Re: Controller TYPE2b

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Alberto,

> I´m relatively new in TRNSYS and the most problems I have had up to now it
> comes from the TYPE2. I´m simulating a  Thermal Solar Energy System with
> controllers for the pumps to each circuit (primary and secundary). The Upper
> input temperature it is the collector outlet temperature and the Lower input
> temperature  it is the Temperature of outlet flow 1 of TYPE60 for the first
> controller and  the Temperature of outlet flow 2 of TYPE60 for the second. I
> have put enough dead band, Upper 10ºC and 1ºC Lower, nevertheless the
> controller work as it likes and normally the simulation end  with a  "stuck"
> percentage.

I sent a reply to a similar problem recently but the message is not in 
the archive yet so here is a slightly edited copy/paste:

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 x% 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. In your case I 
would assume the tank has enough inertia to de-couple the controllers so 
you should not get into that kind of problem.

You may want to check 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 tank can help you understand why controllers 
get stuck.

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). 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
Fax: +1 (608) 262-8464
E-mail: kummert@xxxxxxxxxxxxx

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