PID Help

Logix V30, using PIDE
I have a process with a vfd controlling pressure and a flow control valve that controls flow rate. The vfd pumps water from a tank, through some filters and instruments then back into a downstream process.
VFD loop is closed with a pressure sensor.
FCV loop is closed with a flow meter.

They fight each other.
When my process wants more pressure, the VFD pump speeds up. This causes an increased flow to my FCV which chokes the pipe and causes a pressure spike which is driving the VFD loop nuts. The same is true when my process tries to adjust the flow rate. I have gotten them both to tune fairly well, so long as the other loop is in manual. In the end, i need to control both the pressure and the flowrate of the water in the system.

Any ideas or hints for things to try?

Maybe I shouldn't answer, not used to control logix. But is there a feed forward function in the PIDE?

I'd feed forward the output from FIC into the PIC but I'm used to other controllers. A feed forward x gain that adds straight to the output.

There is, but I'll have to look at the documentation to see how to implement it. Thanks for the reply. I'll give it a shot in the morning.

Maybe a little more info on why you need to control both precisely.

What is the min/max range of pressure the system can stand?

If you use a proportional only control on the pressure with a wide band ( small gain)
and let it float a little as the flow is more precisely controlled. This may help prevent the fighting.

Or the opposite, flow proportional pressure precise.

You can control pressure or flow but you can't control both at the same time unless you know something about pump curves and system curves and where the two intersect.
Both curves can change and that will change where they intersect.
Look up pump curves and system curves.
This is not a simple PID application.

Edit:
http://www.engineeringtoolbox.com/pump-system-curves-d_635.html
The pump curves curve downwards. The pump curve can move up and down as a function of speed assuming you are using a centrifugal pump.
The system curves curve upwards. The more you close the flow valve the sharper the curve upwards.
Study the curves and you will get an intuitive feel for your problem and it isn't an easy one.
It is easy to see you can't change just one control signal at a time. They must be changed in a coordinated way.

i'll try and get some further clarification on the process from my customer tomorrow. i appreciate the responses. I rarely do PID loops, mostly test/assembly automation, so this is a learning experience for me.

travispedley said:

i'll try and get some further clarification on the process from my customer tomorrow. i appreciate the responses. I rarely do PID loops, mostly test/assembly automation, so this is a learning experience for me.

Click to expand...

This is not the kind of application to learn on.
Nothing is linear. A simple PID will not work without lots of augmentation.
Get as much information as you can.
Ask if they have data for system and pump curves.
If not you can't find the operating point as shown in the diagrams that I posted a link to. Sometimes it is better to walk away.

However, If you know how to solve these kinds of problems you will be rare and do very well in the control world.

This maybe a dumb question but I don’t see it answered anywhere in the post
Where is the pressure feedback transducer located ?
If the transducer is located between the pump discharge and the flow control valve (Before the flow valve) then you will never get control of the system

The pressure transducer must be located after the flow control valve and as close to point of use as possible
What will happen if the transducer is located before the flow valve is as the flow closes to regulate the flow to set point the pressure behind it will go up (less flow) the pump will slow down to maintain the set pressure. This would result I the flow out of the flow control valve to go down. The flow control will then open the flow valve to increase the flow to meet the flow demand as set in the system. This increased flow will drop the pressure and increase the pump speed to maintain the set pressure. This will result in an oisolating control loop and you will never get control of the loop.

With the pressure transducer on the downstream side of the flow control and close to process.
As flow demand goes up the flow valve opens and with the increased flow the pressure transducer will signal the pump speed to increase to supply the needed additional flow to meet the needed pressure.
And as the flow demand goes down the flow valve closes the pressure transducer will tell the pump to slow down to maintain the pressure at the set flow.
With the pressure transducer close to the process the pressure to the process will more stable.

Another option would be to control one or the other thru a signal select.

Pressure control with a flow override.

Or flow control with a pressure override.

GaryS said:

This maybe a dumb question but I don’t see it answered anywhere in the post
Where is the pressure feedback transducer located ?
If the transducer is located between the pump discharge and the flow control valve (Before the flow valve) then you will never get control of the system

The pressure transducer must be located after the flow control valve and as close to point of use as possible
What will happen if the transducer is located before the flow valve is as the flow closes to regulate the flow to set point the pressure behind it will go up (less flow) the pump will slow down to maintain the set pressure. This would result I the flow out of the flow control valve to go down. The flow control will then open the flow valve to increase the flow to meet the flow demand as set in the system. This increased flow will drop the pressure and increase the pump speed to maintain the set pressure. This will result in an oisolating control loop and you will never get control of the loop.

With the pressure transducer on the downstream side of the flow control and close to process.
As flow demand goes up the flow valve opens and with the increased flow the pressure transducer will signal the pump speed to increase to supply the needed additional flow to meet the needed pressure.
And as the flow demand goes down the flow valve closes the pressure transducer will tell the pump to slow down to maintain the pressure at the set flow.
With the pressure transducer close to the process the pressure to the process will more stable.

Click to expand...

The transducer is between the FCV and the pump. No way around this in this system. I just talked to my customer and the pressure is more important than the flow response. so i'm going to try tuning the pressure loop and then tuning the flow loop at a much much slower response. hoping that a less aggressive flow loop will help. I'll post my results.
Thanks for the suggestions.

Travis
If the system is as you say then I see no way you can ever get control of either loop
Unless you put one loop in manual control
Pressure and Flow are always interactive.
If you keep the pump speed constant and increase the flow the pressure will go down and of course the reverse is also true.
If you keep the flow constant and increase the pump speed the pressure behind the flow control valve will also increase and the reverse is also true.
When you have 2 or more control loops with their feedback the placement of the sensors is critical and must be given careful thought.
I see no need to be concerned about the pressure between the pump and the flow valve it will be what it is as a result of the other variables. The point you need to be concerned about the pressure is at the point of use.
I always hate it when a client doesn’t understand the interaction of controls and puts together a system that will not work and then gives it to us to control and insist we can make anything work and we are not allow to make the changes necessary.
You will have to pick one and control around it but keep in mind the pressure you measure at the pump discharge is not going to be the pressure at the point of use, if you control the flow ( Increase flow decrease pressure at the point of use I don’t see a way around that.

I once drove 900 miles to work on system that was messed up and the in-house guy could not get it working. When I arrived the first thing the in-house guy said was that under no circumstances was I going to be allowed to make any changes to his system. I went to the boss that brought me in and laid it on the line if I can’t make the necessary changes the is no point in me being there and I was going back home. He had a talk with the in-house guy, 2 days later the system was running. I spent a few more days cleaning up the bad code and the operator interface. From what I was aware of the system ran untouched for about 8 years after that.
Sometimes you just need to say NO and walk away.

Update.
Tuned the vfd with a pretty quick response, then tuned the FCV to be very slow.
vfd adapts at about 5 seconds, 5 minutes on the FCV.
Seems to be working to their satisfaction at the moment, but things will change quickly when the machine gets actual process media in the field. Thanks for the tips and tricks.