Feed Forward

T1tu$

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Join Date
Feb 2016
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louisiana
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2
I have a startup issue in need of a solution, the problem involves 3 valves on a evaporation system. The three valves are a evaporator feed water level valve, evaporator feed water temperature valve and the evaporator pressure valve. The level and pressure PID's are fast loops the temperature is a slow loop all are tuned for steady state running of the system. The problem that is seen during startup is as follows the feed level drops, causing the level valve to open to maintain the running water level, because of the slow tuning of the temperature valve it can't get the temperature of the water up to setpoint fast enough this causes the pressure to drop drop rapidly and shut the system down.

The old control philosophy was to set the low limit of the temperature valve to a percent greater then zero for a configurable amount of time. This would ensure that the temperature valve would not close during startup of the system. At one point in the past both the time and CV percent were configurable by the operator. Over the years the CV limiting also became complex with the percent open having 3 different setpoints the one used dependent of the percent open of the feed level valve the greater open the level valve the greater held open the temperature valve. The percents open value varied from unit to unit and programmer to programmer, it was all pretty much a guessing game.

I have started to use the feed forward of the PID to assist the temperature and pressure valve. For the pressure valve I use the percent error of the level valve as the feed forward control value. For the feed temperature I use whichever percent error is greater of the level and pressure valves. Because of the nature of the percent error once the system is stable the value is near zero and has little effect on loop it is being used as the feed forward on.

Using the feed forward allowed me to eliminate the need to use the startup CV limiting on the temperature valve, this worked great on a unit with larger/perfectly sized steam valves. On a system recently with valves that were slightly undersized and the temperature valve during steady state was operating in the upper rages of it's CV we end up with startup issues again. This lead me to have to add a configurable startup CV that is sent to the feed temperature valve for 1 second upon the system starting up.

What I'm looking to do is get away from the guessing game of a startup CV. I do not want 2 different tuning parameters either since startup only last about 5 minutes.

I've looked at cascading ratio but in all of the applications I've seen it used in it is 2 different process values dependent on 1 valve. I have 3 different process variable and 3 different valves.

Thoughts and ideas would be greatly appreciated.
 
1) Not sure what you mean could you please elaborate

2) Almost 25 years worth of the same equipment design, with the same startup issue using controllers from SLC to Controllogix and Siemens PLCs. Size of the systems change and specked valves but the general design has not changed much since around the mid 1990's. In a nutshell yes repeatable.
 
1) Not sure what you mean could you please elaborate

2) Almost 25 years worth of the same equipment design, with the same startup issue using controllers from SLC to Controllogix and Siemens PLCs. Size of the systems change and specked valves but the general design has not changed much since around the mid 1990's. In a nutshell yes repeatable.

Sorry, I guess I didn't make that very clear. I had food burning. What I mean is, can you clearly understand and describe the startup sequence/process. It sounds like you can and you are very familiar with the process.

One thing I try to avoid is making PID controls any more complex than they have to be. Not that you are, just stating that I have seen situations where I needed to make a change and it was difficult to remember all the different relationships between the PID loops and why I did what I did. Also coming in to a project and having to learn what someone did.

Is it possible to make a discrete startup sequence where you move set points, and params into the controllers? Maybe even consider manual mode for some of the loops during startup. If your interlocks and shutdowns are squared away, That shouldn't be an issue. I don't know your process though. An actual sequence might make it easier to understand and modify as needed.

Basically if there are "states" where you can capture the process as "good to proceed" to the next state is what I'm getting at.

Also In my experience, look up tables can be a big help here. That is, if you have a certain PV, you output a certain value. The Controllogix has an FBD instruction for this. I can't remember the name. XY or f(x) You can find it.

Hope that helps some.
 

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