Hello guys,
I have one not well defined question about one control loop. I'm not interested in details,
but general cause of the following behavior:
Let's say there is a control loop for controlling air pressure in some kind of pipe's ring.
There are three lines as inlet to the air ring, on each there is an air flap that is controled by the
controller. Controller is some kind of black box, but I know for sure that is PI. Three air flaps are
actuators, and air pressure is maintained by opening or closing these flaps.
If one flap is switched from auto to manual and maintained open, for example, 80% and other two is left
on automatic mode, interesting behavior is happening.
For example, if set point is set to 20 mbar, controller is acting in such way that PV (actual pressure)
is oscillating in range from 18,5 to 19.5 mbar and causing two air flaps constantly to open and close
alternatively by 5 %. Practicaly whole system is in slight oscillation state. Air flaps are constantly opening
and closing. I'm not sure why this is happening, since when all three flaps are on automatic mode, everything
is working fine, after some time flaps are open in such way to produce steady air pressure in the ring.
(without steady state error due to PI control).
What can cause this behavior, generally speaking? What are possible remedies to this problem? Maybe to
try to increase dead band on the controller, because 19,5 mbar if 20 is setpoint is not a big deal?
Can you give me any recommendations?
Thanks
I have one not well defined question about one control loop. I'm not interested in details,
but general cause of the following behavior:
Let's say there is a control loop for controlling air pressure in some kind of pipe's ring.
There are three lines as inlet to the air ring, on each there is an air flap that is controled by the
controller. Controller is some kind of black box, but I know for sure that is PI. Three air flaps are
actuators, and air pressure is maintained by opening or closing these flaps.
If one flap is switched from auto to manual and maintained open, for example, 80% and other two is left
on automatic mode, interesting behavior is happening.
For example, if set point is set to 20 mbar, controller is acting in such way that PV (actual pressure)
is oscillating in range from 18,5 to 19.5 mbar and causing two air flaps constantly to open and close
alternatively by 5 %. Practicaly whole system is in slight oscillation state. Air flaps are constantly opening
and closing. I'm not sure why this is happening, since when all three flaps are on automatic mode, everything
is working fine, after some time flaps are open in such way to produce steady air pressure in the ring.
(without steady state error due to PI control).
What can cause this behavior, generally speaking? What are possible remedies to this problem? Maybe to
try to increase dead band on the controller, because 19,5 mbar if 20 is setpoint is not a big deal?
Can you give me any recommendations?
Thanks
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