Latest design trends in modal accelerometers for aircraft ground vibration testing

Kurzfassung

Accelerometers are widely encountered in structural analysis applications such as modal analysis with vibrational or impact input excitation and operational modal analysis. This paper aims to outline design trends and requirements for acceleration sensors in order to insure optimal structural analysis measurement results. Key parameters for a performing modal sensor are: sensitivity, mass, noise level, amplitude and phase frequency response, as well as thermal transient response, thermal sensitivity response, transverse sensitivity (cross axis), base strain and survivability which will be taken into detailed consideration in this paper. Nowadays three IEPE (Integrated Electronic Piezo Electric) sensor designs can be considered: piezo-ceramic shear, piezobending beam and piezo-crystal shear mode sensing elements. Unfortunately, none of the sensor technologies available on the market today will allow for the best of all parameters mentioned earlier. Advantages and disadvantages have to be considered in order to make the optimal choice. Even though Variable Capacitive (VC) MEMS sensors can be used in cases of operational modal analysis at ultra-low frequencies, such as Bridge Structural Testing or Monitoring, only IEPE technology will be in this study. Besides the technical properties of an accelerometer, the handling qualities during installation and removal are extremely important for high channel count systems. Installation time, error rate and reliability for more than 10 years during several tests a year are of special interest for the user. Among the considerations made here, easy monitoring and sensitive axis alignment compared to the overall coordinate system will be examined. The German Aerospace Center (DLR) will illustrate the applicability of accelerometers in context of industrial testing such as Ground Vibration Testing (GVT) of aircraft structures or structural and modal testing of wind turbine blades where innovative methods such as allowing one free adjustable degree of freedom around one rotational axis in order to freely orient the sensitive axis.