There
are two primary types of velocity transducers, the moving coil type and
the piezoelectric type.
Moving Coil
Type -
Internally is
made up of a coil supported by springs and a permanently fixed magnet.
Coil movement
relative to the magnet generates a voltage proportional to the velocity
of that movement.
Generally require
no output signal amplifiers.
Moving
Coil Velocity Transducer
They are larger
than other types of transducers.
They are not reliable
for low frequency measurements without the use of a correction factor because
the coil tends to follow the magnet at low frequencies.
Balancing at lower
frequencies ok since the relative amplitudes recorded will be consistent.
Moving coil type
velocity transducers are also affected by magnetic fields that may be present
around electric motors or other magnetic sources and may require magnetic
shielding in order to be used in such an environment.
Piezoelectric
Velocity Transducer -
Similar to the
piezoelectric accelerometer in design with the obvious difference of the
output charge being proportional to the velocity of the transducer.
The charge is
created by the piezoelectric disks responding to the stress of whatever
applied forces there may be.
Generally require
a built-in amplifier due to the extremely small signal generated.
Piezoelectric
velocity transducers are generally not affected by magnetic fields and
can also measure accurately down to 60 cpm or less.
There are thermal
sensitivities and ambient temperature limitations involved.
