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In rotating machinery, variations of modal parameters with rotation speed may be extremely important in particular for very light and undamped structures, such as helicopter rotors or wind turbines. The natural frequency dependence on rotation speed is conventionally measured by varying the rotor velocity and plotting natural frequencies versus speed in the so-called Campbell diagram. However, this kind of analysis does not give any information about the vibration spatial distribution i.e. the mode shape variation with the rotation speed must be investigated with dedicated procedures. In several cases it is not possible to fully control the rotating speed of the machine and only coast-down tests can be performed. Due to the reduced inertia of rotors, the coast-down process is usually an abrupt transient and therefore an experimental technique, able to determine operational deflection shapes (ODSs) in short time, with high spatial density and accuracy, appears very promising. Moreover coast-down processes are very difficult to control, causing unsteady vibrations. Hence, a very efficient approach for the rotation control and synchronous acquisition must be developed. In this paper a continuous scanning system able to measure ODSs and natural frequencies excited during rotor coast-down is shown. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the rotor surface, in order to measure the ODS, and to follow the rotation of the rotor itself even in coast-down. With a single measurement the ODSs can be recovered from the LDV output time history in short time and with huge data saving. This technique has been tested on a laboratory test bench, i.e. a rotating two-blade fan, and compared with a series of non-contact approaches based on LDV: - traditional experimental modal analysis (EMA) results obtained under non-rotating conditions by measuring on a sequence of points on the blade surface excited by an impact hammer, - continuous scanning LDV measuring the ODS of the structure excited by an impact hammer modulating the laser output, - tracking laser Doppler vibrometry (TLDV) operating at different rotation speeds under stationary conditions, - tracking continuous scanning laser Doppler vibrometry (TCSLDV) operating at different rotation speeds under stationary conditions. EMA and TLDV have been performed over the same grid of points sufficiently dense to have ODSs with adequate spatial resolution, it requiring long measurement time. The application of different techniques allowed us to completely characterize the tested bladed rotor and to validate the continuous scanning application to transient rotator processes.

A powerful technique for monitoring and analysing mechanical systems is the application of laser Doppler vibration measurements. When the object to be analysed rotates, the measurement becomes difficult. That is, when a steady laser beam is used, the presence of a tangential motion of surface determines a noisy disturbance. The speckle pattern produced by the roughness of the analysed surface is moving with the surface itself, and this phenomenon is observed as a signal with a frequency depending on the rotational speed. For these reasons, the aim of the present work is to show the possibility of Eulerian approach to the vibration measurement of rotating objects. A Laser Scanning Vibrometer has been used for the tracking of the same point in the complete circular motion. The capabilities of this system are tested on the measurement of vibration map of blades of a model of naval propeller working in water: a comparison has been found with the static analysis, in water and air, of the same propeller. Some results are shown.

The paper discusses the complexity of calibration of strain gage full bridges applied to measure mechanical loads in large wind turbines, when direct application of calibration loads is not feasible. In particular, at first, it presents a generalized static‐dynamic mechanical model which allows to calibrate the strain gage full bridges using its own unbalanced masses to generate known reference inputs. Then, the paper discusses the uncertainty associated to such a calibration, according to the ISO/IEC Guide 98‐3:2008 “Guide to the Expression of Uncertainty in Measurement”. The uncertainty of the reference input and the following calibration is discussed, which is often larger than the target set by the standard IEC‐61400‐13 used for wind turbine type certification. The paper comments on the attainable range of calibration which is rather limited with respect to expected load range in operation. Even if calibration should take place in isothermal effects, this is not always the case in real world practice. Therefore, the thermal effects on strain gage bridges are also discussed, putting into evidence its influence on calibration uncertainty both for full bridges in T configuration and in parallel configuration.