Abstract
The paper focuses on two basic problems for the control engineer: the separability, as a
fundamental assumption to frame correctly the system connections and the concept of proportional
transfer element used as a model for many theoretical developments. Some fundamental
connections and the concepts of the input and output signals are analysed, as well as some aspects
regarding the control and the load disturbance signals. The problem of proportional elements and
proportional feedbacks is developed both from theoretical and practical point of view, with its
main advantages and disadvantages. A lot of examples give a deep understanding on the discussed
aspects.s
