Brand New to the PCS7 Block World: Tank level control

[riggs33x]

I'm coming from a background of PLC ladderlogic/rockwell products. I am new to blocks. Can you point me in right right direction?

What I would like to do: control tank level
**All hypothetical for educational purposes**

Let's say there is a PID block controlling a valve position (filling a tank) with a setpoint of 75 in full Auto mode. When the tank level hits >90 I want the PID control to be disabled while in Auto operation and the valve goes to 100%

When the tank level <85 I want the PID block to take back control.

What suggestions to accomplish this? I am seeing some compare blocks with >,< type logic for the block nubs.

Lastly: what does nub stand for? I realize it's the connection points from the various blocks to others. Thanks!

 

[drbitboy]

Lastly: what does nub stand for? I realize it's the connection points from the various blocks to others. Thanks!

It does not stand for anything i.e. it is not an acronym; cf. e.g. here.

 

[Ken Moore]

Are you using the APL library of blocks? If so, you can use one of the PID alarm output nubs (with messaging disabled), the alarms have configurable deadbands, so alarm at 90.1, reset of 15, will do it. Connect the alarm output nub, to the Force MV input nub. By using the alarm nub, you can can change the alarm point and reset via the faceplate in WinCC.

 

[Peter Nachtwey]

I'm coming from a background of PLC Let's say there is a PID block controlling a valve position (filling a tank) with a setpoint of 75 in full Auto mode. When the tank level hits >90 I want the PID control to be disabled while in Auto operation and the valve goes to 100%

This doesn't make sense. If the level is at 90, why do you want to open the valve to 100%. Since the valve is filling it would make the level evn higher.

You don't need a PID just a simple proportional band where the filling valve is 100% open at 75 and closed at 85. Ratio how open the valve is between 75 and 85. No integrator or derivative is required. Simple. The Siemens programmers can show you how to make a simple proportional band controller.

 

[drbitboy]

This doesn't make sense.

"... misunderstanding all [we] see ..."

Let's say there is a PID block controlling a valve position (filling a tank) with a setpoint of 75 in full Auto mode.

@riggs33x:

Where does the PV (Process Value) input to the PID come from?

• Tank Level measurement?
• Valve Position measurement?
• Some other measurement?

Where does the CV (Control Variable) output from the PID go to?

• Target Valve Position?
• Target Valve Position signal i.e. to a valve positioner?
• Some other signal?

What is the PID Loop Update Time, and how are evaluations of the PID instruction scheduled (fool me once ... )?

 

[Peter Nachtwey]

"... misunderstanding all [we] see ..."

A PID isn't necessary for tank level control within a band.
The only gotcha is that when the valve is 100% open it better be able to flow as much as what is going out or the level will drop.