Hi Reza,

Thermal processes are not difficult to control. They are typically very high capacity processes, which means that the control circuits will usually be waiting on the process to respond to their input. Still, if I were doing a control system I would use actual opamp circuits for the controls. Remember that opamp is short for operational amplifier. They were designed to do mathematical operations, in particular linear summation (Proportional), integration (Integral), and differentiation (Derivative). You could limit the role of the µ-controller to receiving and controlling set points, and let the opamps control the process.