In addition to the main control goal, some additional goals or constraints may be specified. A typical example is the "small control" requirement: the control function should have little power or should require a small expenditure of energy. Such a restriction is needed to avoid "violence" and preserve the inherent properties of the system under control. This is important for ensuring the elimination of artefacts and for an adequate study of the system. Three types of control are used in physical problems: constant control, feedforward control and feedback control. Implementation of a feedback control requires additional measurement devices working in real time, which are often hard to install. Therefore, studying the system may start with the application of inferior forms of control: time-constant and then feedforward control. The possibilities of changing system behavior by means of feedback control can then be studied.