Adaptive Local Out lier Probability for Dynamic Process Monitoring☆

Yuxin Ma,Hongbo Shi*,MenglingWang

Key Laboratory of Advanced Control and Optimization for Chemical Processes of Ministry of Education,East China University of Science and Technology,Shanghai200237,China

Adaptive Local Out lier Probability for Dynamic Process Monitoring☆

Yuxin Ma,Hongbo Shi*,MenglingWang

Key Laboratory of Advanced Control and Optimization for Chemical Processes of Ministry of Education,East China University of Science and Technology,Shanghai200237,China

A R T I C L E I N F O

Article history:

Received 24 December 2013

Received in revised form 28 January 2014

Accepted 7 February 2014

Available on line 19 June 2014

Time-varying

Complex data distribution

Local outlier probability

Multi-mode

Fault detection

Complex industrial processes often have multiple operating modes and present time-varying behavior.The data in one mode may follow specific Gaussian or non-Gaussian distributions.In this paper,a numerically efficient moving window local outlier probability algorithm is proposed.Its key feature is the capability to handle complex data distributions and in cursive operating condition changes including slow dynamic variations and instant mode shifts.First,a two-step adaption approach is introduced and some designed updating rules are applied to keep the monitoring model up-to-date.Then,a semi-supervised monitoring strategy is developed with an updating switch rule to deal with mode changes.Based on local probability models,the algorithm has a superior ability in detecting faulty conditions and fast adapting to slow variations and new operating modes.Finally,the utility of the proposed method is demonstrated with a numerical example and a non-isothermal continuous stirred tank reactor.

©2014 Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

1.Introduction

In industrial processes,operating conditions are usually affected by some slow variations denoted as time-varying characteristics, caused by some dynamic behavior such as seasonal fluctuation, catalyst deactivation,equipment aging,sensor or process drifting, preventive maintenance and cleaning[1].Generally,effects of the time-varying behavior on the mean and covariance of variables cannot be neglected,so there may be m any false alarm s if conventional multivariate statistical process monitoring(MSPM) methods are applied directly[2].In order to maintain process efficiency for a long period of time,numerous adaptive methods have been developed.Recursive MSPM methods and methods based on the moving window strategy are two alternative widely used approaches[3,4].

Multi modality is another important feature of industrial processes due to changes of market demands,alternations of feedstock or variations of manufacturing strategy.The difference between the characteristics of nearby operating conditions is always significant, so in tensive studies have been carried out with either multiple local models or a single global model[5,6].While it is more practical to accommodate the time-varying behavior and multi mode features together.The developed methods can be divided in to two categories. One is the adaptive clustering methods.Teppo la et al.[7]applied adaptive fuzzy C-means algorithm s on the score values of principle component analysis(PCA)to monitor a wastewater treatment plant.Liu[8]used an adaptive Takagi-Sugeno fuzzy model on PCA subspace to model a large scale nonlinear system containing many operating regions.Since PCA is used as a p reprocessing too l,monitoring results of these two methods more or less depend on and be restricted by the capability of PCA.Petković et al.[9]designed an on-line adaptive clustering method utilizing a generalized in formation potential.Although previously unseen functioning modes can be included by introducing an adaptive expert system,the method suffers from a non negligible detection delay.The other category is adaptive statistical methods.Improved recursive algorithms based on recursive PCA or the signed digraph were proposed with some if-then rules to distinguish process condition changes from disturbances[10-12].Ge and Song[13]introduced the just-in-time-learning strategy to the modeling procedure of local least squares support vector regression and the residuals between the real output and the predicted one was analyzed by a two-step in formation extraction strategy.Xie and Shi[14]and Yu[15]developed two different dynamic fashions of Gaussian mixture model(GMM)separately based on the moving window strategy and a particle filter resampling method.

The problem of complex data distributions in time-varying and multi mode processes has scarcely been addressed.Although the moving window strategy has been proven to be effective,it still encounters some limitations when incorporated with statistical methods such asPCA,partial least squares(PLS)or GMM.Since the variables of an industrial process may satisfy specified Gaussian or non-Gaussian distributions,and high order statistics are usually helpful to reveal more in formation from the data[16-18],adaptive monitoring algorithm s should be developed,which can explore both Gaussianity and non-Gaussianity of process data.Local outlier probability(LoOP)is an unsupervised data mining technique proposed for outlier detection[19].It combines the idea of local,density-based outlier scoring with a probabilistic,statistically-oriented approach,and assigns the probability of being an outlier to all data records.Since a normalization procedure is included,LoOP is independent of any specific data distribution. Therefore,a combination of LoOP and moving window strategy should be potential to tackle these problems.

The main contribution of this paper is to propose a numerically efficient moving window LoOP algorithm for monitoring industrial processes with complex data distributions,time-varying property and multiple operating modes.Some designed rules are introduced and incorporated with a two-step adaption approach to ensure that the monitoring model can be updated at a high speed.To cope with the multi mode features,a semi-supervised monitoring strategy is employed,and an update termination rule is developed to prevent the monitoring model contaminated by faults or disturbances. Since the method is based on local probabilistic models,the accuracy of model is higher and it will be much easier to detect faulty conditions.

2.Adaptive Process Monitoring Based on Moving Window Loop

For low computation burden and practical applications,it is fast and reasonable to only update the in formation of those samples whose neighbors have changed due to the insertion and discard of samples. The key problems addressed in this section are how to find the affected samples and how to update their in formation.

2.1.Offline initialization

To make an initialization and calculate the LoOP value for eachsample χj(j=1,2,…,L)with dimension D in the initial window W1, its k nearest neighbors are found as follows,with its neighborhood set in W1can be recognized as knn1(χj).

Assuming that samples in knn1(χj)are centered aroundχj,then we can define probabilistic set distance as:

whereλis a weighted factor usually taken as2.For estimating the density aroundχj,the probabilistic local outlier factor(PLOF)is defined as follows with function E(.)used to com pu te the expectation of PLOF in the current window.

Finally,by applying the Gaussian error function,the local outlier probability indicating the probability that a sample is an outlier can be calculated as:

where erf(.)is the Gaussian error function applied to obtain a probabilistic value.

2.2.On line updating and process monitoring

By applying the moving window strategy,a two-step adaption procedure is introduced to update the monitoring model.Some more details of the adaption procedure for a window size L are as follows.

Step 1:discard

The effect of eliminating the oldest sampleχifrom the previous window Wion the mean and variance can be evaluated as follows.

where diag(.)is the function used to calculate the diagonal matrix. Eq.(6)describes the updating of the variable mean while Eqs.(7)-(9)describe the updating of the variable variance.

After moving all the information a bout χifrom the current monitoring model,a se(i＞1)is constructed to store the samples,in which χiis one of their k nearest neighbors.

where knni(χj)represents the neighborhood set of sampleχjin window Wi.Obviously,if,due to the deletion ofχi,the neighborhood set knni(χj)will change. Step 2:insertion

When a new sampleχi+Lis judged normal and added in to the data matrix,the updated m ean vector and variance in Wi+1are computed as follows.

To achieve normalization,the aggregate value nPLOF1which can be considered as a standard deviation of PLOF values is obtained:

Eq.(11)describes the updating of the mean vector while Eqs.(12)-(14)describe the updating of the variance.However, only for those with new sampleχi+Lam ong their k nearestneighbors,their neighborhood set knni(χj)will be updated to knni+1(χj).Therefore,the setcan be augmented to:

where p≠j andχj∈Wi+1.Since the PLOF of a sample will change not only with the change of its neighbors but also with the change of its probabilistic set distance,two sets˜Si−1and Si−1are constructed as:

where only the samples in Si−1need to update their PLOF values. By incorporating the updated probabilistic set distance,the PLOF and local outlier probability for the new data window Wi+1are computed as:

Since LoOP assumes no specific data distribution,it will not be proper to apply 0.95 or 0.99 as the confidence limit for judging an outlier as that employed in GMM or other probabilistic methods.For those algorithms without assuming Gaussian distribution,kernel density estimation(KDE)is an effective method widely used in estimating control limits[20].However,it will be time consuming to run the KDE algorithm every time after model updating.Obviously,if the local outlier probability of a sample is zero,it must be a normal one and will have negligible effect on estimating the control limit in the current window. However,the monitoring model must be updated every time when a normal sample is inserted,because new normal samples are appropriate representations of current states of the monitored industrial process and they can always bring useful information to guarantee the accuracy of the monitoring model.Therefore,samples with LoOP value 0 will be used only to update the monitoring model but the control limit will not be reevaluated.

To cope with the multi-mode problem,model updating is operated through a semi-supervised switch strategy.If am ode change is previously known to occur,an alternative approach will be enabled to fit the transient stage and the new mode by blindly accepting every new sample as a norm alone.However,there should be a termination rule to make the monitoring scheme switch back to its former state for fault detection as soon as possible to prevent the model from adapting to faulty conditions.During this transition,if the local outlier probability of anew sample is0,there must be enough data to construct an accurate local model in the new cluster.Therefore,when the LoOP value of a new sample is0,it is reasonable to say that the model is ready for monitoring the new operating m ode and the period of blind updating can be terminated.

2.3.Methodology

The flow diagram of the proposed moving window local outlier probability(MW LoOP)monitoring scheme is shown in Fig.1,with the detailed approach as follows.

The offline modeling steps are summarized below:

(1)Collect L samples from the current operating condition to construct the initial window W1.

(2)Based on the standardized samples,an offline model is built according to Eqs.(1)-(5).

(3)Specify a confidence level(1−α)%and app ly KDE to estimate a control limit for the LoOP values in the initial window W1.

By introducing the switch rule,the on line semi-supervised monitoring steps are summarized below.

(1)For a new sampleχi+L,standardization is first done by using the meanμiand varianceΣiof the window Wi,where i＞1.

(2)Calculate the distance fromχi+Lto samplesχi+1,χi+2,…,and χi+L−1in the window Wiand compute LoOPi(χi+L).

(3)If am ode change is previously known to occur,the flag value that is initially 0 should be set to 1 by an operator and every new sample is accepted as a normal one.Until the LoOP value of a new sample is 0,the flag will be automatically set back to 0.Go to Step(5).

(4)If LoOPi(χi+L)＞lim it−LoOPi,where limit−LoOPiis the control limit of the window Wi,χi+Lis judged as an outlier,and then the circulation will go for the next new sample.Other wise,it continues to the next step.

(5)The two-step adaption strategy is adopted to update the current model according to Eqs.(6)-(20)by discarding the oldest sample χiand inserting the newest sampleχi+L.

(6)If LoOPi(χi+L)＞0,the control limit is recalculated by KDE.

(7)If several consecutive samples are judged as outliers,an alarm is triggered.

Fig.1.Flow diagram of processm onitoring scheme based on MW LoOP.

3.Case Study

3.1.Numerical eχample

To demonstrate the superiority of the proposed method in dealing with complex data distribution and time-varying behavior,a numerical example is employed,which is similar to that used by Lee et al.[21].

Consider three source variables as follows:

where k is a sampling index and k=1,2,…,2000.

Totally 2000 samples are generated with the following system:

where e=[e1,e2,e3,e4,e5]Tare zero-mean white noises with variance 0.02 and y=[y1,y2,y3,y4,y5]Tare the monitored variables.The first 1-1000 samples are normal ones,while at the 1001st sample,a slow d rift 0.001(k−1000)is added to A(1,2)and A(2,2)to simulate the time-varying behavior that should be adapted by the monitoring methods.Then a step bias ofχ2with magnitude 3 is introduced at the 1501st sample.

In the moving window strategy,choosing a proper window size is a difficult task for compromising computational efficiency and model accuracy.A smaller window size means a lower computation load while a larger window size means higher model accuracy.As for the number of nearest neighbors,a large value will diminish the difference between normal samples and outliers while a small value will lead to inaccurate expression of local density.To verify the effect of the two parameters on Type I error,two tests on the 1-1500 samples are conducted and the results are shown in Fig.2.Fig.2(a)demonstrates the variant tendency of k versus Type I error with a window size of750 while Fig.2(b)shows the variant tendency of window size versus Type Ierror with the number of neighbors of30.By compromising the trade-off between computation speed and model accuracy,the window size is chosen to be 750 and the number of nearest neighbors is 30 through trial and error method.

The moving window PCA(MWPCA)applied in this paper is the algorithm proposed in[22],and the number of principle components is determined by the cumulative percentage variance(≥85%).For all methods applied in this paper,the confidence of control limit is set to be 99%.

A fault occurs due to a step change in the non-Gaussian source variable χ2.As shown in Table 1 and Fig.3,moving window PCA fails to raise an alarm as the fault occurs since it cannot figure out the changes in variables with complex distributions,while conventional LoOP can show an obvious difference between normal and faulty conditions but it makes too m any consecutive false alarm s from the 1300th sample because it cannot hand le the time-varying behaviors.In contrast,the proposed moving window LoOP algorithm has an acceptable Type I error and the best Type II error.The fault is rapidly detected without missing alarms.

3.2.Non-isothermal continuous stirred tank reactor

Fig.2.The effect of k and window size on Type I error.

Table 1Monitoring results of the numerical example

The proposed method is compared to moving window PCA and conventional LoOP by simulating a non-isotherm alcontinuous stirred tank reactor(CSTR).The process is shown in Fig.4.It is a first order reaction, reactant A premixing with a solvent to product B.It should be noted that only the PI control loop for temperature T is active in this simulation. More details about the simulation condition should be referred to[23]. The nine monitored process variables are:T=outlet temperature, C=outlet concentration,FC=cooling water flow rate,T0=inlet temperature,TC=cooling water temperature,CAA=concentration of pure A,CAS=concentration of solvent,FS=solvent flow,and FA=flow rate of constant A.

Fig.3.Monitoring results of the numerical example.

Fig.4.Diagram of CSTR process.

Table 2Monitoring results for the three faults of CSTR

Consider a very slow drift in reaction kinetic constant k0to rep resent the time-varying feature of catalyst deactivation.With simulation time 5000 with 2500 samples are generated.The slow d rift is introduced from t=2000m in as k0=k0initial(1−(t−2000)×10−4),while three kinds of fault are introduced from t=4000 m in:Fault 1:a step bias of cooling water temperature sensor with a magnitude of 1.5 K;Fault 2: a random noise of cooling water temperature that obeys uniform distribution U(−4,4);Fault3:a d rift in the sensor of CAAand its slope is d CAA/ d t=0.001 kmol·m−3·m in−1.

According to the empirical guidance described in Section 3.1 and through trial and error method,the window size is chosen to be 700 and other parameters are the same with those in the numerical sample in Section 3.1.Monitoring results for the three faults are shown in Table 2.Type I errors of conventional LoOP for three faults are as high as 16.20%because it lacks the capability of adapting to time-varying processes.Com pared with moving window PCA,Type I error of MW LoOP is acceptable but it perform s better in Type II error.The reason lies in that with limited data samples,the Gaussian distribution assumptions of PCA cannot be fully satisfied,while the proposed method,which is free of distribution,can achieve a more accurate model.As a result,it is more sensitive to faulty conditions.

Fig.5.Monitoring results for Fault2 of CSTR.

Fault 2 is a random non-Gaussian noise added in Tc.Fig.5 shows that the monitoring results of conventional LoOP start to crash after t=3200 m in because it cannot deal with the time-varying behavior in the CSTR process.The SPE statistic of moving window PCA shows an acceptable result in Type II error,but T2does not work well,because PCA is designed for extracting the Gaussian in formation in to its feature space,and the remaining in formation including non-Gaussian features and disturbances will be separated into SPE.For the proposed method, a more accurate model is built with out assuming any specific distribution of data.It seems that a few normal samples are judged as outliers, but they are d is continuous,so no alarm is triggered.Table 2 shows that Type II error of moving window LoOP is the most satisfactory. Therefore,a conclusion can be d raw n that the proposed method is the most effective one compared to moving window PCA and conventional LoOP.

Nex t,the ability of the proposed monitoring scheme to deal with mode changes is tested through Scenario 1 described below. The whole operation period consists of three stages.In the first stage t=1-2000m in,the process is operated under mode 1,with the outlet temperature T setting at 368.25 K.In the second stage t=2000-4000 m in,the reaction kinetic constant k0changes as k0=k0initial(1−(t−2000)×10−4).Then the set-point of T changes to 370 K from t=4000 m in and after a while the process reaches steady state and run under mode 2.In the last stage t=4000-5000m in,a step bias of concentration of pure A with a magnitude of2 km ol·m−3is introduced at t=4800m in to simulate a faulty condition.

Fig.6 shows the monitoring results of moving window PCA and moving window LoOP for scenario 1.The vertical lines rep resent the stable time of the process judged by the dissimilarity index[10]and the stable time of MW LoOP judged by the proposed update termination rule.The statistic of proposed method becomes stable much faster than T2and SPE,which means that the risk of adapting to faulty conditions will bemuch lower.In terms of the 100 faulty samples,Type II error of the proposed method is 5%,while that of T2is 32%and SPE cannot detect this fault.Thus,the local model built by the proposed method is more accurate than the global model built by moving window PCA.From this point of view,the proposed method is much more practical,since it needs fewer samples to build a local model than a global one.

Fig.6.Monitoring results of Scenario 1.

4.Conclusions

The proposed moving window LoOP methodology offers many peculiarities,among which its fault detection capability,adaptive on line implementation,and utility for multi mode processes without prior know ledge requirement are addressed in this paper.To cope with the time-varying behavior,a two-step adaption approach is introduced to update the monitoring model while some updating rules are designed to reduce the computation load.Forh and ling multi-mode characteristics,a semi-supervised switch strategy is incorporated and an update termination rule is designed to prevent the monitoring model fouled by faulty conditions. Due to the superiority of local probabilistic models,the proposed method can achieve a more accurate model and the monitoring efficiency can be easily maintained.Through a numerical example and a non-isothermal CSTR process,the flexibility and effectiveness of the proposed method are validated compared to moving window PCA and conventional LoOP. Future work will be focused on how to isolate faulty variables and how to tackle strong nonlinear property.

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☆Supported by the National Natural Science Foundation of China(61374140),Shanghai Postdoctoral Sustentation Fund(12R21412600),the Fundamental Research Funds for the Central Universities(WH1214039),and Shanghai Pujiang Program(12PJ1402200).

*Corresponding author.

E-mailaddress:hbshi@ecust.edu.cn(H.Shi).