Can someone give me some quick pointers on this?
I have a device which mounts in a furnace and is cooled by compressed air. The compressed air (controlled by a proportional valve) blows on a block of steel at the hot end of the sensor. This block of steel has a thermocouple embedded deep inside. A simple single input, single output system.
The problem I have is that PID just can't seem to control the temperature well. I always end up with oscillations of at least 10-15 deg F. I have tried autotuning software (Labview PID vi's), hand coded PID based on opto22 control manuals, manual autotuning procedures and even a dedicated Omega CN9000 autotuning temperature controller. None of these do well at all (the Omega was the best but still had wild fluctutations of air flow rate instead of just a constant rate with small corrections).
The system has a long dead time - on the order of about 20-25 seconds between when the air comes on and when the thermocouple detects a change in temperature of the block (thermal capacitance).
Why is this so difficult to control? Is there something other than PID that would better address this problem? Or can someone recommend a procedure I can undertake to better understand the system?
Any help would be greatly appreciated!