Kristina Ahlström

Future architecture of flight control systems

The development of fault tolerant embedded control systems such as flight control systems (FCS) are currently highly specialized and time-consuming. We introduce a conceptual architecture for the next decade control system where all control and logic are distributed to a number of computer nodes locally linked to actuators and connected via a communication network. In this way, we substantially reduce the life-cycle cost of embedded systems and attain scalable fault tolerance. All fault tolerance is based on redundancy. Our philosophy is to cover permanent faults with hardware replication and handle all error processing caused by both permanent and transient faults with software techniques. With intelligent nodes and use of inherent redundancy we introduce a robust and simple fault tolerant system that utilizes minimum hardware and has bandwidth requirements of less than 300 kbits/s, which can be met with an electrical bus. The study is based on an FCS for JAS 39 Gripen, a multi-role combat aircraft that is statically unstable at subsonic speed.

Redundancy management in distributed flight control systems: experience & simulations

In the design of fault tolerant real time systems, the most important issue is fault handling and redundancy managing. Adding hardware as well as software in order to tolerate faults requires a redundancy strategy to attain and prove the expected as well as the required fault tolerance. This paper presents fault handling strategies of a future distributed architecture for a flight control system (FCS) designed for the JAS 39 Gripen, a modern 4th generation multi-purpose combat aircraft. The results are based on knowledge of and experience from the JAS 39 Gripen, with over 15000 flight hours. Consequently, a highly dependable real time control system is addressed, however, the principles of the distributed system are general and can be applied to other combat and commercial aircraft as well as for other embedded control systems, e.g. in cars, trains etc. The distributed architecture aims to tolerate permanent and transient physical faults, whereas software design faults are not catered for. Simulations give experimental results for validation of the fault tolerance qualities of the distributed control system. The fault handling simulations include transient fault recovery, exploring three redundancy principles and also tests of time limits for permanent fault handling, i.e. system reconfiguration. The results are based on experiments on a simulator validated against the actual aircraft.

Conceptual Design of Distributed by-Wire Systems

A design method for ultra-dependable control-by-wire systems is presented here. With a top-down approach, exploiting the systems intrinsic redundancy combined with a scalable software redundancy, it is possible to meet dependability requirements cost-effectively. The method starts with the systems functions, which are broken down to the basic elements; task, sensor or actuator. A task graph shows the basic elements interrelationships. Sensor and actuator nodes form a non-redundant hardware architecture. The functional task-graph gives input when allocating software on the node architecture. Tasks are allocated to achieve low inter-node communication and transient fault tolerance using scalable software redundancy. Hardware is added to meet the dependability requirements. Finally, the method describes fault handling and bus scheduling. The proposed method has been used in two cases; a fly-by-wire aircraft and a drive-by-wire car.

Evaluation of Timing and Fault Handling Qualities of Distributed Flight Control System

Abstract The next generation of fly-by-wire systems as well as automotive drive-by-wire systems will distribute the control functions. When distributing the control among several computer nodes delays are introduced mainly due to communication between nodes. Here simulations of such a distributed flight control system result in an optimum scheduling of the communication bus from a timing point of view.