Dear all,

I would like to ask for help with my HVAC project.

In my project I need to mantain room pressurization using differential pressure sensor (0-10V), VFD fan (0-10V) and LOGO!. I have created simple program using a PI-block. It works just as needed (mantaining constant room pressure at 10 Pa). However, if an employee enters the room, the pressure drops suddenly and the fan goes to high rpm. Thus, after the person closes door the pressure becomes very high (~60 Pa) for a brief moment, until fan rpm drops. I have tried to add an analog filter or an averaging block between AI and PI block, but it does not solve the problem. How to make PI block non-sensitive to brief pressure drops, mantaining the same rpm during such events?

You may try adding ON/OFF delay before output Q. Make adjustable time delay ON and delay OFF as you like. If the change of PI output not exceed delay time, it will not change output. This could reduce frequent switcjing of output.

However if you use transister output PWM. I do not know how to, let's see advice from other members.

Hi,

the PI-loop FB has an manual value you can configure.

Change the program that in case the door is opend to switch the PI-loop FB to manual mode and configure the value for manual output as needed. 

If the door gets closed change to automatic mode.

Another option is to change the parameter configuration of the PI-loop to a slower set.

Both are just ideas and you need to check if it fits to your requirements.

Hope it helps

Helps an Analog Ramp block you?

Post your programm (in .lsc format and zip it)) so we can try to find a solution

Thanks for the suggestions.

Please, see the project file attached.

I have already tried analog filter block, it seems to work, but most times it confuses PI block so it behaves erraticly, due to bad feedback with actual room pressure.

Hello All,

I have find "sort of" solution that seems working for now. The output value from PI block (to control the FAN RPM) now stored as a constant for 5 seconds, and then refreshed with the new value from PI block. This solution though, in order to work correctly, requires to decrease PI amplification fourfold, in my case from 0.4 to 0.1.

Best regards