Implementing absolute timing for synchronization in aircraft structural testing

  • 15-Aug-2011 09:30 EDT
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Modern solutions such as the industry-standard PXI Express platform incorporate a 100-MHz dedicated system reference clock and built-in timing and triggering buses to synchronize the I/O modules.

For the past 40 years, structural engineers have synchronized their strain measurement systems by stringing coaxial cables between them. Over time, capabilities evolved from simply sharing a trigger to being able to share triggers and a common system reference clock. With this approach, engineers could simultaneously trigger events across multiple systems and maintain synchronization relative to a reference clock signal.

As the cost of GPS receivers goes down and new synchronization technologies enter the market, it’s now affordable and feasible to eliminate the coaxial cables by implementing time-based synchronization. Advanced measurement platforms are leading this growing trend in synchronization that reduces cabling, adds absolute time referencing, and optimizes data correlation for large systems.

Cutting cabling

Static load and fatigue aircraft testing require measurements of hundreds or even thousands of strain gages in response to real-world loads to validate the strength or lifetime of a structure. Since there are so many channels required, full-scale structural test systems typically span multiple hardware systems.

Therefore, precise multisystem timing and triggering are critical to correlate all of the strain gage measurements with each other and with the applied loads. Modern solutions such as the industry-standard PXI Express platform incorporate a 100-MHz dedicated system reference clock and built-in timing and triggering buses to synchronize the I/O modules.

The 100-MHz reference clock is automatically phase lock looped (PLL) to a 10-MHz system reference clock that can be shared through coaxial cable with other chassis to synchronize large systems. This method is often referred to as signal-based synchronization because it physically passes the clocks and trigger signals between systems.

In full-scale aircraft testing, though, it can be costly and cumbersome or even impossible to interconnect testing systems using this signal-based synchronization technique.

As an alternative, structural engineers are now adopting time-based synchronization with time references like GPS that can correlate and synchronize measurements anywhere in the world without a direct connection between measurement systems. Time-based synchronization eliminates cables and reduces costs associated with technician time and maintenance.

Adding absolute time references

Structural engineers can take advantage of time-based synchronization by locking their system clocks to a timing source and generating events, triggers, and clocks based on this absolute time.

Many timing references, including GPS, IEEE 1588, and IRIG-B, transmit time information in a packet so systems can correlate their time to a common reference. PXI timing and synchronization modules, such as the NI PXI-6682(H), can create clock signals for PXI and PXI Express systems with synchronization accuracies up to 10 ns for a variety of timing references.

With this module, engineers can also generate future events and clock signals at specified times and time-stamp input events, all with the synchronized system time. This is particularly useful if control algorithms need to be automated for fatigue testing or if another dynamic event needs to be set up to take place at a particular time in the future. With time-based synchronization, engineers have absolute time referencing and nanosecond level synchronization in structural test applications.

Optimizing data correlation

In large-scale structural testing, it is important to be able to tightly correlate data from multiple measurement systems. This is even more critical in dynamic structural tests, such as aerodynamic, impact, blast, and ballistics testing, where data changes very quickly and small channel skews can translate into significant measurement errors.

One method to overcome these errors is to establish an initial time reference, based on one of the timing protocols, and then use the time delta of the sample clock to reference data points. However, if the sample clock is not aligned with a time reference, the accuracy of the time-stamp data will degrade over time as the clock skews.

With time-based synchronization, the 10-MHz PXI system clock can be disciplined to an external time reference and continuously adjusted to stay synchronized to the reference, reducing error due to drift in comparison to a free running oscillator. The 100-MHz system reference clock is automatically PLL to the disciplined 10-MHz clock and distributes its phase-locked signal to the individual slots.

By continuously phase-aligning the sample clock of the data-acquisition modules with a time reference, cumulative drift is eliminated and the accuracy of the time stamps can be ensured over long acquisition periods.

Though the tightest timing accuracy possible can be achieved with a conventional signal-based synchronization solution, systems are constrained by the length of the cables. Using a time-based synchronization scheme, any compromise in timing accuracy due to the error in references is more than overcome by the freedom to place the measurement system anywhere the clock can be received.

With the ability to add absolute timing and reduce costs and cabling, more and more structural engineers are eliminating the coaxial cables and taking advantage of time-based synchronization.

Elizabeth Smith, Product Manager, National Instruments, wrote this article for Aerospace Engineering.

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