PID control 2 output problem

[elec4fun]

Hi everybody,

I need to design a system controlling the temperature in a 3U rack. The temperature inside the rack must follows the outside temperature +/- 7°C. In the rack it'll have a fans and a heater.

The problem is: How am I going to control the fan ANd the heater with one PID ? I have some ideas...

- I could have a multiplexer. When the PID output is positive the heater is on and fan is off. When the PID output is negative heater off and fan on.

- I could have the fan and the heater coupled. When PID output is 50% both work at 50%. When PID output decreases heater level decreases and fan level increses...

What do you think of these ideas ? Do you have others ones ?
In both case, how can I determinate PID coef ? I know Ziegler Nichols is a good method but here how can I use this method ? I think this problem hs something to do with SIMO systems (Single Input, Multiple Output) but I couldn't find any documentation on it....

Regards,
Franck.

 

[ronyutp]

I think that any of the proposed solution defines a nonlinear system. In my experience I recomend you the first posibility ( heater or fan, but not at the same time).

The tuning process is not easy, due to the fact that the system is not linear. In my opinion it should be empirical or applying nonlinear control techniques.

 

[ace1903]

"I could have the fan and the heater coupled. When PID output is 50% both work at 50%. When PID output decreases heater level decreases and fan level increses... "

This is not good solution. It is pure waste of energy!

Clasicall PID solution will solve problem.

 

[elec4fun]

"I need to design a system controlling the temperature in a 3U rack. The temperature inside the rack must follows the outside temperature +/- 7°C. In the rack it'll have a fans and a heater."

I made a mistake. It is +6°C (+/-1°C).

We decided to apply a PID control on the heater. The heater fan will have a constant speed depending on the outside temperature. So the action of the heater will be effective in every temperature range.
The Main fan will not be use (not for this temperature control...).

ace1903 you are right. The solution is out of sense.

The PID tunning will be empirical for sure.

Best regards.
Franck.

 

[Prabakaran]

Can i have some more project based on PID . i decided to do a project in closed system based om PID control systems. so can anyone help me?