Jonathan Newell discusses the latest technology for acoustic testing in the space industry with Siemens PLM.
Venturing into space has long since established itself in the domain of commercial enterprise rather than the Government run money-no-object behemoths of the past. With commercialisation comes the necessity for controlling costs whilst meeting the highest quality standards and complying with the requirements of certification bodies.
One hotly contested area of space technology is the manufacture, launch and operation of earth satellites, all of which have to survive the traumatic rigours of launch before settling down to a lifespan enduring the harsh conditions of space.
With such destructive launch and operating environments, it’s little wonder that satellites have to go through stringent qualification testing at the end of their development cycle before being certified for launch.
With the acquisition of LMS International five years ago, Siemens PLM Software built on its long experience in dynamic environmental testing and, through its partnerships with major companies in the space industry, has developed new methods of performing cost effective qualification testing for satellites based on its Simcenter Testlab software and Simcenter SCADAS hardware.
To understand more about these methods, I spoke to Siemens’ Bruno Massa, Vice President of Testing Solutions, and Dr Alex Carrella, the company’s Space and Defence specialist.
According to Massa, the partnerships Siemens forms with the space industry represent a crucial element of the way the products develop. “We actively engage in technology advancement in the industry as a partner and not purely as a supplier,” he says.
One important partner is Thales Alenia Space, a European space satellite and payloads manufacturer, with whom Siemens developed a new approach to acoustic testing of satellites.
A departure from reverberation
Acoustic testing subjects the device under test to intense noise levels while measuring its vibration response. This test is performed on both components, such as reflectors and solar panels as well as full systems.
Satellite acoustic testing is traditionally performed in acoustic reverberant rooms. In most cases, these large facilities (sometimes over 1,000 cubic meters to accommodate large spacecraft) are filled with gaseous nitrogen which has a lower sound absorption coefficient than air. The noise is generated by modulators connected to horns placed in the chamber; the result is a noise level that can reach over 150 decibels (dB).
In such a test, the item is subjected to an acoustic excitation with a frequency spectrum that changes according to the launch vehicle. The required uniformity is then guaranteed by the design of the reverberant chamber. Because of this, there is very high capital expense in setting up such facilities. They are safe, reliable and accurate but at high cost and in short supply.
The challenge to the industry was to produce an alternative test environment that is repeatable and more flexible in terms of where it can be set up.
The arrival of DFAX
Now, an alternative means of acoustic testing which uses commercial loudspeakers is in partial use in the industry today for the qualification of North American satellites. This method is referred to in the industry by the type of acoustic field that the large set of loudspeakers produce which is a direct field as opposed to the reverberant field produced in the standard chambers. For this reason this new alternative is known as Direct Field Acoustic Noise or DFAN (this is the acronym being adopted in one of the latest standard being produced and should replace the other different abbreviations used such as DFAT or DFAX),
Modern loudspeakers and amplifiers deliver the required high decibels to obtain the target overall sound pressure level (OASPL). The vibration levels measured on the specimen during the DFAX test are comparable with those measured with reverberant field acoustic excitation. However, apart from the NASA Handbook 7010 which provides some initial guidelines to the test engineers who wants to adopt this technology, there is not yet a commonly accepted standard to perform this type of test. And the space industry, where every test is regulated by standard practices, is now coming together to develop such a standard.
Thales Alenia Space has been conducting experiments using Siemens PLM’s hardware and software to explore and validate new methods for satellite acoustic testing.
With relatively simple equipment when compared to a reverberant room, DFAN lowers overall test expenses, can be performed in more locations and brings more flexibility with shorter test sequences. However, the nature of the sound field in a DFAN test differs from that of a reverberant room test and this difference needs to be accounted for in order to produce realistic test conditions, something Thales Alenia Space and Siemens engineers are engaged in.
At the heart of the work being done at Thales Alenia Space is the Siemens Simcenter suite comprising SCADAS hardware and Testlab software.
The SCADAS hardware is both an output device that can provide signals for excitation and an input data acquisition (DAQ) device for taking sensor signals for measuring the response of the device under test.
According to Massa, the Testlab software can handle the full flow of the test operation from setup through to DAQ as well as the visualisation of vibration profiles. “Having such flexibility within the software provides a powerful ability to perform not only analysis, but in-depth investigations,” he says.
At Thales Alenia Space, the acoustic test system uses SCADAS hardware fitted with a multiple inputs multiple outputs (MIMO) controller and is combined with Simcenter Testlab.
The objective is to reproduce the acoustic environment in the fairing of a satellite launcher. The test setup is designed to generate the high acoustic levels that excite the specimen during takeoff. The setup is comprised of 96 loudspeakers, stacked in 12 columns and adequately positioned in a circular configuration, and 96 amplifiers that deliver the required high power of 4×5 kilowatts (kW). The specimen being tested is placed at the centre of the five-meter cylinder of loudspeaker columns. The challenge is to reproduce a uniform diffuse acoustic field around the specimen and make sure that it is equivalent to that of an acoustic reverberant room.
16 microphones positioned around the specimen analyzes the response and corrects the signal input where necessary. The corrected values are reinserted in the loudspeakers in order to create an homogenous acoustic field.
Using this MIMO closed-loop algorithm, the output matches the reference profile, allowing the successful qualification of the reflector shell of an antenna subsystem as a demonstrator.
For vibration studies, the combination of Simcenter portfolio offers considerable flexibility both as a test and a development platform. Simulation studies can be performed and virtual models created.
By comparing actual vibration testing results with virtual models, simulation parameters can be adjusted to create more accurate predictions. I asked Massa whether there is likely to be a point at which qualifications can be obtained using virtual models. “The industries which use these models are changing too rapidly for this to be viable. A car 8 years ago isn’t the same as ones available now. These changes are too profound to validate a part based on simulation,” he says.
Despite this, a great advantage of Simcenter is its ability to be integrated with other tools both within the Siemens PLM portfolio and with products from other suppliers. Making use of such tools enables the test data to be exploited throughout the design, development, test and manufacturing process.
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