what i going to do is to use 2 or 3 profiles. One is preheating, it will go to below 5C of setpont (since i know that i will get 5C overshoot when running full power), than steady level profile, and for cooling would be same as steady level, since it is easy to hold temperature when it's falling than it's rising. now i am getting 0.1C undershoot and 0.5C overshoot at steady profile, goal is to get zero overshoot since i have pwm control with 15b resolution (1KHz pwm frequency, and no heat on power electronics )
Thermocouple is glued to small pcb, and i will use two different size pcb ( one with 2x2cm, second 4x4cm, and by knowing pcb size i will adjust temperature to get as close as possible to real board temperature, since if i have two different size, i can do approximation of curve with 2 points by saying that relationship is linear, since T is function of s (area) T=C*s )
And even at this PI level, painting function (1pixel/degree) due to float to integer conversion error is painting same line as setpoint