Quality-of-control management in overloaded real-time systems

Transient overload conditions may cause unpredictable performance degradations in computer controlled systems if not properly handled. To prevent such problems, a common technique adopted in periodic task systems is to reduce the workload by enlarging activation periods. In a digital controller, however, the variation applied on the task period also affects the control law, which needs to be recomputed for the new activation rate. If computing a new control law requires too much time to be performed at runtime, a set of controllers has to be designed offline for different rates and the system has to switch to the proper controller in the presence of an overload condition. In this paper, we present a method for reducing the number of controllers to be designed offline, while still guaranteeing a given control performance. The proposed approach has been integrated with the elastic scheduling theory to promptly react to overload conditions. The effectiveness of the proposed approach has been verified through extensive simulation experiments performed on an inverted pendulum. In addition, the method has been implemented on a real inverted pendulum. Experimental results and implementation issues are reported and discussed.