The Accelerometer
The accelerometer is based on the electrical properties
of piezoelectric crystal.In operation,the crystal is
stressed by the inertia of a mass.The variable force
exerted by the mass on the crystal produces an electrical
output proportional to acceleration.Two common
methods of constructing the device to generate a residual
force are compression mode and shear mode
respectively.A residual force is of course required to
enable the crystal to generate the appropriate response,
moving in either direction on a single axis.A shear mode
construction is illustrated below. An accelerometer operates below its first natural
frequency.The rapid rise in sensitivity approaching
resonance is characteristic of an accelerometer,which is
an un-damped single-degree-of-freedom spring mass
system.Generally speaking,the sensitivity of an
accelerometer and the ratio between its electrical output
and the input acceleration is acceptably constant to
approximately 1/5 to 1/3 of its natural frequency.For this
reason,natural frequencies above 30KHz tend to be used.The frequency response curve can be influenced by anumber of factors,mainly the mass,the stiffness and the
degree of system
damping.The resonant peak of the
accelerometer can be eliminated by increasing the
damping.However,increasing the damping introduces a
phase shift in the linear range whereas un-damped
accelerometers have very little phase shift until near the
natural frequency.It is therefore usual to have un-damped
accelerometers with very high natural frequencies so that
the linear range is extended as far as possible.Typical
damping ratios are 0.01 to 0.05.This resonant frequency in combination with theappropriate damping can be utilised to monitor bearing
impact.Several manufacturers,including Sensonics have
developed transducers that utilise the ‘ringing’ of the
transducer to mechanical impulses to measure the healthof
roller bearings.This technique analyses the high
frequency response of the transducer (at resonance) to
determine an ‘impact factor’ normally in dB,which isdirectly proportional to the quantity and force of metalon metal impacts.This factor is normalised to an overallhealth measurement by consideration of bearingdimensions and rotational speed.
Shear mode construction
Typical accelerometer frequency response Shear mode devices which apply a shear force to theinner and outer surfaces of a ring of crystal (as opposed
to a perpendicular force to a disk of crystal),offer a
distinct advantage over standard compression techniques.
When mounting the device to the plant,normally through
a stud & screw arrangement,the mechanical stresses
within the transducer assembly change.Compression
mode devices are particularly affected by these stresses,
which produce low frequency effects,compounded if
further integration is carried out.Sensonics shear mode
range of transducers are unaffected by base strain and
offer a true low frequency performance down to 0.4Hz.Although the
piezoelectric accelerometer is a self-generating device,its output is at a very high impedance
and is therefore unsuited for direct use with most display,
analysis,or monitoring equipment.Thus,electronics must
be utilised to convert the high impedance crystal output
to a low impedance capable of driving such devices.The
impedance conversion electronics may be located within
the accelerometer,outside of but near the accelerometer,
or in the monitoring or analysis device itself.Accelerometers with internal electronics are convenientand can use inexpensive conventional plugs and cable but
they are limited to temperatures of typically 120
º
C.Locating the electronics in a cool location away from the
accelerometer allows the transducer to tolerate higher
temperatures.
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