Wang Shikui (China Institute of Aeronautical Computing Technology, Xi'an 710068, Shaanxi) Fund Project: This article was funded by the Fundamental Aviation Science Fund 00I31002.
One of the most important technologies in information integration technology is the avionics communication technology. Based on the MIL * STD * 1553B bus, this paper analyzes the solutions to several key problems in the design of avionics communication systems. Finally, it focuses on the implementation technology of a 1553B bus communication network of an airborne ACT flight control system.
Overview The development of modern avionics integration technology has greatly improved the performance of aircraft. The most critical foundation of avionics integration is the formation of airborne communication networks. Statistics of domestic and international airborne electronic communication systems. Advanced large-scale civil aircraft, such as Airbus and Boeing, have adopted ARINC429 or ARINC629 to establish avionics communication networks; the vast majority of military aircraft in active service and under development are based on MIL airborne electronic communications system. Regardless of military and civilian aircraft, the avionics communication network can realize the integration of avionics information, and achieve the purpose of avionics information integration. It is worth noting that special indicators such as high real-time performance, maneuverability and reliability of military aircraft have put forward higher requirements for avionics communication systems. This paper analyzes and explains the key problems of military aircraft avionics communication system, and gives advanced and feasible solutions. Finally, taking the 1553B bus communication network of an airborne ACT flight control system as an example, the implementation technology is emphasized.
1 The analysis of several key issues in the avionics communication system is based on the 1553B multiplex transmission bus network. The three characteristics of avionics information integration are: First, multiple physical distributed subsystems are connected into a network by the 1553B bus mode; Second, due to the different working modes, control objects, and real-time requirements for data generation, transmission, and processing of each subsystem, each has special requirements in terms of network layout and information transmission control; third, although each sub The system works asynchronously, but to complete flight and combat tasks, some shared information needs to be processed on a unified time base to be effective, so it is necessary to establish a avionic clock synchronization mechanism. Therefore, the following analysis from the perspective of avionics communication system hierarchy, network topology, communication control scheme and avionics clock synchronization design.
1.1 Hierarchical structure of avionics communication system The sub-communication system is divided into 5 layers: application layer, driver layer, transmission layer, data link layer and physical layer, as shown. The functional division between these 5 layers should be clear, and the interface should be simple, so as to lay a good foundation for the design and implementation of hardware and software.
The application layer is the highest level of the communication system. It implements the throughput response of the communication system and the reliability of the communication system. From the above, the dual-redundancy static bus controller controls the structure and management functions (such as initialization, maintenance, and reconstruction). And interpretation functions (such as describing the meaning, validity, scope, format, etc. of data exchange). The driver layer is the software interface between the application layer and the bottom layer. In order to realize the management function of the application layer, the driver layer should be able to control the initialization, start, stop, connect, disconnect, and start of the self-test of the multiplex bus interface (MBI) in the subsystem, monitor its working status, and control its Data exchange of the system host.
The transmission layer controls the data transmission on the multiplexed bus. The tasks of the transmission layer include information processing, channel switching, and synchronization management.
1553B stipulates that the transmission sequence of each message on the control bus is controlled.
1553B stipulates that the bitstream transmission on the physical medium of the 1553B bus is handled.
The application layer and driver layer are implemented on each subsystem host, and the transmission layer, data link layer, and physical layer are implemented on the MBI.
1.2 Topological structure selection of communication network The topological structure of avionics communication network refers to the physical interconnection structure of each subsystem of avionics. The typical topology of theoretical analysis simulation and actual application verification (such as F * 10, B * 52 and other military aircraft applications) has the following three types: First, a single-level bus topology, in this topology, all subsystems of avionics Both are connected to the same 1553B bus cable. This structure is suitable for avionics systems with a small number of subsystems and low network communication load.
Second, multiple single-level bus topologies. In this topology, the subsystems of avionics are classified according to the similar functions or the frequency of communication and exchange of information between each other, connecting different subsystems to 2 or more 1553B. On the bus. This structure is suitable for avionics systems with a large number of subsystems and heavy network communication load (single-level bus cannot meet). A typical example is to build an avionics communication network into two buses to control navigation and weapon management.
Third, the multi-level bus topology. In this topology, there are at least two levels of 1553B buses with different levels of communication function. Generally, the lower-level bus needs to receive the control commands of the upper-level bus, and at the same time return the working parameters to the upper-level bus. This structure is suitable for a large number of functional units of some subsystems in avionics, and each unit requires 1553B bus (ie, lower-level bus) network communication. Eventually, each subsystem is connected to the avionics system through the upper-level 1553B bus. This structure management is complicated, not only requires designing the hardware gateway between the upper and lower buses, but also organizing the information exchange between the upper and lower buses.
The avionics communication designer should select or combine the best communication network according to the number of airborne electronic devices and the typical network topology.
1.3 Communication control scheme 1553B standard "command response multiplexing data bus standard" not only supports the static bus control scheme of the centralized mode, but also supports the dynamic bus control scheme of the distributed mode.
The static bus control scheme is that a fixed bus controller manages the message communication between all the subsystems on the 1553B bus. This solution has the advantages of simple communication control, easy fault detection, and easy implementation of hardware and software. However, there are fatal shortcomings of communication paralysis caused by the single point of failure inherent in the centralized control network.
The dynamic bus control scheme means that there are several sub-systems on the 1553B bus with a bus controller, but only one is allowed to be a bus controller in a period of time. There are two ways to transfer bus control rights: time division system, that is, each potential bus controller is pre-allocated to a fixed time period to control the bus; cyclic transfer control method, which is based on the communication address of each subsystem Arrangement order handover control, this method is more complicated than the time division system management but high efficiency. The dynamic bus control scheme has the advantages of a distributed control network. The communication network has strong reconfigurability and reliability, but it brings the disadvantages of complex communication control, difficult fault detection, and difficult implementation of hardware and software.
In order to meet the requirements of the avionics overall index and comprehensively analyze the characteristics of the static / dynamic bus control scheme, it is advisable to adopt a dual-redundancy static bus controller as a backup method for each other. In this mode, there are two controllers with bus control capability on the 1553B bus, and the two are backups for each other. When power is on, one of them acts as the active bus controller to manage the bus communication, and the other acts as the backup bus controller; the backup bus controller always monitors the working status of the active bus controller, and replaces it once an unrecoverable fault is found Become the active bus controller to manage bus communication. This scheme not only has the advantages of simple communication control, easy fault detection, and easy implementation of hardware and software, but also avoids the fatal shortcomings of communication paralysis caused by a single point of failure.
Bus controller (BC) BC status line Bus controller (BC) BC status line 1 Third (if the main CpU in the subsystem stops working for the Illish letter control J plan determines that tsFLCC needs it to embed A. type MBI. Its 1.4 Time synchronization mechanism Because each subsystem of the avionics system works according to its own timing clock, there must be a timing error problem. However, in order to achieve real-time tasks and synchronization of transmission information between the subsystems, avionics communication is required The system provides a unified system time. The unity of this time is not only a short time after power-on, but also must be maintained in flight. To achieve this requirement, a avionics time synchronization mechanism must be established. The mechanism works as follows.
Each subsystem of avionics shall have a real-time timer (RTC) with the same clock resolution and length. After the real-time timer of each subsystem is powered on, it will automatically start counting. The avionics bus controller periodically broadcasts its real-time timer value to each subsystem, and each subsystem continuously calculates the error between its own RTC and the bus controller RTC according to this cycle, and handles it with a revised unified system time Real-time tasks. The period value of the bus controller broadcast RTC should be determined according to the requirements of the avionics system and various subsystems for RTC accuracy. When high accuracy is required, the period value should be small; otherwise, the period value should be selected larger.
Using this avionics time synchronization mechanism, the time base of each subsystem of the avionics system is unified, ensuring the high performance of the entire aircraft flight and combat.
1.5 Communication fault handling Communication fault handling is responsible for handling faults and errors that occur during the communication of the system. The fault handling process can be divided into temporary faults and permanent faults. Temporary faults refer to accidental faults due to interference; and permanent faults are faults that exist for a long time or permanently due to hardware failures of subsystems or communication cables.
The bus controller shall firstly retry the communication faults reported by the various subsystems or cables according to the system requirements on the double-redundancy cable. If the fault disappears, it will be regarded as a temporary fault; otherwise, the bus controller will record the fault In the case, it was determined as a permanent failure. The bus controller off-nets the determined faulty subsystem, and only queries the faulty subsystem according to a certain query period; and records the faulty cable determined.
The faults found in the subsystem where the non-bus controller is located are divided into the following three processing methods: first, if the multi-bus interface hardware in the subsystem fails, the terminal flag bit in the status word should be set; second, If a non-MBI failure occurs in the subsystem, and this failure is not a paralytic failure, the subsystem flag bit in the status word should be set; MBI is prohibited from responding to bus controller commands.
2 The implementation technology of the 1553B bus communication network of an ACT flight control system The above-mentioned key problems and solutions of the avionics communication system are a general design criterion. Specific to the establishment of a 1553B communication network within a certain avionics communication system or some special avionics sub-systems, necessary optimization design is necessary. The following uses a 1553B bus communication network of an airborne ACT flight control system as an example to illustrate the specific application technology of the above avionics communication design criteria.
An airborne ACT flight control system consists of four subsystems, which are: flight control computer (FLCC), on-board maintenance BIT (MBIT) device, code generator and flight parameter recording device. Among them, FLCC adopts 4 redundancy control strategies, so it has 4 channels, and each channel requires a 1553B communication interface (MBI). In this way, the 1553B communication network inside the ACT flight control system has a total of 7 nodes, as shown below.
First of all, the ACT flight control system uses a distributed communication system based on the 1553B bus. Therefore, the 5-layer structure of the avionics communication system described above can also be applied. Easy to modify and high reliability.
In terms of network topology, the 7 nodes of the ACT flight control system normally use one as the bus controller (BC) and the rest as the remote terminal (RT). The system traffic is not heavy and there are few nodes connected to the network, so choosing a single 1553B bus topology can meet the communication requirements and can be easily realized.
When choosing a communication control scheme, we must proceed from the ACT flight control system is the most critical and most reliable feature of aircraft electronic equipment. In order to ensure that the FLCC is foolproof, the FLCC as the bus controller adopts a 4-redundancy communication control design scheme that is mutually backup, which is more reliable than the usual dual-redundancy backup scheme. One of the 4 redundant channels works as BC to manage 1553B bus communication, and the rest works as RT (backup BC). Only when the active BC fails, the RT with a smaller address becomes the BC and begins to manage the 1553B bus communication. Problems such as fault determination and handling will not be detailed here.
In terms of time synchronization, first, there is strict time synchronization between the FLCC4 redundancy channels of the ACT flight control system. This is achieved by the FLCC internal clock system, which is a problem within the scope of redundant computer design and between each node of the 1553B communication network. The level of time synchronization is different; second, the transmission of the 1553B bus information between the FLCC, MBIT device, codec and flight parameter recording device is carried out in accordance with the FLCC time period of 12.5ms, and the change of the message is periodic time characteristic ; There is no requirement for real-time clock (RTC) synchronization as described in Section 2 above, so no RTC-based synchronization mechanism is added to each node.
Finally, in terms of fault handling, first, FLCC adopts a redundant design idea to implement a 4 redundancy MBI mutual backup solution, so that MBI failure does not affect the overall performance of the system; second, once the BC of FLCC fails, The other RTs within FLCC can monitor the occurrence of the fault and deal with it in time; third, if any of the MBIT device, vocoder and flight parameter recording device is found to have a communication fault in BC of FLCC, the message is retried first, if the fault disappear It is determined as a temporary fault, otherwise, the determined faulty node is off the network.
3 Conclusion The avionics communication system is a complex airborne distributed real-time communication network, which involves all electronic devices on the avionics multiplex bus. The quality of its top-level design directly affects the performance of the entire aircraft. The hierarchical structure of the avionics communication system, the topology of the network, the communication control scheme, the synchronous design of the avionics clock, and the troubleshooting of communication faults are all key technical issues in the avionics communication system. The 1553B bus communication network design scheme of the ACT flight control system can be used by avionic designers and specific implementers.
Chen Ruoyu. Time division command response multiplexing bus standard. IS Bu GJB289A-97.
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