Which temperature transducer should I use?
I’ve seen a variety of temperature sensors out there:
There are various sensing technologies (http://www.maxim-ic.com/app-notes/index.mvp/id/3229)
• RTD (e.g., http://www.mamacsys.com/TE-701-702ordering_information.htm)
• Thermistors (e.g., http://www.mamacsys.com/TE-701-702ordering_information.htm)
• Thermocouple + transducer e.g., (http://www.pc-s.com/multi/MultitekTRtherm.htm)
• Integrated circuit (e.g., LM35 http://www.ladyada.net/learn/sensors/tmp36.html). I can’t think of a good way to mount an IC.
There are various mounting styles (http://www.mamacsys.com/TE-701-702ordering_information.htm)
There are various resistances (http://www.mamacsys.com/TE-701-702ordering_information.htm) from 100 ohm to 100 kohm. The PLC I’m planning to use takes 0-5V on its analog inputs.
Which of these varieties do you recommend for my project and why?
Where should I put the temperature probe?
At the end of the day, the temperature I care about is the temperature on the surface of the processor.
The most accurate way to measure this I’ve seen is to mill a channel in the processor and put a thermocouple in it (for example, http://www.hardocp.com/image.html?image=MTMxMDczNDE4NkdFU2tUckRQRzlfMV8zX2wuanBn). However, I milling the surface of a $225 processor a bigger risk than I’m willing to take.
So, my best idea right now is to measure the exhaust heat by putting the probe right after the heat-sink.
One of my concerns with this is that it will inhibit the airflow too much. Another concern is that I don’t want air to be escaping from the hole the probe pokes through. Do you have ideas for solving these issues?
What algorithm will take in the set-point and current temperature and calculate the fan power?
Another puzzle that I need to figure out is which control algorithm I want to use in the PLC. I’ve heard about the following (http://en.wikipedia.org/wiki/Control_systems):
• Binary control: if the temperature is above the set-point, turn the fans on. One problem with this is the fans need to be running at a low level all of the time to prevent the processor from overheating. One way around this would be to run the fans at a low level (e.g., 50%) as a default state; when the temperature pierces a certain threshold, I would run them at 100%. I could see the fans cycling on and off at a high frequency with this approach and wasting a lot of electricity.
• Proportional control: the PLC would vary the fan speed in proportion to the difference between the current temperature and the set-point. So, for example, if it is way hotter than it should be, the fans will run at 100%, while if it is barely hotter, they will run at 40% (their minimum speed). Wikipedia talks about downsides to this, but I don’t understand them.
• PID control. The PLC would also consider the rate the temperature is changing and historical temperatures when deciding how much power to apply to the fan. One challenge with this is how to implement the algorithm. I would have to spend some time programming the thing. The lazy (and expensive) solution would be for me to pick up a PLC with the algorithm already built in (e.g., http://www.mcmaster.com/#38615k73/).
Which algorithm do you advise I use?
What is a good online HVAC controls distributor?
I’ve seen a variety of sites that have just a listing of the manufacturers they carry and then a link to CGNA (http://www.cgnacontrols.com/search/), which seems to have a limited selection. One easier to use site I found was controlstop.com. What do you usually use?