C. Santolini

Laser Doppler vibrometry on rotating structures in coast-down: resonance frequencies and operational deflection shape characterization

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.

Diagnostic procedure on brake pad assembly based on Young's modulus estimation

Quality control of brake pads is an important issue, since the pad is a key component of the braking system. Typical damage of a brake pad assembly is the pad?backing plate detachment that affects and modifies the mechanical properties of the whole system. The most sensitive parameter to the damage is the effective Youngs modulus, since the damage induces a decrease of the pad assembly stiffness and therefore of its effective Youngs modulus: indeed its variation could be used for diagnostic purposes. The effective Youngs modulus can be estimated from the first bending resonance frequency identified from the frequency response function measured on the pad assembly. Two kinds of excitation methods, i.e. conventional impulse excitation and magnetic actuation, will be presented and two different measurement sensors, e.g. laser Doppler vibrometer and microphone, analyzed. The robustness of the effective Youngs modulus as a diagnostic feature will be demonstrated in comparison to the first bending resonance frequency, which is more sensitive to geometrical dimensions. Variability in the sample dimension, in fact, will induce a variation of the resonance frequency which could be mistaken for damage. The diagnostic approach has been applied to a set of undamaged and damaged pad assemblies showing good performance in terms of damage identification. The environmental temperature can be an important interfering input for the diagnostic procedure, since it influences the effective Youngs modulus of the assembly. For that reason, a test at different temperatures in the range between 15??C and 30??C has been performed, evidencing that damage identification technique is efficient at any temperature. The robustness of the Youngs modulus as a diagnostic feature with respect to damping is also presented.

Vibro-acoustic analysis of synchronous belts by laser Doppler vibrometry and boundary element techniques

A major problem in directly measuring synchronous belt noise is that, often, the noise coming from bearings, fans or any type of vibration transmitted through the belt drive may be taken for belt noise. In the automotive field, most of the times, the noise generated by belt drives is estimated using microphones in the proximity of belt, crankshafts, idlers...Such a method can be misleading, since it may easily include the contribution of background noise at some frequency. Moreover, a big amount of data is needed to test different layouts and running conditions. This work aims at developing a tool for estimating the acoustic radiation of synchronous belts during the design stage, one of the main objects being that of reducing the high number of tests currently employed.

Vibration transmissibility on rifle shooter: A comparison between accelerometer and laser Doppler vibrometer data

The transmission of mechanical vibrations from tools to human subjects is known to be potentially dangerous for the circulatory and neurological systems. It is also known that such damages are strictly depending on the intensity and the frequency range of the vibrational signals transferred to the different anatomical districts. In this paper, very high impulsive signals, generated during a shooting by a rifle, will be studied, being such signals characterised by a very high acceleration amplitude as well as high frequency range. In this paper, it will be presented an experimental setup aimed to collect experimental data relative to the transmission of the vibration signals from the rifle to the shoulder of subject during the shooting action. In particular the transmissibility of acceleration signals, as well as of the velocity signals, between the rifle stock and the subjects back shoulder will be measured using two piezoelectric accelerometers and a single point laser Doppler vibrometer (LDV). Tests ha...

Diagnostic procedure on brake pad assembly based on Young modulus estimation

Quality control of brake pads is an important issue, being the pad a key component of the braking system. Since the pad - backing plate detachment can alter the mechanical properties of the whole system, the Young Modulus variation could be used for diagnostic purposes, it being a symptom of that impelling damage. The Young Modulus will be estimated from the first bending resonance frequency which will be identified from the Frequency Response Function measured on the pad. Two kind of excitation methods, i.e. impact hammer and magnetic actuation, will be presented and two different measurement sensors, e.g. Laser Doppler Vibrometry and microphone, analysed. A comparison with standard non-contact ultrasonic measurement on a simple isotropic and homogenous aluminium test sample is presented to validate the approach.

Characterization of Rotating Structures in Coast-down by means of Continuous Tracking Laser Doppler Vibrometer

In rotating machinery variations of modal parameters with rotation speed may be extremely important in particular if very light and undamped structures are taken into account, like helicopters rotors or wind turbines. The relation between natural frequencies and rotation speed is expressed in the form of Campbell diagrams. However it could be required to know also the deviation of operational or mode shapes. In several cases it is not possible to fully control the rotating speed of the machine and only coast-down tests can be performed. Such kind of tests is often fast due to the reduced inertia of rotors: for this reason, an experimental technique able to determine Operational Deflection Shapes (ODSs) in short time and with sufficient accuracy, appears very promising. Moreover coast-down processes are very difficult to be controlled, they causing unsteady vibrations. The need to obtain ODSs from coast-down experiments requires the measurement over a large number of points and therefore a very efficient approach for the rotation control and synchronous acquisition must be developed. In this paper a continuous scanning system operating on rotating structures has been developed, that allows to measure ODS and natural frequencies excited in rotating conditions at different rotation speed during a coast down. This techniques has been tested on a laboratory test bench and compared with traditional Experimental Modal Analysis (EMA) results obtained in non-rotating conditions and with data from Tracking Laser Doppler Vibrometry (TLDV) operating in coast down and at consecutive constant rotation speeds (i.e. each measurement was performed in steady conditions). EMA and TLDV have been performed over a grid of points in order to have ODSs with adequate spatial resolution, it requiring long measurement time. However these data has been used as reference to validate the continuous scanning approach.

Dynamic characterization of a centrifugal compressor rotor by a laser scanning vibrometer

The dynamic behavior of a 3D impeller (1100 mm diameter) has been modelled by means of a standard FEM code (ANSYS) and experimentally validated using a laser scanning vibrometer based measurement technique. The theoretical finite element model has been previously developed. Attempts made by using classical measuring techniques has not allowed to obtain the low frequency cyclic modes calculated. Those modes have been obtained using a laser scanning vibrometer. Sine sweep to detect frequencies and sine excitation at resonance frequencies, in order to improve the accuracy of the modal shape, have been used. The keys of success was the ability of the laser vibrometer to detect very low vibration amplitudes (of the order of nanometers) with a high spatial resolution, inside an industrial environment, in very low testing time, all fundamentals for this application. The experimental and numerical activity presented in this work has been carried out from Nuovo Pignone together with the University of Ancona with the final purpose of establishing a new fast procedure to identify, in production industrial environment, the dynamic behavior of large dimensions impellers for centrifugal compressors.