Design and Implementation of an Avionics Stimulator Based on VxWorks System

The avionics system refers to the sum of all electronic systems on the aircraft, and its simulation verification plays an important role in the design and development process of the avionics system. Conducting simulation verification of the avionics system can effectively reduce the risks and costs associated with the integration process of the avionics system and shorten the development cycle. During various simulation verifications, the avionics stimulator needs to provide excitation signals to the simulation components, so the accuracy and stability of the excitation signals provided by the avionics stimulator will directly affect the results of the simulation verification.

Currently, avionics stimulators are mainly developed based on the Windows system, which is slightly insufficient in terms of data transmission real-time performance and reliability, and there are often delays and packet loss issues when providing excitation signals to the simulation components. The VxWorks system adopts a task scheduling mechanism of priority preemption and round-robin scheduling, which has good real-time performance. An avionics stimulator developed based on the VxWorks system can effectively solve the shortcomings of avionics stimulators developed under the Windows system. Therefore, this paper designs an avionics stimulator based on the VxWorks system, which can provide excitation signals with real-time performance and reliability for the avionics system.

The avionics stimulator designed in this paper consists of software and hardware parts. The software part mainly includes the excitation data acquisition module and the data conversion module. The excitation data acquisition module receives flight data during the simulated flight process via Ethernet, including parameters such as the aircraft’s position, attitude, and communication navigation system operating frequency. The avionics stimulator data conversion module mainly implements encoding and decoding of AFDX bus data and ARINC429 bus data, completing the mutual conversion of ARINC429 bus data and AFDX bus data, and transmits excitation data to various avionics simulation components. The AFDX data bus protocol can be found in reference [4], and the ARINC429 bus protocol can be found in reference [5]. The hardware part mainly completes the porting of the VxWorks system on the MPC8270, establishing the hardware development platform. The overall design of the avionics stimulator is shown in Figure 1.

Figure 1 Overall Design of the Avionics Stimulator

From a practical application perspective, the excitation data acquired by the excitation data acquisition module mainly comes from the flight data in the flight simulation software. The excitation data acquisition module achieves memory sharing with the flight simulation software, thereby enabling real-time acquisition of flight data.

The flight data format sent over Ethernet is based on the UDP packet format. After the data acquisition module receives one frame of data packet, it first checks the frame header of the packet, then verifies the correctness of the data frame’s checksum, and then determines the message type and parses the information contained in the message for the packets with correct checksums. By designing the interface function for encoding ARINC429 bus data words, calling the ARINC429 data word encoding interface, the flight data information decoded from the UDP format data packet is encoded to obtain the corresponding ARINC429 data word for the excitation data. After determining the period and channel, this ARINC429 data word is output to the avionics simulation components. The excitation data acquisition process is shown in Figure 2.

The AFDX bus is widely used in advanced models such as the B-787 and A380 due to its excellent transmission efficiency and high reliability. Currently, the backbone network of avionics systems usually uses the AFDX bus to connect various airborne subsystems into a highly efficient and reliable whole, while many subsystems in aircraft still use the more mature ARINC429 bus. Therefore, this paper designs the data conversion module of the avionics stimulator to achieve mutual conversion between AFDX bus excitation signals and ARINC429 bus excitation signals. The mutual conversion of the two data formats is an inverse process. In this section, the avionics stimulator is designed to send and receive ARINC429 bus data words corresponding to AFDX bus data packets, thereby achieving encoding conversion between bus data.

Figure 2 Excitation Data Acquisition Process

By analyzing the AFDX bus data, the AFDX bus message is designed into a general format that can be composed of multiple “messages”. Each “message” in the AFDX bus is divided into three parts, facilitating the filling of the ARINC429 bus data word structure into the AFDX data frame structure. MsgType indicates the encoding of the message type; LengthInBytes indicates the length of the PayLoad field; PayLoad is the content carried by the AFDX bus. The length of the PayLoad is variable, and the specific information format it carries is determined by the value of MsgType. The general format of messages in AFDX bus packets is shown in Figure 3.

Figure 3 General Format of Messages in AFDX Bus Packets

Based on the design of the general structure of the “message” content in the AFDX bus packet, the ARINC429 bus data word structure is filled into the AFDX “message” structure framework, resulting in the AFDX data packet corresponding to the avionics stimulator receiving and sending ARINC429 bus data, as shown in Figure 4. Here, the Versio field represents different ARINC429 message versions; the ChannelIndex field is the component’s channel number; the A429Word field represents the specific information of the ARINC429 data word; the PeriodInMs field represents the reception and transmission period of the ARINC429 data word; the TransmitCount field indicates the number of transmissions of the ARINC429 data word.

The VxWorks system provides developers with a large number of Board Support Packages (BSP), which facilitate the simplification of BSP porting work. Before conducting BSP porting, the appropriate BSP package must be selected according to the CPU model, and the BSP-related configuration information must be adjusted based on hardware data. After completing the BSP configuration, a new VxWorks Image Project is established, and the files necessary for creating the image are created.

Figure 4 Conversion of ARINC429 Bus Data to AFDX Data Packets

The VxWorks module is configured according to the performance requirements of the target board. After the VxWorks system image is compiled, the FTP environment is debugged. By running the FTP Server in Workbench 3.3, the user information configuration in the FTP environment is completed. After completing the above configurations, the VxWorks image is downloaded to the target board MPC8270 for operation. Thus, the construction of the VxWorks system platform is completed.

After the functionality of the avionics stimulator is realized, it needs to undergo verification testing, mainly targeting the avionics excitation signals that the avionics stimulator can provide, namely ARINC429 bus excitation signals, AFDX bus excitation signals, and real-time testing of excitation signals.

The avionics stimulator performs data compliance testing on the received ARINC429 bus excitation signals. Five types of excitation data are obtained from the flight data for testing, as shown in Table 1. One output channel of the avionics stimulator is connected to the ARINC429 bus analyzer, and the excitation data in Table 1 is uniformly set to a transmission cycle of 200 ms and output to the ARINC429 bus analyzer. The excitation data displayed by the bus analyzer is shown in Figure 5.

Table 1 Values of Excitation Parameters

Figure 5 Data Displayed by the ARINC429 Bus Analyzer

By comparing the data information in Table 1 with the results displayed in Figure 5, it is shown that the data information contained in both is consistent, and the ARINC429 bus excitation signals output by the avionics stimulator meet data compliance requirements.

The excitation data acquisition module selects the VOR1 frequency flight data of the simulated aircraft in the flight simulation software for verification. The relevant information of the excitation data VOR1 frequency is shown in Table 2. The avionics stimulator sends the AFDX bus excitation signal through the AFDX board to the switch, and captures the AFDX data packets through Wireshark. The corresponding captured data packet is a hexadecimal number: 3800DC84, as shown in Figure 6. By comparing and verifying, it is shown that the data information in the AFDX bus excitation signals output by the avionics stimulator is consistent with the ARINC429 payload data, and the avionics stimulator has achieved the expected function of converting ARINC429 bus excitation signals into AFDX signals.

Table 2 Configuration Information of AFDX Signals Output by the Avionics Stimulator

Figure 6 VOR1 Frequency Data Packet

This paper relies on the unique advantages of the VxWorks system to ensure the real-time performance of data acquisition. “Real-time” does not mean “fast”, but refers to the determinism of the system’s response time. Time determinism is specifically reflected in whether the avionics stimulator can completely send and receive excitation signals according to the specified period. First, the avionics stimulator under the VxWorks system outputs the ARINC429 bus excitation signals, and the configuration of the output data information is shown in Table 3.

Table 3 Configuration Information of Output ARINC429 Bus Signals

After completing the configuration information, in low-speed (12.5 Kb/s) mode, the avionics stimulator outputs the ARINC429 bus excitation signals, and the oscilloscope tests the 8th output channel of ARINC429. Similarly, using the avionics stimulator under the Windows system to complete the transmission of the data configured in Table 3, the time interval between the starting points of each pair of adjacent waveforms is tested, and the test results are shown in Table 4.

Through testing and verification, the time intervals between two consecutive signal waveform outputs from the avionics stimulator based on the VxWorks system are the same as the set transmission period, which is precisely 30.00 ms, without generating jitter delays. However, the avionics stimulator under the Windows system tends to have time jitter between the time intervals of consecutive signal outputs, with time delays of more than 1 ms between each pair of waveforms. In summary, the avionics stimulator based on the VxWorks system better meets the high real-time performance requirements for simulation verification of avionics systems in terms of time determinism.

Table 4 Single Channel Time Determinism Test Results in ms

This paper proposes an overall design scheme for the avionics stimulator based on the VxWorks system, targeting the MPC8270 platform, and conducts a detailed design to achieve mutual conversion between AFDX bus data and ARINC429 bus data, providing good excitation data for practical avionics simulation systems. Simulation tests show that the designed avionics stimulator achieves the expected results.

(This article is sourced from “Modern Electronic Technology”, authors: Sun Yigang, Chi Wenqiang, authors’ affiliation: School of Aeronautical Engineering, Civil Aviation University of China)

[4] Yang Feng, Hong Yuanjia, Xia Jie, et al. Overview of AFDX Network Technology [J]. Electronic Technology Application, 2016 (4): 4-6.

[5] Zhou Tingting. Design and Implementation of Multi-Protocol Processing for ARINC429 Bus in Aviation Communication Equipment [J]. China New Communication, 2017, 19 (6): 20-22