Braking Systems with New IMA Generation 2011-01-2662 Published 10/18/2011 Stephane Bernard and Jean-Pierre Garcia Messier-Bugatti-Dowty Copyright 2011 SAE International doi:10.4271/2011-01-2662 ABSTRACT First generation of Integrated Modular Avionics (IMA), currently onboard in aircraft type Airbus A380, A400M, or designed for Airbus A350, and whose principle was initially to introduce some common processing resources, had been developed in such a way to reduce the quantity of embedded equipment part number, and to harmonize the nature of the avionic units by minimizing the specificity of electronic equipment: thus, for instance, the number of processing units in the A380 is half that of previous LRU-based avionic generations. This type of architecture had already many interest at aircraft level; nevertheless, some of the designed electronic components were not able to support some performance requirements of the function suppliers, such as Messier- Bugatti-Dowty, which had to limit the deployment of the functions to implement in IMA; For this reason, it was still necessary to execute the fast control loops, as the antiskid algorithms, in specific remote electronics (RDC, RBCU), independently from the standardized CPIOM. Beyond this first generation of avionics architecture, new more optimised concepts are presently studied, through research programs as SCARLETT (IMA of second generation, IMA2G ), where Messier-Bugatti-Dowty takes part to the definition and the evaluation of new modular electronics standard, more powerful and capable to host the most critical and fast control functions, and in such a way to ensure that the performance of these defined generic components (middleware, reduced API653 layer), plus the latency introduced by the fast field bus match well with the high time critical performances required, for example, for the execution of the braking regulation. In particular, the antiskid function needs very low latency and jitter, and full deterministic transmission to operate nominally. Furthermore, actual research study on braking, landing gear and aircraft on ground control laws requires more and more real-time power and fast control loops to provide new functionalities and higher efficiency, and necessary to be considered in the design of these new platforms. The target is, thus, to reach more standardized and scalable avionics, dedicated to the highest critical aircraft functions as the braking control, and standard capable to facilitate the high critical system integration and to harmonize the development and test process, regarding the final common objective (at aircraft and system levels) to reduce the development time and cost, without compromising the trend of more efficient and powerful real time control laws with adaptive behaviour. INTRODUCTION The European Research & Technology program called SCARLETT (SCAlable & ReconfigurabLe Electronics platforms and Tools), in which Messier-Bugatti-Dowty contributes as active partner in the design of the future highly time critical system architectures, provides the main opportunity to progress the state of the art of the embedded avionics beyond the current IMA1G concept, especially regarding the following aspects : Scalability, portability and adaptability Fault tolerance and reconfiguration capabilities Minimum number of types of standardized electronic modules These new platform generations have to be designed in taking into account to cover as much as possible the functional and performance requirements, plus all the technical constraints
Figure 1. IMA2G Braking avionic architecture 1 highlighted by the airframers, and by the time-critical system suppliers, such as Messier-Bugatti-Dowty dedicated to the ATA32 landing gear system design. In particular, the antiskid braking control, one of the most time-critical aircraft systems, included in the ATA32 perimeter, appears as sizing and important to consider for a study of IMA2G time critical platform, especially regarding : Its high criticality level (DALA) Its high processing rate (dedicated to the antiskid control) and its determinism constraints Its variety of Input / Output Its required level of fault tolerance Thus, the IMA2G platform shall offer sufficient material resources (in term of CPU capability, memory size, bus, interface processing) in order to maintain high real-time performance level especially required for the braking control function. DESCRIPTION OF NEW BRAKING CONTROL ARCHITECTURE This new IMA platform is based on avionic architecture concept composed of set of Core Processing Module and Remote Data Concentrator generic components drawing, thus, a segregation (partial or total according to the adopted configuration) between computation resource and Input / Output management stage; this design philosophy aims to reach the use of fully standard electronic bricks (covering maximum functional and interface needs), consequently reducing the development effort and improving the high level of integration of numerous systems. On a strict technological viewpoint, the avionic strategy, such as considered here, reclaims much more powerful and efficient CPU resource, in the main or remote computation core, in such a way even to be capable to execute fast control loops, for instance the antiskid braking algorithms with some fast processing phase running up to 400Hz. On other hand, for this same braking function, the data communication technology, used between the Core Processing Module and the Remote Data Concentrator, is required to operate with total transmission determinism and lowest possible latency and jitter to reach the expected processing performances, widely reducing the choice of the possible solution today proposed. DETAILS OF IMA2G AVIONICS CONCEPT With these different requirement, two concepts of modular avionics are today imagined and highlighted, dedicated to host the high critical functions, such as the Messier-Bugatti- Dowty's antiskid braking control, for the future aircraft generation. Both showed architectures deal with dual command strategy in active/passive mode at CPM level, in active/active mode at RDC level (each RDC ensures the braking control for two wheels); these 2 control architectures are dedicated to the hydraulic braking of four-braked- wheels aircraft type. Control Architecture 1 This first concept, as illustrated in Figure 1, proposes a system cluster composed of 3 types of avionic components: A COMMAND Core Processing Module This component is sized to host a braking COMMAND software partition, capable to compute the high level braking mode, the flight phases, the braking orders on each aircraft hydraulic circuit, and execute the braking activation. The fastest computation algorithm, within this partition, is executed at 50Hz. A MONITORING Core Processing Module This component is sized to host a braking MONITORING software partition, capable to determine the system failure for system reconfiguration and provide information to others aircraft systems; The fastest computation algorithm, within this partition, is executed at 50Hz. An Intelligent Remote Data Concentrator This component is sized to host a REMOTE braking software partition, capable to determine the corrected pressures, servo valve drift,
Figure 2. IMA2G Braking avionic architecture 2 compute the braking servovalve currents, and manage the antiskid protection. The fastest processing, within this partition, is executed at 400Hz. Here, the 3 types of equipment exchange command & monitoring data through an ARINC664 part 7 communication network (AFDX technology), without additional need of fast network to reduce the communication time, because the antiskid control is locally managed by the irdc. Control Architecture 2 The second concept, as illustrated in Figure 2, proposes an architecture with the 3 same types of avionic components, as previously described. However, the difference in this avionic configuration is only the COMMAND Core Processing Module executes all braking control software functions, even including the fast control loops which manage the wheel antiskid control algorithms. Whereas the Remote Data Concentrator (specific and only dedicated to the braking function) becomes a simple passive equipment (without applicative software) which ensures the network processing and the Input / Output management: the generation of the servovalve command currents - the conditioning of the physical measurement feedbacks (wheel speed frequential signals, hydraulic pressure transducer voltage data). In other hand, like in the architecture 1 showed above, the pure computation resources (the CPMs) communicate with peripheral Input / Output resources through ARINC664 part 7 network. However, a faster field bus is here parallel required: the role of this one, here fulfilled by a communication technology Low Capacity AFDX, is to ensure the exchange of the fastest command data between the CPM and the RDC, in such a way, as the antiskid control is implemented up in the CPM COM, not to compromize significantly the global system reactivity with too much communication latency for the acquisition of the wheel speed and the transmission of the resulting current command. PERFORMANCES OF THE BRAKING IMA2G PLATFORM Regarding the design structure, the distribution principle introduced by these new generation of avionic platform is leading to consider new timing constraints, especially depending on: The latency of processing performed by each equipment, at level of the middleware and API layers (CPM + irdc if remote control application implemented) The latency and the jitter of the communication network linking the computation resources and the Input / Output management stage These delays are illustrated for both braking architectures through the Figure 3. In both showed braking architecture, the network processing time (applied to AFDX and Low Capacity AFDX) is taken into account in the network global latency. In any case, it appears that the management of the antiskid algorithms by the Core Processing Module, is going to introduce significantly more delay than the distributed architecture concept 1 or a more classic centralized concept, in requiring to forward the wheel speed data at higher avionics level, to compute and retransmit the correct current references to the RDC. But, whatever the implementation way of the antiskid function in the irdc or in the CPM, the current challenge with this IMA2G is, thus, to optimize sufficiently the processing tasks, the software process (services OS 653 ) on the different avionic bricks, and the performances of the digital transmission network, in such a way to reach, as much as possible, a system reactivity level close to a more centralized architecture solution.
Figure 3. IMA2G avionic performances EVOLUTION TOWARDS ATA32 ELECTRICAL SYSTEMS Regarding the current trend, especially followed by Messier- Bugatti-Dowty, to develop and evolute towards new electric technology, for instance dedicated to the braking function, with multiple possible advantage in term of maintenance operation simplification, reduction of operating costs and, furthermore, proposing new functionalities such as the realtime measurement of heat-sink wear or the smart management of loads affecting each brake (actually implemented to the braking of the Boeing 787 Dreamliner), these new control principles mean also, either an evolution of the present control algorithms, either the introduction of new processing logics, with significant material constraints to consider, as soon as possible, in the design of the generic host avionic platform, in term of : Growth of computation load and memory capability requiring the use of new digital technology Interface capabilities to acquire and condition data coming from new generation of sensor technology: tachometer, wear sensor, torque sensor, or others. Moreover, within an electric braking solution, the control principle differs significantly from a hydraulic braking mode, with a certain quantity of specificities dedicated to the electric actuator control. These specificities could be covered by the introduction of additional electronic equipment type EMAC, provided by the brake supplier, unit especially in charge of the execution of the motor control & monitoring fast loops. This control unit would communicate with the IMA platform via a fast field bus at low latency and jitter. Figure 4. Electric braking control architecture The target is, thus, to reach the design of an IMA2G generic platform, capable to respond, indifferently for a same function, to a need of hydraulic or electric control nature, sometimes with variable real time power constraints or own interface requirements. SMART SENSOR CONCEPT In any case, within these new control architectures, and in addition of the IMA2G, Messier-Bugatti-Dowty studies also the concept of smart sensor (as illustrated in Figure 4), specific component which could be located in aircraft main landing gear bottom, with high reliability level on electronic and mechanical components in harsh environment, and whose main functional role is to acquire and condition locally the brake parameters, such as the wheel speeds, afterwards provided in the fast local network, towards the ATA32 computation units (generic or not), data too used by other aircraft systems like the flight control. This technology is today strongly highlighted, regarding its main interest to be capable to digitalize locally the data, thus to reduce the number of wires along the main landing gear (significant weight gain), and to optimize the noise level in the transmission line.
However, about the real-time performance aspect, specific care is required to take into account and optimize the additional latency time on the local processing made by the smart sensor and the data transmission time in the local field network. IMA PLATFORM, FULL OPENNESS AND GENERICITY IMA2G time-critical platform, as studied in SCARLETT, is accessible through standardized Application Programming Interface; this API layer is following an ARINC653 part 1 specification standard, equipped with a reduced process management (limited to up to one periodic process) in such a way to optimize the compatible Operating Software performances and determinism. Regarding this interface context, the braking application is adapted and optimized to satisfy the requirements of this reduced API653, limited in term of proposed software services; in particular, all the functional tasks of each braking control application are rescheduled around only one periodic process, executed per partition. aircraft, and in order to keep equivalent system performance levels. This challenge is widely considered and is presently as the common care of any actors of the aeronautical world, such as Messier- Bugatti-Dowty which contributes actively to the IMA2G study in such a way to adapt and optimize its future control systems with this new generation of avionic components, fully generic and scalable DEFINITIONS/ABBREVIATIONS AFDX Avionics Full-DupleX switched ethernet API Application Programming Interface ARINC Aircraft Radio INCorporated AS Application Software ATA Air Transport Association Figure 5. Braking Control SW adaptation to Reduced API A653 standard These braking control software bricks are developed in a total generic matter, capable to be interfaced with any type of electronic equipment, offering the same API653 standard, and obviously sized to support the real time performances characterized by this aircraft system type. SUMMARY / CONCLUSIONS Regarding the growth of the embedded electronic functions, furthermore required to be always more optimized and efficient, the increase of safety constraints meaning more stronger process of design, validation and certification, sharing of new avionics platform standard appears today as essential for the airframer and the system integrator in such a way to facilitate the design effort, reduce the development cycle and the resulting costs. BCS CPM CPU IMA Braking Control System Core Processing Module Core Processing Unit Integrated Modular Avionics LC AFDX Low Capacity Avionics Full-DupleX switched ethernet RBCU Remote Braking Control Unit However, this IMA2G standard, to be successful, will have to be sufficiently sized, optimized, and performant to absorb more and more computation power and real-time constraints, especially within the future generation of more electrical RDC Remote Data Concentrator
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