Temperatur​e control system using thermocoup​les and cooling fans

Greetings labview community,
I'm fairly new to the labview program and though I have watched the tutorial videos and tried to search these forums I couldn't find a solution to my problem.
I need to write a program in labview that controls the temperature via use of fans inside a case with heatsinks and various electronic parts.
In particular I want to use two thermocouples, in order to measure the temperature of two transistors (one in each side of the case), where heatsinks are placed on top of them.
When the temperature is above 25 degrees, a fan should activate. The fans speed should be faster, the longer the temperature stays above 25 degrees, until it drops down to 25 degrees. 
How can this be done? I thought of using a "mask and limit testing" for activating it, but I can't figure out how to control the speed of the fan in a loop automatically based on the temperature.
Here are some of my specifications:
USB-6009 (this was provided to me and is therefore my only option)
Labview 2011
NIDAQmx 9.5 driver
2 thermocouples (one for measuring the temperature for each transistor and activating one fan)
2 fans 0-5V 
Below you'll see an attachment of the program I've wrote so far (just for one set of thermocouple-fan) and I'm using a slider to simulate the input from the thermocouple. If there is anymore information needed, tell me. 
Thank you very much.
Attachments:
Temperature sensors and fan activation.vi ‏130 KB

Hi Lomedil
What you have is a general control problem. This statement:
"The fans speed should be faster, the longer the temperature stays above 25 degrees, until it drops down to 25 degrees. "
Is an indication that you at least need a PI-controller, or you could use a PID-controller. The I in PID stands for integral and that means that the longer you have the same value different from your reference the larger the control signal will be and thereby the faster the fan will spin.
I would recommend you to have a look at the following article to learn the basic principles of control theory:
PID Control
http://zone.ni.com/devzone/cda/tut/p/id/6440
To find the coefficients in your design you can either choose to make a mathematical approximation of your test setup and then calculate the three constants Kd, Ki, Kp using this model, or you can choose to estimate the parameters on a "trial-and-error" basis until you have the desired response time / overshoot i.e.
In general you should note that designing a control application on a General Purpose O/S as Windows will not give you deterministic timing, thus your control loop can be more unstable than expected, design your controller with this in mind! For the fan operation it should not be a problem but for more advanced control purposes your would always use a Real-Time system like the singleboardRIO, compactRIO or setting up a standard PC as a LabVIEW Real-Time Target (Converting a Desktop PC to a LabVIEW Real-Time Target - http://zone.ni.com/devzone/cda/tut/p/id/2733)
Best Regards
Anders Rohde
Applications Engineer
National Instruments Denmark

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