posted on 2013-04-22, 13:22authored byIain D.C. Tullis
Torque measurement of a rotating shaft is a method of monitoring machine performance.
Steady transmission of mechanical power from the prime mover to
the load is vital to avoid gear and bearing wear, shaft fatigue failure, bearing
and coupling failure and noise. Mean and fluctuating torque are fundamental
quantities of interest.
The laser torquemeter is capable of providing a non-contact measurement of
time-resolved torque through a driven system. The laser torquemeter depends
upon laser speckle produced from coherent light illuminating a diffuse object and
the speckle pattern may be used in determining the angular position of a rotating
object. When the object rotates the backscattered speckle pattern, which changes
continuously but repeats exactly with every revolution, is sampled by a suitably
positioned photo detector. The photo detector output signal is periodic and one
period is recorded in memory as a reference and the angular position of a shaft
can then be determined by a comparison of this recorded reference signal with
the current photo detector output signal. The speckle pattern from two axially
separated points on the shaft are monitored and under low or, ideally, zero torque
the photo detector outputs are recorded into the laser torquemeter electronics.
The laser torquemeter then tracks the live photo detector output and determines
the angle at the two points on the shaft. Relative angular displacement in the
two angle outputs appears when torque is applied and the shaft twists. When the shaft is displaced, for example by vibration, the backscattered
speckle pattern changes on the photo detector and the similarity between the
recorded, reference signal and the live, current signal is reduced. In this thesis,
the cross-correlation of the real-time photodetector output signal and the
recorded reference signal as a function of shaft position is examined. The effects
of various shaft motions - rotation, axial translation, pitch and yaw, and radial
translation are theoretically and experimentally examined and the results can
then be used in the design of an optical head for the laser torquemeter.
A review of the current torquemeter technology allows for discussion of the
broad spectrum of typical torquemeter operating conditions. The optical head
of the laser torquemeter may vary significantly for various torque measurement
scenarios. A design procedure for the optical head of the laser torquemeter is
summarised.
The holy grail of torquemeter manufacturers is to produce a cheap, easy to use,
robust, accurate, reliable and non-contacting torquemeter. The laser torque meter
has great potential to meet these requirements.
History
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Mechanical, Electrical and Manufacturing Engineering