Rslogix 5k pid

I need to maintain a liquid level in a small (200 gallon) tank. There is a pump that is taking liquid out of the tank at a variable rate, and no matter how much or little that pump takes out of the tank I want to maintain a 50% tank level using a modulating valve that takes a 4-20ma signal. I started to set it up with a PID instruction, but am kind of lost with what settings to put into the PID. It is a very simple application, but I need a little help!!!

1. Why does the tank have to be exactly 50% level?
2. Can the tank level be in a range of say 40% to 60% instead. If so, then you could just set up a staggered level control to keep the tank at that range. In this case you wouldn't need a PID loop.

50% is just a number I chose, it could be 40 or 60, butf f I used a staggered level control, my mod valve would be opening and closing constantly because I am moving a lot of liquid out of the tank. Its only about a 200 gallon tank, and the pump is moving over 100 gallons per minute out of it.

Is it just the tuning parameters you are needing? You're not going to need the derivative term, so I wouldn't worry about that. If the outflow is highly variable, it sounds like you will be at least slightly above and below the setpoint quite often. So I would say the integral term might not be needed either. That leaves us with proportional only. Also I would recommend a deadband of some amount maybe 10%, otherwise you will be constantly moving the control valve. With such a small tank and high and variable outflow, you could see the valve move a lot anyway.

A two zone staggered control might work also. An inner zone and an outer zone with different control valve percentages.

Forgot to ask what the inflow GPM is. That is important.

I would use step control on the inflow valve if exact level control is not needed.
If the tank is 0% full the valve is 100% open. Tank level is 30% valve is 50% open, ect. You will have to tweak the numbers to match your flow flow rates, but since exact level control is not needed, a PID is just over complicating things. With a step system the valve will only move to predefined setpoints as needed saving wear and tear on the valve, and greatly simplifying the programming.


Bubba.

What are you using for a level sensor? Pressure transducer with 4-20 ? probe? Using a PID loop is not always the best way to go. Sometimes just managing a set point with delay fill timer is perfect. You could have it start filling at any point and stop at some higher level after if you have no return. IF there is a return (like a CIP sys) then write in -to drain logic when full to avoid overflows. When I set up a PID loop i use a graph (part of RSLogix)and start with proportional, then derivative, after i fine tune integral.........somehow at 47 i ended up back on nights....damn it.

I would keep it simple. I would open the valve 100% when the level is 40% and close the valve to 0% when the level is 60%. Basically, only a proportional band is required.

This assumes that the valve open at 100% can keep up with the pump running at 100gpm.

If you want to get tricky you can open the valve based on the pump speed too. This is kind of a feed forward. This should permit control with less deviation from 50%.

If an integrator is used the time constant would need to be about 0.2 minute. Usually the time constant is something divided by the rate of change of something. In this case the something is the level and that is always changing and so is the rate of change in the level so a time constant would be constantly changing. I would just stick with the feed forward and proportional band.

Yes I am using a 0-5psi transducer that puts out a 4-20ma signal, I think I will try the step control method, seems like that might be a lot more simple! Thank you all

Erik,

What are you struggling with exactly? Also, I'd recommend setting up your PID using a function block instruction called PIDE. Here's an example on how to set it up, tune it, etc.

http://www.plcdev.com/getting_started_with_the_logix5000_pide_function_block

Vlad