ADVANCED VIBRATION PROCESSING TECHNIQUES FORCONDITION MONITORING AND QUALITY CONTROL IN I.C. ENGINESAND HARVESTING MACHINES (2009)

The topic of this thesis is the development and the implementation of
advanced vibration processing techniques for machine condition
monitoring and diagnostics with two fields of applications: the quality
control of I.C. engines by means of cold tests, and the monitoring and
control of harvesting processes.
The cold test, i.e. the final test after the assembly line and before
shipping the engine to the customer, consists of the final quality control
of the engine in a non-combustion state. Techniques for engine condition
monitoring based on the analysis of vibration signals are widely used.
However, these techniques are often applied to engine tests in firing
conditions. This thesis addresses the use of several signal processing
tools as a means for the monitoring and the diagnosis of assembly
faults through the cold test technology. Firstly, an approach based on
the use of Symmetrized Dot Patterns for the visual characterization of
vibration signatures is proposed in order to obtain reliable thresholds
for the pass/fail decision after the cold test. Secondly, the fault
identification is discussed on the basis of the cyclostationary modelling
of the signals. The first-order cyclostationarity is exploited through the
analysis of the Time Synchronous Average (TSA). Subsequently, secondorder
cyclostationarity is analysed by means of the Wigner-Ville
Distribution (WVD), Wigner-Ville Spectrum (WVS) and Mean
Instantaneous Power (MIP). Moreover, Continuous Wavelet Transform
(CWT) is presented and compared with the WVD and WVS. The choice
of different wavelet functions and some methods for the CWT map
optimization (i.e. purification method and the average across the scale
vi
method (TDAS)) are also considered. Moreover, the capabilities of the
Instantaneous Angular Speed (IAS) in detecting assembly faults have
been tested.
It is worth noting that the cyclostationary and time-frequency technique
capabilities have been verified for both simulated and real signals.
The experimental results indicate that the image correlation of
Symmetrised Dot Patterns is a good solution that can be used in the
cold test technology in order to increase its efficiency and fault detection
capability. Moreover, it will be proved that the first order
cyclostationary analysis is able to identify the presence of assembly
faults but it is not appropriate to localise the faults. The second order
analysis overcomes this problem indicating the angular position of the
mechanical part affected by the fault. This is achieved by means of a
correlation between the results obtained from the cyclostationarity
analysis and the angular position of the mechanical events. Concerning
the time-frequency analysis, the WVS as well as the CWT, using both
Morlet mother wavelet and TDAS method can be considered good tools
to characterise the transients due to the fault events in the timefrequency
domain. Thanks to this research study it is possible to
understand which of the above-mentioned techniques is effective for an
easy and fast quality control and for the diagnosis of the considered
assembly faults. Moreover, the limits and drawbacks of both monitoring
and diagnostic procedures are shown.
The originality of the first part of the research mainly concerns the use
of vibration measurements for the quality control of engines at the end
of the assembly line while the greater part of methods used for cold test
applications focuses on pressure and torque measurements.
The second part of this thesis concerns the analysis of relationships
between the harvesting process parameters relative to a nonconventional
harvesting machine and its vibration response.
Common and uncommon features extracted from a segmentation
analysis have been correlated with the harvesting process efficiency in
order to define the optimal monitoring feature subset. Moreover, the
Discrete Wavelet Transform method is performed in order to find the
vii
frequency range mostly characterised by impulsive components. In
addition, some outlines obtained through the vibro-acoustic analysis
performed in the angular domain are also given.
Two different indoor and outdoor test rigs have been built to test the
machine under different setting conditions in order to evaluate their
influence over the vibration response of the threshing unit. The test
results are used to identify how the vibration generation is linked to the
crop distribution during the threshing process.
Good correlations have been obtained by analysing the concave middle
radial signal and by calculating the relationships that exist between
some time domain features and the efficiency parameters. These
features can be assumed as good indexes in explaining the crop
distribution between the rotor and the concave and, consequently, the
efficiency of the process. Moreover, it will be shown that the vibroacoustic
features selected are well-connected to the different sources of
the concave excitation.
The main original contribution of this second part concerns the use of
the vibration signal as an effective way to monitor the harvesting
process. It can also be considered as a proper quality control indicator
for the user during field operations.