Thanks for your replies for auto tunning pid but its a bit difficult to do it in a short time i think time proportioning control is enough for my system.I study a bit for this subject i will determine a proportional band and in this band i will give the energy proportional, i will give 100% energy, from start to SP-(PB/2),then i'll give energy proportionally towards the set point, at the set point the energy will be 50% then at the SP+(PB/2) the energy will be 0%, am i true?But my problem is how can i determine the %90 %80 etc...pwm pulses? how can i determine the pwm period? can anybody help me with this problem?