Using NI PXI-Controlled SCXI Strain Gauge and Accelerometer Modules in Complex Aerospace Testing

Author(s):
James L. Slemp - Radical Systems, Inc.

Industry:
Aerospace/Avionics

Products:
LabVIEW

The Challenge:
Providing a cost-effective large-scale, high-speed data collection solution for complex data sets including acoustic, strain, pressure, vibration, temperature, and heat flux inputs with "near real-time" remote viewing and data reduction.

The Solution:
Using National Instruments LabVIEW, PXI, and SCXI technologies to achieve a tightly coupled, large-scale system that is both expandable and compact.

System Specifications
The Boeing Company, who is developing the ground-based missile defense (GMD) system, required an advanced data acquisition system that could acquire and analyze simultaneous high-speed (20 kS/s) and medium-speed (5 kS/s) inputs and display them in “near real-time” to a remote computer in the launch control bunker. As a result, we, at Radical Systems, Inc., used PXI and SCXI products in a parallel data acquisition system to maintain the data throughput and required precision. The system was controlled via an embedded NI PXI-8176 controller running Windows 2000. Using good programming practices and an advanced data acquisition software architecture, we were able to exceed all of the customer requirements. We remotely accessed the embedded PXI controller throughout the test via an Ethernet interface both controlling and observing the data prior to, during, and after test completion.

The system was required to provide the following features:

• 24 high-speed (20 kS/s) input channels for strain gauges and/or bridge-type transducers with simultaneous sample-and-hold capabilities
• 16 high-speed (20 kS/s) input channels for accelerometer and/or acoustic transducers with simultaneous sample-and-hold capabilities
• 48 isolated digital inputs and outputs for discrete I/O control and synchronization
• 128 medium-speed (5 kS/s) input channels for isolated analog inputs including thermocouples, bridge transducers, pressure transducers, heat-flux, and various other transducers
• 16 user-definable custom calculations based upon combinations of the acquisition channels
• Embedded IRIG time data collection and data synchronization
• Near real-time analog graphs and tabular displays for all channels
• Postprocessing analysis that includes time-based analysis, statistical analysis, spectrum analysis, octave analysis, cross-correlation, autocorrelation, cross-power, convolution, deconvolution, ratio analysis, delta analysis, frequency analysis, power analysis, harmonic analysis, and the ability to design and build a variety of digital filters to be applied to the collected data
• User-definable and recallable test setups
• User login and access control
• Scalable and redundant data logging
• User-definable channel scaling and calibration
• User-definable channel alarms with or without alarm latching
• User-definable channel groups and displays
• Embedded graphing and printing
• User-definable graphical panels including user-definable JPG or BMP backgrounds
• Embedded bridge nulling and calibration

Data Acquisition System Design
We developed the RAD-DAQ system for advanced missile and rocket testing. For this effort, the system collected data in a continuous-acquisition mode for a total of 168 channels, 40 simultaneous high-speed (20 kS/s) and 128 medium-speed (5 kS/s) channels.

We selected the PXI and SCXI technologies to achieve both the data collection parallelism and synchronous high-speed sampling via the PXI trigger bus and the NI SCXI-1520/1531 simultaneous sample-and-hold circuitry. The SCXI-1520 and SCXI-1531 modules located in SCXI Chassis 1 were connected to an NI PXI-6052E, collecting the data in parallel at a 20 kS/s rate. By using the PXI trigger bus, we synchronized each of the high-speed PXI-6052E modules to the same clock, which created simultaneous sampling for all of the high-speed channels.

To use the SCXI-1520 and SCXI-1531 modules in parallel mode, we developed low-level SCXI register-level programming (RLP) control drivers to place these modules into a parallel acquisition mode that was not available using traditional NI-DAQ. Implementing SCXI RLP control, we also could add features to detect open connections on the SCXI-1520 inputs, which could result in multiplexer failures at higher speeds. We were concerned about the saturation of channels due to a transducer input opening because this was a one-shot missile test that could destroy transducers during launch. By using the RLP control, we could disable the open channels when an open input was detected without data loss in the adjacent channels.

The second and third SCXI chassis operated in a traditional multiplexed mode for each of the SCXI modules. Each of the multiplexed chassis had a SCXI-1520 located in slot one, connected to a PXI-6052E for system redundancy, speed, and optimization of code reuse. We also synchronized these PXI-6052E modules via the PXI trigger bus so all of the slower speed channels could be collected in a simultaneous fashion. We then used a common PXI trigger to synchronize both the start and stop of the test between the high and medium-speed sections of the data acquisition system to the IRIG time decoder. This synchronization design created a data set that had less than 0.01 ms of skew between the related points.

Achieving Major Cost Savings
Our development of the RAD-DAQ system is a superset of functions and previously developed products including TopCAT and the InterSCADA data acquisition and control packages. By building on these works and using a modular design, we optimized code reuse, and the total development time for the initial prototype was less than 45 days. The final version of this system not only showed a cost saving greater than 30 percent compared to the predecessor, but it was also able to perform its function more intuitively and with greater versatility. When we compared this system with all of the included features, it was found to cost 70 percent less than similarly configured systems.

For more information, contact:
James L. Slemp
Radical Systems, Inc.
3313 Memorial Parkway
Suite # 150
Huntsville, AL 35801
Tel: (256) 883-9791
Fax: (256) 883-4030

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