you considered an electric solenoid valve with LVDT feedback?

You can't P gain you way out of a PID problem. The P gain can only be increased so much. After a point the settling time can not be decreased. This limit on the effectiveness is dependent on the system.

Where is the trend? A trend is worth a lot more than 1000 words. It can save big $$$$.

Have you considered using a variable bias or feedforward?
Peter.. The pump mentioned (10HP) is only for the cleaning of tank..for the rest of the 20 odd hours it is a different pump that is programmed to supply the machine with 15PSI of milk..I am trying to set my system up on the worst case..I will know where i stand tommorow night..Hell this set up might not work..if not its back to the drawing board but i feel i should try every thing possible before i tell them its not going to work the way they want..
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Old December 12th, 2005, 10:47 PM   #32
Lancie1
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Darren,

I think that you have mixed up the TWO different methods that I have mentioned here. In the second method, you read in a new pressure every 1 second, or whatever time period that you think is needed for accurate control. Take the old average and multiply it by 3. Take this number and add it to the new pressure reading. Divide the total by four and save this as the new average reading.

Average = ((3 x Old Average)+ new reading) / 4

You can use a different multiplier and divisor other than 3 and 4 (for example, 4 and 5). I used 3 & 4 because I found that I can usually make this work with only 16-bit integer numbers, and some PLCs only have 16-bit integers.

Last edited by Lancie1; December 12th, 2005 at 10:52 PM.
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Old December 12th, 2005, 10:49 PM   #33
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o.k. that makes more scence to me now..!!
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Old December 13th, 2005, 02:57 PM   #34
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Even with the problem of the foaming, it would be simpler to use the analog output to set a pre-determined speed based on the following.

if one pump is running, pre-set the analog output to give a flow a little above the flow of the delivery pump or pumps running, then if 2 set another flowrate etc.
i.e. 3 presets for combination of the pumps pumping out the liquid & another for CIP.
I agree about the average method of sampling the analog pressure transducer, then again it would have been cheaper to use 3 level probes (although not if the medium is like oil that would require capacitive probes).
Simple is often the best!.
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Old December 14th, 2005, 02:47 AM   #35
darrenj
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Well i got it figured out.. I think Ken was the closest to guessing the problem..I should have figured it out as soon as i saw the -14 reading..When the tank started to overflow and i entered a smaller number to close the valve..the valve closed..HOWEVER The overflow line was full of liquid and tried to drain..This caused suction in the tank giving me a negative reading..(Dont forget the pressure sensor is only 0-1psi)..Once i figured that out it was only a case of programming the drain valve to open for a second or 2 to break the suction and everything was good..

I got the PID working o.k. but not great..But that will take some time changing values and seeing what works best..

Thanks all for the help..

Lance1 the average worked great, but caused a bit of a lag in the PID..I tried it without and seems to work good now that i have the overflow sorted..

Now i just have to adjust the program to work ith the new sensor..

One more quick question.. The operator can close the inlet valve by the HMI. When he does this i want the Output from the PID to go to 0 Can i write a 0 to the CV to force it off??

Thanks
D
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Old December 14th, 2005, 02:55 AM   #36
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Why don't you write the CV to a float, and pass a 0 to that float when you need the output off?

As well as shut off the PID loop so you don't build wind up a very large integral.
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Old December 14th, 2005, 02:57 AM   #37
darrenj
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Quote:
Originally Posted by CroCop
Why don't you write the CV to a float, and pass a 0 to that float when you need the output off?

As well as shut off the PID loop so you don't build wind up a very large integral.
Good point..why dont i do that!!

Sometimes you just look right over things..

Cheers
D
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