Vibration Measurements and Modal Analysis on Space Telescope Components

The European Southern Observatory is an intergovernmental organization established in 1962 and supported by 16 Member States - Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland, and the United Kingdom - the host country Chile, and strategic partners.

ESO’s headquarters are located in Garching close to Munich in Germany but ESO brings together over 750 staff from more than 30 countries, and countless more collaborators worldwide to serve more than 22 000 users from over 130 different countries with technologies and data.

For the ESO and the ELT, Dewesoft has implemented specific measurement data acquisition and data evaluation routines for vibration analysis of active ELT components such as pumps, fans, and modal analysis of mirror supports.

Dewesoft designs and manufactures versatile and easy-to-use data acquisition, test, and measurement instruments that are used in the world's most advanced laboratories. The measurement equipment supplied for this project included:

Hardware

• 1x SIRIUSie - 8 channel isolated SIRIUS High-speed EtherCAT slice

• 1x DS-IS-20 - 20 N inertial shaker with external amplifier

• 1x DS-IS-IS-10 - 10 N inertial shaker with external amplifier

• Multi-axis accelerometers

• Impact hammer

Software

• DewesoftX - data acquisition (DAQ) software

• DEWESOFT-OPT-MODAL-TEST - Modal test option (frequency response function)

• DEWESOFT-PLUGIN-MODAL-ANALYSIS - Modal analysis plugin

• DEWESOFT-OPT-FFT-ANALYSER - Dewesoft FFT Analyzer option/frequency domain

• DEWESOFT-OPT-C++-SCRIPT - Dewesoft C++ script  

• DEWESOFT-OPT-FG-MUL - Multichannel function generator

In this case, the applied methods were developed by an interdisciplinary group of ESO engineers and technicians from the fields of control engineering, mechanics, and structural dynamics. Calculations of the vibration behavior using the finite element method (FEM) are the basis for the metrological analysis of the actual components.

For both the vibration measurements and the modal analysis, the working group first examined the respective task using a dynamic theory model and from this derived a measurement strategy. Before the actual measurements, the expected system behavior was then simulated by structural mechanics, and, based on this, guideline values ​​and component tolerances were defined.

Seeing the fringes of the visible universe

In the future, the ELT will be used for top-level European research in the field of astronomy. The location is Cerro Armazones - a Mountain with a height of 3,060 meters (10,040 feet) in the Chilean Atacama Desert.

The giant telescope should help to answer many of the unsolved questions about the universe. With its primary mirror – having a diameter of 39 meters and made up of 798 hexagonal mirror elements – it can collect 13 times more light than the largest optical telescopes in existence today. It will provide images 16 times sharper than those of the Hubble Space Telescope.

From a research point of view, it will support studies of a wide range of topics from basic physics to cosmology. The telescope will observe our solar system as well as extrasolar planets, nearby galaxies, or distant observable objects at the fringes of the visible universe.

As a modern high-performance telescope, it observes space in the visible and infrared frequency spectrum, in which even the smallest disturbing movements of the telescope components. Even movements in the range of a few nanometers can have a major impact on the observation and image quality. Therefore, very high-quality demands are placed on the ELT mechanics and components.

Preventive analysis to avoid disruptions

The background for the early testing is that, especially in large telescopes, the number of critical components that could lead to a reduction in performance due to possible malfunction is very high. In addition, special tests in the installed state often cannot be carried out, or at least not easily, due to the complexity of the system.

After all, subsequent corrections would lead to enormous delays in commissioning simply because of the number of components. The Dewesoft measurement technology makes it possible to check potential disturbance variables - their causes and effects - on prototypes or end products throughout the project and before the components are assembled in the ELT.

Vibration measurement: introduced forces recorded indirectly

Active components such as pumps or fans can cause dynamic forces that trigger oscillations and vibrations in the entire system and impair the observation quality of the ELT. To measure these forces, the company Dewesoft has developed and set up corresponding test arrangements with precisely defined boundary conditions. The forces are measured according to the principle of gravitational mass.

For this test, the engineers and technicians mounted the test system, including any elements intended for damping, on a heavy optical table with known dynamic properties in terms of mass, stiffness, and deformation modes. Coupling vibrations between the test object and the table are minimized due to the heavyweight table, which is kept suspended by controlled air springs, and at the same time keeps the test system decoupled from floor vibrations.

In this test procedure, forces lead to an acceleration of the table. This acceleration is recorded and specifically measured allowing the measurement software to calculate the forces introduced. In this way, vibration measurement points at the component level make it possible to identify any improvements needed at an early stage – and not when all components are installed in the overall system and may be very difficult to access for measurements and correction work.

Modal analysis: simple and efficient quality check 

Each of the 798 hexagonal mirror elements that make up the ELT's large primary mirror is mounted on so-called kinematic mirror supports. These are used to lift and adjust the very rigid elements. The behavior of this mechanism had to be described and characterized dynamically using modal analysis (excitation by vibration shaker or impact hammer).

The calculated and simulated behavior during excitation is compared with the actual measured modal response. This allows conclusions to be drawn about assembly errors or damage (due to structural vibration) to structural parts - long before the mirror elements are even assembled. When the telescope goes into operation such errors could affect observations.

To create stable boundary conditions for the modal analysis and the determination of possible solid motions of the mirror, a very stiff support structure was designed and built, and was anchored to a heavy concrete floor. Several multi-axis accelerometers were mounted on the mirror support to be tested. Exciting the structure of a test object is done with the help of a vibration shaker – whereby both the location and the direction of the excitation are varied.

In the evaluation unit of the test setup, the vibrations are superimposed and evaluated regarding strength, phase, and coherence. In addition, the Dewesoft measuring system used calculates and visualizes the mirror support as a 3D modal form. The representation on the display has been adapted to the typical requirements of this modal analysis, which means the quality and validity of the measurement can be very quickly recognized.

Like this, the automated modal analysis enables a simple and at the same time efficient quality check of the mirror supports before assembly in the telescope - a reliable control of thousands of installed complex components. This effort pays off quickly, detecting hidden errors such as faulty screw connections or damaged parts early and reliably.

Conclusion

In both vibration and modal analysis, the solutions from Dewesoft achieved a very good match between simulations and measurements. This was made possible by the theoretical considerations in advance, the careful preparation of the test setups, and the high quality and accuracy of the measuring systems.

Such metrological methods are not limited to the ELT, but also find a wide range of applications in other areas of research and development. In particular, the measurement of the introduced forces is of interest for applications in which sensitive system components are tested, regardless of their installation environment.

An example is the dimensioning and efficiency measurement of damping elements or fan mounts. With this type of modal analysis, a wide range of applications opens up because its use has become much more flexible thanks to new modal analysis software modules from Dewesoft.