A General Method for Module Automatic Testing in Avionics Systems

The traditional Automatic Test Equipment (ATE) systems are insufficient to cope with the challenges of testing more and more complex avionics systems. In this study, we propose a general method for module automatic testing in the avionics test platform based on PXI bus. We apply virtual instrument technology to realize the automatic testing and the fault reporting of signal performance. Taking the avionics bus ARINC429 as an example, we introduce the architecture of automatic test system as well as the implementation of algorithms in Lab VIEW. The comprehensive experiments show the proposed method can effectively accomplish the automatic testing and fault reporting of signal performance. It greatly improves the generality and reliability of ATE in avionics systems.


INTRODUCTION
Along with the development of electronics and information technology, the avionics systems are becoming more and more complicated, leading to a much higher requirement of automatic testing for system reliability (Allen and McNeil, 1994;Craig, 2002;Ross, 2005;Dong and Li, 2009;Steadman et al., 2002).There are two major directions for the automatic test system, the generality and the openness.The primary goals include the decrease of maintenance cost during its lifespan, the extension of applications, the improvement of adaptability and the improvement of fault diagnosis accuracy (Malesich, 2008;Michael and David, 2003;Droste and Guilbeaux, 2009).
The performance of the avionic equipments is easily affected by environmental factors because the particularity of working environment.If the temperature is high, low or changing rapidly, these equipments might lose accuracy.The environmental factors also include water vapor, rain, snow, wind, sandy dust, sunlight, vibration and corrosion etc.Therefore, the signal performance such as the rise time or fall time of impulse signal should to be tested thoroughly so as to increase the reliability of avionic equipments (Truebenbach, 2005).
So far, the three-level maintenance of avionics is evolving toward the new one with only two levels (Kornreich, 1989;Mullen and Wchter, 1993).The new maintenance imposes new requirements on the testing in all stages.The two-level maintenance not only brings the revolution of avionic technology, but also determines the trends of future automatic test equipments.The three-level maintenance is composed of the Organizational level (O-level), the Intermediate level (I-level) and the Depot level (D-level).The testing in O-level mainly targets at the isolation of system faults to the Line Replaceable Unit (LRU).I-level needs to isolate the system faults of LRU to the Shop Replaceable Unit (SRU).D-level should be able to isolate the faults of SRU to the shop replaceable subunit (SSRU) that makes use of much more accurate and specialized test equipments.In the two-level maintenance, the components that cannot be repaired in O-level are delivered to D-level, while bypassing Ilevel (Beat, 2002;Hu and Ying, 2006).
According to the philosophy of two-level maintenance, the LRU of avionics will be gradually replaced by the Line Replaceable Module (LRM).The modularized architecture requires that the ATE is capable of bus testing and systematic simulation (Beat, 2002;Hu and Ying, 2006;Michael, 1993).The main function of ATE transforms naturally from the testing of entire equipment to that of a module.It should isolate the faults to the components inside LRM or even smaller units and perform the fault diagnosis accordingly.Meanwhile, the avionic systems are more and more complex nowadays.The simulation and excitation of complex signals are inevitable.The arbitrary wave generation also becomes an important function of ATE (Ziomek and Jones, 2009).
Under the three-level maintenance, the traditional automatic test equipments usually target at LRU.They are mainly used to test the entire equipment, instead of focusing on the testing of individual modules (Orlet and Murdock, 2002).Thus, it is very difficult to locate the faults into the components or even smaller units.Besides, the types of equipments that can be tested are limited.
Aiming at the insufficiency of ATE and the future trend of automatic test technology, we propose a generalized automatic test method for avionics modules, with application to the avionic test platform based on PXI bus.We utilize the virtual instrument technology to realize the automatic testing of signal performance in modules.Taking the avionics ARINC429 bus as an example, we build the automatic test system to test the signal performance of ARINC429 bus.This system has the merits of generality and reliability.It has reference significance in the future development of ATE of avionics systems under the two-level maintenance.

SYSTEM ARCHITECTURE
The general method of module automatic testing employs the virtual instrumental technology.It is applied in the avionics test platform based on PXI bus.The architecture of this method is shown in Fig. 1 which is composed of the embedded measurement and control computer module in the main testing box, the power control box, various testing resources, signal condition box and the interface of testing arrays.
The embedded measurement and control computer module is responsible of the management of testing resources.It provides the excitation signals for modules under test calling of testing resources by PXI bus; it collects, analyzes and records the output signals generated by module to test the performance of the signals.The testing resources mainly include the bus communication module, the digital oscilloscope module, the digital millimeter module, the arbitrary waveform generator module and the relay module.
The power control box is used to supply power for the modules and the signal condition box.It adopts relays to attach the output power to test array after isolation.The relay is controlled by PXI host computer.The voltage and the current of each channel of power are displayed by the digital watch.
The signal condition box is responsible for the condition of the original signals, such as the enlargement, the isolation, the filtering and the transformation.The digital signal test system, analog signal test system and all kinds of bus test systems can be built by replacing the bus communication module and signal condition box.The test system has the ability of automatic test, process and analysis of digital, analog and bus signals in the module under test.It has the generality of same type of signals in variant modules and no limitation to the type of module under test.It also improves the generality of ATE.
This system is concentrated on the performance testing of various signals in the modules.The gathering of signals and their analysis as well as simulation and excitation are the key technologies.In this study, we take the avionics bus ARINC429 as an example to construct a general automatic test system for ARINC429 bus signals.
Measure the items under following output conditions: 1~31 bits of the data word are respectively all 0, all 1, 0 alternating with 1 and 1alternating with 0 Send the same data circularly to stabilize the waveform 100 kbps±1% 12~14.5 kbps±1% Pulse rise time T R 1.5 μs±0.5 μs 10 μs±5μs Pulse fall time T F 1.5 μs±0.5 μs 10 μs±5μs Signal bit width T Y 10 μs±2.5% (1/R) μs±2.5% Signal pulse width T X 5 μs±5% TY/2±5% Requirements of performance test for arinc429 bus: The bus system is responsible for the information exchange and sharing of all sub-systems.Therefore, it is very important in the avionics systems.The reliability of the bus is the key factor to determine the stability of the whole system.ARINC429 is one of the mostly used buses in the avionics systems.ARINC429 Digital Information Transmission Specification (DITS) is figured out by ARINC Aeronautical Radio Inc composed of American avionics manufacturers, the Airline Company, the airplane companies and other nation's Airline Companies.It stipulates the requirements of digital information transmission of avionics and the related sub-systems.
ARINC429 bus adopts bipolar return-to-zero code, as shown in Fig. 2. The basic information unit is composed of a 32-bit data word, transmitted through a twisted shielded pair of wires.The transmission rate has two types, a high speed one of 100 kpbs and a low speed one of 12.5 kpbs.According to statistics of substantial avionics tests, we choose the following electrical characteristics to test.
• Signal amplitude: The difference between positive (negative) level and zero level V P (V N ), namely positive (negative) output amplitude.• Zero level range V Z : The level range of bus port with the state of bus waveform return to zero or no output data in the bus interface.The output electrical characteristics mainly include positive output amplitude, negative output amplitude and zero level range, the requirements is as shown in Table 1.Here, A and B refers to two terminals of   (Kuenzi and Ziomek, 2007).ARINC429 bus signal that satisfies the amplitude and rate requirements can be generated by the arbitrary waveform generator and waveform editing software to realize signal imitation and stimulation of Unit Under Test (UUT).The signals are analyzed and processed to realize the performance test using Lab VIEW.The test platform of ARINC429 signal is shown in Fig. 3.
The test resources of ARINC429 bus module signal test system include ARINC429 bus communication module, digital oscilloscope module, digital multimeter module, arbitrary waveform generator module and relay module.PXI system controller uses the dual-core processor Intel Core2 Duo T7400, the dominant frequency is 2.16 GHz and the memory is 512MB.
The digital oscilloscope module with 8-bit resolution offers an input voltage range from ±0.8V~±30V, the sampling rate is 250 MS/s.The resolution of digital multimeter module is 7.5 bits with 20000 reading samples per second.The arbitrary waveform generator with 16-bit resolution offers an input voltage range from 0.564V~12V, the sampling rate is 100 MS/s, the band width is 43MHz and the buffer is 32MB.

IMPLEMENTATION OF ALGORITHM
The software is developed in Lab VIEW, the visualized virtual instrument integrated development environment of NI.Lab VIEW can integrate the development, analysis, visualization, simulation of many algorithms.It can also perform advanced signal processing, generator of applied programs, Signal Express software package, report generator for Microsoft Office and other functions.It is able to satisfy many performance requirements in the signal testing.Lab VIEW programs are called Virtual Instrument (VI).When a VI is run, values from controls flow through the block diagram, where they are used in the functions on the diagram and the results are passed into other functions.
The entire testing software is composed of seven modules, whose structure is shown in Fig. 4. The key techniques in the performance testing of bus signals include: 1. Signal gathering of ARINC429 bus by using the digital oscilloscope module 2. The testing of output characteristics 3. Testing of input characteristics 4. Analysis and display of test data In 2), we test the signal amplitude, pulse rise time, pulse fall time, signal bit rate, signal bit width and signal pulse width etc.In 3), ARINC429 bus signal that satisfies the amplitude and rate requirements are generated by the arbitrary waveform generator along with waveform editing software.We use these signals to realize the signal imitation and stimulation of UUT, so that the input characteristics can be tested.
Signal acquisition: ARINC429 bus signal is classified as differential signal.Positive and negative signals are acquired by two channels of digital oscilloscope respectively.We obtain the differential signal from the positive and negative signals.The sampling rate of oscilloscope is set based on the incidence rate of bus signal.The vertical parameter, horizontal timing and trigger edge should be configured before signal acquisition.The positive and negative signals contain the time and scale information of waveforms.The time interval of two samples in the waveform acquired can be separated from positive and negative signals.The waveform signal Y can also be obtained from positive and negative signals.Time and waveform information yield the ARINC429 bus signal.

Output characteristics test:
The key characteristics of ARINC429 bus signal include signal amplitude, zero level range, pulse rise time, pulse fall time, signal bit width, pulse width and bit rate.The signal processing VI in Lab VIEW performs the function of signal generating, digital filter and data windowing and spectrum analysis.Here, waveform measurement VI performs the function of time domain and frequency domain measurements, such as direct current, root mean square, frequency/amplitude/phase of single frequency, harmonic distortion, signal to noise and distortion ratio and average FFT measurement.The application VIs for the system has been developed using Lab VIEW-based graphical development environment to test the output characteristics and time characteristic of waveform.

Signal amplitude measurement:
In our sub-VIs, the waveform signal is taken as the input signal.We define the measurement method of high level and low level of waveform in state setting.We choose "Peak" method to search maximum and minimum heights in the whole waveform and to measure amplitudes.We take the positive, negative and differential signal as the input signal of sub-VI to measure positive level amplitude, negative level amplitude as well as line-to-line amplitude and to output the corresponding amplitudes.
Pulse rise/fall time measurement: In our sub-VIs, we take the signal waveform under test as the input signal and set the polarity as a rising edge or a trailing edge.The rising or falling time can be obtained from the transient duration.ARINC429 is a differential signal that adopts bipolar return-to-zero code, which is different from the single end signal, e.g., RS232.The reference level must be set in the very beginning to configure the high, low and intermediate reference levels of transient interval.According to the characteristics of ARINC429 bus signal waveform, 90, 10 and 50% of the high level amplitude are chosen as the high, low and intermediate reference levels, respectively.
Signal bit rate measurement: ARINC429 bus signal is not a pure impulsive signal.It actually adopts bipolar return-to-zero code.In the actual application, "zero" always drifts, so that we increase the amplitude of differential signal properly.A set of zero-crossing points' X coordinates of positive and negative signals are obtained, as shown in formula 1 and 2: ( ) The maximum X coordinates of positive signal The principle of measuring signal bit width is similar to measuring bit rate, where the details are omitted in this study.
Signal pulse width measurement: Figure 5 shows the diagram (code) of the pulse width measurement VI.We take the acquired positive or negative signal as input signal.The signal pulse width can be obtained from the pulse duration.Also, ARINC429 bus signal is a differential signal that adopts bipolar return-to-zero code.The high, low and intermediate reference levels are chosen to be 90, 10 and 50%, respectively of the high level amplitude.

Input characteristics test:
Traditional ATE does not have the function of signal emulation.Thus, it is not able to produce complicated waveforms to carry out input characteristics test.To perform performance tests, we should generate more complicated waveforms so that arbitrary waveform generating is inevitable.
One important item of input characteristics test is to measure signal amplitude range.Generic ARINC429 communication module can only generate normal bus signal, while it cannot change output signal amplitude to test input characteristics.We carry out the simulation of signal waveform by using the arbitrary waveform generator.We develop a binary file format of ordinary ARINC429 signal that can be recognized by the arbitrary waveform generator.We then change the amplitude range and generate the waveform by the arbitrary waveform generator and finally output the ARINC429 bus signal with changeable amplitude in the  6.The high level range of output signal is +6.5~+13V, the zero level range is 2.5~-2.5Vand the low level range is -6.5~-13V.
Data analysis and display: Lab VIEW contains a comprehensive set of tools for data analysis.It can analyze the gathered and measured data using various mathematical algorithms, including curve fitting and data model building, differential equation, internal interpolation and external interpolation, linear algebra, nonlinear system, optimization, rooting, mathematical statistics and random processing.Meanwhile, it affords multifarious report generating tool kits that acquire result and information from the data and generate test reports subsequently.With the help of tool kits in Lab VIEW, we analyze the data via a series of algorithms.The test report is generated and displayed on the front panel.

EXPERIMENT RESULT
We test various modules using signal test system of ARINC429 bus.The test report generated by data analysis and display modules gives the maximum, minimum and average values of 1000 test results.The test interface and the test data report are shown in Fig. 7 and 8.The left side of interface is the display window

•
Pulse rise time T R : The time interval between the 10 and 90% voltage amplitude on the leading edges of the pulse.• Pulse fall time T F : The time interval between the 10 and 90% voltage amplitude on the trailing edges of the pulse.• Signal bit width T Y : The time width of logic "1" and logic "0" of signal waveform.• Signal pulse width T X : The signal pulse width excluding signal trailing.• Bit rate R: Bit transmission rate of the bus.

Fig. 3 :
Fig. 3: Signal test platform of ARINC429 bus, (a) Schematic diagram of test platform, (b) Picture of test platform

Fig. 4 :
Fig. 4: Software structure diagram of module automatic test system

Fig. 7 :
Fig. 7: Test report of ARINC429 bus signal with high speed The signals generated by the tested resources are sent to the interface of test array via the signal condition box and are matched with the signals of the modules under test.It sends multi-channel signals to the oscilloscope and millimeter via matrix switch.The test array interface supplies mechatronics of test unit and adapter, including test slot and signals connected to module under test and electric connection matched with test, control and power interface of test unit.Module under test is connected to the test platform via test adapter; test platform tests the performance of the same types of signals.It can connect to variant modules and test performance of multiple signals by replacing variant test adapters.

Table 1 :
We target at the bus signals under test, gather the signals, analyze the input/output characteristics.The testing results are also presented in this study.Test items and requirements of output electrical characteristics Requirements Fig. 1: Structure diagram of module automatic test system

Table 2 :
Test Items and requirements of output time characteristics Requirements

Table 2 .
It cannot gather bus signal to test output electrical characteristics and time characteristics.It cannot change the level range dynamic to test input characteristics either.To satisfy the need of performance test, digital oscilloscope and arbitrary waveform generator of National Instruments (NI) are chosen.Digital oscilloscope is a very important tool of test and diagnoses in ATE.It gathers and analyzes ARINC429 bus signal sent by module under test