The facility for ground vibration tests can be used and adapted for structural dynamic tests on structures of various sizes and complexity within the field of aerospace and energy. The areas it can be used in range from hammer impact tests on small models and components, through to ground vibration tests on aircraft. The modular construction allows for two tests to be carried out simultaneously. The infrastructure of the facility consists of an LMS data acquisition system with in-house developed signal conditioning and cable routing, as well as vibration exciters of various sizes, sensors for different measurement tasks, as well as test facilities for storing aircraft and helicopters.
Data acquisition system
A key part of the facility for ground vibration tests is a mobile data acquisition system by the company LMS, with a total of 384 analogue input channels. The entire system consists of four individual SCADAS III front-ends. Each front-end provides 12 slots for data acquisition boards. Each front-end slot is used for synchronising one module with the others, with one circular data bus per fibre optic cable.
In addition to the analogue input channels, the data acquisition system is also capable of producing a total of 16 analogue output signals. They can
emit both standard signals (sine, swept sine, real and periodic signals, chirp, burst random, burst chirp), as well as user-determined signals, such as for optimising the energy input of the exciter, taking into consideration the dynamic behaviour of the test structure.
Sensors for vibration measurements
Vibrations are measured with accelerometers. Sensors that are designed in a different manner, or that have different measurement principles, are available for the respective applications.
For very low frequency ranges, piezoresistive sensors by the company Entran are used. In total, approximately 350 sensors of this kind are available.
For high frequency ranges of up to 10kHz, piezoelectric sensors by the company PCB are used, which operate in the measuring range of ±50g and the frequency range of 0.3-12,000Hz. In total, 230 sensors of this kind are available.
Electrodynamic vibration exciters of various sizes are available for vibration excitation.
System Control Software
The ground vibration testing facility is controlled using the Windows-based software Test.Lab by LMS. The software communicates with a front end via a KAT6 Ethernet cable with a RJ45 connector. Both the transmission of the control parameters and the collection of measured values are done via this cable. This is possible for distances of 100 metres or more between the control computer and the data acquisition system. The software specifications include modules for measuring frequency responses using different standard signals (step and swept sine excitation, noise and impulse excitation). In addition, there is a software module for phase resonance methods, which enable the simultaneous control of several vibration exciters. The identification of natural vibration parameters using measured frequency responses is carried out with a program module for modal analysis in LMS Test.Lab. For this, there are several modal parameter estimators (time range and frequency range).
Database for Ground Vibration Test data
The last, but by no means the least, important step in carrying out a ground vibration test is the provision, classification and evaluation of the collected data. The Department of Structural Dynamics and Aeroelastic System Identification has, in recent years, developed a solution, which can be operated independently from commercial measurement hardware and software. The measured data is stored in an SQL database together with identified modes. The software Correlation Tool, which was developed specifically for structural dynamic tests, offers direct access to the database with the aim of evaluating and classifying the large amount of identified modes. With this, all results can be visualised in various ways and compared with each other. At the same time, customers have 'read-only' access, enabling them to view the acquired data, and track test progress.