Low cost acceleration measurement system as a retro-fit solution for vibration monitoring of wind turbines in operation

Wind turbines are subjected to a combination of various loads during operation and must also withstand extreme weather conditions. In some conditions wind turbines exhibit vibrational problems due to unknown circumstances. In these cases, it would be helpful to have an easy installable measurement system to monitor the vibrational behavior of the rotor blades or even the whole wind turbine. The standard measurement technology using rugged measurement hardware including a measurement system and acceleration sensors is quite expensive. It is usually only applied for prototype testing but not for long term monitoring of vibration and operational conditions. Therefore, a development of low-cost and low-power measurement hardware has been initiated at the German Aerospace Center (DLR) institute of aeroelasticity based on MEMS acceleration sensors, such as those used in millions of smartphones or other mobile devices. But not only the cost for sensors has to be reduced. Also, a low-cost solution has to be found for data acquisition hardware and signal processing hardware. Based on the MEMS sensor concept, a measurement system has been developed that also relies on low-cost electronics for all other components. In its current state it is capable of measuring up to 25 sensors of the type ADXL362 without phase shift at a sampling rate of 200 Hz. Currently the German research wind farm WiValdi in Krummendeich is built up by the DLR. Within the DFWind research project supporting WiValdi a rotor blade has undergone a modal test on a test rig at Fraunhofer IWES in Bremerhaven with heavy external and internal instrumentation of accelerometers. During this test, the MEMS based measurement system were extensively tested and benchmarked against high cost prototype testing equipment. MEMS sensors has been added to 14 of 16 internal measurement positions so that a direct comparison of the standard system, an imc Chronos Flex with IEPE accelerometers and the DLR low-cost MEMS system has been accomplished. In total, data from 13 different measurement runs and three excitation points have been recorded with the MEMS system. These are then examined in the frequency domain and with operational modal analysis. The resulting modal parameters, such as the eigenfrequencies, damping ratios and mode shapes are used for the comparison between the standard and the low-cost MEMS system

Embedded flight vibration testing system for online flutter monitoring of UAVs

Unmanned Aerial Vehicles (UAVs) have brought technical innovation with unique structural dynamic challenges to the aerospace sector. The increasing number of UAV applications have resulted in a large variety of structural layouts. In order to maintain the high safety standard presupposed in the aerospace field, measurement of the structural dynamic stability in flight has become even more important. However, online monitoring of aeroelastic stability has just been developed for regular vibration test equipment, such as used in ground or flight vibration testing of large aeroplanes. On board application to small scale UAVs is not feasible with this equipment. To this end an embedded flight vibration testing system based on Micro-Electro-Mechanical Sensors (MEMS) and Single Board Computers (SBCs) for real time flutter monitoring was developed. The system is based on the latest miniaturized electronic hardware and designed to be lightweight, robust and highly configurable. The software was developed in house and optimized for real time performance. State of the art system identification and machine learning techniques are used to identify and monitor modal parameters for determination of flutter stability of aircraft in flight and forecasting of the occurrence of flutter critical flight conditions

Hardware-in-the-loop testing of a miniaturized real time flutter monitoring system for UAVs

Unmanned aerial vehicles continue to drive innovative aircraft designs, introducing new challenges to the aeroelastic stability of aircraft. Hardware-in-the-Loop HIL systems provide a powerful platform for testing complex real time embedded systems. The challenges of identifying and tracking modal parameters as an indicator of flutter are still unresolved. In this work a HIL system is used to test a miniaturized real time FLUtter monitorinG system FLUG. The hardware consisted of custom electronics and mini computers, a real time controller was used to run the aeroservoelastic model. An onboard computer was integrated to perform signal processing, modal analysis and tracking and send results via telemetry to a ground station. The system was demonstrated during a virtual flight test campaign. Eight modes were identified and tracked throughout the flight. A clear trend of decreasing damping with increasing flight speed and altitude were observed which can be used as a flutter indicator when approaching the edge of the flight envelope

Flight Vibration Testing of the T-FLEX UAV using Online Modal Analysis

Flight testing of the UAV demonstrator T-FLEX in the EU project FLIPASED has provided a platform to demonstrate the capabilities of an online modal identification system based on miniaturized hardware. The system uses data from Micro Electro Mechanical System MEMS sensors via a real time interface to the Flight Control Computer FCC. The signal processing, modal analysis and mode tracking using machine learning are performed using state of the art algorithms in Python and run on the Onboard Computer OBCII which is a Raspberry Pi 4. The data is encoded and transmitted via radio frequency RF telemetry to a ground station, where it is decoded and plotted in an interactive GUI. The natural frequencies and damping ratios can be visualized as a function of time, Mach number or altitude. This allows engineers on the ground to assess the flutter stability of the aircraft in real time during flight testing. A flight test campaign at the DLR airport Cochstedt served as the first deployment of the flutter monitoring system. The system was demonstrated to function robustly without bugs or system failures and was capable of running in real time. The modal parameters were identified with high certainty and could be accurately tracked throughout the flight envelope. The integration of the online modal analysis system with the onboard flight control system for active flutter control will be the next logical progression of the developed system