Aerospace Structural Dynamics Test System

Dr. Lee Peterson, Department Chair and Professor of Aerospace Engineering at the University of Colorado has been an avid LabVIEW programmer since it first appeared on Macintosh. His current research on the structural mechanics of large precision spacecraft at the Structural Dynamics & Control Laboratory (SDCL) at the Center for Aerospace Structures led him to build a complex measurement, control, and analysis application in NI LabVIEW.

Introduction

The SDCL uses very specialized equipment to do the research. This includes a custom designed enclosure called the Thermal-Acoustic Stabilization Chamber (TASC), shown in Figure 1 (courtesy of the SDCL). The TASC is able to keep temperatures extremely stable, varying by only “+/- 0.1 K over several days and +/- 0.005 K over several hours.” Vibration level is also tightly controlled with “less than 10 μg RMS over 1 kHz.”

Spacecraft structures for NASA and others are placed inside the TASC and equipped with sensors including accelerometers, LVDTs, and laser interferometers. The tests that are then done are able to detect movements of less than a nanometer.

Transducers used:

• Accelerometers
• LVDTs (Linear Variable Displacement Transducer)
• Laser Interferometer
• Cameras
• Thermocouples

This TASC was originally instrumented and controlled using a combination of VME hardware and National Instruments SCXI. The VME system was used to acquire the dynamic signals including the vibration signals. The signal conditioning for the accelerometers was external for all channels. The NI SCXI system was used for low speed measurements including temperature monitoring. The software used in the system was a combination of LabVIEW, C, FORTRAN, and The MathWorks, Inc MATLAB®.

This instrumentation and control system had a number of limitations that Dr. Peterson sought to overcome. One of these was to create a synchronous and phase locked timing system. For the tests, it was critical that data acquisition timing between the laser interferometer and the rest of the sensors be tightly synchronized. Another shortcoming was the lack of a flexible data storage system to hold setup, calibration, and test data for a variety of test scenarios. In all, Dr. Peterson identified six critical requirements in a test system that give him the ability to validate theories and models of spacecraft structures:

1. Parallel data processing
2. Reliable processing performance
3. Tight timing and synchronization
4. Flexible data storage
5. Accurate instrumentation performance
6. Adaptive modeling capabilities

An updated system using National Instruments PXI was able to overcome these challenges. The current instrumentation and control system includes a wide variety of PXI modules. Figure 2 illustrates the overall system.

Instrumentation and control system components:

• (2) NI PXI-1045 18-slot chasses
• (16) NI PXI-4472B 24-bit, vibration-optimized, dynamic signal acquisition modules
• NI PXI-6052 Multifunction DAQ
• NI PXI-6733 16-bit analog output module
• NI PXI-1409 IMAQ module
• NI PXI-6602 counter/timer Module
• NI PXI-6653 multi-chassis timing and synchronization module
• NI PXI-7344 motion controller
• NI MXI-2 interface to VXI
• NI PXI-1045 18-slot chassis
• NI PXI-8186 embedded controller running LabVIEW Real-time
• (8) NI PXI-7831R reconfigurable I/O modules
• NI PXI-6602 counter/timer Module
• (2) NI SCXI chasses for temperature
• VME chassis for 8 channels of laser interferometry

The first two PXI chasses are used for vibration signal acquisition, motion control, image acquisition, and control of the VME laser interferometers. The SCXI chasses are controlled via the multifunction DAQ modules for the low-speed, high-accuracy temperature measurements. The third PXI chassis is a recent addition to the system, running LabVIEW Real-time with eight reconfigurable I/O modules for high speed control.

Parallel data processing

To handle more advanced modeling, Dr. Peterson needed a way to overcome the traditional sequential approach to structural test, necessary because of intensive processing requirements. He greatly reduced test-cycle time by extending parallel data processing as steps are run rather than after a complete test. Dr. Peterson uses LabVIEW to launch multiple processing applications and is developing a method to execute distributed analysis on the fly on 12 dual G5 Macintosh computers over Gigabit Ethernet as tests are run.

Reliable processing performance

Because of the cost and complexity of the structures being tested, Dr. Peterson’s system required reliable data acquisition, motion control, and dynamic user interface loading. He achieved this through LabVIEW multithreaded and event-based programming functionality. According to Dr. Peterson, it is the “inherent structure of LabVIEW that makes this type of programming much simpler than C or The MathWorks, Inc MATLAB®.” For example, the multithreaded capabilities of LabVIEW were essential for the dynamically loaded user interfaces for specific test scenarios. In the software, a primary thread controls processing and other critical activities while a secondary thread controls an appropriate user interface. In addition, processing improvements are being realized through event based programming to handle individual activities such as data acquisition and motion control.

Tight timing and synchronization

The TASC system's complex timing strategy includes 5 separate clocks that are triggered simultaneously and 2 synchronized chassis of dynamic vibration measurements. To maintain and control the clocks, Dr. Peterson used a single NI PXI-6602 counter/timer module as a master clock that distributed all of the timing signals. The two chassis are synchronized with an NI PXI-6653 synchronization module that ensured less than 0.1 degree of phase shift between 128 channels of vibration data.

The 5 system clocks are:

• “Fast Data Acquisition (DAQ)” for vibration signal acquisition
• “Slow Data Acquisition (DAQ)” for temperature measurement
• Laser interferometry
• Image capture (IMAQ)
• Signal generation and motor control

  Flexible data storage
  

Because tests are often done for government sources, a high degree of traceability was required. However, a highly adaptable framework was needed for constantly changing test setups. Dr. Peterson met these criteria by developing a LabVIEW database with terabyte hard drives to contain calibration, user notes, sensor location, and test data.

Accurate instrumentation performance

To adequately characterize the structural performance, Dr. Peterson needed a system sensitive to 60 μN of force. He found the 110 dB dynamic range of the National Instruments PXI-4472B signal acquisition module had the accuracy to make these measurements. Dr. Peterson also achieved an unexpected benefit – by using the NI PXI platform, he reduced his system size from three 19 in. racks to one 19 in. rack.

Adaptive modeling capabilities

The complex structures tested by Dr. Peterson required high-speed control to simulate boundary conditions. The high-precision large space structures tested in the TASC need more prediction and better modeling to reduce risk and cost. The National Instruments PXI-7831R module provided the necessary response time for control and permitted experimentation in virtual testing. This made it easier for Dr. Peterson to simulate complete system test environments and reduce risk and cost.

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