This paper presents a comparative study between accelerometer and laser vibrometer measurements aimed at on-line
quality control carried out on the universal motors used in washing machines, which exhibit defects localised mainly in the
bearings, including faults in the cage, in the rolling element and in the outer and inner ring. A set of no defective and
defective motors were analysed by means of the acceleration signal provided by the accelerometer, and the displacement
and velocity signals given by a single-point laser vibrometer. Advantages and disadvantages of both absolute and relative
sensors and of contact and non-contact instrumentation are discussed taking into account the applicability to real on-line
quality control measurements and bringing to light the related measurement problems due to the specific environmental
conditions of assembly lines and sensor installation constraints. The performance of different signal-processing algorithms
is discussed: RMS computation at steady-state proves effective for pass or fail diagnosis, while the amplitude of selected
frequencies in the averaged spectra allows also for classification of a variety of special faults in bearings. Joint
time–frequency analysis output data can be successfully used for pass or fail diagnosis during transients, thus achieving a
remarkable reduction in testing time, which is important for on-line diagnostics.
r 2005 Elsevier Ltd. All rights reserved.