Some PID Questions...

Not disagreeing with you rich but dont forget there is a 2" overflow tube on the tank..Even under full overflow being pushed by the 10hp pump there isnt even 1 psi in the tank..

Lance you made some good points..I will check into those and see what i come up with..

Thanks all
 
This thread doesn't make sense

That is a big pump for such a small tank. I think this is the part of the problem.

How much power does it take to pump 1 GPM at even 60 PSI?

Is the tank 24 inches high or 24 feet? How does one get 60 PSI with only 24 inches of fluid?

I would think that a very simple control scheme would work for such a simple application.
 
I think you misread peter. The 60psi is for the air valve that controls the inlet..The tank gravity feeds/ small piston into the cartons..hence why the level in the tank is important.. The 10hp pump is for the CIP system..during production the fillers are fed from a different pump that keeps a constant 10-12 psi in the lines..But i want to set the system up on the worst case..If i can get it to work with the CIP pump the production pump would be a piece of cake..

D
 
Instead of all this fooling around controlling level with valves and sensors and prevent foaming of milk

WHAT IF use a VFD to control level in tank. More out level goes down sensor sends signal to pump to speed up.

Could use scale to measure tank + content weight control off the weight. Would not even be in contact with milk.

Would need a level switch to ensure the fill pumps taking suction out of tank would not run dry. A setting from the scale could supply this function.

Since this is milk and food requirements require easy cleaning components need to be selected with this in mind.

Fewer components = less complexity, breakdown, cleaning.

Dan Bentler
 
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Dan this is basically what happens..The level switch is a pressure sensor..this opens and closes the valve..There has to be a valve somewhere..
The fewer parts less breakdown thing is what they are trying to do..If you could see the levelmatic system running this at the moment you would understand..It is complex with lots of moving parts,pressure switches and lots of headache to set up..
 
darren,
Lancie1 makes a good point about calibrating the PV sensor scaling. I would suggest calibrating the output device as well.
The type of valve actuator on the inlet valve can affect the scaled output (CV) from the PID. A 0-60 psi rotary actuator for a "butterfly" type valve will require a certian amount of "cracking" pressure (usually 10-12 depending on spring). A diaphgram type linear actuator will need a different amount of "cracking" pressure(3-5). We use both types on steam and or product lines. Most of the I/P transducers will have a zero & span adjustment. The 0 to 60 requirement is usually a rotary type. You can either scale the CV or adjust the zero setting on the I/P transducer to eliminate this "dead zone". I refer to deadzone as a change in CV that doesn't cause any change in the process. This may get you a little closer to your goal. There may be a deadzone on the upper end of the scale as well. For example if the PID sets the CV at 75% of 4-20ma, you get 16ma. If this 16ma is = to 40 psi, and the valve is wide open at 40psi, then the next 4ma sent from the PID won't have any effect on the process. Hope this makes sense! I guess the main point is; if the I/P is not tuned to the particular control valve actuator you will have an unstable result.
Another problem I see is that your level sensor (PV) seems really sensitive. The oscillation (-14 to overflow) you saw may have been due to the liquid splashing about when the fill valve closed rapidly. It is difficult to predict the frequency of the splashing you will see. I would try the averaging methods on the PV sensor mentioned by Lancie1 to reduce this problem. It was sugggested to me once to not make drastic changes in the setpoint if possible. Instead the setpoint should be ramped from one setting to another slowly. For example; instead of changing the setpoint from overfill to 50% instantly, it would be better to have the setpoint change from 100% to 50% over a time period of say 5 seconds. This can do done with a timer and add or sub instruction. Ramping the setpoint may serve to reduce the oscillating.

Good Luck
BD
 
darrenj said:
Dan this is basically what happens..The level switch is a pressure sensor..this opens and closes the valve..There has to be a valve somewhere..
The fewer parts less breakdown thing is what they are trying to do..If you could see the levelmatic system running this at the moment you would understand..It is complex with lots of moving parts,pressure switches and lots of headache to set up..

Darren -
I forgot the 10 HP pump feeds more than one tank.
Each of these tanks would need a feed reg (level control)
UNLESS they were all at the same level and at the same distance.
-- that is a big if

Dan
 
The more I think of this , I'm really surprised that the end use allows the use of a DP cell , which would be very difficult to clean to a food grade standard . Most of the food customers I deal with wouldn't allow anything like this .

It would have to go non-contact , eith ultrasonic , and it could be fitted to the existing setup , or possibly to load cells , and weigh the tank .
Ultrasonic is probably the best way to go , with simple on off control and a bit of a dead band to prevent cycling .

I don't think you will get what you have to work very well , there is miles too much disturbance in the tank to get a good valid pressure signal , and it also seems that it is not particularly hygenic , though of course this is your customers cal .
Where did you say it was ......
 
last week i spoke to a supplier about some ultrasonic transmitters I had to use for a application.
He told me that you could get a ultrasonic that could measure trough steel/metal so you dont have to make any hole in the tank.
But unfortunally the ultrasonic is an On/Off with configurable low/high level.
But if you are interested I could get talk to my supplier and get the brandname of the ultrasonic.

Dariusch
 
10baseT said:
The more I think of this , I'm really surprised that the end use allows the use of a DP cell , which would be very difficult to clean to a food grade standard . Most of the food customers I deal with wouldn't allow anything like this .

There are a lot of dairy factories in NZ with a lot of milk silos that use DP cells for level indication. I personally don't know the details of installation, however, they wouldn't be used if they couldn't be cleaned.
 
darrenj said:
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?
The integral and derivative terms are calculated as a function of time. The update time that you specify to the instruction is the basis of the calculation. The instruction expects and assumes that it is being executed at intervals equal to the update time. The minimum value in CLX is 10 msec. You should consider how fast the level can change due to flow out and/or flow in and set the update time to be faster - most likely 100's of msec. If you execute the instruction every scan, then any integral or derivative calculations will be hugely distorted. The easiest way to get the execution interval correct is to put the instruction in a periodic task whose interval time is equal to the update time.

I agree with AutomaticLeigh that you probably only need proportional control - probably with some bias. The level error, after all, represents the integral of the difference in flows into and out of the tank.
 
Thanks Gerry and all..i do think that lances idea of and "average" and everyone else suggestions of only P and I will work..
I agree with everyone ultrasonic would be the way to go however it just wont work in this application..Load cells also would be a pain becouse of the different hookups..Not to mention re-inventing the wheel( Remember this is an existing application and no one want to start cutting and welsing the SS tubing..)

As for Pressure sensors..they are used all over the place in dairy applications.. They are 316 SS and are made with a sanitary Tri clover Flange.. I know all 6 Raw silos and all 4 pasturised silo's use them for level, Not to mention the constant level tanks for the HTST system...
 
Darren,

If you are interested in averaging your Level (Pressure) reading, here is another way that I have used. Store your average value in a memory location, say N7:0. At some periodic time (once a second perhaps), multiply the old average N7:0 by 3, take a new reading and add it to 3 x N7:0, divide by 4, save the new average back ito N7:0, and use this value as the Process Value input into your PID block.

This method weighs the new value at only 1/4 of the total, so that the average pressure changes much slower. This really helps when the pressure is jumping up and down around some average value.
 
O.K. so let me see if i got this straight..
First.. I take a reading every say .5 seconds for 2 seconds.. (4 readings..add them up and divide by 4..store them in "average" wait 1 second Then take "average" x 3 + "New reading" / 4 = "Average"

So basically i am doing the average twice..to stabalise readings?

Thanks

Darren
 
Consider

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?
 

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