In
engineering - generally, the unit of ‘dB (decibel; dB)’ is used to express the ‘size’ of noise, vibration, electricity, etc. If
this value is large, it means that the value of noise and vibration is large.
Why were these units created? You can just call them pressure (pa),
acceleration (G), and voltage (V) as they are... The reason is that the unit dB easily represents the 'range of change in sensation.' So, strictly speaking, dB cannot be said to be a unit because it is a
value that varies depending on how the standard is set.
Weber-Fechner's law
The
change in human sensory quantity (P) can be best expressed as the ratio of the change amount to the reference stimulation quantity (S).
A sound source is a source that generates sound power.
If sound sources are classified in the form of poles, they can be classified
into monopole, dipole, and quadrupole sound sources. Among them, a
point source is a source that is radiated forming a spherical wave from a small
sphere that periodically expands and contracts as a result of a change in
volume, and is also called a monopole source.
In addition, when countless point sources form a straight line, this form like
a train is called a line source.
The power radiated from the sound source decreases
linearly in the area that can be assumed to be a free sound field (an area that
is not very close to the sound source and the sound pressure decreases
uniformly as the distance increases). '. In other words, the farther the distance, the
quieter the sound. However, the attenuation ratio of the sound power according to
the distance is different depending on the shape of the sound source. That is,
points decay quickly and lines decay slowly. explain this
distance attenuation according to the
sound source
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point, line and plane are explained by speaker, train
and wall.
Unlike temperature measurement, where you can select
only Celsius or Fahrenheit and press a button, you need to know a lot about
'vibration' before measuring. How to select units and how far is the frequency range? What
are the types of sensors and how to attach them? Also where? in which
direction? Everyone should know how to measure. As a metaphor, 'knowledge
is something that is easy for those who know it, unlike wisdom that is
difficult even if you know it.' If you know about the vibration, it is easy to
measure the selectivity of the instrument.
Distinct difference between vibrometer
(vibrometer and frequency analyzer FFT vibration analyzer)
Recent vibrometers that can perform simple spectrum
frequency analysis and trend storage are being sold at low prices, but in
general, vibrometers do not have as precise or diverse functions as FFT
(vibration frequency analyzer). Therefore, it is necessary to know the
practical optimal knowledge that can identify these differences, and refer to
the following.
item
vibrometer
Frequency
analyzer (FFT, multi-channel vibration analyzer)
Appearance
output
Types and uses of instrumentation
Magnet-attached or probe-type machine
surface vibration measurement, machine condition management
Select
the number of simultaneous measurement channels, whether to measure FRF
(resonance), machine diagnosis, long term recording (DAQ), rotor dynamics,
front-end or hand held, for Labs and machine analysis
Sensor selection
-The magnet attached type is better than
the probe stick type because measurement stability and high frequency are
possible.
-Sensitivity modifiable instrument (high sensitivity,
high impact measurement possible)
-Sensitivity
modifiable instrument (high sensitivity, high impact measurement possible)
-For low
frequency detection (using displacement sensor and DC sensor, or using
acceleration of 500mV/g or higher)
-General
type acceleration sensor (100mV/g)
-High
impact acceleration sensor (10mV/g)
Unit
This data is very
realistic and it is the result confirmed in the actual domestic industrial
field, but it can be subjective and biased, so please refer to it.
When analyzing the frequency, it is the first step to
distinguish the relationship with the number of revolutions. Among them, harmonics that are compared to
synchronous components and are not calculated as integer multiples of TS
(turning speed) are called asynchronous(Non-synchronous) components. It refers to a frequency
component that is both related to the number of revolutions and has nothing to
do with the number of revolutions at all. Among them, the case related to the
number of revolutions is a frequency generated by rolling bearings, belts,
chains, etc., and is an asynchronous component because it does not fit into an
integer multiple. On the other hand, frequencies that are not related at all to
rotational speed correspond to frequency groups caused by resonance, electrical
faults, fluid excitation, and friction, which are also asynchronous components.
Non Synchronous faults
For patterns and signals of defects whose frequencies
are not integer multiples of TS or are not related at all, just look for those
that are not related to rotation events. That is, the inner and outer rings of
rolling bearings do not fit in integer multiples because they continue to
rotate according to a certain pattern formula when the shaft rotates once. Also, in the case of
other shafts connected by a belt or in the case of the oil whirl, although it
is related to the increase or ......
Observation of asynchronous components is generally not
good.
In particular, frequencies below 1X TS are not normal at all. Oftentimes, it is
a defect in itself. It is important to know how it was in the past and how it
compares to similar machines, but it is important to analyze and find the cause
because it can be a serious matter that needs to be proven through additional
tests.
In the spectrum, which is a graph for vibration
analysis, “How many Hz should the maximum frequency be set?”, “What is the
Fmax?”, “How much is the sample frequency?” If you are hearing these questions,
you need to understand what the Max frequency is. Inquiries related to this can
be divided into two types. One may ask what the term 'maximum frequency' itself
is, and the other will mean how much to set the maximum frequency. Here we will
try to answer your questions related to this.
The maximum analysis frequency (Max
frequency) should be placed so that the frequency of interest is centered...
Fmax=2.56*Fs
maximum frequency; The maximum analysis frequency is
expressed as ‘fmax’ and is t............
For systems (online,
offline) used for the purpose of efficiently performing maintenance
(maintenance) by monitoring the state, reducing safety failures, and predicting
the time of failure, a sensor that observes the state must be selected. There
are many transducers (sensors) that convert these physical characteristics into
electrical quantities, such as temperature sensors, pressure sensors, force
sensors, flow sensors, position sensors, ultrasonic sensors, and current
sensors. It is very important because it contains information, and it can be
seen as the first gate that plays the role of the five senses (eyes, nose,
mouth, touch, taste) that can check its stability or abnormal state. Therefore,
if
this sensor is not properly selected, the state cannot be represented and the alarm setting
is also in error, so unnecessary and important values cannot be trusted.
Considerations
when selecting a vibration sensor for condition monitoring
The installation location and direction of the vibration sensor
and the installation method are important, but first of all, the selection of
the sensor is very important. It is basic to select a sensor according to the
object to be monitored, but looking at other considerations, it can be
summarized as follows.
1. It is not a high-speed, large-scale, critical facility, it is
frequently disassembled and assembled, contains sufficient intermediate frequency,
and it is convenient to select a magnet-attached acceleration sensor that measures the housing.
An accelerometer is the easiest
transducer to measure vibration. Since the measurement direction of this
transducer has already been determined, the sensor must be placed in the
desired direction for measurement. It doesn't really matter if it's up or down.
This is because vibration is repeated and if you want to know the relative
movement, you have to use two or more sensors that measure it at the same time anyway.
In this case, the concept of a phase is only additionally required. However,
compared to a single axis vibration sensor, a triaxial vibration sensor
literally means a sensor having the ability to measure the magnitude of
vibration in three axes at the same time. It is
easy to measure and you will be able to identify vibrations in all directions
in a short time.
Advantages and disadvantages of
Triaxial Accelerometer
Triaxial is described as each three-dimensional orthogonal direction
in Z, X, and Y directions (H, V, A directions in the case of vibration
diagnosis of rotating equipment) that are perpendicular to each other. The axes
in each direction have an initial phase difference of 90 degrees from each
other, and the 3-axis vibration acceleration sensor outputs the amount of
vibration acceleration in each direction as voltage. Therefore, it can be said
that the 3-axis vibration sensor has the following advantages.
First of all, it is possible to measure the vibrations in all directions at the same time. It measures the
vibration in each axis direction at the same time without the need to measure
it sequentially with several different sensors or at different times, so it is
possible to prove the simultaneous state at the time and shorten the measurement
time.
Second, it is possible to measure -----
On the other hand, the 3-axis vibration acceleration sensor is at
least twice as high expensive as the combined
price of all three single-axis sensors, and the average probability of failure (MTBF) of the single-axis sensor is shortened to 1
year compared to the manufacturer's recommended 3 years. do. However, in fact,
the piezo-type accelerometer is more robust and less durable than expected, so
it is not easy to check the broken state. Recently, like a gyro sensor, a
modularized sensor capable of measuring 6-axis vibration has already appeared,
and like MEMS, which basically employs 3 axes,
new-technology vibration sensors with clear advantages
of low frequency have already started to lead in price. The day when
MEMS sensors will be used more often than piezos can be seen in the near
future.
