Coordinated Control - RSLogix 5000

Hello everyone,

I need to control a discharge pressure in our pipeline and there's two ways of doing this:

1. By controlling the speed of the pump (RPM)
2. By controlling the position of a recirculation actuator (4-20 mA)

At the beginning I was using the PI Function Block to control the Pressure (PV) by modifying the Pump Speed (CV). But the problem is, sometimes the pump runs at its minimum capacity which is 2500 rpm (Low Limit) and the Pressure is still higher than what we want.

At this point is when we need to open the recirculation actuator to a fixed position (Manually) to allow the pump to run faster and still meet the desired discharge pressure.

Tech note 54859 mentions the following: "Coordinated Control (CC) – Controls a single process variable by manipulating as many as three different outputs."

I found a manual explaining this function:

http://integrated.cc/cse/ControlLog...Control Instructions - Continuous Process.pdf

My doubt is, when performing the Auto Tune function, is the CV going to variate the pump speed to its maximum capacity? Same with the actuator?

This is my first time using it, have anyone of you had used it before? Any recommendations or things I need to consider?

Thank you for your time.


PLC = 1769-L33ER Compact Logix 5370

I don't see how auto tuning just once will work. The pump will need to be auto tuned for different recirculation set points because this changes the system. The problem is pump and recirc actuator can 'fight' each other if not careful. Does the discharge pressure change quickly due to external factors?

Your control of the recirc valve can target a pump speed/ pump control output.

Example would be that you would set the recirc control to be reverse acting on pump speed feedback. Target a pump speed of 3000rpm.

As the pump speed slows down, the recirc valve opens more, the pump speeds up and the valve closed.

g.mccormick said:

Your control of the recirc valve can target a pump speed/ pump control output.

Example would be that you would set the recirc control to be reverse acting on pump speed feedback. Target a pump speed of 3000rpm.

As the pump speed slows down, the recirc valve opens more, the pump speeds up and the valve closed.

Click to expand...

This is the way I would first consider doing it.
This approach is known more generally as "valve position control" or "midranging control" and is a simple form of optimizing control; in your case, energy usage (pumping costs) is minimized while maintaining your pressure setpoint.
Two PI controllers are required: control pressure with pump speed, control pressure controller output position with recric valve (this is tuned for slower response relative to the pressure controller).

I imagine the coordinated control instruction could be made to work too though I've no experience with it.

Split the range of the cv output. Such that 0-50% controls the valve (50 % = fully closed). And 50 to 100% controls pump speed ( 100% = full speed)