Vibration measurement is of fundamental importance in many machinery applications
including for the development and monitoring of rotating machinery. In such applications,
measurement of the vibration transmitted from the rotor into a non-rotating part of the
structure is the most common arrangement but this cannot always be relied upon because
vibration transmission may be low. In such cases, the use of a non-contacting vibration
transducer capable of measuring vibration directly from the rotor itself is desirable.
Laser Doppler Velocimetry (LDV) is a non-contacting vibration technique capable of such
measurements but vibration measurements on rotating structures using LDV have been
shown to be ambiguous. The sensitivity of the measured velocity to other rotor vibration
components can be significant enough to mask the intended vibration measurement
entirely. This thesis examines the use of LDV for vibration measurements on rotating
structures more comprehensively than in any previous study.
A new and completely general theory is developed to allow the velocity sensitivity of LDV
measurements taken from rotating structures to be described for laser beam incidence in an
arbitrary direction on a target element requiring 6 degrees of freedom to define its
vibratory motion fully. Extension of the' theory to optical configurations incorporating
multiple laser beams is also included with a number of useful instrument configurations
The theory enables some fundamental questions regarding the use of LDV on rotating
structures to be answered. Of particular importance is the confirmation that direct
measurement of radial or pitch and yaw vibration is not possible because the
measurements will always be unavoidably cross-sensitive to other motion components.
Resolution of these components is possible, however and a new method of resolving
steady state, non-synchronous radial, pitch and yaw vibrations is presented enabling a
range of measurements to be made for the first time using LDV. Several of these
measurements were made on a running IC engine and of special note are the angular
vibration measurements made using a novel instrument incorporating 3 beams, the laser
angular vibrometer, designed specifically for the task.
Errors within the resolution technique are considered in detail and, looking forward, a
number of promising means by which to reduce error magnitudes are introduced and
recommended for further investigation. LDV has great potential for rotating machinery
diagnostics and such developments are key to achieving this potential.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.