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Combining Process and Condition Monitoring Data
M.Sc. (Eng) Esko Juuso and Prof. Kauko Leiviskä
University of Oulu, Control Engineering Laboratory, Finland
Abstract
Early detection of fluctuations in operating conditions and fault detection can be done with
similar methods. Signal processing is needed for the condition monitoring measurements, and
interpolation is some process measurements and especially laboratory analysis. Effective time
delays are very important in process data. Feature extraction uses statistical analysis, and the
methods can be based on generalised norms and moments. Intelligent condition and stress
indices are calculated from these features by nonlinear scaling. The new scaling approach,
which also uses the norms and moments, improves sensitivity to small fluctuations. In the
condition monitoring cases, the condition indices are consistent with the vibration severity
criteria, which originate from VDI 2056. Only one norm was needed in the cavitation
analysis, and the resulting index can be used in power control. The same methodology
provides good results in detecting fluctuations of flavour ingredients in brewing, in predicting
web break sensitivity in paper machines and in intelligent analysers of the process conditions
in wastewater treatment. Linguistic equation (LE) models of the normal case suit for detecting
fluctuations, and case-based reasoning (CBR) is used if specific case models can be
developed. The overall procedure includes following steps: (1) select informative features, (2)
scale the features, (3) calculate intelligent indices, and (4) combine indices in models.
Keywords: Intelligent methods, statistical analysis, fault diagnosis, condition monitoring,
process industry
1Introduction
Production of high quality products, more efficient use of energy and raw materials, and
stable operation in different operating conditions require advanced automatic systems for
detecting faults and fluctuations in process operation. Interactions between process variables
Revised manuscript sent to congress materials
and various time delays depend strongly on operating conditions and can dramatically limit
the performance and even destabilise the closed loop system. Paper machines can be run at
desired speed with the least possible amount of breaks and good uniform quality when
runnability is good [1]. Efficient use of fermentation vessels is crucial in brewing economy
since fermentation is the most time consuming step in the production of beer, especially
continuous fermentation requires fast and precise response to any changes in process
conditions seen as fluctuations of flavour ingredients [2].
In general, recognizing the most important machines for production and theirs appropriate
health monitoring bring in considerable benefits through more operating time and fewer
shutdowns. Early detection of fluctuations is important as overhaul before breakdown is in
many cases more effective than run to failure method. Automatic fault detection provides
more time to identify root causes when a system takes care of some routine tasks [3].
Measurable changes can happen e.g. in machines vibration, temperature, power consumption
and number of particles in lubricant. Generally, vibration measurements are the most widely
used methods. Generalised moments were calculated for a specific sample time in [4]. The
root mean square (rms) values and peak values are most widely used features, see [5] for more
application experiences. Generalised norms, which were introduced in [6], have been used in
scaled form in condition indices [7]. Nonlinear scaling is an essential part of many
applications [8, 9]. Recently, the scaling methodology has been improved [10], and the new
methodology has been extended to normal process variables [11].
Theoretical development and industrial applications on fault diagnosis are continuously active
in many areas. Faults in sensors, actuators, controllers, control loops, process equipment,
process parameters, as well as operator-induced faults have been described various ways, and
also a wide variety of methods have been used in fault detection [12]. Functions and features
can be realised with various methodologies, and there are also numerous ways to combine the
overall system [13]. The fault detection problem is, in principle, a classification problem that
classifies the state of the process to normal or faulty. This is based on thresholds, feature
selection and classification and trend analysis. Fault localisation starts from the symptoms
generated in the previous stage and it either reports fault causes to the operator or starts
automatically the corrective actions.
Detecting operating conditions can be done with similar method as fault detection although
the classes do not necessarily correspond to any fault [13]. Connection between vibration and
machines health can be used for fault diagnosis by using advanced signal processing and
feature extraction methods. Increasing number of measurement points and more demanding
problems require automatic fault detection. Model-based approach is used in complex
systems. Final goal is to use these indicators in control. Indirect measurements have been used
directly in control, e.g. fuel quality in lime kiln control [8] and water quality in water
treatment [14]. Simulation tests on using cavitation indices in power control of a water turbine
are promising for combining condition monitoring and control [15].
The frequency range of phenomena studied in fault detection varies tremendously; from
milliseconds in vibration analysis to hours and days in analysing the slowly developing
process faults. Especially for slow processes met in chemical, pulp and paper and biotechnical
industries, temporal reasoning and trend analysis are very valuable tools to diagnose and
control the process.
This paper presents a nonlinear scaling approach which can be used as indirect measurements
and extended to control and case-based reasoning. Results of applications in condition
monitoring and process analysis are summarised.
2Signal processing and feature extraction
Features can be defined by
,)
1
()( /1
1
)(/1 p
N
i
p
pp
p
p
i
x
N
MM å
=
==
a
a
t
a
t
(1)
where
Â
Î
a
is the order of derivation, the order of the moment
Â
Î
p is non-zero, and
t
is
the sample time. The sample has Nsignal values. This norm, which has the same dimensions
as the signal )(
a
x, is defined in such a way that
.
¥
<
£
¥
-
p
The norm (1) is a Hölder mean,
also known as the power mean. The norm values increase with increasing order, i.e.
,)()( /1/1 qqpp MM
a
t
a
t
£(2)
if
q
p
<
. The increase is monotonous if all the signals are not equal. The absolute mean, the
rms value and the absolute harmonic mean are special cases where the order
p
is 1, 2 and -1,
respectively. When the order 0
®
p, we obtain from (1) the absolute geometric mean. The
norm (1) represents all the norms from the minimum to the maximum, which correspond the
orders
-¥
=
p
and
¥
=
p
, respectively. [6]
The sample time
t
is an essential parameter in the calculation of norms. The sample time and
the number of samples should be chosen on the basis of process and faults. Short sample
times and relatively small requirements for frequency ranges make this approach feasible for
on-line analysis and control. The computation of the norms can be divided into the
computation of equal sized sub-blocks, i.e. the norm for several samples can be obtained as
the norm for the norms of individual samples. The same result is obtained using the norms:
[ ]
,)(
1
)(
1/1
1
/1
1
/1
p
K
ii
p
S
p
K
i
p
p
i
p
S
p
p
KSS
SM
K
M
K
Mú
û
ù
ê
ë
é
=
þ
ý
ü
î
í
ì
=åå ==
a
t
a
t
a
t
(3)
where S
K is the number of samples
{
}
N
i
i
x1
)(
=
a
.
In condition monitoring, the signals are velocity )1(
x, acceleration )2(
x and higher derivatives,
)3(
xand )4(
x. The other signals are obtained from acceleration through analogue or numerical
integration and derivation, see [6]. Variables can be selected by comparing health indices,
denoted by SOL. These indices are calculated for each feature by dividing the reference value
by the feature value obtained for the case under consideration. The reference value is the
corresponding feature value in the good conditions. This is done separately for each rotation
speed, and the decisions are based on the average.
The norm (1) was introduced for signals in [6], but it can also be used for normal process
measurements [11]. The derivation is not used for the process data if the measurements are
not very frequent. In addition, data acquisition systems normally use the norm (1) with 1
=
p,
and the averages for longer intervals are calculated by (3). If all the signal values are positive,
also the norms generalised to any real-valued order produce real-valued features. To obtain
dimensionless features, these norms must be normalised.
3Detecting operating conditions
Process and condition monitoring data is combined in detecting operating conditions (Fig. 1):
normal process measurements are directly used in feature extraction, signal processing is
needed for the condition monitoring data, and some infrequent measurements need to be
interpolated.
Figure 1. Detecting operating conditions and faults.
3.1 Nonlinear scaling
The basic idea of the linguistic equation (LE) methodology is the nonlinear scaling developed
to extract meanings of variables from measurement signals. The scaling function scale the real
values of variables to the range of [-2, +2] which combines normal operation [-1, +1] with
handling of warnings and alarms. The scaling function contains two monotonously increasing
functions: one for the values between -2 and 0, and one for the values between 0 and 2. Both
expertise and data can be used in developing the mapping functions (membership definitions).
Functions consist of two second-order polynomials )( j
Xf j
- and )( jj Xf +, which are defined
by points
{
}
.)2),(max(),1,)((),0,(),1,)((),2),(min( jjhjjlj xcccx -- (4)
The analysis of the corner points is based the norms (1) and the skewness of the data. The
value range of j
x is divided into two parts by the central tendency value j
c and the core
area, [ jhjl cc )(,)( ], is limited by the central tendency values of the lower and upper part.
There are problems when the value range is very wide or the distribution is very concentrated.
The new approach introduced in [10] is based on the generalised moment
(
)
,
)(
k
X
k
p
p
k
MXE
s
ga
ta
ú
û
ù
ê
ë
é-
=(5)
where X
s
is calculated about the origin, and kis a positive integer. Since this analysis is
done for the features, derivation is not used, i.e. 0
=
a
. The central tendency value is chosen
by the point where the skewness 3
g
changes, i.e. 0
3=
g
. Then the data set is divided into two
parts: a lower part and an upper part. The same analysis is done for these two data sets. The
estimates of the corner points,
(
)
j
l
cand
(
)
j
h
c, are the points where the direction of the
skewness changes. The iteration is performed with generalised norms.
Only monotonous, increasing functions can produce realisable systems to be used in a
continuous form. In order to keep the functions monotonous and increasing, the derivatives of
the functions should always be positive. The monotonic increase of the scaling functions is
satisfied with
))(()()max(
),)(()min()(
jjhjjhj
jljjjjl cccx
ccxc
-=-
-=-
+
-
a
a
(6)
if the coefficients -
j
a
and +
j
a
are both in the range ú
û
ù
ê
ë
é3,
3
1. Corrections are done by changing
the borders of the core area, the borders of the support area or the centre point. The local
linearity requirement is taken into account, if possible. [10]
3.2 Intelligent indices
A condition index can be based on several features, which are all scaled to the range
[
]
2,2-:
()
[
]
),()max( )(1
1
1
1
a
a
ta
kk
mn
nk kk
p
k
n
kkC FfwMfwI -
+
+=
-
=åå += (7)
where k
w is the coefficient and 1-
k
f the scaling function of the feature k. Features include
maximum norms
(
)
k
p
M
a
t
max and other features )(
a
k
F, e.g. bins of the histograms.
Features can have specific frequency ranges. Index (7) is obtained from the features of the
signal )(
a
x, but an index can also combine the features of different physical signals. The
condition index )(
a
C
I is a number between -2 and 2: high values correspond to good
conditions, and -2 means not allowable conditions.
The index (7) can also represent a stress index )(
a
S
I, where high values correspond to high
stress conditions, and -2 means very low stress. High-stress operating conditions should be
avoided in order to keep the process in good condition.
3.3 Model-based case detection
Linguistic equations (LE) models provide a flexible environment for fault diagnosis
applications, software sensors, risk analysis and detection of sensor failures [16, 17]. Only
linear equations are needed in LE models, since the nonlinear effects are handled with the
scaling functions. Complex systems also use case-based reasoning (CBR), which is a problem
solving paradigm for finding out the solution to a new problem by remembering a previous
similar situation and by reusing information and knowledge of that situation. In structural
point of view, CBR is a cyclical method that stresses reuse of solutions to similar problems,
where solutions are maintained in carefully indexed memory [1].
4Applications
The first level in detection of operating conditions is to find out if the process is in normal
operation. Deviation from the good operation can be detected by condition and stress indices.
In complex systems with several faults, specialised case models are needed to identify the
fault cases. Degrees of membership for the activated cases can be used for estimating some
process or product quality features.
4.1 Water turbine
The new scaling function improves the sensitivity of the cavitation index for short periods of
cavitation (Fig. 2). The previous methods provided three levels: (1) cavitation-free, (2) short
periods of cavitation and (3) cavitation. The new scaling approach divides level 2 into two
levels (Tab. 1). These results are consistent with the vibration severity criteria, which
originate from VDI 2056 [18, 19]. The cavitation-free situations and strong cavitation are
detected with the relative )max( 75.2
4
3M. The new index provides a clear indication when
the short term cavitation becomes stronger. The levels presented in Table 1 can also be
defined with fuzzy membership functions. [10]
Figure 2. The cavitation index for a Kaplan water turbine [10].
Table 1. Cavitation index and vibration severity criteria [10].
Cavitation index Cavitation level Severity
1
)4( -<
C
ICavitation-free Good
01 )4( <£- C
IShort periods of weak cavitation Usable
10 )4( <£ C
IShort periods of cavitation Still acceptable
1
)4( ³
C
ICavitation Not acceptable
In the simulation test [15], power control minimises the cavitation risk by dividing the load
between three turbines, whose conditions are normal, bad and very good. Each turbine has
three operating modes: low, normal and high power. In the normal area, a cavitation free
power level is taken as an operating point. The low and high operating areas are defined by
local minima of the cavitation indices. The control system has a feedforward controller, which
allocates the load to the turbines by means of cavitation indices, and a feedback controller,
which is based on the LE approach. Each turbine has a P type LE controller which is adapted
to the operating conditions by the scaling functions. The cumulative time in the strong
cavitation provides an indication of possible damage to be used in selecting the turbine for
low power operation. The characteristic curves are adapted to the recent indices in order to
handle the changes in the condition of the turbines. For power stations with many turbines,
alternatives to reduce cavitation risks are evaluated by simulation to optimise maintenance
actions.
4.2 Lime kiln
The scaled norms ))(max( 1
1
4
151 Mf - and ))(max( 25.4
25.4
4
151 Mf - are highly sensitive to
faulty situations in the supporting rolls (Fig. 3). Surface damage and alignment problems are
clearly detected and in the present system also identified with two norms of different order
obtained from the signal )4(
x. An early indication of friction increase is also achieved. The
data set covers the following cases: (1) surface problems, (2) good conditions after grinding,
(3) misalignment, (4) stronger misalignment, (5) very good conditions after repair work, and
(6) very good conditions one year later. Maintenance was done for one of the supporting rolls.
All the rolls can be analysed using the same features throughout the data set.
The overall condition index )4(
C
I can be defined as an average of the scaled features. All the
very good cases are close to the lower corner
[
]
2,2
-
-
:.1
)4( -<
C
I The index is below zero
for the good cases and also for cases with small fluctuations. These cases are clearly usable.
For still acceptable cases, the index is below one. All the faults and clear friction cases are in
the area where ,1
)4( >
C
I i.e. the severity level is ‘not acceptable’. The upper corner
[
]
2,2 is
related to the case where temperature was reduced by cooling. The very high signal levels are
caused by noise, but the hidden faults can be detected by stopping the cooling for a moment.
Figure 3. Scaled norms, p = 1 and 4.25, to different condition of the points 1-16: surface
damage (t), misalignment (), friction (n), very good (Ï), good (), small fluctuations
(È), and special cases (Ø) [10].
4.3 Experimental systems
Machine condition monitoring presented in [20] was based on models developed for normal
operation and nine fault cases including rotor unbalance, bent shaft, misalignment, and
bearing faults. Five different rotation speeds were used, and measurements were taken with
seven separate accelerometers simultaneously in a test rig. Two sensors measured axial
vibration and five accelerometers radial vibration in vertical direction. The classification
results of the experimental cases were very good and logical in [20]. Also a special artificial
immune system (AIS) algorithm, the AbNET, provided comparable results [21].
A large number of equations and features are not necessarily needed. The large number of
features was reduced by genetic algorithms in [22]. The number of variables and sensors
required to diagnose each fault can be reduced considerably by selecting the most sensitive
features [23]. Misalignment and bearing faults were detected by the peak value )3(
p
x obtained
from two sensors (Fig. 4). More features are needed to identify the faults. In this case, three
sensors are needed from seven, and totally seven features are used to calculate eight health
indices. Good conditions and all six fault types can be identified by seven rules, and the
strength of misalignment (Fig. 4(a)) and unbalance [23] can be calculated. The results are
further improved by using the rotation speed is used in the fault models.
(a) Misalignment (b) Bearing faults
Figure 4. Health indices (SOL) calculated for misalignment and bearing faults in a test rig: the
minimum-maximum variation of the index values is caused by the rotation speed [23].
4.4 Continuous brewing
A model-based system has been developed detecting for monitoring and diagnostics of
fermentation and flavour formation in immobilized yeast fermentation process has been
presented in [2]. Some fluctuations of flavour ingredients were detected for only short time
periods as the process was stable (Fig. 5). Time span of these fluctuations was usually too
short for development of specialised models. A more detailed model with seven equations for
all the prereactor measurements indicates clear differences in the end of the test period. By
creating grounds for prediction of quality factors the models increase options to control the
product quality in different cases.
Figure 5. Degree of membership for the normal model of continuous brewing [2].
4.5 Web break sensitivity
The web break sensitivity stands for possibility of web to break, predicting the amount of
breaks during one day from the measurements collected from the paper making process before
the actual paper machine. The paper making process is typically nonlinear with many and
long delays, numerous process feedbacks at several levels, several closed control loops,
interactions between physical and chemical factors, and various unmeasurable factors. Delays
and interactions change in time and with process conditions. [24]
The case-based reasoning (CBR) is needed in the identification of different operating
situations in paper machines. Because similar break sensitivities can result from a multitude
of dissimilar cases, the case base uses a division into categories which correspond to different
levels of break sensitivity. The system is based on LE models and fuzzy logic. Each equation
provides a new fact with a degree of membership, and the resulting set of facts is used in the
fuzzy reasoning of process cases and break categories. The break category is defined by the
case with the highest degree of membership. Although, the case base is fairly small the results
from the on-line tests were relatively good compared to real break sensitivity. The predicted
break sensitivity is an indirect measurement, which provides an early indication of process
changes. The list of variables in the active cases can be used to avoid harmful operating
conditions. [1]
4.6 Wastewater treatment
Wastewater treatment within Finnish pulp and paper industry is most commonly done in an
activated sludge plant, which is a complex biological process, where several physical,
chemical, and microbiological mechanisms simultaneously affect purification results. Limits
of the emissions are defined by authorities. A lot of process measurements are available, but
measurements do not include sufficient information on special features of the influent nor on
microbial composition of the sludge. Populations of microorganisms are highly important, e.g.
sludge bulking cause especially poor treatment efficiency results when biosludge escapes
from secondary clarification.
Figure 6. Effect of the operating conditions on the treatment result of an active sludge
treatment plant: reduction of chemical oxygen demand (COD) and diluted sludge volume
index (DSVI) [11].
Biological water treatment depends strongly on changes in inlet water quality. Load and
nutrient should be balanced since both an exceptionally high load and excess nutrients cause
problems. The operating conditions are modified by oxygen, temperature and flow. Much
slower changes in biological state influence drastically on the purification result and
subsequent process phases. The nonlinear scaling approach presented above were used in [11]
for the variables obtained from process measurements and laboratory analysis. Interpolation
was needed for some variables. The balance of the load and the nutrients was evaluated by the
difference of the corresponding scaled values. The worst case with low reduction and settling
problems arise, when there multiple warnings and alarms (Fig. 6). Correspondingly, good
reduction and very good settling was achieved when there were very few warnings. As it
takes some time to lose good conditions and also recover from problematic conditions, the
intelligent indices are useful for process control.
5Conclusions
Early detection of fluctuations in operating conditions and fault detection can both be based
on generalised norms and moments. Signal processing is needed for the condition monitoring
measurements, and interpolation for some process measurements. The new scaling approach,
which also uses the norms and moments, improves sensitivity to small fluctuations. LE
models of the normal case suit for detecting fluctuations, and CBR is used if specific case
models can be developed. The overall procedure includes following steps: (1) select
informative features, (2) scale the features, (3) calculate intelligent indices, and (4) combine
indices in models. As the analysis is based the same methodology in all these applications,
monitoring of the machines can be combined with process data. Smooth operation and high
quality of products is the main goal of all these applications, and this can be achieved by
combining these indicators with process control in the same way as it has been done for
smaller indicators used in lime kiln control and water treatment.
References
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Complex Nonlinear Systems’, Proceedings of the 17th World Congress of the
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