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Focus: Advances In Vibration Qualification Testing
by Noel Brown, Peter Sims, Julian Simpson and Kim Boldt of Brüel & Kjær

Since World War II, vibration testing has gone hand-in-hand with aerospace and space development. The latest developments in this technology have led to synchronization with data acquisition and has benefitted satellite qualification.

FocusBrownFig1 Satellites appear to have a serene life — floating gracefully in orbit, their forms seem untroubled by the Earth-bound stresses of gravity and vibration. However, that same mass of technology has to endure being stowed as the unhappy payload of a launch vehicle. There, the satellite must endure the noise and subsequent vibration of the ~145 dB interaction between the rocket engines and launchpad environment, the jarring transonic climb phase, pyroshock as stages separate, turbulent boundary layer excitation, and more besides. These forces can induce fatigue in resilient metal structures, not to mention the sensitive electrical components of satellites.

Given these huge stresses, and the fact that damaged satellites cannot be easily repaired once deployed, it’s vital they are thoroughly tested before their violent ride into orbit. Qualifying the durability of satellites is a critical stage in their development, helping to prevent sending useless objects on one of those expensive trips into space. Consequently, the space industry probably has the most demanding requirements for vibration testing, and indeed, the development of vibration testing techniques has been closely connected to the aerospace and space industries.

FocusBrownFig2 The concept of vibration testing as we know it today is relatively new and has been continuously developed since its origin during World War II when the impetus was the desire to test parts and equipment for use in aircraft. Even then, structural and mechanical failures due to vibrations were not the only problems, as the use of complicated electronic and electro-mechanical equipment made control systems and communication instrumentation sensitive to the vibrations encountered during mobile operation.

Space Applications
Depending on the stage of a project, different testing regimes are adopted to establish the robustness of components, subsystems, and fully assembled satellites. Design qualification tests are usually carried out on a structural model — a complete physical replica — during the development phase, in order to demonstrate that the design enables the equipment to withstand the vibration levels it will see during launch, as well as a qualification margin. The tests also allow verification of the spacecraft’s mathematical model by measuring motion at ‘resonant frequencies’, at which elements of the spacecraft structure are prone to self-vibrate once vibration is initiated. Then, acceptance tests are carried out on the actual flight model of the satellite, in order to verify workmanship and ensure the equipment does, indeed, operate satisfactorily in its final configuration and will not degrade during launch.

FocusBrownFig3 Test Hardware
Vibration tests are conducted with mechanical shakers. These shakers come in a variety of sizes and operating configurations, ranging from small, permanent-magnet types to the larger, electromagnetic units. Small or medium shakers can be cooled using ambient air, while larger shakers require a water-cooling system. In Brüel & Kjær’s LDS V900-series of shakers, water-cooling is applied to the field coils, resulting in quieter operation and a cooler body temperature that minimises the temperature effects on the equipment under test. This makes water-cooled shakers ideal for applications requiring high forces, or large payloads tested for short durations. The absence of air blowing around the shaker and test equipment also makes water-cooled shakers particularly appealing in clean-room environments. Combining four large shakers in a custom quad setup is a typical way to test the largest assembled satellites.

FocusBrownFig4 Controlling the mechanical shakers is a power amplifier that provides the current and voltage. In turn, a vibration controller governs the signal that is sent to the amplifier, while interfacing with a computer that allows the operator to enter test parameters and observe channel information. The controller provides a low voltage drive output to the power amplifier by using a closed-loop control method. Through this, it constantly monitors and modulates the output drive signal, ensuring it meets the programmed specification.

Meanwhile, accelerometers measure the applied vibration levels on the actual shaker. This serves the purpose of controlling the test by supplying feedback to the controller, so that any difference between the output drive signal and the physical vibration performance of the shaker can be compensated for.

New Control Architecture
When dealing with complex test objects such as satellites, controlling tests accurately is difficult. The core concern for vibration qualification testing is the safety of the test object, so the large amount of different modes in which the satellite can vibrate require monitoring and control feedback to be effected from as many points as possible. After all, breaking the structural model during testing would be a significant setback — however, breaking the flight model would be a disaster. Consequently, the latest generation of vibration controllers, such as Brüel & Kjær’s VC-LAN, offer many channels for control and limiting of diverse points on the satellite.

BruelKjar_ad_SM0911 With a modular concept such as the VC-LAN has, these controllers can be combined to allow as many as 64 channels for control and limiting of complex structures, with more than 1,000 channels of abort monitors possible. Such allows the test to be instantly stopped by a single overload at any one of those separate points. This channel count comes from advances in co nnectivity that allow integration with data acquisition hardware via a standard LAN connection. Brüel & Kjær’s PULSE data acquisition has similar modular architecture, so with the two systems a scalable solution is achieved, with essentially as many abort monitors as necessary.

Setup Simplified
Setting up tests is a laborious part of satellite testing that typically takes far longer than the test itself. The operator is responsible for correctly attaching the unit under test to the shaker, attaching accelerometers, and the general preparation of the setup. Finally, the operator programmes the controller and observes the vibration test to completion. Understandably, the setup process is very open to errors, meaning that user-friendly simplicity is highly prized.

Integrating data acquisition and vibration control brings great benefits throughout the vibration testing industry, but it comes into its own in the field of satellite testing by providing concurrent and integrated data acquisition with hundreds of input channels. Since testing and development centres on data, the ability to combine data collection with the same setup that controls the test has the potential to save a huge amount of time and effort. Setup time, resources and errors can subsequently be drastically reduced, while the capital investment is limited to one intelligent system that can be reconfigured at leisure.

FocusBrownFig5 Advanced Technology
The modern drive to save time and simplify testing procedures has caused developers of the new generation of controllers to invest a significant effort in technological solutions. Consequently, setup problems such as signal under-ranges and overloads can now be virtually eliminated. This is thanks to dual, parallel analogue-to-digital converters that work in harmony to deliver an exceptionally wide 130 dB dynamic range for the input channels, without multiple-input voltage range circuitry.

Consequently, setup errors are greatly reduced and initial ranging can be completely eliminated. For tests with multiple input ranges this brings a significant benefit, as testers previously had to run a preliminary low-level test to make sure all of the input ranges were properly set for the full-level test, and wouldn’t result in overloads. In the past, getting round this problem by setting all of the inputs to full-scale was not an option either, as it would result in a loss of data resolution on low level signals. With the new technology however, right-the-first-time data capture is automatically taken care of, reducing the testing stresses that satellites have to undergo, as well as the risk of producing unreliable data — and possibly having to repeat the test.
Controlling a vibration controller is usually the domain of a dedicated PC, and the modern controllers interface via a standard LAN connection. Via a router, they can subsequently be operated wirelessly, if need be, or otherwise located very close to the shaker. In addition to reducing cabling, which can present a significant problem, the entire test can thus be analysed and run remotely. In fact, the VC-LAN features full stand-alone capability with a built-in battery backup to safeguard against power failure. Besides this, the controller can operate without a PC, allowing tests to be programmed and then operated without a PC attached — further insuring against unwanted actions.

Ultimately, it’s all about data, however, so an embedded database makes it quick and easy to recall test setups and data, featuring keyword searching. Using the ASAM-ODS industry standard file format is important to make it easy to share data with other users, and interface with standard programs like Microsoft® Excel®. Simplified reporting is another factor that is hugely appreciated by testers, who can thus spend their time doing what they do best, while the software takes care of the rest.