PID to slow, Level Control

Hi friends

I have a L24ER-QBFC1B in a Level Control that you can see in the picture.

Tank1 is the process tank. Level sensor is the process variable.
Control valve is the control variable.
The pump is filling Tank1 when Setpoint > ProcessVariable. This pump is controlled just by digital output, not by the PID block.

All the signals are scaled from 0-100%, Tank capacity is 100 liters aprox.

The valve opens from 0 to 100% in 2 minutes aprox., when the setpoint is reached the valve is 100% opened, the pump stops, the valve begins to close but it needs 2 minutes to close so, the level decreases below the setpoint and the valve open again, and the valve never closes,sothe level never is constant.

Proportional Gain is 1
Integral 0.0014
Derivative 0.0014

I am novice in PID controls, which parameters i need to modify to set the PID stable? Do I need to change the control strategy?

Any comment would be appreciated.

Thanks in advance

Who ever designed your system should be fired.
It isn't the PID that is slow. It is the valve.

It is easy to see the pump reacts instantly but the valve is way too slow.
So here is the question. If the pump is left on can the flow through the valve exceed the flow from the pump? If not then the pump must be turned on and off so the average flow matches what the valve can do.
To maintain level the pump flow and the valve flow must be equal. If this is not possible the level will always be going up and down.

It looks like your valve is position controled and has a position feedback. You will need to model the valve flow relative to position at the target tank level (you might need to apply a correction based on the actual level).

The flow rate from tank 1 to tank 2 will be govered by the position of the valve so: The set position of the valve governs flow and the pump must match this flow to maintain the level in tank 1. This implies that the PID loop should control the pump and not the valve.

You will also need to know the flow rate of your pump and do some PWM as it is a digital control. E.G. if your pump runs at 40 L/min and your valve position required 20 L/min then the pump must run 50% of the time so if you set it on a 1 minute cycle then it would be on for 30 seconds and off for 30 seconds (many PLCs have built in functions that can do PWM from PID).

Scale everything to real units (L/Min) rather than % and use your PID output to sum with your predicted pump flow based on the valve position.

I agree with Peter about the design of the system but then it looks like a college asignment question to me which, rarely relate to the real world in any way.

Nick

I know that is not the better design, but it is what i have and i need to work with this.
Do you recommend control the pump with the PID?

The pump is controlled by a Powerflex 4 drive, so i can control the pump with analog signal 0-10v.
What would control the valve? another PID?
Can you make a simple diagram of the PID?




I really appreciate your comments.

Thanks a lot