Seamlessly Integrated MTS FlexDAC Data Acquisition Systems
The Challenges of Implementing Data Acquisition into Aerospace Structural TestingDan Van Horn, Senior Software Engineer for MTS, has focused on integrating data acquisition systems into complex aircraft structural test setups since 1999. In this Q&A, he explains the challenges data acquisition (DAC) can pose for test labs and how MTS is introducing new ways to solve them.
Q: What are some of the recurring challenges you face when implementing the data acquisition portion of aerospace structural tests?
Van Horn: Data acquisition challenges typically fall into two categories: those related to system integration and those resulting from limitations in DAC unit flexibility. System integration problems often stem from mixing DAC units, system controllers and test management software purchased from different vendors; these often manifest as inadequate or inferior integration between DAC units and controllers, resulting in unacceptable data skew or jitter. Limitations in DAC unit flexibility can hinder testing capacity, narrow test system architecture options, and dramatically drive up costs, specifically in cabling needed to connect to other DAC units and strain gages.
Q: Why is it an issue if controllers, DAC units and test software come from different vendors?
Van Horn: The primary problem is that you’re assembling a collection of components that are not specifically designed to work together. This creates challenges for system integration right out of the box. If the components are from different providers, customers can’t be certain they will all arrive at the same time. They haven’t been tested to work together. They have been developed by different teams, commissioned through different processes, manufactured to different quality standards.
Q: How do these issues manifest themselves in the test environment?
Van Horn: System integration problems manifest most dramatically when it comes to troubleshooting. For example, if there’s uncertainty as to when a test has reached a specific end level, does this indicate a problem with the system controller? Or did the DAC unit take the wrong measurement? Effectively resolving the situation might require meeting with all vendors simultaneously to exhaust the possibilities, which definitely takes time. Meanwhile, the problem persists. This is one of the primary reasons why we developed the MTS FlexDAC™ 20 data acquisition system, which integrates seamlessly with MTS FlexTest® controllers and MTS AeroPro™ software. A test system comprising MTS FlexDAC 20 units and FlexTest controllers represents the only truly synchronized solution in the industry.
Q: Why is it important that the controller and the DAC unit be synchronized?
Van Horn: Because eliminating skew between the control and DAC systems removes a major source of uncertainty in a test. If a system utilizes two different internal clocks — which is always the case with components from different vendors — data will contain some degree of skew. Typical aerospace systems can be off by 25-50 milliseconds, and it’s not always apparent if a sample is ahead or behind. This uncertainty can be especially problematic in the event of a test article failure. Aero test articles are extremely expensive. So when one fails, test data are analyzed closely to understand exactly what occurred. If the data from the controller and the DAC system are not synchronized, it can look like simultaneous events happened at different times, bringing ambiguity into the investigation. Until now, the technology was not capable of fully eliminating the skew so it became widely accepted as a necessary evil. To minimize this inherent skew, test engineers must oversample. But this requires more processing power and the repeated adjustment of time stamps between the system controller and DAC data sources, which in turn introduces jitter.
Q: Is there any way for a lab to eliminate inherent skew and avoid the need for oversampling?
Van Horn: We purposefully designed our new MTS FlexDAC 20 units to employ the same clock source and the same trigger source as our FlexTest controllers. They are truly synchronized. Customers can be absolutely certain that the time stamps of every sample will align with zero milliseconds of skew. If the controller says the actuator is moving, then the control and DAC systems sample simultaneously. There is no ambiguity. We believe this will give test engineers greater confidence in their methodology. Furthermore, the same filtering scheme is employed by both systems, eliminating phase shift due to unique filter methods and frequencies. If you see a phase shift, you can identify the cause and resolve it more quickly because you can rule out clock synchronization as the culprit. You can spend more time analyzing results than questioning their accuracy.
Q: Does having an existing inventory of DAC equipment prevent a lab from adopting this new technology?
Van Horn: Absolutely not. In the past, labs that made significant investments in thousands of channels of DAC were locked in. If they needed to add capacity, they had to purchase the exact same model; if that specific unit was discontinued, they had to start fresh and completely replace their inventory with the latest model. As a result, test teams were limited by the purchasing decisions made years prior. To overcome this problem, we designed our new DAC offering and software to integrate with DAC units from other providers. This gives labs the flexibility to build their MTS FlexDAC capacity whenever they want, in only the specific increments they need at a given time.
Q: Are there other ways labs can improve flexibility in the test setup?
Van Horn: Yes, by avoiding DAC solutions that require proprietary cabling between the DAC units. These cables tend to be very expensive and if you need to make a connection beyond 20 meters, you may need a proprietary signal repeater, which is also expensive. This is why many aerospace manufacturers avoid implementing distributed DAC architectures. It is simply too costly to run the cables.
Q: What are the advantages of a distributed DAC architecture?
Van Horn: It’s much more convenient, because you can position individual DAC cabinets separately around the test article, much closer to where actual measurements are occurring. This is made feasible by using a DAC solution like the MTS FlexDAC 20, which employs off-the-shelf Ethernet cables - instead of proprietary ones - to span greater distances between DAC units. Ethernet cables are relatively inexpensive and available in much longer lengths, so you can also avoid the need for signal repeaters. More important, distributing the DAC cabinets around the test article means you can run significantly shorter transducer cables between DAC units and thousands of strain gages, which is eminently more manageable, reduces expenses and actually helps reduce signal degradation.
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