PID Loop

EET

Member
Join Date
Sep 2010
Location
Manitoba
Posts
4
Hi,

Im finding this site very helpful. Everyone is very quick to respond with information. I am new to the whole Allen Bradley, RSlogix world.

I am trying to set up a control loop which will control the amount of process water entering a tank by using the flow rate in cubic meters per hour leaving the tank.

Any Suggestions?
 
There is a measure on the inflow. But its level needs to be controlled based on the outflow. It needs to increase or decrease depending on the cubic meters per hour leaving the tank.
 
Originally posted by EET:

There is a measure on the inflow. But its level needs to be controlled based on the outflow. It needs to increase or decrease depending on the cubic meters per hour leaving the tank.

Well, yea, kinda.

We need to make sure we are all clear on the system. The way I read it you have a controlled infeed to a tank. You have an uncontrolled but measured outfeed from the tank. you want to control the infeed flow to maintain a level in the tank. Is this correct or is this backward?

In any case, the item you want to control is tank level. In pure terms what you are referring to when talking about controllong the inflow to account for the outflow is disturbance rejection. And if you can use it you should.

In a perfect world, if you command the inflow to equal the outflow at all times your level will never change. The integral of the difference in flow rates equates to the change in tank level. So, you could attempt to calculate tank level based on flow rates but that is an incrememntal exercise (you don't really know absolute level) and, even if you seed the level variable with an initial value, it is subject to the integral of any offset errors in your measurement.

Is it possible for you to add a level sensor?

Keith
 
Diagrams win. This seems fairly simple with a bit of math, and a good set of parameters of operation.

SFC seems like a good choice for language for this application.
 
Originally posted by Honga:

This seems fairly simple with a bit of math, and a good set of parameters of operation.

Systems always work pretty well in simulation. It's when you actually start moving physical material through them that it gets a little difficult.

Generally speaking a model won't guarantee that a system will work. It will only tell you that it won't. Said another way, assuming the model is substantively correct, you will never control better than your model says you will.

Keith
 
Use a control valve on the tank inlet controlled by a level controller on the tank. The level controller will close the material balance such that the inlet flow will match the outlet flow as per your requirement.
 

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