High-precision multichannel PXIe systems realize more efficient wind-tunnel testingStory
August 11, 2014
Simulation tests of aerodynamics are conducted to improve the physical designs of a variety of products so as to enhance overall safety, comfort, and performance. The larger the object being tested, the more complicated the test will be. Test and measurement solutions based on PXI Express (PXIe) architecture can help to simplify and speed up system installation.
Military aircraft are commonly put through wind-tunnel testing to analyze hardware and software response in combat or other mission-related simulations. In the initial stage of a wind-tunnel test, a scale model in miniature is often used for simulation in a small laboratory, with design improvements continually made based on data acquired from the tests. However, the airflow volume measured in a small wind-tunnel laboratory is not enough to determine more subtle changes in airflow behaviors; real-world tests, at scale, in a large wind tunnel are still required at the final testing stage.
For instance, testing a true scale model of an aircraft requires a large number of sensors mounted across the length of the tested object in order to acquire data related to critical components such as airfoils and engine, or to determine the disturbance of high-speed airflow caused by various airfoil shapes. The increased number of sensors and extended test scale add to the difficulty of achieving accurate test and measurement.
In the past, traditional test and measurement solutions used data loggers to acquire data from sensors. Different kinds of sensors require different data loggers, however, which increases the difficulty in achieving integrated data analysis and developing a sensing network. PXI modular instrumentation platforms based on industry-standard computer buses have become an alternate option for customers looking for more flexibility and efficiency in their testing scenarios without incurring additional cost. A highly efficient, precision test and measurement solution based on PXI Express (PXIe) architecture – with multiple channels and fully integrated synchronization – can connect with sensors for measuring different physical quantities and can help to simplify and speed up system installation.
Challenges in wind-tunnel testing
Complicated relationship between sensors and data loggers
Testing a large object to actual scale requires different types of sensors for measuring correlations among surface pressure, airflow velocity, and vibration acceleration. Due to the difficulty in integrating sensors of different types and brands, various data loggers are used to continually acquire and record data from the sensors according to the system time set separately in each data logger. Users are required to install electrical wiring for each data logger and additional signal wiring for drivers of each sensor, adding steps that can be complex and time consuming.
Difficult data integration due to nonsynchronized triggers
Simultaneously triggering data loggers in the field is also challenging. A traditional solution is to connect the data loggers via the bus interface designated for synchronization and provide an external clock reference. However, this setup complicates signal wiring; moreover, loggers can be safely retrieved only when the testing is over and data can be gathered and analyzed in a data center only during periods when the system is operating and loggers are scheduled to log data.
Lack of unified system clock causes drifts in data precision
Another testing problem is maintaining accuracy and precision in measurement. In a conventional test and measurement system, each data logger works on its own system clock and acquires data regularly. However, users may find that the clock settings of an individual data logger has nuances, which result in minor drifts in data precision and then accumulate into greater inaccuracy over a longer measurement time. Theoretically, aerodynamic experiments require data capture of material changes under surface pressure, airflow velocity, and vibration acceleration during exactly the same period of time; nonsynchronized data recording would forfeit their reference value for analysis.
High-efficiency, multichannel PXIe systems meet requirements for accurate wind-tunnel testing
PXIe is a modular electronic instrumentation platform designed to meet all the previously mentioned challenges and provide a more efficient solution for building test and measurement equipment. ADLINK is just one of several vendors to provide high quality PXIe equipment for test and measurement and is a sponsor member of the PXI Systems Alliance (PXISA), an organization dedicated to progressing PXI specifications and supporting interoperability testing to enable effective multivendor solutions.
Supports cross-sensor data acquisition
Today’s PXIe dynamic signal acquisition (DSA) modules can provide multiple 24-bit analog input channels of tight synchronization in a single module. All channels have independent signal amplifiers and analog/digital converters to allow for simultaneous data transfer between channels, enabling these modules to support data acquisition from various sensors. Modules can also include a high resolution and high dynamic range suitable for recording physical signals that require quantified measurement from low frequency to high frequency, such as vibration, pressure, and fluid volume.
Simplifies electrical wiring
PXIe platforms consider support for standards such as IEPE (Integral Electronic Piezoelectric), which can apply – for instance – in support for IEPE-compliant sensing devices via analog input (AI) channels. ADLINK’s PXIe-9529 DSA module’s front-end circuit was designed to provide power to activate connected IEPE sensors; therefore, users can employ interfaces to directly connect sensing devices without the need for additional wiring. This structure helps to greatly reduce wiring complexity in test/measurement instrumentation (see Figure 1).
Figure 1: PXIe-9529 dynamic signal acquisition (DSA) module x17 provides flexible expansion to as many as 136 channels for data acquisition in a single chassis.
(Click graphic to zoom by 1.9x)
Reduce measurement inaccuracy with AC/DC coupling
AI channels in PXIe DSA modules are equipped with AC/DC coupling. Because sensors provided by different manufacturers vary in their lengths of wiring, which may produce input/output impedance mismatch with the connected module and cause drifts in signal leveling, the use of AC coupling can help to reject DC signals resulting from impedance mismatches and eliminate measurement inaccuracy.
Synchronization enabled by PXI Trigger structure
PXI and PXI Express chassis can provide a PXI Trigger bus on the backplane to allow users to program the deployment of master/slave modules. When the master module receives a trigger signal for data acquisition, it will immediately pass the signal to all the slave modules in the chassis via the built-in trigger lines. All wires for synchronization are already built into the system, so users do not have to deploy additional wiring or prepare for an external trigger signal source.
Unified system clocking
Again, a PXIe chassis includes a system clock to provide reference timing for PXIe modules with phase-locked loop (PLL). This system-timing signal is precisely transferred to all PXIe module slot pins to serve as an external reference clock for certain data acquisition (DAQ) boards or high-speed digitizers designed with PLL. The PLL built into DSA modules can switch time reference from an internal clock to the system clock provided by the chassis, which allows test and measurement system designers to lock the timing of all the boards and modules to the 10 MHz reference clock provided by the chassis. In this way, all boards and modules in the system can acquire accurate, synchronized data in unified periods (see Figure 2).
Figure 2: The PLL built into DSA modules can switch time reference to the system clock provided by the chassis, which enables test and measurement system designers to lock the timing of all the boards and modules to the 10 MHz reference clock to provide synchronized data in unified periods.
(Click graphic to zoom by 1.9x)
Real-time data processing
A PC-based test and measurement system inherits the high-speed and high-efficiency data transfer performance of PCIe architectures, enabling real-time data transmission to the control center during the course of data acquisition and simultaneous data storage to storage media, or even data transfers and updates among databases of different testing stations.
To implement precision wind-tunnel test and measurement on a true scale), large objects require the deployment of high-density measurement points and a variety of sensors for measuring different physical quantities. More simplified wiring, more computing power provided by high performance architecture – in this example, x86 architecture – and excellent network connectivity are key factors to reduce the time needed for system installation and data analysis. PXIe systems are aimed at fulfilling these requirements, so as to ensure the accuracy of data acquired from different sensors and provide precise test and measurement results over an extended length of time.
Andre Hsieh is the Technical Leader, Measurement and Automation Product Segment, at ADLINK Technology. Andre has more than 11 years of experience in the technology industry and is currently responsible for the technical planning and support of ADLINK’s measurement products. He has a Master’s degree in mechanical engineering from National Taiwan University of Science & Technology.
ADLINK Technology 408-360-0200 www.adlinktech.com