Even if there are measuring instruments and
vibration sensors, if you want to measure the vibration of a target structure
or facility (machine), where and in what direction should you attach the sensor
and acquire data? It can be overwhelming. In fact, there must be prior
knowledge with a lot of theoretical and empirical basis. Selection and setting
of measuring instruments and sensors are important, but data that is not measured
in the right location becomes the basis for error diagnosis. To summarize, the
answer would be ‘a point where the highest vibration state
(position, direction, behavior) can be read well, so evaluation is possible and
it is safe.’
계측기와 진동센서가 있더라도 막상 대상 구조 또는 설비(기계)의 진동을 측정하자고 하면 어떤 위치에 어떤 방향으로 센서를 부착하고 또 데이터를 취득해야 하는 것인가? 막막할 수 있다. 여기에는 사실 많은 이론적이며 경험적인 근거가 있는 선행지식이 있어야 한다. 계측기나 센서의 선택과 설정도 중요하지만 제 위치에 측정이 되지 않은 데이터는 오류진단을 내기에 근거가 되기때문이다. 줄여서 답을 말해보라면 ‘최고진동상태(위치, 방향, 거동)를 잘 읽을 수 있어서 평가가 가능하고 안전한 지점’이라면 정답일 것이다.
Displacement means the maximum distance that has changed around
the reference line, so it is easy to intuitively judge the maximum amplitude of
vibration, and since the amount can represent and replace the magnitude of
stress, the main unit of vibration used in various standards (displacement,
speed, acceleration). In particular, it is widely used for direct deflection and
vibration of structures, vibration monitoring of pipes, and ‘online monitoring’
of shaft behavior using sleeve bearings. This is also the reason why
low-frequency acceleration sensors are widely used. The sensor used at this
time is accurate and good for evaluation if it directly outputs the
displacement, but on the other hand, there are many cases where its use is limited due to
limitations in its installation, orientation, and frequency.
Understanding the diversity and
weaknesses of displacement sensors
Type of Displacement sensor
Image
Disadvantages and Features
Eddy current principle ,
Proximity probe, Gap sensor
- Responds only to conductors
- Interference between nearby sensors and nearby conductors (a gap of at least 3 times the diameter of the sensor must be provided)
-Vibration measurement below the unit of mm
Linear measurement below -1000Hz
LVDT
-Linear variable differential transducer (measuring the amount of movement of protrusions, bar type, wire type, )
- When measuring absolute displacement, it is necessary to manufacture a jig for the reference position
- Suitable for low frequencies (below 50Hz)
Strain gauge
Measuring micro displacement, vulnerable to noise
Laser doppler
High-frequency measurements (40 kHz) are very expensive
The person in charge of vibration
technology is bound to hear the question, “What is
the current vibration state of the object in question (equipment, structure,
building, etc.)?” from the manager or the person in charge of
other technologies. You can even ask about the results when you have only
measured and delivered the data. So always have an answer ready for your
question.
Basically, evaluating the state of an
object requires comprehensive judgment based on absolute, relative, and mutual
standards, so it is not easy to answer at the time. . The
evaluation of the state of this absolute standard is mainly
divided into good / bad / unacceptable, and the various expressions and
standards have been summarized.
Level display of vibration evaluation
First of all, the method expressed in overseas standards
and reference standards using graphs is that the British type is mainly marked
with ABCDE 5 grades, and ......
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.
In selecting a transducer, the most important priority
would be sensitivity and 'Frequency range'.
To
explain the vertical axis in the graph of amplitude and frequency, it would be
nice if one sensor could measure all amplitudes, but it can never be done
because it depends on the sensitivity. To explain the horizontal axis in this
way, if all sensors can measure all frequencies, there is no reason to select a
sensor. This is because each type of vibration sensor, further subdivided, each type of vibration sensor
(including an acceleration sensor) has its own frequency range of an area where
measurement is accurate. This is called the frequency range.
Frequency range
The sensor indicates the range of measurable frequencies
(e.g., acceleration sensor: 0.5 to 10 kHz), and the definition of this range is
slightly different depending on the user, so that the maximum and minimum frequency
ranges corresponding to the 'accurate zone' of the measurement can be selected.
This
frequency range is 'a reliable area where the sensor can output a properly
matched signal; It means 'the range of frequency response accuracy according to
the frequency sensitivity deviation', and although the meaning is slightly different from the
non-linearity of the sensor, this reliable area can be expressed as a linear
area.
Looking at the criterion of error amplitude related to
the frequency domain, select the rate of change of the amplitude limit that can
be judged by the presence or absence of an error, eg) ±3dB, ±5%, ±10%, etc.,
and the applicable frequency at this time is ..........
If
the desired 'unit' of vibration is selected as a result for the evaluation, the
next step is to select an appropriate 'sensor'. First of all, it is necessary
to check whether contact attachment is possible at the location to be
measured... or whether the attachment method is appropriate. For example, in
the case of high frequency (more than 5000Hz), the magnetic attachment method
is not appropriate, and if you want to measure the behavior of an axis, it is
difficult to use a contact sensor. Also, if you try to measure the speed and
measure 5 Hz using a coin type speed sensor, you will get an amplified error
signal. This is because the characteristics of each sensor are different depending
on the amplitude band, frequency band, resonance band, etc.
Selection of displacement, velocity, and acceleration sensors
such as amplitude units
Since the output voltage or current is proportional to
each unit, the selection of the unit is not very different from the selection
of the sensor. Sensors mainly used for diagnosing and monitoring equipment
1. Eddy current type displacement sensor (Proximity) that
directly measures the behavior of a shaft supported by a sleeve bearing in a
non-contact manner.
2. Accelerometer, which propagates shaft vibration to
rolling bearing and measures it indirectly by contact method outside the
bearing housing (indirectly transmitted to the housing by impact of the bearing
connected to the shaft)
3. There is a velocity transducer that works without power.
However, among these, the speed sensor is very precise,
but has a weakness limited to the range of 10 to 1000 Hz because it has a
natural frequency in the upper and lower frequencies, which is why displacement
sensors and acceleration sensors are widely used. (If it is out of this frequency
range, an erroneous or amplified value is output.)
A sensor (transducer) is one of the components
of a system that is mainly used by companies that use sensors to research,
diagnose or manufacture monitoring equipment. Since the manufacturer has
selected a sensor that fits a specific principle, the manufacturer has
accumulated a lot of engineering grounds for this. In many cases, the level of
engineering is considerably deeper than that of academia because it must be
required and the reliability of the measurement needs to be verified.
Above all, this principle can be considered as the most
basic sensor selection method. The reason why a displacement sensor is
called a displacement sensor and an acceleration sensor is called an
acceleration sensor is that each sensor generates an electrical output
'proportional to the amplitude unit'. It's because you do it. For example, since the value
converted to displacement by outputting acceleration vibration with an
acceleration sensor and integrating twice is not very accurate (especially when
it is not a sine wave), it is better to measure acceleration vibration with an
acceleration sensor, and displacement vibration It is basic to measure with a
displacement sensor. However, there are cases where it is absolutely necessary to
evaluate the health of a machine or the vibration of a building with the
'velocity' value, which is used as the most evaluation unit in academia and
industry, so this only allows integration from the acceleration sensor once.
Because the speed sensor isn't cool...
