Applying Bayesian Networks in Making Intelligent Applications For Static and Dynamic Unbalance Diagnosis
Applying Bayesian Networks in Making Intelligent Applications For Static and Dynamic Unbalance Diagnosis
Applying Bayesian Networks in Making Intelligent Applications For Static and Dynamic Unbalance Diagnosis
Dedik Romahadi1,2, Muhamad Fitri1, Dafit Feriyanto1, Imam Hidayat1, Muhammad Imran3
1
Department of Mechanical Engineering, Faculty of Engineering, Mercu Buana University, Jakarta, Indonesia
2
School of Mechanical Engineering, Beijing Institute of Technology, Beijing, China
3
School of Material Science and Technology, Beijing Institute of Technology, Beijing, China
Corresponding Author:
Dedik Romahadi
Department of Mechanical Engineering, Mercu Buana University
Jl. Meruya Selatan No. 1, Kembangan, Jakarta Barat 11650, Indonesia
Email: dedik.romahadi@mercubuana.ac.id
1. INTRODUCTION
The vibration occurs due to damage to shafts, bearings, gears, lack of connection tightness, lack of
smooth lubrication, and the imbalance of rotating machine elements or unbalance [1]–[5]. Vibration in a
machine is very important to pay attention to because, from a vibration, many errors arise and damage the
machine's components. In the past, vibration analysis required full-spectrum instruments to identify frequencies
at which vibrations were dominant [6]–[8]. The operator then compares the peak frequency with the operating
speed and converts it to a graph to determine the possible causes. One of the advantages of the method is that
the operator can gradually learn how a piece of equipment vibrates and why certain problems occur at the same
multiple rotational speeds [9].
The latest generation of vibration meters has more capabilities and automatic functions than its
predecessors. Many units simultaneously display the full vibration spectrum of the three axes, providing an
idea of what is happening with a particular machine [10]–[13]. Although today's vibration meters offer many
automated features and capabilities, they still require a basic understanding of vibration analysis to use them
effectively. Each parameter in the vibration data collection can indicate the type of damage in the machine. For
this reason, proper analysis can produce appropriate recommendations for handling any engine failures that
occur [14]–[16]. Many vibration measuring instruments do not have intelligent diagnostic features, especially
for the type of unbalanced damage. Users must understand the basic concepts of vibration analysis for types of
damage. An expert also still needs time in the process of diagnosing unbalanced damage, because most tools
only provide unbalance damage parameters and users still need to carry out a series of data analysis processes
until they can decide the unbalance damage that occurred [17].
A Bayesian network (BN) is a probability graphic structure that depicts a causal relationship between
interrelated variables. There are four things that BN can offer as a method: first, BN can easily deal with
incompleteness or problems with data [18]. Second, BN allows one to learn about causal relationships. The
learning process becomes important when we try to understand the domain of the problem. Third, BN can
facilitate the combination of domain knowledge and data. Lastly, BN offers an efficient and principled
approach to avoiding overfitting the data [19]–[24]. Modeling in BN involves two steps, namely creating a
network structure and estimating the probability value of each node. One of the programs that can be used to
build the BN algorithm is to use matrix laboratory (MATLAB). Seeing the advantages that BN has as a
decision-making tool and supported by several journals on the same topic, BN was chosen as a method for
making intelligent applications for the diagnosis of static and dynamic unbalance [25]–[29].
Several studies have been conducted related to this research topic. Research to analyze unbalanced
damage, as well as other damage that may arise when the motor is operating [30]. Research that aims to detect
the location of damage that occurs in the classifier by using vibration signal analysis and measuring the
magnitude of the vibration and presenting it in the form of a frequency domain (spectrum) using the fast Fourier
transform or FFT [31]. The engine condition assessment refers to the ISO 10816-3 standard in velocity and
acceleration modes. Based on the data spectrum analysis, the dominant damage lies in the unbalance rotor.
Associated with expert systems in the field of vibration analysis, Cobb and Li [32], Li et al. [33]; Amrin et al.
[19]; Sahu and Palei [34] emphasize that BN is highly reliable when dealing with uncertainty. BN is a suitable
method for developing intelligent applications in the field of vibration diagnosis.
Based on some of the problems that mention on the second paragraph and the advantages of the BN
method. The author assesses the importance of making an intelligent system to analyze a malfunction in a
machine. For this reason, through this study, the author aims to create an intelligent system to analyze one of
the common malfunctions that occur in a machine, namely unbalance using BN modeling which will be
implemented in the MATLAB application.
2. METHOD
The implementation of this research can be divided into several stages. In general, the stages are
preparing the concept of an algorithm scheme, modeling BN according to the concept of unbalance diagnosis,
making a program in MATLAB, and testing the system that has been made. BNs are constructed using a
statistical method known as Bayes' theorem. This theory employs conditional probability, which is the
likelihood of an event A if it is known that event B has already occurred. The symbol for conditional probability
is P(A|B). The conditional probability can be derived from (1). In addition, there is a joint probability, given
by (P(A∩B)), which represents the likelihood that events A and B will occur. In (2) shows the joint probability.
𝑃(𝐵|𝐴)𝑃(𝐴)
𝑃(𝐴|𝐵) = (1)
𝑃(𝐵|𝐴)𝑃(𝐴)+𝑃(𝐵|𝐴̄)𝑃(𝐴̄)
The network topology can be obtained by encoding the expert domain's subjective knowledge. One
arrow can connect the greatest sequence of nodes to the lowest sequence of nodes if a condition is met. Each
arrow is prevented from connecting the lowest node order to the highest node order by the algorithm. If the
scheme is applied to other variables, the network topology may change. Many topologies are possible due to
the fact that multiple arrows can connect various pairings of nodes. Instead, the variable may be subdivided
into many causative variables, with arrows then connecting each cause variable to the corresponding effect
variable.
The BNs network inference can be shown in (3). A basic example involving a three-node network,
X → Y → Z. If we have a proof for the root node, X = x, updating in the same direction as the arc is a
straightforward application of the chain rule based on the network's implied assumption of independence.
Based on the input data obtained, the probability value will be calculated through several stages using the BN
method, starting from determining the parameter value for each damage symptom, then determining the
conditional probability value, after two values are obtained, the system will calculate the combined and
posterior probability values for each type of damage. unbalance adjusted for the BN structure and the posterior
probability value is used as a probability inference of the type of unbalance damage. BN generates relational
Applying Bayesian networks in making intelligent applications for static and dynamic … (Dedik Romahadi)
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information and conditional probabilities through bidirectional propagation between input and output nodes.
Also, in real case implementations, it is common to use multistate nodes. So, from the consultation carried out
by the user, then get the type of unbalance damage that occurred on the machine and the error percentage.
Data preparation in the form of machine specifications and vibration data. This data is prepared as
input and training data when creating the system. Then describe the components of the vibration spectrum to
obtain output data that can be read as input for BN. The entered spectrum data is then processed to find the
frequency of each machine component. Machine specification data will be the reference for frequency
calculations and will be adjusted to the spectrum. If the calculated frequency value is found in the spectrum, it
will be input for evidence to BN, while if the frequency value is not found in the spectrum, it will be input for
no evidence to BN. Then the input data for BN are grouped into 2 types. The frequency line that is proven to
exist according to the frequency calculation becomes input type 1 (true). This input has the requirement that
the frequency amplitude must be 2 mm/s in root mean square (RMS). Meanwhile, for data that is not proven
to have a frequency line according to the calculation or the value of the frequency amplitude < 2 mm/s RMS,
it becomes input type 2 (false). Furthermore, the engine specification data and the input type grouping process
are then entered into the BN calculation so that it will provide information on the diagnosis results such as
vibration status and type of unbalance.
Vibration data was obtained from the measurement results of the demo machine. Measurements are
made on the horizontal, vertical, and axial axes at the drive end and non-drive end bearing locations. When
taking the overall vibration value and spectrum, the demo machine is conditioned to experience static and
dynamic unbalance problems. The measurement and reference data will be used as a reference in building the
BN system. The finished system will be tested using some random demo machine measurement data. Validate
the results of the system diagnosis against the entered vibration data and look for the cause of an incorrect
diagnostic result is found.
(a) (b)
Figure 3. Condition of unbalance setup, (a) static unbalance and (b) dynamic unbalance
Applying Bayesian networks in making intelligent applications for static and dynamic … (Dedik Romahadi)
178 ISSN: 2252-8938
NDE H with NDE V). After finishing building the BN structure, the next step is to determine the probability
value of each combination of event components that occur. The conditional probability table (CPT) for static
unbalance and dynamic unbalance nodes can be seen in Table 2 and Table 3.
computational result of the BN algorithm without any evidence provided. It can be seen without proof that the
static unbalance probability value is 26.8% and produces a dynamic unbalance probability of 22.7%. All
probability values are still less than 50%, this indicates that there is no damage to the centrifuge.
In ensuring that the prediction results are correct based on the concept of vibration analysis, a series
of experiments are carried out and validate the results. Figure 6 shows the renewal of confidence because new
information is provided with evidence of large amplitude symptoms at the 1X frequency and the phase on the
horizontal-vertical axis is more than or equal to 70º and less than or equal to 110º. The probability value of
static unbalances increases to 75% and dynamic unbalances to 41%. So, it can be concluded that the machine
has a static unbalance problem because it has a larger percentage compared to the dynamic unbalance
percentage value. If we review the results of the system from the inputs given, the results are same by theory
and manual analysis. Probability updates due to the new information provided with evidence of large amplitude
at 1X frequency, impeller thickness smaller than or equal to impeller diameter, and phase in DE-NDE bearings
smaller than or equal to 20º and more than or equal to 340º as shown in Figure 7. It can be seen that the
probability value of static unbalance increases to 80% and the percentage of dynamic unbalance becomes 40%.
From these results, it can be concluded that the damage to the machine is static unbalance.
Applying Bayesian networks in making intelligent applications for static and dynamic … (Dedik Romahadi)
180 ISSN: 2252-8938
value of 5 mm/s RMS, an impeller thickness of 20 mm, an impeller diameter of 100 mm, the phase between
the two bearings on the same axis is 12º and the phase between the horizontal and vertical axes is 98º. The
column of the spectrum graph shows a large amplitude at 1 time the impeller frequency. Then the system
produces a diagnosis that the problem that occurs is static unbalance with a percentage value of 90%. The
system recommends balancing using single plane mode.
The application operation experiment using the data from the measurement results of the demo engine
which is set to experience dynamic unbalance is shown in Figure 10. As seen in the input data column, the
vibration value is 4.7 mm/s RMS, the impeller thickness is 120 mm, the impeller diameter is 100 mm, and the
phase between the two bearings on the same axis is 45º and the phase between the horizontal and vertical axes
is 15º. The column of the spectrum graph shows a large amplitude at 1 time the impeller frequency. The system
produces a diagnosis that the problem that occurs is dynamic unbalance with a percentage value of 90%. The
system recommends being balanced using the two-plane mode. The results of all experimental data in detail
can be seen in Table 4.
Applying Bayesian networks in making intelligent applications for static and dynamic … (Dedik Romahadi)
182 ISSN: 2252-8938
The demo machine before a series of measurements is set up with several conditions and will be
compared with the predicted problem results issued by the system, as shown in Table 4. The system diagnosis
results are by the actual state of the demo machine. In the fourth experiment, the results of the system diagnosis
are not the same as the actual conditions, this is because the system is only prepared to read the unbalance
problem. Even so, the system can still recognize that misalignment damage is another type of damage.
Therefore, it can be concluded that the system that has been designed can predict the state of the machine based
on vibration data quickly and accurately.
4. CONCLUSION
BN-based software for the prediction of unbalance damage has been developed. The application of
BNs in intelligent system design begins with making network modeling. Then the percentage of unbalance
values is made based on the value of each parameter using the MSBNx software. The percentage results that
have been collected will be used as a reference when creating the BN algorithm in the MATLAB application.
In addition, a function is also made to enter data that will be used as the basis for calculating the percentage of
unbalance in the system. Then an application was made with the AppDesigner menu in MATLAB so that the
program that had been created could be installed on the computer and operated easily. BN Modeling was
successfully built based on the concept of vibration analysis. Four nodes have been defined as input parameters
and two nodes as output. The six nodes are connected according to their function so that it becomes a system
that can predict the type of unbalanced damage to the machine. From six trials using different data, all data
entered can be predicted by the system quickly and accurately. All software features also work properly.
ACKNOWLEDGEMENTS
The authors sincerely thank the financial support provided by Mercu Buana University, contract No.
02-5/535/B-SPK/III/2022.
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BIOGRAPHIES OF AUTHORS
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184 ISSN: 2252-8938