|LIST OF APPLICATION
|Ground Loop Causing Problem in
50 Hz interference in measured data; input stage damaged in the
equipment.....you may have experienced similar problems
in measurement when a few instruments are connected together; or when
transducers are installed on a AC powered machine.
typical measurement problem can easily be illustrated by the example of
large machinery or a production line. The following figure depicts a
situation where two measurement points (e.g. two motors in a paper
mill) are 50 meter apart from each other. A user would most likely
place the measurement system in the middle and run cables to the two
both devices under test (DUT) are connected to ground and the data
acquisition system (DAQ) too is grounded by its mains connection. One
would assume that the potential of each ground connection is identical.
This assumption is unfortunately not always true, mainly due to
load-switching combined with inappropriate ground wiring (too small
wire diameter, bad connections, etc.). The potential of the ground
connections at the different measurement points can change for a short
period of time.
these differences may occur only for a short period of time and with
only a few volts potential difference, the effect produced is still
significant. When two points with different potential are connected
using low-impedance cable (e.g. measurement cable), a current starts to
flow and the potential difference is equalized. Assuming a voltage difference
of only 1 Volt and a measurement cable with a resistance of 0.1 Ohm, a
current of 10 Amps will flow.
phenomenon is called ground loop and has the potential to damage
measurement equipment and DUT as well as to interfere with sensitive
of Dynamic Trigger
in Noise Monitoring
the day, the sound of a barking dog may be masked by background noises
such as passing cars, whereas during the night, when the
background noise is lower, this would more likely be perceived as an
performing unattended noise monitoring using a fixed trigger level which
is appropriate for capturing the higher level events, the dog
barks would be missed since they are below the trigger level, as shown
in the figure below.
is a technique in which event triggering tracks the background level;
and the event trigger level is equal to the background level plus an
the dynamic trigger method, we can select to track the L90 level
(background noise) and trigger a noise event when the measured level
exceeds the L90 level plus an offset, e.g. 15 dB. This permits the
capture of lower level noise events which occur during periods of low
background noise, as shown in figure below.
|LARGE DC OFFSET
Integration from Acceleration into Velocity or Displacement
may have experienced problem in large DC offset after integrating
accelerometer signal due to the DC offset in the accelerometer. The
problem is even more annoying when integating 2 times to displacement. When
Overall RMS or Crest Factor are calculated, the DC offset completely
distorts the values.
By using High
Pass Filter after integration, the DC offset can be
removed in subsequent post-processing calculation - RMS, Crest Factor,
FFT..... An example is illustrated in the workflow diagram below (using
nCode GlyphXE software.)
vibration monitoring techniques that are used on rotating machinery
have been unsuccessfully applied to reciprocating engines and
compressors for many years. The reason is that many typical faults on
reciprocating machinery are characterized by mechanical looseness,
which results in impacting or shock events in the machine. Since
impacts generally have little effect on the overall vibration level,
these faults are not detected at an early stage. As a result,
abnormalities are not diagnosed until damage has occurred and it is too
late to take simple corrective measures.
The monitoring technology of Reciprocating Machine Protector (RMP) is
based upon the detection and counting of mechanical
shock events that occur in or near the machine’s cylinder
assembly. The RMP compares the impact vibration against two predefined
threshold vibration levels, Alert (low) and Alert (high). The diagram below uses
impacts over Alert
(low). Impacts over Alert
(high) are shown as circles, which
are significant shocks caused by mechanical looseness.
frequency of occurence of shocks over Alert(high) and Alert(low) are
counted separately as
N(high) and N(low) respectively. Weighings are applied to these counts
but with greater
weighting on N(high) due to its higher severity level. Then the two
weighted counts are summed up to give the Reciprocating Fault Index (RFI),
which is a better and more reliable indication of machine health than
conventional impact transmitters based only on vibration level.
RFI = N (high) * Weighting (high)
+ N (low) * Weighting (low) + constant
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reading full paper of IMI RMP....
RMP Data Sheet
| Diagnosis of slow rotating machine
Rotating Machine for crane, cable car, escalator,
mill...may run in slow speed that
makes vibration analysis a challenging job.
The followings are the measurement requirements in general:
- In order to capture enough cycles of
the signal, long sampling duration
may be needed (e.g. 16 seconds)
- High sampling rate
(e.g. 50kHz) is used to
capture the vibration impact due to e.g. bearing fault (which means
900,000 data samples in 16 sec)
- To remove the dominated
low frequency signal due to rotor, gearbox.., High Pass
Filtering is needed (e.g. 500 Hz HP)
A slow rotating rotor (<40 rpm) driven
by electric motor
gearbox was found defective in bearing, which leads to high impacts in
vibration due to change in tooth meshing (impacts are
detected in figure below using ACMT
<Adash Compressed Time>, which can meet the above
Figure : Time Signal of good bearing using ACMT <Adash
: ISSN 1392 - 1207. MECHANIKA. 2011. 17(1): 71-77
Bearing Condition by gENV
|Fault in rolling element bearing
will generate vibration spikes, that can be deteced in early stage by
measuring Peak value of the bearing vibration (acceleration). However,
Peak value is extremely sensitive; and its low repeatability sometime
leads to false alarm and wastes time in investigation.
Other methods of detections using bearing resonance frequency to
amplify the vibration have similar issue as the Peak method.
represents the energy of the Envelope
Acceleration of the bearing vibration, that plays a
balance between early fault detection and repeatability. By trending the value of gENV,
bearing fault can be detected in early stage so that unscheduled
breakdown and secondary damage can be avoided.
Why 3-wire is preferred than 2-wire in 1/4 bridge measurement?
1/4 bridge measurement, if 2 wires are
connected to the strain gage (refer to the diagram below), the
resistances of the 2 cables become part of 1/4 bridge circuit. The
cable resistance can cause big offset
(especially for long cable) which can be compensated
in the amplifier, but not the measurement error due to
resistance change in varying temperature.
using 3 wire circuit (refer to diagram below), the change in
cable resistance will cause the same variation on both 1/4 bridges
(green and yellow); and the balance of resistance maintains.
Therefore the ouput voltage (Umeas)
is independent of the cable resistance and its variation due to
is some loss of sensitivity due to the cable
resistance, but can be compensated by increase of excitation
voltage through feedback control in amplifier.
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Why 6-wire is preferred than 4-wire in full bridge measurement?
cable to strain gauge sensor causes measurment error as 'excitation'
votage drops due to resistance of long cable. By using 'Sense'
leads which do not carry current, the
amplifier measures the feedback voltage in real-time; and increases
the excitation voltage until the 'sense' voltage reaches
the level required. This ensures accuracy in measurement especially
when cable resistance changes with varying temperature.
Below is an example of 6-wire
full bridge measurement circuit. For quarter-bridge measurement, 4-wire
circuit is preferred for the same reason.
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|Why using Carrier Frequency
Amplifier in bridge
environment, there are strong and frequent electromagnetic interference
from the surroundings - line voltage and its harmonics, high frequency
pulse... or even cellular phone, Carrier Frequency
can systematically and effectively mask the
intereference frequencies. By means of Amplitude
Modulation, amplifier filters and measures
only the signal within the narrow sidebands around the carrier
frequency, so interferences, e.g. due to thermal voltages in the
circuit are fully eliminated.
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