Page 17 - Prosig Catalogue 2005

HARDWARE PRODUCTS
MADE TO MEASURE

Made To Measure accelerometer is not exceeded at any
point during the test.
What types of measurements can I make Microphones
with a P8000 system? Microphones are used to measure
variations in atmospheric pressure.
he purpose of this article is to introduce the different types of Variations in pressure that can be detected
transducers that can be used with the Prosig P8000 series data by the human ear are considered to be Training & Support
Tacquisition system. The article deals with the design and function of sounds.
the different types of transducer and the applications they are normally Acoustics is the science behind the study
associated with. of sound. Sound can be perceived as
Accelerometer pleasing to the ear or to be undesirable.
An accelerometer is a device that measures acceleration. It is normally The human auditory system normally has
attached directly to the surface of the test specimen. As the object moves, a maximum range of 20 Hz (or cycles per
the accelerometer generates an electric current that is proportional to second) to 20 kHz, although this range generally decreases with age.
the acceleration. Acceleration and vibration are similar but not the Sound pressure variations in that frequency range are considered to be
same. If a material or structure has a vibration then it will be subject to detectable by the human ear. A microphone must be able to detect all of
certain accelerations. The frequency content and the magnitude of these these frequencies and in some cases more. Sometimes sound pressure
accelerations are directly proportional to the vibration. variations outside that frequency range can be important to design
engineers as well.
The main type of accelerometer is a piezo electric type. The official name
for this type is IEPE, which stands for Integrated Electronic Piezo Electric. The main types of microphone are the condenser microphone, carbon Condition Monitoring
This is often referred to by product names such as Piezotron®, Isotron®, microphone, magnetic microphone and the piezoelectric microphone. The
DeltaTron®, LIVM™, ICP®, CCLD, ACOtron™ and others, which are all condenser microphone is the most widely used in situations where quality
trademarks of their respective owners. The acquisition equipment must and accuracy are required. It is capacitive in its design and it operates
include suitable power supplies and signal conditioning to work with the on the transduction principle in which a diaphragm that is exposed to
internal electronics of these transducers. pressure changes moves in relation to the pressure fluctuations. Behind
the diaphragm there is a metal plate, usually called a back plate. This
IEPE transducers are usually based on quartz crystals. These transducers back plate has a voltage applied across it and is effectively a capacitor.
normally have the crystal mounted on a mass. When the mass is subjected As the diaphragm moves closer or further away from the back plate the
to an accelerative force a small voltage is induced across the crystal. This charge on the plate changes. These changes in charge are then converted
voltage is proportional to the acceleration. to voltages.
There are also many other types of accelerometer in the market: A vibration can be considered to be a rapid motion of a particle or a
capacitive types, piezoresistive, hall effect, magnetoresistive and heat fluctuation of a pressure level. NVH studies are concerned with the study
transfer types. of vibration and audible sounds. These studies focus on reducing the Software
Accelerometers normally come in two distinct packages, side and top excitations and thus the amplitudes of the sounds, and reducing the
mounted. The side mounted package has the interconnecting cable transitions between large changes of frequency or shocks. Microphones
are used in much wider fields, however, and in some cases they can be
or connector on the side and the top mounted package has them on
the top. Different circumstances require different mounting methods. used to acquire a sound so that it can be amplified later. Microphones are
generally used in any area of testing where the precise input to the human
Accelerometers can come in one of two different types: single axis and
triaxis. The single axis accelerometer measures acceleration in one auditory system needs to be measured, for example environmental noise
direction, where as the triaxial accelerometer measures the acceleration or inside an aircraft cabin.
in the classical 3 dimensional Strain Gauges
planes.
A strain gauge is a transducer used for determining the amount of strain,
Mounting methods are very or change in dimensions in a material when a stress is applied. When the
important for an accelerometer. transducer is stretched or strained its resistance changes.
It is normally best not to rely
on just one attachment method The most common type of strain gauge is the foil gauge type. This is Hardware
from a reliability point of view. effectively an insulated flexible backing that has a foil pattern upon it.
The mounting method for an This foil pattern usually forms a 2 wire resistor. The gauge is attached to
accelerometer is effectively the structure under test by way of an adhesive. As the structure under
an undamped spring and the test is deformed the foil is also deformed. This deformation causes the
frequency response effects of this length of the foil pattern lines to change. This change in turn affects the
spring on the frequency related gauges resistance.
magnitude of the measured In order to measure this very small change the gauge is normally
vibration must be considered. Consequently, there is no single attachment configured in a Wheatstone bridge.
method that is best for all cases. The most widely used attachment is a
sticky wax, although super glue is also very popular. Both have different Strain gauges are very sensitive to changes in temperature. To reduce
frequency transmission characteristics as well as other advantages and the effect of this potential corruption a Wheatstone bridge is used with
disadvantages. voltage supply sensing. This reduces the effect of temperature changes.
Accelerometers are probably the most widely used transducers in any Strain gauges are also available as thin film types and as semiconductor System Packages
investigative work on a structure of any kind. NVH (Noise, Vibration & types. The thin film types are used in higher temperature applications
Harshness) testing is just one of many fields that make use of them. where they are applied directly on to the surface of the structure. This has
If there is a requirement to find the frequency of a vibration then an the additional advantage of not disrupting airflow in aerodynamic design.
accelerometer would be the obvious choice. It is important to make sure The semiconductor types of gauges are referred to as piezoresistors.
the frequency that is being investigated is within the usable range of These gauges are used in preference to the resistive foil types when the
the accelerometer and that the maximum amplitude capability of the strain is small. The piezoresistors are the most sensitive to temperature

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