Re: Type 2, controller was stuck

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Timo,

> during the simulation of a control strategy for an archive repository I
> face a problem concerning a controller which is meant to control the
> relative humidity of the type 56 zone which represents that storage
> module. An equation determines the relative humidity of the air flow to
> that zone using the control signal of another type 2 controller (one
> controller for humidifying, one for dehumidifying purpose):
>
> phi_in = 50+yB*10+yE*-20
>
>
> Several trace outputs for the controller type 2 following by the warning
>
>   ***** WARNING ***** UNIT  8 TYPE  2
>     CONTROLLER WAS STUCK  12 PERCENT OF THE TIMESTEPS
>     DURING THE SIMULATION
>
> are printed to the list file. Actually, the controller does not perform
> any action though it has to. What problem does that warning indicate?
> How can I handle it? Is it problematically to use Type 2 for controlling
> the humidity of an air flow as I do in this project? The problem does
> not occur when I don't apply the moisture capacitance model for the
> concerning zone of the type 56 building.

Oscillations are normal for an ON-OFF controller considering that you 
are using a fixed time step and iterating during the simulation. Having 
the controller "Stick" to one state is necessary in a simulation to 
prevent endless oscillations.

The fact that the controller(s) are stuck during 12% of the time is not 
worrying per se because in most cases you can hope that errors will 
cancel each others in average. Of course, some times you may end-up with 
a controller stuck in the bad state for long periods of time, especially 
if you have 2 controllers acting on the same variable. You have to think 
about what happens during the iterations of one time step taking into 
account the component order: Using a TRACE command on the two 
controllers and the building can help you understand why controllers get 
stuck. You could also try not using both controllers at the same time 
(e.g. allow dehumidification OR humidification only, based on some 
variables that you would get from the previous time step in order to 
prevent oscillations).

Sometimes, reducing the time step can help you. If you go down to a 
timestep much shorter than the time constant of your system you should 
get rid of all oscillations and the controller should never stick 
(again, this may not be true if 2 controllers are acting in opposite 
ways on the same variable).

In your case, the modeling of the humidification/dehumidification device 
seems to be very simple, so may be you could use the internal humidity 
control in Type 56. Another option is to use a proportional (actually a 
PI or PID) controller.

I hope this helps,

Michaël Kummert

--
_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
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E-mail: kummert@xxxxxxxxxxxxx

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