Fail-safe spacecraft operation guaranteed
To proof the space-worthiness of spacecraft assemblies and instruments, the European Space Agency, ESA, executes qualification and acceptance tests at its technical center ESTEC in Noordwijk, The Netherlands, using a modular LMS Test.Lab Environmental system. The 552-channel system – with double channel count compared to previous system – helps ESA to more confidently trace the spots on complex test specimens that are most sensitive to extreme noise and vibration excitation. LMS vibration control software and four independent LMS SCADAS III front-end stations, with LMS Test.Lab acquisition and on-line processing software, enable ESA to protect test specimens, maximize data security and perform multiple tests in parallel. When deploying all processing stations in one single test setup, ESA is able to qualify Europe’s largest space assemblies, such as the Automated Transfer Vehicle (ATV), a space ship designed to periodically re-supply
the International Space Station (ISS).
The most challenging spacecraft ever built in Europe
To endure its inaugural launch and flight in 2006, the unmanned 20-tonne ATV module – installed in a dedicated Ariane 5 launch vehicle – must be able to withstand extreme vertical g-forces and a deafening boaster noise. Upon removing the fairing panels and main cryogenic stage approximately nine minutes after lift-off, the rocket’s upper stage will carry the cylindrical ATV to a circular orbit at an altitude of 260 kilometers. Once a ballistic trajectory describing half an orbit around the Earth is completed, the ATV separates and becomes a fully automated space ship
with multiple solar arrays. Then, the bussize cargo ship autonomously navigates towards the ISS and automatically docks to the Russian Zvezda module, providing propellant, spare parts and experiment equipment as well as cloths, food and towels for ISS crew. The space journey of the ATV ends up to six months later, when it will destructively re-enter the Earth’s atmosphere.
Obviously, before spacecrafts are launched, mock-ups, test models and the real flight assembly are tested inside-out according to very strict measurement procedures, to make sure they are able to live up to high expectations set. At its ESTEC technical center, ESA makes use of specialized test facilities to subject space assemblies and components to a wide range of experimental mechanical test campaigns. Besides environmental vibration test facilities including large electro-dynamic shaker setups and a huge hydraulic shaker system, ESA houses the Largest European Acoustic Facility (LEAF), which serves to reproduce relentless launch and flight noise loads.
Test specimen surrounded by real-time processing stations
Two years ago, in replacement of its 15-year old data acquisition system, ESA upgraded to a new state-of-the-art environmental test system. This complete test solution combines LMS Test.Lab Environmental software and LMS SCADAS III hardware, altogether providing 512 data acquisition and 40 vibration control channels. Its purpose is to qualify spacecraft structural assemblies for mathematical model verification, to execute noise and vibration acceptance tests on spacecraft flight models, and to run deployment and shock tests. The overall test system operation is coordinated by a master control workstation, while up to four stations – positioned as isles around the object under test – process and analyze the massive incoming data streams. Each isle independently manages the continuous data processing of 128 acquisition channels, displays measurement results in real-time, and delivers digital and paper reports shortly after completing the test.
According to Gaétan Piret, Test Manager at ESA’s Mechanical Engineering Department, the use of separate transportable 128-channel data processing stations positively impacts flexibility, productivity and data security. “Depending on test specifications received, we divide the LMS processing stations over multiple simultaneous tests, or have all stations participate in a single high-channel-count test campaign. This approach speeds up the many tests we perform on smaller spacecraft and instrument subassemblies and components, while the doubled channel count accommodates more detailed full-satellite investigations. Another advantage of the system’s four individual processing stations is that they limit worst-case frequency data loss to only 25% in case of a serious hazard or breakdown, where other systems potentially put all the data at risk. In addition, we are able to re-process any lost information by means of LMS Test.Lab’s parallel throughput capability, a reliable feature that made the considered installation of digital tape recorders obsolete.”
Streamline pre-test procedures and safeguard test specimens
The lengthy test campaigns performed on spacecraft assemblies and components are easily captured by LMS Test.Lab’s intuitive and process-centric workbook concept, providing straightforward and repeatable test procedures and maximum data consistency. Already in the pre-test phase, the system facilitates and accelerates the work of test personnel to prepare the test setup for accurate measurement operation. Gaétan Piret explained, “During a so-called “tap-test”, an operator conveniently runs an automated procedure that verifies the transducers’ response to a single-tap hammer excitation. This procedure is significantly faster than in the past, when it took two operators to tackle this task on a channel-by-channel basis.” LMS Test.Lab finalizes the instrumentation stage by automatically reading out and processing TEDS (Transducer Electronic Data Sheet) sensor information, including sensor name, orientation, serial number, sensitivity and calibration validity.
“By applying LMS Test.Lab’s user-friendly pre-test features, we are able to take several hours up to even a full day off of the total pre-test elapse time,” Gaétan Piret stated. “To further streamline test preparation activities, we also adopted the concept of patch panels, a connectors-based interface between the test specimen’s instrumentation wiring and the test system’s processing (and vibration control) stations. The flexibility of this straightforward concept enables us to quickly connect LMS processing stations to any instrumented test object by just plugging in a handful of patch panel plugs.”
Once ready for testing, the test specimen mounted on a shaker system is subjected to a specific excitation pattern. Through the LMS Test.Lab closed-loop vibration control station, the excitation pattern is precisely matched to a predefined load profile. At any time during the test, it is able to detect and accurately measure the sharpest resonance effects in the assembly, to make sure maximum protection for mission-critical specimens is guaranteed. By closely watching the vibration levels of critical assembly components in real-time, LMS vibration control prevents over-testing and avoids specimens to receive damage in any other way. If needed, the test can be aborted by initiating a security excitation pattern that achieves a safe standstill.
Top-class measurement accuracy and data consistency
The continuous stream of measurement data entering the system is processed by LMS Test.Lab’s accurate on-line reduction software for sine, random, acoustic or shock signal processing in parallel with time data throughput. It is impressive that the throughput performance remains continuous at a 25.8 Kilohertz sampling rate with all channels in use. To make sure no mismatching errors occur between the test system’s vibration control and processing, the vibration control status information is systematically shared among all processing stations. This information serves as “real-time labels” for all acquired and processed data, and automatically resets averaging calculations of data reduction procedures when starting the next level of a random-excitation test campaign. This automation feature avoids manual interaction and processing inaccuracies, and eliminates the need for additional re-processing after the test.
Another feature that increases data consistency is the charge-sensor and bridge-sensor conditioning that is integrated in LMS SCADAS III. In combination with LMS Test.Lab, which oversees sensor conditioning setup and all related data management, this capability saves the burden of dealing with numerous external conditioning units. “The excellent quality and performance of the LMS acquisition and on-line processing capabilities along with top-quality sensors and enhanced sensor mounting practices helped us realize a 50% overall increase in measurement accuracy,” Gaétan Piret commented.
LMS Test.Lab increases test flexibility multiple magnitude
To support efficient decision-making, engineers standing next to the test specimen have the opportunity to monitor (calculated) on-line result displays that relate to subassemblies manufactured by their respective firms. Once a test run is completed, the result databases of all participating processing stations are consolidated and predefined digital batch reports become available in PDF format. LMS Test.Lab is also capable of printing reports on the spot at three pages per second, allowing all paper reports to be produced within minutes after ending the test. For special test assignments, ESA uses LMS Test.Lab’s flexible Microsoft Office compliant features to quickly generate reports combining result data comparisons with superimposed graphic data representations. To move away from paper-based reporting, ESA plans to connect a number of computer workstations that offer subcontractor representatives direct access to any kind of digital information concerning ongoing and completed test runs.
Based on two years day-to-day experience with the new LMS Test.Lab Environmental test system, Gaétan Piret summarizes how the system strengthens ESTEC’s mechanical testing operations, “There is the conclusive observation that with the new system, we are able to reduce testing throughput times, while handling twice as many measurement channels than before. Reductions in test duration immediately translate into test assignments that can be executed at a lower price. The doubled channel count extends our measurement reach considerably, as it enables us to handle larger test specimens, execute more detailed test campaigns and run multiple tests in parallel. The flexibility of LMS Test.Lab takes us even one step further. We can quickly switch between acoustic and vibration tests, and directly connect to non-standard test instrumentation configurations. Overall, our LMS test system helps us efficiently execute environmental test operations with outstanding accuracy and data consistency, altogether delivering higher test confidence and productivity in support of our future European space projects.”
