Dave Scheibenhoffer - G Systems
Michael Fortenberry - G Systems
Dramatically Reducing Test Cycle Time
At NI Partner G Systems Inc., we upgraded VME-based equipment with a more robust, compact, and reliable data acquisition and real-time control system in less than four months. Lockheed Martin engineers now can configure their PXI-based system 10 times faster than their previous VME equipment while doubling their channel count. Also, the Portable Digital Data Acquisition System (PDDAS) reduces test cycle time from 2 sec to 50 ms all for less than it would have cost to upgrade only a portion of the VME-based test system.
Improvements to jet engine designs require precise characterization of engine operating parameters. To accomplish this, design engineers examine the jet engine inlet air pressure profile to observe the air flow pattern distortions across the engine turbine inlet. To acquire this data requires an accurately scaled model of the aircraft and a transonic wind tunnel to simulate actual operating conditions.
Engineers at Lockheed Martin use scale models of equipment in the testing of the F-35 Joint Strike Fighter. Because their previous VME-based test system proved too difficult to configure and upgrade, engineers at Lockheed Martin commissioned the new PDDAS system that we developed to control and acquire data fro their wind tunnel tests.
The PDDAS includes 128 channels of simultaneously sampled dynamic pressure measurements based on two PXI chassis, equipped with a total of 16 NI PXI-4472 Dynamic Signal Acquisition modules. At first glance, two chassis may seem to unnecessarily complicate the system; however, by linking the two chassis using the NI MXI fiber-optic extension, no additional complexity was introduced. MXIbus basically performs as a PCI bridge to the second chassis. From a software point of view, the boards in the second PXI chassis appear as though they reside in the first chassis. Using PXI also provides high enough data transfer rates (132 Mbytes/sec) at a competitive price for future expansion.
Also included in the PDDAS is the NI SCXI-1520, which connects to a full-bridge Kulite pressure transducer for strain gauge signal conditioning. With LabVIEW Real-Time, we can achieve the deterministic response time needed to both acquire air pressure data and to provide control signals back to the wind tunnel to vary operating conditions.
With The PXI architecture, we can handle the large data sets acquired in the PDDAS system, and LabVIEW Real-Time handles what is known as the “buzz” calculation (approximately 450,000 floating point calculations per 50 ms), that looks for a resonance condition in the engine inlet. At certain operating parameters, the air to the inlets may be in phase. If allowed to reach full resonance, the resulting forces can damage the engine. To prevent this, the PDDAS system constantly monitors for approaching buzz conditions and provides feedback to the wind tunnel control system to change tunnel operating parameters as required.
With the large data volume and computationally intensive calculations, the embedded PXI-8176 Pentium controller does not have enough remaining bandwidth to store all the acquired data to disk for permanent storage. To resolve this challenge, we used a VMIC reflective memory card in the PXI chassis. NI provided a LabVIEW Real-Time driver to support the setup and configuration of the reflective memory board. With this solution, we can “reflect” the acquired data to a Pentium host workstation running Windows 2000. This workstation uses an off-the-shelf Fibre Channel board and driver to write the data to a Redundant Array of Independent Disks (RAID) as a secondary task. Reflective memory proves a simple, yet elegant solution to resolve a potential system bottleneck.
Because the PDDAS system is used at a number of wind tunnels throughout the country, Lockheed Martin engineers need a ubiquitous mechanism to provide real-time feedback to the individual wind tunnel control systems. Therefore, we used User Datagram Protocol (UDP) for this application. Though UDP is not a deterministic protocol, it is a readily available interface at each wind tunnel facility. By carefully selecting the LabVIEW task priorities, the PDDAS can send UDP packets at a deterministic rate of 50 ms.
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