The output of a level loop have to open or close two valves to add water and at the same time control the temperature by adding cold or hot water.

One valve is cold water and the other is hot water.
Merci in advance.

 

Without having more information this seems like a simple application that has the potential to become somewhat less simple.

Does the tank have any kind of mixer? minimal dead time? Otherwise this is going to be very hard to control.

--
Bob

 

The "simple" solution would be 1* level PID, 1 * Temperature PID and 1 * Ratio Calculation.

1. The level control sets the total water demand and sends the output to the ratio controller.

2. The Temperature Controller sets the Ratio of Cold Water to Hot Water. Output Range is 0.0 to 1.0 and is also sent to the ratio controller.

3. The output of the ratio controller sets the Hot water valve = (Level Controller OP * Temp Controller OP) and the Cold water valve = Level Controller OP * (1 - Temp Controller OP) So the Hot and Cold water valves work in opposite directions, but the total valve opening = output of the Level controller.

There are some potential problems of course and this control strategy will not work in all cases.

1. The hot and cold water flow control valves should be the same size so that 50% hot water flow is the same volume as 50% cold water flow. If not you need to adjust the ratio calculation.

2. Depending on how you tune the level controller, the total water flow will vary. This means the gain of the temperature control will vary. If the level control tuning is aggressive and makes large water flow changes you may need adaptive gain settings on the temperature control. Lower Water flows will need higher temperature Gain settings.

I would try the above strategy and see how well it works in your plant before doing anything more complex.

Am I right in thinking you are already an Expertune user? You can check their library here http://www.expertune.com/ArticlesFull.html#PIDOpt

Rob
www[.]lymac.co.nz