Control

Pagina 1

FLIGHT DYNAMICS, TOPIC 4 CONTROL 4.1 Classification of control: closed-loop control The aim of a controller is to provide robustness to a dynamic system. The robustness is meant to be the ability of the system to have a desired response even if some changes occur, due to disturbances or non voluntary actions of the user, which is the pilot for the flight dynamic case. A first classification of the control systems regards their architecture. In fact, it is possible to distinguish two main categories: •Feedback controllers: they are closed-loop system (they are error-based controllers); •Feedforward controllers: they are open-loop system (they exploits a model based on disturbances). The feedforward controllers are very complex, since the control constant varies with time. As a matter of fact, the closed-loop systems are more utilized. They automatically adjusts the system inputs to reduce errors, moreover they are employed to improve the stability of unstable system and to enhance the robustness against external disturbances. Finally they produce reliables and repeatable performances. On the other hand, the closed-loop system analysis is more difficult then the open-loop system analysis, specially if more than one feedback is present. Furthermore, if the gains of the controller are too sensitive, unstable situations may occur. Among them, the PID control is the most famous and widespread. It features three gain constants: ! is a proportional term (which mainly reduces the rise time), ! is an integrative term (which mainly decreases the steady state error) and ! is a derivative term (which improves stability). The PID transfer function is: ! It shall be noted that the proportional term is a gain which acts in phase, the integrative term is destabilizing and it provides a…