Gap Control With PID

m looking for a way to have "gap" control with PID it was called. I want to have a flow control into a tank where the flow is at a constant set point when the level is Plus or minus five percent. If the error gets outside of this percent error then the PID controller will take over . Has anyone heard of this type of control scheme. Where can I find more information on this programming scheme in RSLogix 500?

Let me make sure I understand this. You have a pump that pumps in at a constant flow rate and you want it to turn on at 5%under and turn off at 5% over a specified level. Is this correct?

If so, you can use greater than (GRT)and less than (LES) arguments to turn the pump on and off if the fill time is longer than a couple seconds. A PID loop would be overkill.:nodi:

I am pretty sure they want one PID controller to control flow until level gets out of whack the have a level PID controller take over to bring it back within the 5% error range.

I think you wan to implement a hi/lo selector or override control

http://modelingandcontrol.com/2011/02/override_control/

I have only ever done it on the 5000 platform and in a dcs with prebuilt blocks designed for that purpose

I would think you could run the 2 pid with conditionals deciding which CV gets fed to your final element, if you only have one which is what I am assuming. You will need to take into implement windup reset, setpoint tracking or output limiting on the disabled controller to keep it from making huge jumps when it comes online.

A cascade loop may also work in your case since you will technically be using flow to control level anyway and just have the inner loop run normally in auto and switch to cascade when level drifts, but I am not sure how well the 500 platform supports cascade control.

http://www.controleng.com/single-ar...control/bcedad6518aec409f583ba6bc9b72854.html

Clarification

Clarification
The Pump will always run. There is a bypass loop. There is a flow control valve which controls how much flow goes into the tank. There is a level valve which controls how much process leaves the tank. I want to have a set point for the flow control valve which will control when the tank level is within 5% of the level setpoint. When the setpoint gets outside of the 5% error window, I want the PID controller to override and quickly bring the level into the 5% error window. The level valve controller will also be controlled by the level.

Ok so you have 2 valves one on a flow in line and one on a flow out of the tank

-the level will be controlled by the outlet valve
-flow controlled by inlet valve

when level gets out of whack you want the level loop to essentially take control of the flow loop and increase or decrease substantially based on the direction of your error.

I am a bit confused because if you have a controlled constant flow in and are controlling level on the outlet, "flow out", when in equilibrium nothing should cause you to have to leave equilibrium. or are there more than one inlet/outlet on the tank

If the process is what I think it is you shouldn't need anything but the level PID controller to keep you level at setpoint or is there a reason your level controller can't compensate for the error itself by using the outlet valve, ie do you have a min and max flow out?

override or high low selector will work and what you will essential have is a pid adjusting the input and output of your process simultaneously this may affect your tuning constants and process stability.

Yes you are correct, I will have other flows going in and out of this tank as the process changes. If some other process needs to go to this tank, I would like it to quickly compensate.

I have exactly the same project, and a second one similar with pressure!

Any help would be great!!!

Hey guys, the reason this thread isn't going anywhere is because the OP is not providing information required to make a good decision. Tank level control is easy, at least it should be. I see too many threads like this.

I doubt this application requires a low select. It can probably be done with a proportional band.

What controls the setpoint to the flow control valve currently? Just set the setpoint = desired level, use a +-deadband, bias the output so that its "at setpoint output" (0 error) = 50%

As the level decreases the valve will open more and vice versa

Yes I agree tank level is easy. But I am no master programmer either. We are doing a school experiment and everyone has been told to look whenever to find the answer. We want the flow in to be constant around setpoint. We want the level control valve(outward flow on the tank) to control the level. We then want to have other in ward and outward flow to and out of the tank. When the level is out of the error allowed, the flow valve will response to adjust the level quicker while the level control valve is constantly controlling