Characterizing Sound Profiles for a New Airbus Aircraft Using NI PXI

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"We chose a National Instruments system because we had good experiences in the past with NI, and the NI modules had the capabilities we needed."

- Johan de Goede, National Aerospace Laboratory (NLR)

The Challenge:
Acquiring data at high sample rates from more than 200 microphones and pressure sensors that are distributed in a large wind tunnel while keeping the signals synchronized within 1 µs.

The Solution:
Using the PXI and PXI Express synchronization bus and the external synchronization possibilities to demonstrate close synchronization of the signals, even if their cabling lengths differ by 100 m.

Johan de Goede - National Aerospace Laboratory (NLR)
Rob Zwemmer - National Aerospace Laboratory (NLR)

The NLR is an independent, nonprofit research institute based in the Netherlands that conducts contract research for national and international customers. At the NLR, the department of avionics technology helps install instrumentation in wind tunnel test models for commercial or military aircraft and space vehicles. We develop and install instrumentation systems to conduct avionics temperature and vibration testing as well as electromagnetic compatibility (EMC) testing. We also develop and install instrumentation systems on aircraft models that will be tested in the DNW large low-speed wind tunnel (German-Dutch Wind Tunnels).

Pressure and strain are important parameters to measure, but acoustic measurements are also needed. For acoustic measurements, data from a large number of sensors typically has to be acquired. For this reason, the inside of the wind tunnel is equipped with microphones, but in some cases, the number is not adequate when more channels are needed; therefore, additional sensors or an additional measurement system is developed for some tests.

For the last two years, we have been working on a project for Airbus that involves a new and innovative type of aircraft, with open-rotor propulsion, as part of the European Clean Sky program. Airbus built a scale model for the wind tunnel and asked us to develop an instrumentation system for the model. The tests on the model will be performed in the DNW-LLF wind tunnel facility and in other wind tunnels in Europe. The new experimental model has two contra rotating open rotor engines at the rear of the aircraft <source:>. For this system, measuring and reducing the noise of the propellers is very important. The model is surrounded by microphones placed at various distances. The model itself is equipped with microphones, pressure transducers, and other sensors for making acoustic measurements and profiling the pressure distribution across the model and the wings.

Specific needs to characterize detailed sound profiles

The wind tunnel already had a system with microphones installed, but these were designed to measure the average noise and sound pressure of the model. Instead, we needed to measure the sound pressures in relation to the angular positions of the rotating propellers and their pitches. We needed a system that was capable of synchronizing all signals within 1 µs (10E-6). Also, the sound pressure needed to be measured at many different locations to create a map or profile of the sound distribution. Inside the model, almost 150 microphones and pressure transducers are installed. These are connected to an NI PXI Express system located in the fuselage of the model. This system is called the Model Dynamic Measurement System.

Additionally, we installed microphones outside the model. There are 48 inflow microphones that are installed on a traversing fixture, to create a 2D sound profile plot from nearby.

Next, we needed to acquire pressure, force and temperature data measured in the engine and rotating parts. These acquisition devices were not available on the market because they needed to operate in harsh conditions and limited space, so we developed them in-house. NLR developed an advanced rotating telemetry system using wireless power and data transfer that fits inside the propeller rotors. Four of these systems are installed in the engines. The data from these sensors is also integrated in the measurement system.

We chose a National Instruments system because we had good experiences in the past with NI, and the NI modules had the capabilities we needed. For our system, we needed to acquire the data at up to 200 kS/s and capture all harmonics that may be present in the signals. Another major requirement was that all captured signals were synchronized to 1 µs to relate the signals to the position of the propellers and the position of the airplane. NI provided off-the-shelf solutions to synchronize DAQ modules and PXI racks together.

The Hardware

For the Model Dynamic Measurement System, we installed an 18-slot NI PXIe-1075 chassis inside the fuselage of the aircraft model. To this chassis, 144 microphones and pressure sensors were attached using two NI PXIe-4498 16-channel high-accuracy DAQ modules and 14 NI PXIe-4331 8-channel bridge input modules.

Outside the model, we used an 8-slot NI PXI-1042 chassis with 24-bit NI PXI-4498 DSA modules for the inflow signals. Additionally, we had a DAQ module for measuring the position of the traversing fixture. This chassis acted as the master timebase for all acquisition modules. We used an NI PXI-6652 system timing and control module and NI PXIe-6672 system timing controller module for PXI Express together with an NI PXI-7951R NI FlexRIO module to synchronize both chassis and the NLR telemetry systems. We conducted tests to validate and demonstrate the synchronization over both chassis.

All data and synchronization connections are fiber-optic lines to prevent EMC problems. A data storage computer stores the test runs. A command and display computer supervises the system, which we use to view results. The synchronization with the telemetry systems in the engines is done with an FPGA module that is linked to the synchronization timebase. Through the FPGA, we can shift the phase of these signals to compensate for delays over long fiber-optic cables.

A major requirement for this project was that all components in the measurement system be tightly synchronized. This included the two PXI systems and the custom-built acquisition systems. To synchronize two PXI systems, the NI synchronization and timing modules were a perfect solution; however, the external custom acquisition system also needs to be in sync with the PXI hardware. Therefore, we used the NI PXI-7951R FPGA module for NI FlexRIO. The advantage of this module was that we could generate multiple clock signals, based off the timing and synchronization module, and introduce minor phase delays to compensate for lead wires (sometimes in excess of 100 m) and delays introduced by the sigma-delta analog-to-digital converters.

The Software

The LabVIEW software on both PXI controllers launches when the system is booted, and it tries to connect with the control/display and storage computers. Once a connection is established, measurements are taken and data is sent to the computers over TCP/IP. On the control and display computer, a user can check the data and sensors, and see whether overloading or clipping occurs. From this system, a sync command can be sent to both the PXI controllers, which then synchronize the clocks and acquisition. Once synchronization is established, the measurements can start. In general, a measurement’s time is limited (up to four minutes). During the actual measurement, data is sent in a binary format over TCP/IP at a rate of almost 50 MB/s to a data storage computer for offline analysis. The software also monitors the temperature of each of the modules to ensure that the electronics do not overheat, especially in the fuselage and engines of the model, where the temperature might rise due to restricted ventilation.

Our Experiences With the System

A typical test can take up to four minutes, and a typical test day can generate more than a terabyte of data. All data has to be streamed as it is acquired through fiber-optic Ethernet to the data server. A part of the processing, such as limited fast Fourier transforms (FFTs) and force calculations, is done immediately to check whether the test was completed successfully, and a more detailed analysis is completed offline.

We have used the system for more than a year, during which it performed as we expected, and we have upgraded only the software. To increase the reliability, we mount the PXI system in the fuselage on vibration dampers, and we monitor the temperature to ensure that the system does not overheat.

By using off-the-shelf NI hardware with software developed by NI Alliance Partner KW Automation, we completed a measurement solution in a short time. After completing the test at the DNW wind tunnel, the system will travel with the model to other wind tunnel facilities.


Author Information:
Johan de Goede
National Aerospace Laboratory (NLR)

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