Up, Up, and Away With Data Acquisition

Aeronautical data speed and volume challenge most acquisition systems. Maybe you need a custom solution.

This year’s sales-warrior award goes to Joe, who successfully met all 20 of his Atlanta Hartsfield connecting flights during two consecutive weeks of business travel. (Applause)

You may not have Joe’s hectic schedule, but anyone who travels by air knows that the real award winners are the airlines. In addition to offering speed and convenience, from a safety viewpoint, you are better off in a commercially operated plane than most other types of vehicles (Figure 1).1

The actual fatality rate for passenger cars per billion miles was similar to that for light trucks and U.S. air carriers in the year 2000—about 12 fatalities per billion miles. Only buses and large trucks came out significantly better.

However, there were 2,052,000 injuries to car passengers vs. 27 for plane occupants, a ratio of 70,000:1 although the total miles traveled by car are only 200 times greater than by air. The much higher incidence of injury in car travel clearly emphasizes the relative safety of flying.

It’s not just a lucky coincidence that airplanes are so reliable. On the contrary, in addition to rigorous maintenance and pilot training, continual design refinement aided by thorough testing plays a major role.

Providing accurate test results in a meaningful form is the job of data acquisition systems. A few examples illustrate the wide range of aeronautical applications served.

Jet-Engine Intake Modeling

A critical part in the development of a new military aircraft is the design of the jet-engine air intake. Ideally, jet engines should be presented with air at a constant pressure across the entire inlet area.

An array of 40 to 80 sensors positioned in the inlet area of the wind tunnel provides pressure data for later analysis. Evenly distributed flow is the goal, even though air is entering the engine from many different directions and some asymmetry is unavoidable. On the other hand, turbulence leading to resonance can destroy the airplane. After numerous computer simulations, it’s the turn of wind-tunnel testing to determine the actual performance of a scale model.

At an overall cost of thousands of dollars per minute, you want to make sure the tests run smoothly and the data is reliable. Equally important, you certainly don’t want to wreck the only scale model, nor can any damage to the wind tunnel be allowed to occur.

Rather than modify an earlier 64-channel custom-engineered data acquisition system—actually a jet-engine inlet flow distortion assessment system—Lockheed Martin Aeronautics, the aircraft developer, chose to buy a new one from G Systems. Using off-the-shelf PXI components and National Instruments’ LabVIEW and LabVIEW Real-Time software, G Systems produced the required 80-channel system in less than four months. The cost of the new, more capable system was comparable to the estimated cost of adding the required 16 additional channels to the old one.

In addition to synchronously sampling each of the channels at 20 kS/s with 24-b resolution, the system also calculates a total pressure value 20 times/s (approximately 500,000 floating-point operations every 50 ms). This is a critical value that is fed back to the wind-tunnel controller. Without the real-time feedback signal, it would be impossible to detect the critical resonant conditions for higher resolution testing and analysis.

Storing all the data in real time and deriving measurements from it take more bandwidth than the Pentium-based system has available. Rather than opt for a custom solution, G Systems fell back on the company’s experience and designed in a reflective memory network.

The test data is acquired and analyzed locally, but simultaneously it is written via a VMIC PCI reflective memory card to a remote server with a reduntant array of independent disks (RAID) storage. As the VMIC website stated, “Reflective memory is a ring-based, high-speed, replicated shared-memory network. It allows multiple computer systems of different bus structures and different operating systems to share real-time data at a high speed and at a deterministic rate.” Writing to the reflective memory causes the local data on all the nodes to be automatically updated. Data is transferred at up to 140 MB/s over fiber-optic cables.

Reflective memory solves another very practical problem: the lack of I/O driver support from LabVIEW Real-Time to RAID storage. Dave Scheibenhoffer, director of sales and marketing at G Systems, said, “The lack of drivers for an embedded solution was avoided by reflecting the acquired data to a Windows XP PC. A choice of drivers then was available to support RAID storage. With the reflected-memory approach, hardware takes care of interrupts and DMA control, and it’s protocol independent.”

High-Altitude Jamming

NAVSYS designs and manufactures a range of aircraft navigation systems. A recent project involved determining the susceptibility of an electronically steerable 16-element global positioning system (GPS) antenna array to ground-based jamming signals.

“Flight testing with GPS jammers is not a trivial matter,” said Alison Brown, president and CEO of NAVSYS. “Not only is authorization required from the FAA to avoid any potential for interference with commercial aviation, but the range time is costly, and instrumentation is required to precisely measure the actual received jammer levels at an aircraft or other test platform.

“It’s one thing to have a single omnidirectional antenna element and a simple receiver. But it’s quite another to have 16 steerable elements being moved around in space with signal processing trying to adjust on-the-fly to changes in attitude and heading,” she continued, “not to mention the effects of jammer signal reflections from the aircraft body. To optimize the GPS receiver anti-jam performance in this complex environment, we needed a source of repeatable, real signals.”

NAVSYS built a prototype system in the form of a standard-size pallet that could fly on an Air Force C12J aircraft. The actual testing cost about $80,000 for a two-day flight, not including pre-flight confirmation of the pallet’s flight safety and decompression behavior.

A customized GPS data recorder, based on the Conduant StreamStor™ recorder, was built to record the complete GPS sampled spectrum from each element of two antenna arrays installed on the receiver. The raw GPS samples were recorded onto hard disks to store the 1.2 TB of data generated during the flight.

The digital data storage system uses special-purpose GPS electronics to sample the GPS signals from the multiple antenna elements and format the data into a 200-MB/s data stream. The data is recorded across eight 160-GB hard disks running at 7,200 rpm.

Data recording is controlled by NAVSYS-written Visual C++ drivers that allow recording start and stop as well as periodic data commits. Commits are important because they ensure that a power hiccup, for example near the end of a four-hour flight, can’t cause the loss of all the previously recorded data.

In addition to recording the raw GPS data from the NAVSYS digital storage receiver (DSR) pallet, data also was recorded by the Air Force Test Squadron using its communication high-accuracy location system (CHALS) test equipment. CHALS provides a precise record of where the aircraft is as well as its attitude and heading in three dimensions. By recording all the information, engineers can play back the expensive flight test signals again and again to perfect the GPS system design.

21st Century Test Stations

Traditionally, avionics manufacturing test stations have been defined by the types of instruments required to test specific components. An example is a pneumatic-valve test station that might include several pressure gauges, valves, and a means of controlling a compressed air supply. In a recent project for Honeywell Aerospace, Averna Technologies took a fresh look at test-station configuration.

According to Steeve Allard, Averna’s technical leader for test, measurement, and automation, “Our engineering approach was to address the similarities among test-station requirements first and exceptions last. What resulted was a philosophy of interchangeable test stations with common core hardware and software, augmented as required by application-specific signal conditioning.”

Figure 2 illustrates the multilayer solution the company developed. The Proligent maintenance platform holds a local copy of all the specifications and test procedures relevant to the test being performed. This software application, which also is available as a separate product from the company, guides the technician through the tests and automatically produces a report upon completion.

Layers 2 and 3 comprise the National Instruments-based data acquisition system. Layer 4 provides a graphical user interface (GUI) customized for the specific type of component being tested. Layer 1 represents the special signal-conditioning hardware required for each type of component.

The test stations are linked by Ethernet to a central server with a database that maintains a library of test specifications and records of test results. The Proligent maintenance platform communicates with this server and presents an operator with up-to-date test limits and correct prompts leading from one test to another.

Compared to the previous entirely manual method, the new test stations show a throughput improvement of up to 6×, depending on the type of component being tested. Equally important, the centralized database ensures that testing is performed uniformly and to the latest revision of the specification. Using manual test methods, uniformity was especially difficult to ensure given the very large number of component types, relatively few well-trained operators, and the volume of printed test instructions that had to be maintained.

Reliable Missile Re-Entry

Although most of us don’t have cause to think much about this country’s intercontinental ballistic missiles (ICBMs), should these weapons ever be used, they must survive re-entry to work correctly. The acquisition and storage of test data relating to re-entry vehicle vibration were key aspects of the application addressed by a recent B & B Technologies’ project.

A total of 160 channels were required, each with a top sampling rate of 102.4 kS/s and the capability to store 120 minutes of data. The signal-conditioning and acquisition part of the job was solved by using 20 National Instruments PXI-4472 dynamic signal acquisition (DSA) modules. Each DSA module provides eight simultaneously sampled input channels with 24-b resolution, a DC to 45-kHz bandwidth, and the required sampling rate. Synchronization of the modules is via an NI PXI-6653 timing module and the PXI star trigger lines.

After much testing, it was determined that two 1.5-GHz, dual-processor workstations were required to accommodate the 50-MB/s data rate that might be generated by the DSA modules. Ten of the DSA cards and a PXI-6653 timing module are housed in each of two 18-slot chassis. A PXI-MXI-3 interface module in each chassis transfers data to the associated workstation.

A number of operational possibilities are provided by this architecture. For tests involving 80 or fewer channels, each workstation and its associated chassis can be used independently. Or, a single workstation can communicate with both chassis, storing data for up to four hours, but at a maximum scan rate of 40 MS/s. Finally, both workstations and chassis can be synchronized to perform as a seamless 160-channel system with the full 102.4 kS/s sampling rate. Data storage is provided by three 73.5-GB SCSI hard drives attached to each workstation.

Summary

Standard data acquisition products available from a wide range of manufacturers often are well suited to aeronautical testing. However, as shown in the examples discussed, sometimes a custom solution is required. The most satisfactory approach in this case may be to contact a systems integrator who already has experience related to your type of application.

Reference

  1. www.bts.gov/publications/nts/html/table_02_22.html

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Published by EE-Evaluation Engineering
All contents © 2003 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

January 2003

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