The long-term operation of any system, large or small, requires that there be a mass/energy balance between input and output. If a process is operated at equilibrium at all times, control would be simple. Since change does exist, the critical parameter in process control is time, i.e. how long it takes for a change in any input to show in the output. System time constants can vary from fractions of a second to many hours. For linear processes, where the process characteristics do not change significantly with time or load conditions, then utilizing the autotuner to set fixed PID Process Control or PI parameters will possibly be adequate to ensure effective control.
To measure output or deviation form, what is dreamed is to measure difference (error). The most common step of reducing and measuring any fault is through feedback, a technique of measuring the output and feeding it back to the controller. The function of the process controller is to adjust a process variable input to eliminate that error. The PID process control is most often the type of controller chosen to do this.