Abstract:
A self organizing learning Petri net modeling a system using a large number of training samples performs consecutive trainings using the training samples with a pre-known output value with respect to an input, and when following training samples are applied to a first system parameter created by pre-tested training samples, begins to create the system according to a method of creating a distinct system parameter when an error between an output value of the system and a pre-known output value of the following training samples is larger than a critical value, and adding the new system parameter to a pre-organized first system parameter. A final system parameter is determined by consecutively learning the large number of the training samples in an organized system again. Through this self organizing process, system modeling can be performed more accurately, and a learning process much faster than a back-propagation learning process using a unified CPN and LPN in a general neural network can be achieved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Patent Application No. 2002-5749, filed Jan. 31, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a Petri network, and more particularly, to a learning Petri net capable of modeling a system using a large number of training samples.  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, soft computing models, such as neural networks, wavelet networks, fuzzy systems, Bayesian classifiers, and fuzzy partitions, are widely used in various fields. Although started on a different basis, these soft computing models are gradually being recognized that they are similar and inter-connectable to each other. For example, a RBF (Radial Basis Function Network) and a class of a fuzzy system are regarded as functionally identical. Having a functional equivalence, a WRBF (Weighted Radial Basis Function) network unifying a neural network, a wavelet network, a fuzzy system, and other traditional methods, such as linear controllers or fuzzy and hard clusters, is provided (Leonardo M. Reyneri, “Unification of neural and wavelet networks and fuzzy systems,” IEEE trans. on neural networks, Vol. 10, No. 4, pp. 801-814, 1999). The WRBF network suggested by Leonardo M. Reyneri has an ability to merge all technologies of the neural network, the wavelet network, the fuzzy system, and other traditional methods within the same system together with unified controlled and uncontrolled training algorithms.  
           [0006]    Meanwhile, Carl Petri of Germany suggested Petri nets capable of modeling various conditions in 1960. A Petri net includes two types of nodes, which are places and transitions and connected to other types of nodes through an arc. A transition is called a function generating an output signal in response to an input signal, and a place is a space storing particular input/output signals. A place is marked with a token and acquires a meaning through a location mark and a firing rule. The firing rule is that transfer is possible only when all input plates in the transition have at least one token and the transition can be fired when the transfer becomes possible. When the transition is fired, each input place of the transition loses the token, and each output place of the transition earns another token. In other words, the tokens in the Petri net can transfer through fired transitions. It means that tokens can be transferred via a particular route instead of all network routes and the Petri net has a capability of a distribution function.  
           [0007]    However, until recently, the Petri net has been regarded merely as a graphical and mathematical modeling tool applicable to a domain completely different from a soft computing area. That is because the Petri net is hardly found in a case where it is applied to modeling and controlling of a nonlinear dynamic system and has no learning ability unlike a neural network.  
           [0008]    In order to solve the above problem, a new style Petri net having a learning ability, called a learning Petri net (LPN), is suggested recently (Kotaro Hirasawa, et al., “Learning Petri network and its application to nonlinear system control,” IEEE Trans. Syst. Man Cyber, Vol. 28, No. 6, pp. 781-789, 1998).  
           [0009]    [0009]FIG. 1 is a diagram showing a basic learning structure of a conventional LPN. Each transition excluding an input transition in FIG. 1 has a predetermined number of input and output places. Each different transition is regarded as not having the input and output places at the same time for simplicity. Although FIG. 1 shows a limited number of transitions and places in practice, more transitions and places may be connected with each other in rows or series in a larger variety of forms.  
           [0010]    In order for the LPN to have a characteristic of a dynamic system, a firing signal is introduced to the LPN. The firing signal is processed in a fired transition and propagates in a network through a token transfer. When the firing signal is assigned in the input transition together with a value of an input signal, and an output signal is earned as the firing signal of the output transition, the LPN can be used to realize an input/output mapping. Meanwhile, input arcs connecting the input places with the transition have weights, h ij . The weights constrained to be positive integers in a general Petri net are extended to be continuously adjustable in order to obtain the learning ability. Output arcs connecting the transitions with the output places have time delays, D jk , which are zero or a positive integer at a sampling time.  
           [0011]    In the LPN, the input/output mapping is realized by the firing signal propagating from the input transition to the output transition through the token transfer. The LPN and the Petri net are discrete in a way that the tokens transfer the firing signals in the LPN. The tokens in the LPN are generated in a transition and provided with a value of the firing signal of the transition until the tokens are extinguished in the place. The value of the firing signal of the transition can be defined as the following equation 1.  
                   h        (       T   j     ,   t     )       =     f        (     ς   j     )              
            ς   j     =         ∑   i            h        (       P   ij     ,   t     )            h   ij         +     β   j           )           EQUATION                 1                               
 
           [0012]    where h ij  is a firing weight on an arc between a place P ij  and a transition T j , i is an index of a place connected to an input side of the transition T j , β j  is a threshold value for the transition T j , ƒ(ζ j ) is a nonlinear function, and h(P ij ,t) is a value of the firing signal of the place at time t defined by a sum of values of the firing signal transferred by tokens in the place. When the place is empty, h(P j ,t) is given a value of zero.  
           [0013]    Although the LPN can have an ability to learn and realize the neural network, it is different to the neural network as it has a capability of a distribution function and at the same time parameters of the LPN are determined by experiences of an expert as in normal neural networks.  
           [0014]    However, there was a problem that the output value may be more or less inaccurate because a number of connections of all transitions and places between an input layer and an output layer are predetermined by the experiences of a user in the conventional LPN. That is, in [the] a real application of the LPN, a problem may occur since a whole system needs to be modified in relation to a degree of an error of an output value as an accuracy of the output value lacks when a structure suitable for the application does not exist, although an accurate output may be expected when it does.  
         SUMMARY OF THE INVENTION  
         [0015]    An object of the present invention for solving the above and other problems is to provide a self organizing learning Petri net capable of obtaining most suitable parameters of the net from input samples and improving a capability of a fast learning speed and a more accurate modeling.  
           [0016]    Additional objects and advantageous of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
           [0017]    A self organizing learning Petri net (SOLPN) to achieve the above and other objects includes training a large number of training samples with a pre-known output value in relation to an input, comparing an error between an output value created by applying following training samples to a first system parameter produced by a pre-tested training sample and a pre-known output value of the following training samples with a critical value, creating a separate system parameter when the error is bigger than the critical value, self organizing an initial system by creating a new other system by adding the separate system parameter to the first system parameter, and determining a final system parameter through consecutive learning of the large number of the training samples in an initial system established based on an initial system parameter.  
           [0018]    The self organizing of the initial system parameter includes determining the first system parameter through a first training sample among the large number of the training samples, applying a second training sample to a first system organized through the first system parameter, comparing a second error produced as a result of applying the second training sample to the first system with a second critical value, and creating a second system parameter organizing a second system when the second error is larger than the second critical value after comparison. When the second error is smaller than the second critical value after comparison, the self organizing learning Petri net further includes applying the following training sample to the first system. When the second error is larger than the second critical value after comparison, it further includes applying the following training sample to the second system. The initial system parameter is determined by repeating consecutive training of the large number of the training samples a predetermined number of times.  
           [0019]    The determining of the final system parameter includes consecutively applying the large number of the training samples to the initial system organized by the initial system parameter, comparing each basic error created in the large number of the training samples with each basic critical value, and amending the final system parameter determined by preceding training samples when the basic error is larger than the basic critical value after comparison, and the large number of consecutive training is repeated until the final system is stabilized.  
           [0020]    Since the self organizing learning Petri net according to the present invention performs the learning not through the system learning by a user&#39;s experience but through the system created by the sample training, a learning process becomes faster and more accurate modeling can be achieved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    These and other objects and advantageous of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:  
         [0022]    [0022]FIG. 1 is a diagram of a conventional basic LPN structure;  
         [0023]    [0023]FIG. 2 is a diagram of a 2-input self organizing learning Petri net (SOLPN) structure according to an embodiment of the present invention;  
         [0024]    [0024]FIG. 3 is a flowchart describing a self organizing process of the SOLPN shown in FIG. 2; and  
         [0025]    [0025]FIG. 4 is a flowchart describing a learning process of the SOLPN shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described in order to explain the present invention by referring to the figures.  
         [0027]    In describing the present invention, cited reference papers are listed at the end of the “Description of the preferred embodiment” with numbers. Each number is marked in round brackets, ( ), and, hereinbelow, descriptions of some technologies will be replaced by providing the number of the relevant reference paper.  
         [0028]    [0028]FIG. 2 is a diagram of a 2-input self organizing learning Petri net (SOLPN) structure according to an embodiment of the present invention. The SOLPN has an input layer, a fuzzy rules matching layer, and an output layer.  
         [0029]    The input layer has two input transitions and two input places.  
         [0030]    The fuzzy rules matching layer may have a number of transitions and a number of places. In the fuzzy rules matching layer, signals propagate based on the following Equation 2.  
               h        (       T   j     ,   t     )       =     exp   (     -         ∑     i   =   1       n   1              (       h        (       P     i   ,   j       ,   t     )       -     h   ij       )     2         σ   j   2         )             EQUATION                 2                               
 
         [0031]    where h ij  is a firing weight on an arc between a place P ij  and a transition T j , i is an index of a place connected to an input side of the transition T j , and h(P ij ,t) is a value of a firing signal of the place at time t defined by a sum of values of the firing signal transferred by tokens in the place. When the place is empty, h(P ij ,t) is given a value of zero. σ is a first system parameter. Exp( ) is an exponential function. In practical application, h(T j ,t) can be any kind of suitable nonlinear functions, and it is not limited just to exponential function.  
         [0032]    The output layer has a single transition and a single place to obtain a single output. In the output layer, signals propagate based on the following Equation 3.  
               h        (       T   j     ,   t     )       =         ∑     i   =   1       n   1              h        (       P     i   ,   j       ,   t     )       ·     h   ij             ∑     i   =   1       n   1            h        (       P     i   ,   j       ,   t     )                   EQUATION                 3                               
 
         [0033]    [0033]FIG. 3 is a flowchart describing a self organizing process of the SOLPN shown in FIG. 2. In order to establish a system, first of all, a first training sample of a sample set of which an input value and an output value are already known is consecutively trained in operation S 310 . That is, after establishing a first system parameter through the first training sample in operation S 320 , a second training sample of the sample set is trained in the system established by the first system parameter in operation S 330 . And then an error between an output value of the system according to the second training sample and the already-known output value of the second training sample is produced and compared with a critical value. If the error is smaller than the critical value, a third training sample of the sample set is consecutively trained in the system. On the other hand, if the error is bigger than the critical value, a second system parameter is produced according to the second training sample in operation S 350 .  
         [0034]    For the following training samples consecutively input to the system, a new system parameter is produced according to the error between the output value of the system according to the second training sample or the following training samples and the already-known output value of the second training sample or the following training samples which are continuously trained in the predetermined system parameter. Finally, after detecting a training completion status of a last sample in operation S 360 , the self organizing process is completed by repeating the consecutive training of the training samples a predetermined number of times in operation S 370 . The following training samples are applied to the new system parameter created by a combination of a pre-system parameter established by a pre-tested training sample, a following system parameter, and a previous system parameter.  
         [0035]    [0035]FIG. 4 is a flowchart describing a learning process (i.e. an optimization phase) of the system self organized through the above process. First of all, the first sample in the system self organized through the self organizing process shown in FIG. 3 is trained in operation S 410 . The error between the output value of the system as a training result and the output value of the already-known training sample is obtained and compared to the critical value in operation S 420 . If the error is smaller than the critical value, the second sample is trained. On the other hand, if the error is bigger than the critical value, the system is amended through back propagation learning in operation S 430 . The following samples are trained in the amended system in operation S 440 . Through the above described processes, completion of a last sample training is detected in operation S 440 , and training of the sample sets is consecutively repeated until the system is stabilized in operation S 450 . When the system is stabilized by the training samples, the learning by the training samples is ended in operation S 460 .  
         [0036]    Hereinafter, the above self organizing and learning processes are re-described referring to the cited papers.  
         [0037]    The present invention utilizes a self-organizing counter propagation network (SOCPN) equation shown in reference paper ( 15 ) in order to establish a system. In a simple case, an improved unsupervised SOCPN (IUSOCPN) employing a flexible fuzzy division is used for sampling information from training data shown in reference papers ( 4 ), ( 15 ). The IUSOCPN may be used as an example of a system self organizing phase of the system.  
         [0038]    A system self organizing process based on the above IUSOCPN is as the following:  
         [0039]    At first, as a first operation, a winner unit, J, having a minimum distance D between the space and the transition in the fuzzy rules matching layer in relation to a current input, {overscore (x)}, is determined by Equation 4.  
               D        (           H   _     j          (   t   )       ,     x   _       )       =       min       j   =   1     ,   …              ,   M                         d   j          (         H   _          (   t   )       ,     x   _       )                 EQUATION                 4                               
 
         [0040]    where {overscore (H)} j (t)=(h 1j (t), . . . ,h Qj (t)), Q is a dimension of the current input {overscore (x)}, and d(•,•) is a metric distance.  
         [0041]    In equation 4, d(•,•) can be defined as [the]  a  Euclidean distance as in equation 5 below:  
                 d   Euc   j          (       x   _     ,       H   _     j       )       =       [       ∑     i   =   1     Q            (       h   ij     -     x   i       )     2       ]       1   /   2               EQUATION                 5                               
 
         [0042]    In a second phase, a winner is determined using the following rules:  
         [0043]    If D({overscore (H)} J (t),{overscore (x)})≦δ, then the unit J is the winner. If D({overscore (H)} J (t),{overscore (x)})&gt;δ, then create a new unit. δ is a predefined system parameter.  
         [0044]    In a third phase, if J is the winner according to the above rules, a parameter equal to equation 6 is maintained.  
             {                        n   j          (   t   )       =           n   J          (     t   -   1     )       +     1                     α   J          (   t   )           =     1       n   J          (   t   )                                    δ   J     =       δ   J     +   η                                  H   _     J          (   t   )       =           H   _     J          (     t   -   1     )       +         α   J          (   t   )            [       x   _     -         H   _     J          (     t   -   1     )         ]                                  p        (   t   )       =     P        (     t   -   1     )                                z   J     =   1                     EQUATION                 6                               
 
         [0045]    If the new unit is created, the parameter is amended as equation 7 below:  
             {                      p        (   t   )       =     P        (     t   -   1     )                                    H     _     P          (   t   )       =     x   _                            δ   P     =   δ                              n   P          (   t   )       =   1                            z   P     =   1                     EQUATION                 7                               
 
         [0046]    0&lt;a J (t)&lt;1 is a gain sequence decreasing together with time, and η is a constant expansion rate.  
         [0047]    In a fourth phase, once the fuzzy rules matching layer is stabilized through the processes above, {overscore (M)} J  becomes fixed, and an output layer starts to learn a desired output y s  for each fixed weight vector by adjusting a connection weight h j  from a J th fuzzy rule unit to an output unit. An update formula at the output layer is as Equation 8 below: 
           h   j ( t )= h   j ( t −1)+β[ y−h   j ( t −1)] z   j   EQUATION 8 
         [0048]    An update rate β is a constant within a range between 0 and 1, and y is a corresponding pre-known output.  
         [0049]    One thing to note is that all K(K&gt;1) number of transitions will be fired at [the] a normal performance phase or the optimization phase of the SOLPN, whereas only one transition is fired in the self organizing process. It means that the number of firing transitions is limited to one in the self organizing phase and the K number of the transitions will be fired in the normal learning performance or the optimization phase. In other words, the number of the firing transitions is different in each phase. In addition, fixing the number of the firing transitions as one makes a calculation in the self organizing phase (process) convenient.  
         [0050]    A general structure of a fuzzy system given by a determination of the number of rules gains an initial parameter of the system through the above described self organizing process. However, the system can be regarded as a rather rough model and is not sufficient to be an accurate modeling. Therefore, in the present invention, a unified learning algorithm for a weighted radial basis function (WRBF) network based on a gradient descendent method is used. A normal supervised gradient descendent method for the WRBF network is shown in reference paper ( 17 ).  
         [0051]    An error function is defined in the following Equation 9:  
               E        (         X   _     ;     C   _       ,     W   _     ,     Θ   _       )       =       1   2            ∑   j            (       y   j     -     t   j       )     2                 EQUATION                 9                               
 
         [0052]    where y j  and t j  are a response of a J th neuron to the input {overscore (x)} and a corresponding target value taken from the training sample set, respectively. Generalized learning rules for a WRBF neuron are as the below equations 10 through 14:  
               δ   j     =         ∂   E       ∂     y   i         =     (       y   j     -     t   j       )               EQUATION                 10                               
 
                     Δ                   w   ji       =       -     η   w       ·     δ   j     ·       ∂   E       ∂     w   ji                       =       -     η   w       ·     δ   j     ·       F   ′          (     z   j     )       ·     [       z   j         n        (     z   j     )       n       ]                     EQUATION                 11                 ΔΘ   j     =       -     η   Θ       ·     δ   j     ·       F   ′          (     z   j     )       ·     [       z   j         n        (     z   j     )       n       ]               EQUATION                 12                 Δ                   c   ji       =     (       η   c     ·     δ   j     ·     F        (     z   j     )       ·     [       z   j         n        (     z   j     )       n       ]     ·               w   ji     ·     [       n   ·       D     n   -   1            (       x   i     -     c   ji       )       ·     sgn        (       x   i     -     c   ji       )       ·     x   i       -     c   ji       )       ]                       EQUATION                 13                 δ                   x   i       =       ∑   j            δ   j     ·       F   ′          (     z   j     )       ·     [       z   j         n        (     z   j     )       n       ]     ·     w   ji     ·     [     n   ·       D     n   -   1            (       x   i     -     c   ji       )       ·     sgn        (       x   i     -     c   ji       )                       EQUATION                 14                               
 
         [0053]    where η w , η c , and η Θ  are the three learning coefficients and z j  is an argument of an active function, F(•). δ j  is a back-propagated learning error at an input normalization layer.  
         [0054]    In the above system optimization phases, the present invention has two differences from the WRBF learning algorithm. A first difference is that not all parameters are optimized in the whole network. This is because only the K number of the transitions is fired according to LPN firing rules. The first difference enables a process time in the learning phase to be reduced. A second difference is that the gradient descendent method is basically slow. However, the parameters have been “intelligently” initialized in the self organizing process having “intelligence.” In order to further optimize a result of the system, another method described in reference paper ( 12 ) is used. The method described in the reference paper ( 12 ) utilizes a concept of decreasing errors and Equation 15. 
         ƒ m+1 =θƒ m +(1−θ) d   m   EQUATION 15 
         [0055]    where d m  is a partial derivative of an error in relation to a weight at an epoch m, and θ is a learning constant.  
         [0056]    A learning rate for the weight, an equation governing [the] a changing value e may be written as below:  
               e   m     =     {             e     m   -   1       +   k               d   m          f   m       &gt;   0                 e     m   -   1       ×                               d   m          f   m       ≤   0                       EQUATION                 16                               
 
         [0057]    where k, and φ are learning constants.  
         [0058]    When e is determined, an actual weight change c m  is amended as following Equation 17: 
           c   m   =−e   m   d   m   EQUATION 17 
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         [0086]    The SOLPN according to the present invention can model a system more accurately and perform learning much faster than the back-propagation learning using a unified CPN and LPN in a general neural network by enabling a system to be self organized through training data.  
         [0087]    Although the preferred embodiments of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiments. Various changes and modifications can be made within the sprit and scope of the present invention as defined by the appended claims and their equivalents.