Abstract:
The invention discloses a method for building adaptive soft sensor. The method comprises the following steps. The input and schedule vectors are constructed, and a novel learning algorithm that uses online subtractive clustering is used to recursively update the structure and parameters of a local model network. Three rules are proposed for updating centers and local model coefficients of existing clusters, for generating new clusters and new models as well as for merging existing clusters and their corresponding models. Once verified, the online inferential model can be created to generate the predicted value of process. Thus, it does not need much memory space to process the method and can be easily applied to any other machine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for building an adaptive soft sensor, and more particularly to a method for building an adaptive soft sensor, which uses a recursive regression algorithm to predict output values of an industrial process. 
       BACKGROUND OF THE INVENTION 
       [0002]    A soft sensor is an inferential model that uses software techniques to estimate the value of a process quality variable using process sensor measurements. Inferential model may be built by using various data-driven methods such as: principle component regression (PCR), partial least squares (PLS), artificial neural network (ANN), support vector machines (SVM), neuro-fuzzy systems (NFS), etc. 
         [0003]    It is well known that all those inferential models are built offline by training data which collected from processes. In certain situations, those training data cannot cover the entire input domains. If new situations arise, the building process has to be reactivated. Techniques such as recursive PCR/PLS, adaptive PCR based on moving windows, exponential weighted PLS, and time-lagged PCR, may be used to overcome this problem. However, in all these adaptive approaches, a window of recent operating data must be memorized, and past experiences are lost as the process evolves and the model is updated. 
         [0004]    U.S. Pat. No. 6,546,379 discloses a cascade boosting method for boosting predictive models for resolving the interpretability problem of previous boosting methods and mitigating the fragmentation problem when applied to decision trees. However, this method may cause coarse transitions from the large number of models. These coarse transitions may reduce the accuracy of the overall predictive model and may also cause confusion for the users of the overall predictive models. 
         [0005]    U.S. Pat. No. 7,505,949 discloses a process model error correction method by integrating two models to generate the desired values of the plurality of the output parameters. This method is disadvantageous in that: it needs a lot of memory to save the data from process; and the online computational burden is large enough. 
         [0006]    U.S. Pat. No. 6,243,696 discloses an online model to generate the predicted value for transferring to the control system. However, this algorithm also needs a historical database. On the other hand, it needs to check whether the variables&#39; names are still valid when the system is running. If not, it is necessary to rewrite the various variables to the database. 
         [0007]    In summarization of the foregoing description, the methods as described in the above-mentioned patents either require relatively large temporary storage space, or cannot be referenced from the required parameters in previous manufacturing processes, or cannot be used to effectively combine predicted data and actual data. Hence, there is an urgent need for a method capable of predicting output values of industrial processes and being referenced from previous and next output values by regression analysis. 
       SUMMARY OF THE INVENTION 
       [0008]    A method for building an adaptive soft sensor is disclosed. In this method, an online learning algorithm for developing a novel soft sensor based on local model network (LMN) is proposed. A subtractive cluster is adopted to determine the number of clusters required, and the parameters of the local models were identified recursively. Only the center, covariance matrix, and parameters of the local model need to be stored as knowledge in the model. 
         [0009]    In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a method for building an adaptive soft sensor, which solves the problem that the previous and next output values cannot be effectively referenced and used when output values are generated from the measurement results from industrial and manufacturing processes. 
         [0010]    According to an object of the present invention, there is provided a method for building an adaptive soft sensor. The method for building an adaptive soft sensor comprises the following steps: operating an online subtractive cluster algorithm to divide online data into a plurality of local regions according to a schedule vector, using input vectors to construct an inferential model according to the local regions, presenting three rules to update existing clusters, add new clusters, or merge two nearest clusters, operating a recursive regression algorithm to update local models corresponding to the local regions, and creating the inferential model by weighting the local models. 
         [0011]    The schedule vector includes a sub set, wherein the sub set is constructed by read and normalized sampled-variables or sampled-variables of a system. 
         [0012]    The recursive regression algorithm is a recursive least square algorithm. 
         [0013]    The selected local model is updated by the recursive least square algorithm using the input vector. 
         [0014]    The center, covariance matrix, and parameters of the local model are stored as knowledge in the inferential model. 
         [0015]    The type of local model is linear. 
         [0016]    The weight of the local model is determined by the distance between the center of the local region and the schedule vector. 
         [0017]    As described above, the method for building an adaptive soft sensor according to the present invention may have one or more of the following advantages: 
         [0018]    (1) This method for building an adaptive soft sensor features a recursive regression algorithm to update the predicted model with reference to previous output values or the latest output values of a process, thereby further improving the accuracy of predicting the outputs and reducing the number of online computations of an operating system. 
         [0019]    (2) In accordance with the method for building an adaptive soft sensor, the most significant parameters in a process can be stored by adding, updating and merging clusters without consuming much memory to establish all the parameters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates a flow chart of a method for building an adaptive soft sensor according to the present invention; 
           [0021]      FIG. 2  illustrates a detailed flow chart of a method for building an adaptive soft sensor according to the present invention; 
           [0022]      FIG. 3  illustrates a schematic view of predicted outputs of a process of a method for building an adaptive soft sensor according to the present invention; 
           [0023]      FIG. 4  illustrates a flow chart of one embodiment of a method for building an adaptive soft sensor according to the present invention; 
           [0024]      FIG. 5  illustrates a diagram of changes in the flow rates in an embodiment of a method for building an adaptive soft sensor according to the present invention; 
           [0025]      FIG. 6  illustrates a diagram of changes in the temperature profile in an embodiment of a method for building an adaptive soft sensor according to the present invention; 
           [0026]      FIG. 7  illustrates a diagram of changes in the number of clusters in an embodiment of a method for building an adaptive soft sensor according to the present invention; 
           [0027]      FIG. 8  illustrates a diagram of changes in the number of clusters in an embodiment of a method for building an adaptive soft sensor according to the present invention; 
           [0028]      FIG. 9  illustrates a plot of real IPB impurity versus estimated IPB impurity in the simulation example of a method for building an adaptive soft sensor according to the present invention; and 
           [0029]      FIG. 10  illustrates a plot of real IPB impurity versus estimated IPB impurity at temperature sampling rates in an embodiment of a method for building an adaptive soft sensor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Referring to  FIG. 1 , there is illustrated a flow chart of a method for building an adaptive soft sensor according to the present invention. In  FIG. 1 , a method for building an adaptive soft sensor  1  comprises the following steps:
       (S 11 ) operating an online subtractive cluster algorithm to divide online data into a plurality of local regions according to a schedule vector, using an input vector to construct an inferential model according to the local regions;   (S 12 ) converting the online subtractive cluster algorithm into a recursive regression algorithm;   (S 13 ) updating parts of existing clusters, adding new clusters, or merging two nearest clusters according to a density function of the schedule vector based on the online subtractive cluster algorithm;   (S 14 ) operating the recursive regression algorithm to update the inferential model into a plurality of local linear models according to the local regions; and   (S 15 ) obtaining a predicted output value by weighting the local linear models to create the inferential model.       
 
         [0036]    Referring to  FIG. 2 , there is illustrated a detailed flowchart of a method for building an adaptive soft sensor according to the present invention. A distributed control system (DCS)  210  collects various sampled-variables in a system or system variables such as temperature, flow rates, etc., as online data that form input vectors to an inferential model. The runtime preprocess  220  performs various preprocessing algorithms. This data is referred to as the raw data. The data may reflect characteristics of input parameters and output parameters. Once the data records are obtained, the runtime preprocess  220  may remove approximately identical data records and/or remove the data records that out of a reasonable range in order to be meaningful for model generation and optimization. 
         [0037]    Referring to  FIG. 3 , there is illustrated a schematic view of predicted outputs of a process of a method for building an adaptive soft sensor according to the present invention. In  FIG. 3 , the input vectors  302  are represented by X in the formulas and the schedule vectors  304  are represented by φ in the formulas, which are sub sets of the input vectors and are composed of the preprocessed data  260 . Turning to  FIG. 2 , the online subtractive cluster algorithm  230  is converted into a recursive regression algorithm to calculate the density of the current schedule vector  304 . 
         [0000]    
       
         
           
             
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         [0038]    where k is a time interval, γ(κ)=φ(κ) T φ(κ), 
         [0000]      η(κ)Σ j=1   κ−1 φ( j ) T =η( k− 1)+φ( k− 1) T ,
 
         [0000]      σ(κ)=Σ j=1   κ−1 φ( j ) T φ( j )=σ( k− 1)+φ( k− 1) T φ( k− 1),
 
         [0039]    Three rules are proposed to classify the current schedule vector φ, respectively: 
         [0040]    i) Rule I—Adding a New Cluster 
         [0041]    If there is sufficient distance between the new datum and all existing centers, and the potential of the new datum is higher than the potential of the existing centers, then the new datum is accepted as a new center. 
         [0000]    
       
         
           
             
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         [0042]    This means that the current point has a more powerful description than other existing cluster center and a new operating regime or new condition appears. 
         [0043]    ii) Rule II—Updating the Cluster Center 
         [0044]    If the new datum is sufficiently close to the old center of the i th  cluster, but has a higher potential than this old center, the old center is replaced by the new datum. 
         [0000]    
       
         
           
             
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         [0045]    Rule II means that the current point has a more powerful description than its old center, which indicates that the operating regime of the system has changed adaptively. Using a new cluster to denote this change could result in a lot of clusters, so the current datum should be used to replace this cluster center. 
         [0046]    iii) Rule III—Merging Two Nearest Clusters 
         [0047]    If two cluster centers are closer than a threshold, they will be merged as follows: 
         [0000]    
       
         
           
             
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         [0048]    This means that the two nearest clusters should be merged into a new cluster. Using this strategy, the redundant cluster can be eliminated online and the cluster configuration can be maintained fairly compactly. 
         [0049]    The local linear model identification  280  is provided with a standard recursive least square (RLS) algorithm to identify the local model. If those cluster centers are not merged, the local model can be updated by the input and output vectors corresponding to schedule vector which are classified by online subtractive cluster. 
         [0050]    Or else, two local models belong to the two nearest clusters are merged into one model. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0051]    Turning to  FIG. 3 , the scheduling vector  304  is the subset of input vector  302 . The type of local model  306  is linear and is responsible for a certain region of the input vector space. The ρ of the scheduling function  308  is a local validated function and may be defined as: 
         [0000]    
       
         
           
             
               
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         [0000]    where c i  is the center of the i th  cluster. The weight value of local linear model may be calculated as: 
         [0000]    
       
         
           
             
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         [0052]    The predicted output value ŷ(k) can be estimated by weighting local linear models  310 : 
         [0000]    
       
         
           
             
               
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         [0053]    In order to describe the algorithm in detail, an industrial o-xylene distillation column is taken as an example. 
         [0054]    Referring to  FIG. 4 , there is illustrated a flow chart of one embodiment of a method for building an adaptive soft sensor according to the present invention. Also referring to the following table, there is sequentially shown the composition of feed, distillate and bottom in a nominal operating condition. Distillate and bottom flow rates are 805.4 kmol/h and 113.66 kmol/h, respectively. The diameter of the tray-column is 3 m. The isopropylbenzene (IPB) impurity in the distillate must not exceed 0.5 mol % to prevent the poisoning of the catalyst located in the next unit. In addition, the measurement also suffers from operating perturbations within the column, which result in an uncertain indication of the average quality. To improve IPB control quality, real-time estimation of the IPB impurity is required. As illustrated in  FIG. 4 , a number of temperature sensors TT1-TT7 were sequentially installed at stages  1 ,  13 ,  23 ,  31 ,  44 ,  55  and  56  in the column to monitor overhead IPB impurity. The distillate flow rate was used to control the temperature differences between tray number  23  and tray number  1 . In the event that the online gas chromatographs (GC) detect an IPB increase in the top product, the operator would manually increase the heat duty to reduce the impurity of the top product. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Feed 
                 Distillate 
                 Bottom 
               
               
                   
                 Components 
                 (mol %) 
                 (mol %) 
                 (mol %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 n-Nonane (Q 1 ) 
                 0.07 
                 0.17 
                 9.16 × 10 −5   
               
               
                   
                 p-Xylene (Q 2 ) 
                 0.06 
                 0.14 
                 1.02 × 10 −5   
               
               
                   
                 m-Xylene (Q 3 ) 
                 2.1 
                 0.51 
                 5.42 × 10 −5   
               
               
                   
                 o-Xylene (Q 4 ) 
                 41.34 
                 98.91 
                 0.55 
               
               
                   
                 Isopropylbenzene (Q 5 ) 
                 0.63 
                 0.25 
                 0.90 
               
               
                   
                 n-Propylbenzene (Q 6 ) 
                 1.69 
                 5.87 × 10 −3   
                 2.88 
               
               
                   
                 1-Methyl-3-ethylbenzene 
                 8.69 
                 9.34 × 10 −3   
                 14.84 
               
               
                   
                 (Q 7 ) 
                   
                   
                   
               
               
                   
                 1-Methyl-4-ethylbenzene 
                 3.92 
                 1.51 × 10 −3   
                 6.70 
               
               
                   
                 (Q 8 ) 
                   
                   
                   
               
               
                   
                 1,3,5-Mesitylene (Q 9 ) 
                 6.44 
                 5.59 × 10 −4   
                 11.00 
               
               
                   
                 1,2,4-Mesitylene (Q 10 ) 
                 21.62 
                 5.62 × 10 −5   
                 36.94 
               
               
                   
                 1,2,3-Mesitylene (Q 11 ) 
                 7.61 
                 0.00 
                 13.00 
               
               
                   
                 1,2,4,5-Tetramethyl- 
                 1.37 
                 0.00 
                 2.34 
               
               
                   
                 benzene (Q 12 ) 
                   
                   
                   
               
               
                   
                 Naphthalene (Q 13 ) 
                 0.80 
                 0.00 
                 1.37 
               
               
                   
                 1-Methylnaphthalene 
                 5.32 
                 0.00 
                 9.09 
               
               
                   
                 (Q 14 ) 
                   
                   
                   
               
               
                   
                 n-Decane (Q 15 ) 
                 0.22 
                 1.02 × 10 −4   
                 0.38 
               
               
                   
                   
               
             
          
         
       
     
         [0055]    IPB impurity dynamics are simulated using the industrial standard dynamic simulator, ChemCad. The Soave Redlich Kwong (SRK) equation of state is used as the thermodynamic model. The feed, product, and column specifications are the same as those in an industrial case. 
         [0056]    Referring to  FIG. 5 , there is illustrated a diagram of changes in the flow rates in an embodiment of a method for building an adaptive soft sensor according to the present invention. In  FIG. 5 , the flow rate disturbances of four components are shown as Q 4 , Q 7 , Q 10  and Q 11 , respectively. 
         [0057]    Referring to  FIG. 6 , there is illustrated a diagram of changes in the temperature profile in an embodiment of a method for building an adaptive soft sensor according to the present invention. The flow rate Q 4  of o-xylene and the flow rate Q 11  of 1, 2, 3-mesitylene change randomly from −10% to +10% of the nominal steady state values (i.e., Q 4  is located in the interval [7671.76, 9376.59] and Q 11  is located in the interval [1598.83, 1954.13]). The flow rate Q 7  of 1-methyl-1-3-ethylbenzene and the flow rate Q 10  of 1, 2, 4-mesitylene were given as a step change at different time intervals, and the flow rates of the other eleven components were maintained in the steady state. Using a fixed step size T=1 min, 6000 sets of input/output data were collected for the simulation. 
         [0058]    The temperature measurements from the top to the bottom of the distillation column are labeled as x 1 , x 2 , x 3 , x 4 , x 5 , x 6 , x 7 , respectively. Since the IPB impurity was measured by an online GC every 5 min in the actual industrial column, the local inferential model can be represented as: 
         [0059]    where Y(k)=−ln y(k) is the logarithm of the IPB impurity. The input vector is represented as: 
         [0000]        X ( k )=[1 , x   1 (5 t− 1), . . .  x   7 (5 t− 10)] T , 
         [0000]    and the scheduling vector is represented as: 
         [0000]      φ( k )=[ x   1 (5 t− 1), . . .  x   7 (5 t− 10)] T  
 
         [0060]    The subtractive cluster parameters were set as r 1 =r 2 =0.21 and r 3 =0.105, respectively, and the width of the basis function S i  was set to 0.3. Moreover, the input vectors, scheduling vectors, and output vectors were is normalized to have zero mean and near unit variances. To calculate errors, the logarithm of the IPB impurity was converted into an actual mole fraction of IPB. 
         [0061]    Referring to  FIG. 7 , there is illustrated a diagram of changes in the number of clusters in an embodiment of a method for building an adaptive soft sensor according to the present invention. From  FIG. 7 , it can be seen that the step changes in the flow rate of 1-methyl-3-ethylbenzene (Q 7 ) and 1, 2, 4-mesitylene (Q 10 ) can often cause changes in the number or centers of online clusters. During the simulation period, the cluster centers moved 47 times, the final number of clusters (number of local models) increased to 5, and one cluster merging event occurred in the 541st sampling event. 
         [0062]    Referring to  FIG. 8 , there is illustrated a diagram of changes in the number of clusters in an embodiment of a method for building an adaptive soft sensor according to the present invention. In  FIG. 8 , at the time interval k=541, the No. 3 cluster can be merged into the No. 1 cluster and disappear forever.  FIG. 8  shows that the proposed algorithm can indeed closely track the dynamics of IPB impurities even with strange nonlinearities in the distillation column. 
         [0063]    Moreover, the present illustrative example employs an evaluation index to determine the prediction accuracy, wherein the evaluation index adopts the mean square error of prediction (MSEP) and average relative error of prediction (AREP). The formula is shown as follows: 
         [0000]    
       
         
           
             
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         [0064]    As mentioned above, we developed this adaptive soft sensor model according to the sampling rates of IPB impurities (every 5 min). To test the performance of this model, the intermediate samples can be predicted using the proposed algorithm. 
         [0065]    Referring to  FIG. 9 , there is illustrated a plot of real IPB impurity versus estimated IPB impurity in the simulation example of a method for building an adaptive soft sensor according to the present invention. The comparison between the real and estimated IPB impurities is provided as shown in  FIG. 9 . 
         [0066]    Referring to  FIG. 10 , there is illustrated a plot of real IPB impurity versus estimated IPB impurity at temperature sampling rates in an embodiment of a method for building an adaptive soft sensor according to the present invention.  FIG. 10  ( a ) shows that the model can track the IPB impurity dynamics with 1-min resolution. Moreover, the MSEP was 16.9×10 −7 , slightly higher than the result shown in  FIG. 9 . The detailed result for the sample interval [3301, 3400] is shown in  FIG. 10  ( b ). 
         [0067]    The method for building an adaptive soft sensor can eliminate the recording of redundant data by adding, updating and merging clusters. Thus, it does not need much memory space to process the method and can be more flexibly applied to various industrial processes. 
         [0068]    The above description is illustrative only and is not to be considered limiting. Various modifications or changes can be made without departing from the spirit and scope of the invention. All such equivalent modifications and changes shall be included within the scope of the appended claims.